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Gas-in-place Estimate of the Habiganj Gas Field
Using Material Balance
A thesis is submitted to the Depaltment of Petroleum and Mineral Resources
Engineering in partial fulfillment of the requirements for the degree of
MASTER Of' SCIENCE IN ENGINEERING
m
PETROLEUM AND MINERAL RESOURCES
by
MOHAMMAD SHAHIDUL ALAM
DEPARTMRNT OF PETROLEUM AND MINERAL RESOURCESENGINEERING
DHAK,.\, BANGLADESH
MAY 2002
11111111111111111111 m11111:11l9696~
f"
•
Bangladesh Univer:sity ofEIigineering and Technology
Faculty of Engineering
Recommendation of The Board of Examiners
The undersigned certify that they have read and recommend to the Department ofPetroleum and Mineral Resources Engineering for acceptance, a thesis entitledGAS-IN-PLACE ESTIMATE OF THE HABIGANJ GAS FJELD USINGMATERIAL BALANCE submitted by MOHAMMAD SHAHIDUL ALAM inpartial fulfillment of the requirements for the degree of MASTER OF SCIENCEIN ENGINEERING in PETROLEUM AND MINERAL RESOURCES
Chairman (Supervisor)
Member
Member
Member (External)
Date: May 18, 2002
/~4/DrMohammad TamimProfessor and HeadDepartment of Petroleum andMineral Resources EngineeringBUETDhaka, Bangladesh
~~~Dr dmond GomesProfessorDepartment of Petroleum andMineral Resources EngineeringBUETDhaka, Bangladesh.
df.'f ~-4<,(Mr,uiiq;;ar Ali ~Assistant ProfessorDepartment of Petroleum andMineral Resources EngineeringBUETDhaka, Bangladesh .
.k~Dr, Ijaz HossainProfessor and HeadDepartment of Chemical EngineeringBUETDhaka, Bangladesh.
•
,..•.:" .-"
Dedicated to my parents
Anwara Begum
and
Jamal uddin Ahmed
Whose love, affections and blessings are the main
strength behind every success
••
ABSTRACT
Habiganj gas field is located approximately 75 miles north of Dhaka in cast-central
BMgladcsh. It is perhaps the second largest gas field in Bangladesh after Titas.
Therefore, it is extremely important to estimate the correct gas-in-place of this valuable
reservoir. Thi, field first started producing in 1969. A total of 10 wells have been drilled
till now, For the estimate of the gas-in-place (GIP), many ,tudies have been performed
by different organiz~tions The reserve in the Habiganj gas field was estimaled by
volumetric and material balance method. By the measurement of moving gas-waler-
contact, it was established that the field is under moderate to strong bottom-water drive,
Although in one of the studies, water inllux was considered lor simulation, most of the
matcrial balance calculation inclL1dedno water influx, Only one study estimated the
water influx in material balance calculation Thi~ important term omittcd from the
calculation raise; doubts on the accuracy of these reserve cstimates, Morcover, the
official cstimate of the rescrve was made about len years back The prescnt study
allempts to establish the drive mechanism through correct theoretical approach and thcn
to estimate the gas-in- place of the Habiganj gas field using material balance including
necessary calculation of the water -influx into the reservoir
ACKNOWLEDGEMENTS
T would like to appreciate the support, inspiration and valuable guidancc that were
received from my supervisor Dr. Mohammad Tamim, Professor and Head of the
Department of Petroleum and Mineral Resources Enginecring T also thank him for
assigning me such an important and practical topic on "Fstimate of gas-in-place in the
Habiganj gas field Ui>lllgmaterial balancc" as my research work. 11was a great
expcrience lor me to get an opportunity to work with such a dcdicated and seriousresearcher.
I am grateful to Dr N M. Anisur Rahman, a former faculty member of BUET, under
whose supervIsion I gathered a lot of experiences and understandings of research, His
assistance helped me a lot to proceed inmy present works
I am indebted to our faculty, Professor Dr Edmond Gomes for his valuable lectures and
demonstrations during our class hours that helped me to understand the concepts of
Petroleum Engineering
I also aelrnowledge the necessary assistance, co-operations received time to time from
Mr. Zulfiquar Ali Reza, Assistant Professor ofPMRE,
I would like to thank my employer Padma Oil Company Limited, Chittagong for
nominating me for graduate studies and also granting me the necessary leave of study I
also thank the Bangladesh Petroleum Corporation (RPC), Chittagong for their necessaryco-operation.
I am very much proud of my family members for their extra-academic support without
which my graduate studies might have been impossible. I appreciate the patience, co-
operations from Lina, my newly married wife, during my graduate studies. Finally, I am
indebted to all my friends, colleagues and well wishers for their nece.ssarysupports.
ii
Chapter
ABSTRACT._
,, ,'
TABLE OF CONTENTS
Page number
ACKNOWLEDGEMENT,. ,
TABLE OF CONTENTS
LIST OF TAllLES." "--- --.".", .. ---".,
"_..in
__,VI
LO
LIST OF FIGURES."
NOMENCLATURE., ,
INTRODUCTION._
___ ,.,,'" __ VlI
'--- ---"''''-- ---".,
I
2,0 LITERATURE REVIEW .. ,.. " ,., ..3
2, I
2,2
Study Report ", ...
Literature . ", ... ,__,,3
"''' ... 7
3,0
4,0
OBJECTIVES OF THE STUDY
PROPERTIES AND OAT A ANALYSIS OF TIIE HABIGANJGASFIEW. ,, ", .. ,, ".,
__ ... " .. 10
II
4, I
4.2
Introduction ", ... ,
Reservoir Descriptions .. ,
4.2.1 Upper Gas Sands
4.2,2 Lower Gas Sands__
. " .11
II
____""''' .". "II
__,.,12
4 2.3 Gas-water Contact and Gas Column _,.... ,.,' .. ,.," __.13
4.3
4.4Reservoir fluid Compositions and Properties."
Production and Pressure Data Analysis __... ".
4.4.1 Production Dala History.
__,.15
.19
,.19
4.4 2 Pressure Data History ..... ".,.
iii
__..... 20
50 STUDY OF :MATERIAL BALANCE EQUATION (I\.1BE) ___28
5 1
5 2Introduction.
General Material Balance Equation
5.2.1 Addition ofWaler Influx Term
5 2 2 Conditions for the Application of MBE,.
, " .28
28
31
_" , .31
5,2,3 Necessary Data for the Calculation ofMBE.. 32
5.3 Material Balance for Gas reservoir " _.. , .. ,' _____",,,.32
5.3.1 Application ofHavlena and Odeh Method for MBE of
Gas Reservoir
5.3.2 plZ Interpretation Method._
.33
.. ,,35
6,0 ESTABLISHMENT OF DRIVE MECHANISM IN THEHABIGANJGASFIEW. " ... ",., " .. " .. " ..... .39
6 1
6.2
Introduction._
Techniques for Establishing Drive Mechanism.
6.2,1 Havlena and Odeh Approach. _,
62.2 P!Z Interpretation Technique.,
. ,,39
,.. " ... .39
.40
.. 41
7.0 AQUfFER MODELS FOR WATER INFLUXCALCULATION 45
7.1
72
7.3
7.4
Introduction., ,
Carter- Tracy Water Influx Calculations."
Steady-State ModeL.
Unsteady-State ModeL.
7.41 Edge-WaterDriveModeL
7.4.2 BoundedAquifer" ... ,.,.,
7.4.3 Infinite Aquifer
7.4 4 Bottom-Water Drive ..
45
,.,"", ... ,., ..45
." " ..47
." .47
..48
.. 52
'".... 53
..... ,,,56
7.5 Suitability of Aquifer Model for the Habiganj Gas Field " ..60
..8,0 ESTIMATE OF GAS-TN-PLACE IN THE
HABIGANJ GAS FIELD WITH AQUIFER FITTING,., ____62
8 1 Introduction, ,. __,.. ",.".,., .62
8.2 Aquifer Fitting Using the Havlena and Odeh Method" __62
8,2,1 Calculation and Graphical Representation .. " .. "., ,.63
8.3 Sensitivity ofTnput Parameters for Reserve Calculation .. __" ..... 72
8.4 Recovery Factor (RF),. _" .. " .. ,.75
8.4.1 Calculation ofRF for Volumetric Gas Reservoir __.76
8.4,2 Calculation ofRF for Water-driven Gas Reservoir ,.77
85
8.6
Determination of Drive Indices (DJ) ..._,... ".,', ..
Results and Discussions, ...
,.... ,.,79
.81
8,7 Suggested Production Strategies for the Habiganj Gas Field ..,,85
9.0 CONCLUSION. 87
9.1
9.2
Conclusion." ..
Recommendations
_.87
,__.88
REFERENCES.
APPENDICES. ,
,,89
__.92
•
LIST OF TABLES
Page
Table 4.1 Gross Sands, Depths and Thickness (IKM: 1991)
Table 4.2 Gross Sands, Depths and Thickness (Beicip 2000) ..
..... ",.",,13
... 14
. Table 4 3
Table 404
Comparison of gas compositions by fluid analyses of the upper gas
sand.. . .... " ...
Gas properties calculation of the upper gas sand,.
.., 16
.. 17
Table 4 5 Gas properties calculation of lower gas sand.
Table 4 6 (a) Production history of the Habiganj gas field,
Table 4 6 (b) Production history of the Habiganj gas field.
Table 4.6(e) Yearly average water production ofHabiganj gas field
. ..... " 18
' ... 20
21
22
Table 4.7 Shut-in pressure data of the Habiganj gas field. ., ..... 27
Table 4.8 Selected pressure points of the Habiganj gas field ... 27
Table 6.1 Material balance prediction of the Habiganj gas field using
Havlena and Odeh equation. 40
Table 6.2 Material balance prediction of the Habiganj gas field using p/Z
interpretation technique " ... , ... ,',.. Al
Table 7.1 Values of regressions co-efficients., .46
Table 8.1 Determination of /" values ... 68
Table 8.2 Calculation of water influx values using Allard and Chen method ... 69
Table 8 3 Values for the application offulJ Havlena and Odeh. ..70
Table 8.4 Determination of Drive Indices showing the correctness of
material balance calculation. 80
Table 8.5 Comparison ofGIP and Recovery Factor for the Habiganj
gas field.. ,,, .. ,, ,, , ,....,, ,, .. , 83
LIST OF FIGURES
Page
Figure 4.1 Cumulative gas and water production of the Habiganj gas field ..... 23
Figure 4.2a Yearly average water production of the Habiganj gas field .. 24
Figure 4 2b Yearly average water production of the Hablganj gas field. .25
Figure 5, I Volume changes in the reservoir associated with a finite pressure
drop.
Figure 5.2 Diagnostic gas material balance plot to determine the GIP and to
29
define the drive mechanism .. , ,. " ..34
Figure 5,3 Gas material balance plot for depletion water drive reservoir 36
Figure 5.4 p/Z plots for a water drive gas reservoir ofHabiganjUpper Gas Sand"
Figure 6 I Plot to determine the drive mechanism and apparent GTP
of Habiganj Upper Gas Sand.
" . 3 7
" .. .40
Figure 6,2 p/Z plot for a water drive gas reservoir across the full range of
p/Z (0-3000) .42
Figure 6 3 Enlarged p/Z plot for a water drive reservoir over a reduced range
Figure 6.4
of p/Z(2400-2600) .
p/Z plot for the Habiganj gas field indicating the correctvalue of OTP..
...",,, .. 43
44
Figure 7,2 Circular reservoir inside a circular aquifer.
Figure 7 3 Linear aquifer geometry .
Figure 7.4 Matching a continuous pressure decline at the reservoir
aquifer boundary by series of discrete pressure steps
vii
" .49
..52
.54
•
Figure74 Linear aquifer geometry. ".,54
Figure 7.5 Idealized flow model for bottom-water drive system.. .57
Figure 8, 1 Typical position ofGWCi and GWCn. ., ,65
Figurc 8 2 Estimation ofGIP by Havlena-Odeh plot with aqllifer
fitting".. .71
Figure 8.3a Sensitivity ofGIT estimate due to change in horizontal to vertical
permeability ratio, Fk= 0.1 ... " ... ,.. ",.", .. ,.. ,,73
Figure 8.3b Sensitivity ofGIT estimate due to change in horizontal to vertical
permeability ratio, Fk ~ 0.5. ..... " .. ,73
Figure 8.4a Sensitivity ofGTP estimate due to change in permeability,
k=260mD" ....... 73
Figure 8Ab Sensitivity ofGIT estimate due to change in permeability,
k=100mD. 73
Figure 8.Sa Sensitivity ofGTP estimate due to change in aquifer thickness,
h=300fL ... ",74
Figure 8.Sh Sensitivity ofGTP estimate due to change in permeability,
k = 200 fi" .74
Figure 8.6a Sensitivity ofGTP estimate due to change in pore compressibility,
..... 74
Figure 8.6b Sensitivity ofGIT estimate due to change in pore compressibility
Cj-=20,lxlO.6." ".. .. 74
Figure 8,7 Relative contribution of each drive.
viii
. 81
B,
F
F,fG
h
k
NOMENCLATURE
Gas formation volume factor, fe/sef or bbllsef
Initial gas formation volume factor, ft3/sef or bbLiscf
Oil formation volume factor, bbllstb or fe/stb
Initial oil formation volume factor, bbl/stb or ft' /stb
Water formation volume factor, bbllstb or ftJ/stb
Formation isothermal compressibility, psi'!
Aquifer isothermal compressibility, psf'
Water isothermal compressibility, psi-'
Total aquifer isothermal comp'ressibility, psi-'
Rate of water influx, bbl/d [mJ/d]
Expansion of the connate water and reduction of the porespace, rcf/scf
Underground gas expansion, ref/scf
Total gas and water productions, ref
Vertical to Horizontal permeability
(encroachment angle)"/360"
Gas initially in place, MMscf
Cumulative gas production, MMscf
Aquifer thickness, feel
Reservoir thickness, feet
Permeability, mD
L Length of 1M linear aquifer geometry. feet
m Inilial gas ClIpvolume to the initial oil volume
N Initial reservoir oil, 5th
N Cumulative produced water, sib,~ Critiall pressure, psia
p, Initial pressure, psi
P(ln) Dimensionless CTR solution of the diffusivity equation
P'(t!) Time derivative of P(ID)
Pot Bottom hole 00•••.;08 pressure, psi!
p Average prcuure, psi!.
'" Pressure drop, psi!
H Producing (ill5lantlllleous) gas oil ralio, !.Cf)'slb
", Cumulative gas oil ratio, scOstb
", Solution (or dissolved) gas oil miD, sd7sth
". Initial solution gas oil fIItio, sd7sth
" External boundary l'!dius, feet
,~ Dimensionless extCf1lllIboundary r1Idius, feel.
'. Reservoir radius, feet,,
S. Water saturation, fraction
S_ Initial connate water salul'lI.lion,fmelion
S, Residual gas saturation, fraction
, Time
•
".':'0 •
T
u
w,
w,z
Greek Symbols,B
Dimensionless time
Absolute temperature, K
Critical temperature, K
Aquifer constant, bbllpsi
Net bulk volume ofreservoir, cubic feet
Pore volume, cubic feet
Connate water volume, bbl or cubic feet
Width, feet
Dimensionless cumulative water influx
Cumulative water influx, bbl
Cumulative water produced, bbl
Gas deviation factor or gas compressibility factor, ratio, uniUess
Vertical distance coordinate
Dimensionless vertical distance coordinate
Dimensionless thickness constant
Porosity, fraction
Water viscosity, cp
Contact angle, deb>TeeS
"
•
•
Abbreviations
BOGMC
bbl
brcf
bscf
'PCIDA
CTR
GIP
GGAG
GWCi
GWCn
fIB
HCPV
IKM
lvIJl,fSCF
PVTru
Bangladesh Oil, Gas and Mineral Corporation
barrel
bi Ilion reservoir cubic feet
billion standard cubic feet
centi poise
Canadian International Development Agency
Constant terminal rate
Gas in place, cubic feet
German Geological Advisory Group
Initial gas water contact, feet
New gas water contact, feet
Habiganj well
Hydrocarbon pore volume, cubic feet
Intercomp Kanta Management
Million standard cubic feet
Pressure, Volume, Temperature
Reservoir barrel
Reservoir cubic feet
Standard cubic feet
Standard barrel
TeF
TVD
uos
Trillion cubic feet
Troe vertical depth, feel
Upper gas sand
xiii
•
•
INTRODUCTION
""hapte.:1 - , - """~ •. r;_.'..,.....--~....,~~.IS:'$- .9G=7E?~"""~"•...[ 'I, "" _ ....•1
~ \._~~'2-0/~/M,.:-/~~ ~~'-,--~-"*-4~n-",""'t,'",.•\
The Habiganj Gas field is located approximately 75 miles north of Dhaka in east-
central Bangladesh. Natural gas reserve was discovered in the Habiganj Gas Field by
Pakistan Shell Oil Company with the drilling of the well, Habiganj NO.1 (HB-I) in
1963. This well penetrated the Upper Gas Sand and Lower Gas Sand. Flo", tesls
were conducted in the Upper and Lower Gas sands through 2-7/8 inch tubing to
establish the productive potential and characteristics of each reservoir, The Upper
Gas Sand was found at a depth of approximately 4500 ft SS and contains a lean gas
comprised of97 7 percent methane. According to Well drill Ltd. (1991), 99% ofthe
total GJP is in the Upper Gas Sand The Lower Gas Sand at a depth of 9850 It 5S
was poorly defined by the seismic report and of minor importancc. The Lower Gas
sand is of the nature of offshore bars deposited in a much lo\ver energy regime than
the Upper Gas Sands. Only two wells (HB-l and deviated HB-S) have pcnetrated
thesc sands, So, the prcsent study mainly focuses only on the Upper Gas Sand,
which is the major sand body ofthe Habiganj gas field. Pakistan Shell Oil Company
drilled a second well to appraise the Upper Gas Sand reserves in 1963. The bottom
hole location of the well, HE. 2 was only 90 feet from HB-l and therefore produced
very little additional information regarding the areal extent of the reservoir Both
IID.1 and HE-2 were left as su~pended wells until final completion operations were
undertaken in 1967, First production from the Habiganj gas field occurred from
these wells in March 1969
Two development wells were drilled in 1984 under a program financed by the
French government. HE-3 was drilled as an appraisal well into the Upper Gas Sand
approximately 3000 feet southeast of the HB-1/HB-2Iocation, Habiganj No-4 was
also drilled as an Upper Gas Sand appraisal well approximately 4300 feet east-
southeast of the HI3-1/HB-2 location.
HB-S was drilled in 1989 as a Lower Gas Sand appraisal well under an Asian
Development Bank project. This well is located approximately 4900 feet south-
,5OUl.heastortlle HB-3 location. HB.S WllSdesigned as a deviated well to encounter
the Upper ~ 5R11d at its Cl-estand the Lower Gas sand. The well, Habiganj No-
6.\\'IS drilled in 1989 under the gas field AppraiSlll project. ~ well is located
approximated 6500 feet soUlh.-sOUlheMtof tile IID-S surface location, along the
strike of the: structures.
118-7 WllSdrilled in \998 lISII.Upper Gas Sand well, HB-8 was drilled in 1998 and
Habigllnj 9 and 10 were drilled in 1999 liSa Upper Gas Sand well. The Upper Gas
Sand is penetrated by all ten wells and rcpn:sc:nts a significant accumulation of gas
reaching a gross thickness of 750 fed above a gu-walcr contact al 4875 feet 55.
Different study groups published their reports on reserve estimates of the H.II.bi8llnj
gas field. Principle studies among them are, IKM (1991). Well drill Limited (1991),
Beicip Franlab-RSCIPetrobangla (2000) and Hydrocarbon UnitINPD (2001), [t is
very much. important to estimate the proper GIP becal,ue it will help the eeonomy of
the country by fOrecllsting the correct remllining reserves or the glls field. On the
basis or this estimllted remllining reserve. the government ClUI take necessary
program ror meeting up the present IllId future demands or the COUntry.finding out
any potentilllity or lIltenlllte use or S!s.
Chapter 2
LITERATURE REVIEW
This chapter has rcviewed the previous study report conducted hy different study
groups and also available literature related to the material balance study of gas
reservoIr.
2.1 Study Report
Several studies have been conducted for different purposes at different development
stages of Habiganj Gas Field. Usually, after a discovery the operating company or
agency estimates the reserve of their discovery. This practice is an on going process.
No estimate of reserves was made by Shell after pool discovery (1963) because of
doubts concerning porosity pinchout to the south due to facies change and about the
termination of the pay sands to the southeast against a fault. On the basis of
additional seismic data in 1963, Shell made the initial estimate of reserves at I.75
TeF for the Upper Gas Sands. Subsequent estimates of reserves were made by
Petroconsultant GmbH (1979), BOGMC (1982) and by the German Geological
Advisory Group (1984). Petroconsultant GmbH estimated the reserve at 3.457 TCF
(maximum) including 3 0201CF in the Upper Gas Sand At 50% probability total
reserve was at 1.28 TCF. In 1982, prior to drilhng of two development wells
Petrobangla re-estimated the reserve and it was 1.257 TCF under proven and
probable category Another 1.045 rCF was assigned to possible category GflP was
2.302 rCF. All these estimates were based on two closely spaced wells. On the basis
of the data from the single-fold seismic grid of Shell and the wells HB-1fHB-2 and
f{B-3 data, the recoverable reserve was estimated by GGAG as 1.437 TCF
DeGolyer and MacNaughton estimated the in-place reserve as 1.704 TCF (proved
plus probable) In 1986, after drilling of two additional wells GGAG and
Petrobangla re-estimated the reserve ofHabiganj and this time GIP was 3.298 TCF
including 2.522 rCF iu the Upper Gas Saud.
•In the same year Hydrocarbon Habitat Study (HHSP. 1986) also studied the reserve
of tile gas fields. According to them the IOlal rt!$Crveof llabigllnj was under proven
category for the Upper GIlS Sand and prollllble category for the Lowei'"Gas Sand.
Proven reserve was 2.677 TCF. Probable Wlel'VC10 the Lower $lind WlS 0.308 Tel<'
only. In 1989 Gllsunie estimated the recoverable reserve ofHllbiganj al 2,60 TCF.
[ntercomp-kt!n!1l Management (IKM, March 1991) retai~ by the ClllllldilUl
IntcrntltiOllll1 Development Agency (CIDA) evaluated six field initially lind later
lotai eight gas fields in Bangladesh for the Gas Field Appmi5ll1 project, Project
Implementation Unil of Bangladesh Oil Gas and Minerals Corpol'ftlion (BOGMC)
fields This report covered the HlI.biganj Field, the fifth of the eight fields under
appraisal (Appendix AI). The emphllsis of IKM study was on lite Upper Gas Sand.
These are the best pay SlIndSof the eight fields !itudy program and all si" Habiganj
wells pcnettllted the Upper GIlS SllI1d,A well-defined W1I.tertable 8tthe base of the
gas column WlISindicated in all the seismie dip lin~ III 1400 1IU= 2-w-!. On the
other hand,. wire line logs of the different wells indicated gaS-wtltcr contaa at
differenl depths within a T1lrJgeof about 20 ft. In spite of localized irregularities of
the GWC due to lithology, it was obvious that a single WIlier table wtlS associated
with the base of the Upper GM Sllllds. After a careful review of all data, tile WIlter
table had been chosen at 4875 ft sub$Cllas the best compromise to define the base of
the pool. The IKM report mentioned that the reservoir fluid of the Hllbiganj
reservoirs WlISnon-retrograde ftI reservoir temperature. The gas sands contain a dry
gas of relatively uniform composition. IKM in their study estimated the reserve of
the Habiganj at 3.669 TCF ofwtlich 3.53 TCF WMfor the Upper Gas Slllld based on
the volumetric reserve estimate. They mentioned recovery from the Upper Gas Sand
in the Habiganj Gas field would be governed by the wtltcr drive mechanism from an
aquifer of effective size ten (10) times tMt of the gas reservoir. They considered the
ultimate. recovery faaor for the Habiganj GM field in between 45 to 52 percent of
the proved and probable reserves. Ifrecovery factor of 5 1% is taken, the recoverable
gas reserve of the Upper sand stllllds becomes 1,85 I TCF. lKM also carried out the
most simplified zero-dimensional material halance calculation for volumetric gas
reservoir, which was e"pressed in the form of a linear equation in terms of prl
versus GI'" It indicated Illl initial reserve of 10.5 TCF. This reserve estimate WllS
,appro:o;imllcly) limes that estimated using the volumetric approach ba~ on the
scismic isopach. They recommended IMI additional pressure surveys were required
to fefint the definition of the WIlIer drive mechanism lind provide additionaL
information on which to conduct material balance veriliu.tion of proved reserves.
Another study of Well drill Limited (November, 1991) after IKM study indicated
thai about 99"10 of the Jlabiganj field gas reserve WIl$in the Upper GllSSand (top
about 4120 ft 55) and it WllSdry gas. with romulll.tivc Ilvcrnge condensate
produaion of 0,45 barrclslMMscf. The cumulative gas produaion up 10 3et June1991, III from the Upper Gas Swtd, was slightly over 200 ber. The Upper Sand i~
Habignnj represents the single best reservoir in Bangladesh, being II relatively
continuous mcked beach and bllmer bar sand sequences, reaching over 750 feet in
thickness and extending seven by three miles aerially al the GWC. They estimated
lMt the Upper Gas Sand contained at least 3.6 TCF of GIP. The lowe:-sand GlP was
e:uimaled at 80 bcf. On the ba~is of pl"C8sureversus production reserve estimate,
Well drill Ltd, also indicaled lhat the aquifer was active and would affect the
reservoir performance significantly. This Upper Sand reservoir was fairly well
defined wilh well log, core and test information and seismic control available. The
lower sand (lOp about 9425 ft. S5) WllSpoorly defined wilh only two wells
penetmting it, and the seismic coverage was wCBkand OIIllIkedby the ma5sive Upper
Oas Sand, The lower $tIJ\dWllSslightly richer ;n condensate with about 1 barrel per
MMscf. The Upper Sand reserves were large enough 10suppon lhe additional wells
recommended by IKM, The lower sand needs further definition and a produdion
outlet. l1ley also mentioned ;n their study thaI the recovcl1lblegas reserve estimate
for the Upper Sand must be based on the volumetric G1P and e:uimated recovery
efficiency. since the MB data is greatly affeCled by the underlying waler leg. It is
surprising to see that after clearly identifying the drive mechanism and the position
of the aquifer, no mempt WllSmade to estimate the GlP from material balancc. The
HavleOlland Odeh (1968) method as well as identifying the denCCl;onpoint from
plZ plot are well established in literatures. Using the more uncertain volumetric
estimate of GIP, they had to use lin artificial high water influx I1lteto match the
history. As a result, the simulation model predicted early water break through and
,lower recovery efficiency. More reCent studies identified the mistake in 1KM (1991)
study and adjusted the values from new volumetric and performance data.
Bcicip Franlab-RSOPetrobangla (2000) mentioned that the Habiganj Upper Sands
Reservoir contains extremely high (97.74%) methane-nched gas of practically nil
propane and heavier components, The reservoir temperature at datum about] 16nF,
the compressibility of the Upper Sands is an immense 508.7,10,6 I/psi, 10,8%
higher than the gas of the 8akhrabad D Lower and 25.7% higher than the Sangu
reservoir gas, They estimated the gas-in-place of Habiganj at 4 623 TeF from log,
core and other test data of 10 wells. Compared to fKM study, this study had
additional information from 4 more newer wells Using cumulative production and
remaining gas left in the zone not invaded by water, the micro recovery factor was
estimated at 736%. They calculated the proven reserve to be 3.236 TCF using a
70% recovery factor.
Hydrocarbon Unit and Norwegian Petroleum Directorate (HCUlNPD, 2001)
reviewed Habiganj data, drafted a new depth contour map on top of Upper Gas Sand
and re-estimated the volumetric reserve. Havlena-Odeh method (H-O) indicated a
GIP of about 5 TCF and also indicated water drive. The plot of plZ vs, Gp indicated
the GIl' at 5.1 TeF. However volumetric analysis indicated a lower figure of 4,69
TCF. For this study Havlena and Odeh method (1968) and material balance analysis
were considered, They used modified van Everdingen and Hurst model for water-
influx calculation For the Lower Gas Sand, lKM's estimate (386 bel) was
considered for the purpose of this report as probable GIl', Total in-place reserve of
Habiganj was estimated to 5 14 TCF, This gave recoverable reserve of 3.85 reFwith a recovery factor of75%.
,2.2 Literature
The review of available literature can be classified as foHows:
2.2.1 Material Balance Calculation
Schilthuis (1935) presented a paper where he derived an equation relating the
quantities of oil, gas and water produced, the reservoir pressure decline auending the
production, the quantity of water that may have encroached into the reservoir
Where sLlfficient and proper data on production, reservoir-pressure behavior and the
properties of the oil and gas are at hand, it is believed that the methods outlined
herein permit the calculation of approximate quantity of oil contained in the
interconnected and permeable parts of a reserVOIr, This material balance equation
has long been regarded as one of the basic tools of reservoir engineers for
interpreting and predicting reservoir performance.
By arranging algebraically, Havlena and Odeh (1968) used the matenal balance
equation for reservoir engineering, which results an equation of a straight line
intersecting the y-axis. The ~traight-line method requires the plotting of a variable
group versus another variable group, with the variable selection depending 011the
mechanism of production under whieh the reservoir is producing. The most
important aspect of this method of solution is that it attaches significance to the
sequence of the plotted points, the direction in which they plOI, and to the shape of
the resulting plot Thus, a dynamic meaning has been introduced into the material
balance calculation in arriving at the final answer
2.2.2 Water Influx Calculation
van Everdingen and Hurst (1949) presented an unsteady-state model of edge-water
drive for the waler influx (W,) calculation which is applicable for large aquifers. The
transient nature of such aquifers suggests that a time-dependent term be included in
the calculations for water influx (W,). This model is based on the radial diffusivity
equation written without a term describing vertical flow from the aquifer.
,Carler and Tracy (1960) presented an equation for water influx calculation at
constant tenninal rale (CTR) solution of the diffusivity equation, This method is
simple and accurate in application and has been coded into several commercial
numerical simulators. Here, the Schihhuis form of the material balance has been
realTsnged into a more useable form. The water influx calculation is simplistic and
does no! consider the size or position of the aquifer.
To account for the flow of water in a vertical direction, Coats (l962} presented a
paper where there was a development and solution of a mathematical model for
aquifer water movements in bottom water drive reservoirs. Pressure gradients in the
vertical direction due to water flow were taken into account. A vertical permeability
equal to a fraction of the horizontal penneability is also included in the model. The
solution was given in the form of a dimensionless pressure drop quantity tabulated
as a function of dimensionless time. This quantity can be used in given equations to
compute reservoir pressure from a known water-influx rate, 10 predict water-influx
rate (or cumulative amount) from a reservoir-pressure schedule or to predict gas
reservoir pressure and pore-volume performance from a given gas-in-plaee schedule
The modcl was applied in example problems to ga..~-storage reservoirs, and the
difference between reservoir performances predicted by the thick sand model of this
paper and the horizontal, radial-flow model was sho1Nl1to be appreciable. The two
major limitations of this model are (1) the given solution applies to the ''terminal-
rate" case, which allows the user to calculate pressure from a known influx rate
rather than the reverse, and (2) the solution is applicable only to infinite aquifers.
Allard and Chen (1988) presented a paper on a new water influx model that differs
from traditional approaches in that it includes the effect of vertical flow at the
reservoir/aquifer interface. The results were presented in the fonn of dimensionless
groups, which makes the model readily applicable to a wide range of systems. This
model mainly overcomes the two principle limitatiOnS of Coats (1962) that were
mentioned earlier. This bottom-water model is a solution to the ''terminal pressure"
case and can be applied to both finite and infinite models They showed that the
"significant errors incurred using radial flow moctei for the bottom-water drive
system.
Olarcwaju (1989) presented a new water influx model that includes edge water
drive, bottom water drive or a combination of both, The 'solutions were presented
graphically in form of dimensionless groups, which make the solutions applicable to
a wide range of systems. Using the classical superposition technique an example
application of the water influx solL1tionto date from a water-drive oil reservoir was
presented.
Walsh (1999) observed that different form of material balance equation has different
level of error tolerance of static reservoir pressures. Some of them are least affected
by presSLIre error, yet others arc so sensitive that those require virtually error"n-ee
measurement. The study only considered equations for volumetric oil reservoirs A
similar study eould be done for gas reservoirs and water driven reservoirs,
The inherent non-uniqueness of aquifer fitting was addressed by Vega and
Wattenbarger (2000), To circumvent a prior knowledge of aquifer properties and
geometry, they suggested a method that did not require any assumption of aquifer
properties and geometry, Using a dynamic iterative method where a normalized
absolute error term was minimi~ed to get the optimum OGIP, ThIS method still
required determining water influx rates using materia! balance and interpolation to
estimate presrure between two measurement time, Although the non-unique aquifer
fitting problem has been addressed, the possibility of not finding a clear minimum
error term was not eliminated. This method can be valuable to support or vahdate
the results found ITomthe regression techniques
"Chllpffr J
OBJECTIVES OF Tim STUDY
3.1Objectives of1he stud)'
The objectives orille study are lIS follows:
• To eslablish the drive mechanism in tile Upper Gas Snnd of the HlI.biganj gas
field using theoreticalllpproaeh.
• To estimate IIdulll gas-in-place in the Upper Gas Sand of the Hllbigonj gas field
using the .e.pproprilllC material balance model lind WIler inf1ul( ctlicullltioll.
3.2 MC'thodo!og)'
The following methodologies are adopted in this study:
• The Hllvlena lind Odeh method and p1Z interpretation techniques an: applied for
the establishment of drive mechanism.
• The suitable aquifer modeillvailllble in the literatures for calculating water inl1uK
would be applied if existence of water drive were identified. The models
considered would be Caner-Tilley WlIter influl< ealculations (1960), VIIn
Everdingen and Hurst model (1949), Coats model (1962), Allard and Chen
model (1988).
• The full equlliion ofHavlena and Odell (1968) would be applied to estimate tile
GIP ofHllbiganj gIlS field
Chapter 4
PROPERTIES AND DATA ANALYSIS OF HABIGANJ GAS
FIELD
4.1 Introduction
Habiganj is spatially adjacent to the Rashidpur gas field, the first ever-frontal folded
belt discovery by Shell in 1963. Twenty km to the west of the Rashidpur ficld the
Habiganj closure is the closest geological and structLlralanalog of Rashidpur From
the surface geologic evidence, the Habiganj structure was believed to be the
structurally lowest component ofthe giant Barmum anticline, the crest of which is
exposed to the south in Tripura, India. Habiganj structure is an entirely separate
closure in its own right that happens to be the northernmost feature along the 130 km
long Barroura trend. Also, Habiganj is the most down-dip frontal folded belt closure
that has the Upper Gas Sand and Lower Gas sand play types, as contrasted against
the TitasJBakhrabad multiple pay play type, further down-dip to the southwest.
4.2 Reservoir Descriptions
Shell covered the Habiganj structure with a single-fold seismic grid in 1962 and
staked the 1963 wildcat location on the basis of the grid The relationship of the
Habiganj closure with the Barmura anticline was not fully clarified in the 1962
vintage of single-fold seismic grid. The possibility of a southern fault terminating
the pay in the Upper Gas Sands of HB-liHB-2 and of a permeability barrier due to
clay-plugging of the sands, were points that were considered in planning for
appraisal wells to the south,
4.2.1 Upper Ga.~Sands
German Geological Advisory Group Upper Gas Sand Map (1984) whieh was
essentially an adaptation of the Shell map, to which the owe was added with
confidence as data from wells HB-3 and HB-4 are conclusive in this respect.
"BOGMC Upper and Lower Gas sand Maps (1982) based onl~ Shell seismic data
of 1962-63 lind well datfl ofHB.11HB.2 indiCllted the: gM-water contact was III 1491
meter or 4890 n subsell. The GGAG Upper GKS Sand map iodiCllted II cremlll
position more or less simillar to the Shell map. The Upper GIlSSands were defined
as marine deltaic $lInds In lhe past lhe idea was lhal the gas ortlle Upper Gas Sands
have lllilted Gwe, wilh meteoric WIlierrecharge. Now the Gwe is interpreted to be
IloriT.ontal, barring loeal differences due to the Cllpillary pressure of the shaly SlIrnb
1I11hebase of the gas column of the Upper Gas Sands. The enormous thickness of
the Upper Gas Sands exceeding 750 ft., their matrix.free nature and lhe occurrence
of accessory glauconite identified in the core report are fully considered with their
beach bar deposition, The Upper Gas Sands of Habigllnj exceeded II gross thickness
of 750 ft., indicating thaI it is II major sand body. [\5 capping shales are more liwI
750 ft. thick in some wells and undoubtedly represent a IlUIjormarine trll.nsgressive
episode. The sands are so cleM as to be unconsolidated, well sorted, with qullrtz as
the dominant constituent. Accessol)' minerllls include feldspars, micas, huvy
minerals, plus glauconite (in tl1lccs) invariably. The most outslmiding tcxtul1l1
ehllraeteristics of the sands is their unconsolidated nature due to the virtual lack of
matrix clay or of Other cementing material. The lack of matrix and good sorting of
the clastics account for the vcl)' high porosity and permClIbility of these massive
sands. According to IKM, average porosity was in the 30"/. range and permCllbility
commonly in the 2 to 4 Darcy rtnge, This nmge of porosity value was also used by
Beicip Franlab-RSClPetrobangla (2000).
4.2.1 1.0»'0 Gas Sand.f
By contrast with the Upper Gas Sands the Lower Gas sand pools are much smaller
and deeper, being in the depth TIlJIgeof9600 ft. to 9800 ft. $lIbsell, The Lower Gas
SIIndswere chllJ'llctcrized by a gross pay thickness of up to 50 ft" porosity of 17 to
18 percent and permeabilities less than 100 mO, The Lower Gas sands constitute a
secondal)' target in the Habiganj field. The Lower Gas SIIndSare of the nature of
offshore bars deposited in a much lower energy regime titan the Upper Gas Sands.
The lateral continuity of the sands are limited, as illustnlled by the faet that
corresponding to three potentially gas-bearing bars in the HB.5 well, there is only
"one in HB-1, The seismic definition of the Lower Gas sands was very poor. Only
two wells (HB-l and deviated HB-S) have penetrated these sands No synthetic
seismograms could be generated as complete logs for HB-l were not available and
HB-S was deviated. Rigorous mapping of the Lower Gas sands was not possible.
Based on the correlation of the continuous gas sands between HB-l and HB-S and
its seismic tie, a tentative pool map ofthe Lower Gas sands had been generated. The
gas produced in the production test of the Lower Gas sands was very lean, the
condensate traction is still higher than in the Lipper Gas Sands, showing a tfend in
the increase in the condensate fraction with depth. Seismic control definitely
indicated the absence of any major faulting in the area, It has disproved the
occurrence of a fault in the southern part of the Habiganj closure. initially surmised
by ShelL Seismic control has definitely indicated the absence of any major faulting
in the area
4.2.3 Gas-Water Con/act and Gas Column
According to TKM study report, the initial owe was at 4875 it TVD, TKM stated
that OWC varies from well to well in the range of20 it and the best possible initial
OWC (4875 ft) was picked for volumetric calculations, Table 4.1 shows gross sands
depth, thickness of 6 wells as studied by lKM (1991).
Table 4.1 Gross Sands, Depths and Thickness (IKM study 1991)
Well Date Reservoir GWCi Gross Thickness
, Drilled Top, it ft (From OWCi, ft)
!lB-l April 1963 4500 4875 375
HB-2 Nov 1967 4500 4875 375
HB-3 May 1985 4156 4875 719
!lB-4 Jun 1985 4275 4875 600
HB-S Aug 1988 4119 4875 756
HB-6 Jan ]990 4309 4875 566
"Table 4.2 Gros~ Sands, Deplhs and Thickness (Bcicip study 2000)
Well Date Reservoir GWen owei Gross Gross
Drilled Top, ft ft ft Thickness Thickness
(From (From
GWen, ft.) GWei, fl)
HB-l April 1963 4504 4783 4888 279 384HB-2 Nov 1967 4504 4783 4888 279 384
HB~3 May 1985 4]60 4783 4888 02] 728HB-4 June 1985 4278 4783 4888 505 610HB-5 Aug 1988 4121 4783 4888 662 767HB-6 Jan [990 4310 4783 4888 m 578HB-7 April 1999 4146 4783 4888 637 742HB-8 Jan 1999 4708 4783 4888 75 180HB-9 July 1998 459] 4783 4888 192 297HB-lO Aug 1999 4260 4783 4888 523 ""In the Beicip Franlab-RSC (2000) study, the initial owe was estimated from the
resistivity logs of well HB-l by observing rather sharp decrease in the resistivity
logs (and is supponed by other well logs). They indicated the initial owe as 4888 ft
TVD mentioning that the variation of GWC occurs over a few feet as opposed to
IKM. Table 4.2 shows the revised values given by Beicip. The table shows the new
GWC as well as initial GWC alongwith the gross thickness of the 10 wells. GGAG
also indicated the gas-water contact was at 1491 metcr or 4890 ft subsea.
According to IKM, the GWC was well defined in HB-l at 9940 ft KB or 9855 ft
subsea, That well-defined water table was at the base of the gas column, which was
indicated in all the seismic dip lines at 1400ms:l: 2-w-t. On the other hand, wire line
logs of the different wells indicated gas-water contacts at different depths within a
range of about 20 ft, In spite of localized irregularities of the GWC due to lithology,
it was obvious that a single water table was associated with thc base of the Upper
. , . 15Gas Sands. After a careful review of all data, lKM diose 4875 ft subsea as the best
compromise 10 define the base of the pool. The present study uses the pressure data
of the Habiganj ga~ field al GWC 4875 ft TVD due to using the same.by other study
groups such as Beicip Franlab-RSC/Petrobangla (2000) and HCUINPD (2001).
From Table 4.2, the minimum gas column was found in the discovery well HB-J,
384 ft according to initial GWC and 279 ft according to new GWC The highest gas
column was 767 ft (from GWCi) and 662 (GWCn) ft in HE-5, A report of lKM
indicated that the recovery from the Upper Gas Sand in the Habiganj gas field would
be governed by the water-drive mechanism from an aquifer of effective size of ten
times that of the gas reservoir Beieip study report showed that to get 3 630 TCF of
GIP, assuming aquifer effective size 10 times greater than reservoir, requires an
impossible pore compressibility of 248 E-06 lIpsi, Thus TKM 1991 estimate of
aquifer volume was grossly in error: so was their water in/1uxco-efficient.
4.3 Resen'oir Fluid Compositions and Properties
Fluid compositional analysis was not available for all wells in the Habiganj gas
ficld. The compositional sources available to lKM were limited to original fluid
analysis /Tom:
• HB-l in 1963(both Upper and Lower Gas Sands)
• HB-T in 1984 (Core laboratory UGS analysis)
• HR-6 in 1990 (from UGS production test samples)
Comparison of the original Shell gas analysis for the Upper Gas Sand with the
subsequent analysis from HB-6 indicates consistency in measured l1uidcomposition
and provides an excellent basis for further study.
4.3.1 Upper Gas Sand Fluid Composition
The composition of all reservoir fluid samples from the Upper Gas Sand is shown in
Table 4.3. Comparison of the component mole fractions indicates agrcement
between the various sampling and analysis data The samples consistently show no
hydrocarbon components heavier than C, with the exception of one Geochem
analysis in HB-6, which found a CJ+ of 0,01 mole percent. A summary of reservoir
gas propcrties at initial conditions is shown on Table 43 for the average fluid
composition used throughout the balance of the st",dy.
Table 4.3 Comparison of gas compositions by fluid analyses of the Upper Gas Sand
(lKM 1991)
lID-l,1963 HB-1,1973 HB-6,I990 HB-6,1990 Avg,
Shell Core Lab Geochem Geochem Composition
Compnt, I 2 3 3
mole frac. mole frac, mole frac. mole frac. mole frac
N, 0,0070 0.0077 0.0087 0.0077 0,0078
I-hS 0,0000 00000 0.0000 0.0000 00000
CO, 0.0001 0.0000 0.0001 0,0001 0.0001
CI 0.9784 . 0.9771 0,9765 0.9775 0.9774
C2 0,0145 00152 0.0146 0.0148 00148
C3 0.0000 0.0000 0.0001 00000 0.0000,C4 00000 0,0000 00000 0,0000 0,0000
,C4 0,0000 00000 0,0000 0.0000 0.0000
iC5 0.0000 0,0000 0.0000 0,0000 0,0000
nCS 0.0000 0.0000 0,0000 00000 0.0000
C6 0.0000 0.0000 0,0000 00000 0,0000
C7+ 0,0000 0.0000 0,0000 0.0000 0,0000
TOTAL 1,0000 1.0000 1.0000 LOOOl 1,0000
Tablt 4." GIISpropenies u!cullltion of the Upper Gu Sand (IKM 1991) "Compon!. , 01. Mol. ••••1,.m ,'m p, . x.Pe T, . "oTe
Fmc,xiN, 0.0078 28,014 0.2185092 493.1 . 3.84618 227.3 1.77294illS 0.0000 34.08 0 1306 0 672.5 0COl 0.0001 44,011 0.004 1070,6 0.1 547.6 0.1Cl 0.9773 16.(4) 15.679 667.8 652,6 343.1 335.3C2 0,0148 30.07 0.445 707.8 10.5 549.8 8.1C3 0.0000 44.097 0.0000 616.3 0.0000 665.7 0.0000
ie4 0.0000 58.124 0.0000 529.1 0.0000 275,0 0.0000'C, 0.0000 58.124 00000 550.7 0.0000 765.3 0.0000iCS 0.0000 72.151 0.0000 490.4 0.0000 369.1 0.0000,C5 0.0000 58.124 00000 488.6 0.0000 845,5 0.0000C' 00000 86.l78 0.0000 445.7 0.0000 888.5 0.0000
C" 0.0000 100.205 00000 396,8 0.0000 972.8 00000Total: 0.0000 16,346509 667.04618 345,2729
. Specific Gravity 0.5642564
Reservoir Tempcr!.wreOp .......•.•...•....•......... 116
Pseudo-reduced Temperature 1.6682456
Initial Reservoir Pressure@4875 fl. TVD 2149,64
Pseudo-reduced Presron: ....•••...•...••...•••...... 3.2226254
7.-flletor 0.834042957
. Initilll Gas Fonnlltion Volume Fllctor (B••) 0.006324579
••
4.3.2 Lower Gas Sand Fluid Composition
Lower zone compositional data is limited to the mitial production test samples
collected at HB-l in June 1963 The fluid composition and initial fluid reservoir
properties of/he Lower Gas sand based on the analysis are shown in Table 4,5, In
comparison with the Lower Gas sand J1uid in the Rashidpur gas field, the gas
composition is too lean and does not contain su/lkient heavy end hydrocarbons
Table 4.5 Gas properties calculation of Lower Gas sand (TKM 1991)
Compont Mol. Mo1.wt"m ,'m p, x*Pc T, x'IcFrac,x
N, o 0038 28.014 0106 493,1 , .9 227.3 0.9H,S 0,0000 34.08 0 1306 0 672.5 0CO, 0,0027 44.011 0.119 1070,6 2.9 547.6 L5CI 09759 16.043 15656 667,8 651.7 343.1 334,8C2 00131 3007 0.394 707.8 9.3 5498 7.2C3 0.0027 44097 0,119 616.3 L7 6657 1.8
iC4 0.0008 58,124 0,046 529.1 0.4 275,0 02nC4 0.0004 58,124 0,023 550.7 02 765.3 0.3iC5 0,0002 72.151 0014 490.4 0.1 369.1 0.1nC5 0,0001 58.124 0,006 4886 0,0000 845.5 0.1C6 00002 86,178 0.017 4457 01 888.5 02C7+ 0,0001 ]00.205 0.010 3968 0.0000 972 8 01Total: 1.0000 16.512 668.3 347.1
Specific Gravity " .. 0.570
Reservoir Temperature'1:.. .. 184
Pseudo-reduced Temperature. 1.855
Initial Reservoir Pressure@4875 ft TVD. .. 4293.5
Pseudo-reduced Pressure. ,6.424
Z-factor " ..... " .. 0.968
Initial Gas Formation Volume Factor (B.,) " .... 0,004] 0
4.4 Production and Pressure Data Analysis of the Habiganj Gas Field
This section will mainly discuss about the prodllction and pressure data history of
the Habiganj gas field. This field went into production from 1969 with two wells of
lorn-I and HB-2. The HE-! was drilled into Upper and Lower Gas sands In the
following sections, a picture of the production data history and the pressure data
history of the Habiganj gas field is given.
4.4.1 Production Data Hv,1ury
The production data history of the Upper Gas Sand of the Habiganj gas field is set
'out in Table 4,6 ll, 46 b and illustrated in Figure 4.1. In the first table, the
cumulative gas production, water production, condcnsate production of the well HB-
1, HB-2, 00-3, HB-4 is described, The second table describes the same parameter of
the well of HB-5, HB-6, HB-9 alongwith Iotal cumulative production of the gas,
water and condensate The HB-1Owell was not under production during the period
under study, Here, it shows the production year from 1969 to 1999 under the
availability of data sources. The total cumulative production of gas from 1969 to
1999 was 751,59 bscfand water production was 55,364 bbl Another Table 4.6(c)
shows the yearly average specific water production for each well. On the basis of
this table some graphs are plotted, FigA 2a and 4.2b show that there is a scattered
trend of increasing specific water production in HB-l and HB-2 upto year 1985,
From 1985 onward the specific water production in all wells are more or less
constant. These rates range between 0,07 bbllMMscf to 0.077 bbllMMscf The
average specific water production rate for each well is 0.074 bbJIMMscf The
production data for fIB-I and HB-2 prior to 1985 seems to have more uncertainty,
Ihe production data quality from 1985 onward for every well seems quite
consistent.
• •
.'.. ..>0
Pressure test for tile lIabiganj gas field was limited to the initial well evaluation
tests, the 1990 IlfId 1999 pressure survey data, The pressure data history was
pro\ided for liB-I, HB.2, HB-3, HB-4, 1iB-5, HB-6, HB-?, HB-8, HB-9, HB-IO for
different time interval. The prC$sufCsurvey data of 1990 and 1999 provided the best
qualily data for pressure tBJISient analysis of the Habigllnj Upper Gas Sand.
Previous testing in the field used low resolution AmCf1l.dagaugC$ for down-hole
prcssure metlsurement.
Table 4.6 (ll) Production history of the Habiganj gas field (Pctrohangla)
HB.' HB.' HB.' HB.
OM C,,", W.ler n."" c,,", w,. OM ""'" WDte, Ou c,", W.1er
'Q' MSCF '" '" "-SO'"'" '" M'O' '" "'" M"" "'" "'"1969 '"'''''' '" " 1921084 ,>0 ""ro 4921252 H-4.1 '" 4S613S04 '" '"'1971 5892246 'M ,ro So4275S6 '" '"1972 7807290 '" no 7109S047 '" '"1973 9504447
"".., SS71714 '" '"1974 10714772 ~,'" 99109J2 '" ..,
"" ll92))IS '" '" 10935143 •••• ..,"" 13002293 .m '" 12061903
~ '"1977 1"20359"'"
,ro 134404118 '"' "1975 17034445 no ,ro 159619911 ." '"'1979 19574S21 on 1039 18701931 ." "'"'''' 22336172 ,." 1222 215114752 '" ,,,.•"" 2751574(, "" 15H 26625028 1125 1531
"" 35106J25 '"'" "'" ))476192 "'" ,..'''' 428S04S19 2012 ".. 4123279S ''', 13m
'''' SI66808S ,,~3134 s0076146 "'" '""'''' S997SS34 2870 3114 57674'168 2130 "" So4J3344 ". 'M 4H1l83 '" '"'''' 66'1!72)] 3229 4231 648557118 "'" '" 1481st139 '" "" 1199S2SS '" '"'''' 74S55298 3652 •••• 725643" ''''' "'" 24272427 1240 1855 2)]98221 "'" "'"'''' 81H9H2 "" "" 7S573127 31S8 S2I9 34127461 1691 '"'' lJ04S3J2 ".. ".."" 8789S614 4243 S826 8S140109 4077 '''' 43976213 2129 31m 42773402 '"" ,.•."" 93383406 448~ 6220 90275S9) "'" "". 53070447 2no 3934 S1332227 "'" "'""" -" 4742 """ -~""" •••• 6141470S 29J7 4579 S911919S 28)9 2819
'''' 104%5S81 ,." "'" IOI\l6lMll 4874 6932 '"""" 3J6S 5232 6329303S ''''' ''''''''' 1147SOO611"., 7831 Ill819125 5130 7610 S071SH4 3SS8 "''' 73631515 3524 3524
",. 1246S1625 '''' "" 12160S011 SSl6 "n 90861493 4361"""
8)659323 4023 (02)
'''' 1339771132 ,m 9307 1J08996S5 6292 9129 10IOOS0422••W 158) 93703810 4Sl7 4531
"" 143856929 "" "•.. 140760018 •• •• "" 1II02lMSS '''' 8346 103627697 """ """'''' 153865232 "" IOSH 150733926 7323 "".. 121359S06 5937 9141 11l8S4666 "" "W
''''' 163750166 7943 llS93 160592857 "'. 11402 111090909 .." 'S" l2J'lH38 6103 6lOJ,,,, 173320119 "" 123H 1101)7215 8321 12129 1413S04673 ""' 10672 1337S1170 "'" "'"
•
•
, , ",...•.
•
Tlhle 4.6 (b) Production history ofthc Habiganj gas field (Petrobangla)
"
HM HM ~-, Total Cumulatl\ .•
'"' """" w.~OM "" c...o Wnte '"' C'"' Wnle OM c...o WnlCf, ,y= """ bb' bb' MSCP
""bb' "SO' bb' bb' MMSCI' bb' ,
"" )916.09 '" """ ']482.606 6)).7 '""" 11l19.802 '" '"'''' 1~916.aJl '" no197J 11I076,l61 ,,, ."1974 206H.7(l.j "'" ,,,1975 2285a,458 ,= IllS21976 25064.196 11]5 Ill'}1977 27ll6O.847 "" ,,~1978 32996 44] 1439 16761979 J8276 HI 16H ,~,,~, 4J920,924 I~'2426'~I Scl1(1).774 HIS l078
I~' 68S8l.l17 lIll "'"m, 84087.J74 3917 "''''''' 101744,23 "" "",~, l276lSJJ "'" "IS
"" l604S6,32 '"00 IO~S"., 19S090.29 "'" 13285
'''' 226998,]6 IllM} "'"1m 259785,4 12Sll 17970
"'" 2!llO61.67 1)779 ''''''"" ll8.S8H6 15074 2205S
''', 900014 '" 'IS J493 12.28 16763 24665
''', 19-444225 ,,, "'" 400393.44 19119 28.(89
'''' 30789982 1(78 2)32 4$1573.43 lI673 ]2424
''', 4JH6101 2l2] 3287 6OS4761 '" '" 503987.5& 2,gg. ~-m-'''' SS12S717 214$ <I" 12999171 '" ~, 567389.99 2~ ~
"" 669)1178 lJ4] "'" 20016539 I~"1528 626791.05 )0714~'''' 787692U ''''' ''', 2681)Q.49 1403 ">I' 258711611 '''' '00 6lI7IU.ll) ))889 "'''''''' 907196l1ll 4Hl "'" )))84730 1729 a52 892'125 '" '" 751594.72 )6181 5""
.,- .~-,-.,...-~'~.,•
• • ••••••
T.hle 4.6{c) Yearly aVCBgt specific water production for each well
"
HB-' H~' HB-' HB< H~' ,.•• H••••y= Water Prd Water PnI, Water Prd Waler PnI, WmerPnl, WaterPnl, Wat •• PnI,
"''''''''''' bblIMMsd bbVM.\tsd" "'''''''''' bblIMM5Cf "''''''''''' bbllMMscf
"" 0,0055 0,00571970 007&6 0.07)\11971 0.02911 O.eMeM1972 0.0569 0.0'"197J 0.OH4 0.eM171974 0.0388 00493In, 0.0323 0,030J1976 0.0287 o,ono -,on 0,0599 0.0682
'''' 0,076$ 0.07061979 0,066' o.ono
"'" O.066J 0.0687
"" 0.0627 0.0649
"" 0.0682 0._"" 0,0621 0,0619
'''' 0,0667 0,0662
"" 0,069l1 0,0703 00707 o.onl
"'" 00759 0.0760 0,0758 O.07JJ
"" O.OllO4 0.079l! 0,0804 O.o.BeM".• 0,0760 0.0764 0.07$1 0.0756
"" 0.0692 0.0691 0."'" 0.0690
"'" 0.0718 00722 0.0719 0.0720,W, 0.07(,3 0,0426 O.077J 0.0771'W, 0.076J 0.2126 0.0753 0.074B 0,075J,W, 0.07H 0.0749 0.0749 0,07.50 0,0749
"" 0,0769 0.0168 0,0770 0,0761\ 0,0769'W, 0.0162 0.0161 0,0763 0.01'9 00761 O.OnB
'''' 0.0766 0.0764 . 0,0762 0.0766 00761 0,0763'W, 0.0770 0.0769 0,076\1 0.0769 0.0768 0.0768
'''' 0.0767 0.0764 0.0767 0.0769 0.07M 00767 0.0773
'''' 0.0764 0.0762 0.0765 0,0763 0.0763 0,0765 0.0760
<,
- -- • •
"I--GIn -- -o-wa-. p, '",,'on I
Fig, 4.\ Cumul~tiyegas 800 water produdion of the Habiganj gas field
For the pressure survey, a single qtwtr.-capacitance (Panel!:)electronic gauge v.uu$ed in !lUrfaeereadout and memory mode to detCfTllinethe bottomhole pressure
through out tnc testing program. This gauge type was standard equipment fOTall
well testing in the gas field appraisal project and has a resolution of 0, I psi. The
pre5$ure 5111Veytest design included a single 12 hour dnI.",'Ciownfollowed by an
extended build-up period of the SlIme duration. The It!$! was designed 10
cm.raCIer17.edthe individual gas reservoirs, estimate r~ir capacity and
pem!elIbility. determine the wellbore skin and determine the nature of outer
boundary responses.
Im_1
•.'~,0900
0.0100 L~~~~~~~~~~~191919191919191919191919191919192069 J1 J3 7577 7981 B3BS&7&991~391 9799 01
/"077'~J! ~.0700 • ••- ..... ., V.
<0 QQSOO /' •i ..0,0300
I
,:< 0.1100
~.•
." ..• •••
G.090~'J ~08~~,~.
O,~700 .0,0(,00
;l; 00100
i. oo~oo.•o ')Joo
1: 00200.g 00100;t; ""000
19'91010'01919'919'91919'9191919206971 7375 7779RI i1 31 i7 3991 93 9,97 99~1
" ""c, 0,Q8 •,,~,07 •, •0."0,
" ""'J "0<1 om",om,
" ",'.'1 10" I." '"' 19.3 109, ,m '"' lOOIy,,,
oO'!,O.OB •, •
i. om • ••0,06
1 0,0,
~M~• Ml
"• "m,~.oJ
19BI 1987 1989 1991 100] 1•• ' ' •• 7 1999 lGOI
".
Fig 4 2 a Yearly average specific water production of the Habiganj gas field
~, IU~6
o 0" ''",''" ID" 'i~,07 '"'"0 " 0001 ,
0,05 • om, ,,'"' I'"','"' o "'
~,
'0; ,'"0.0 I '"',"' 1904 ,""6 '"' 2000 ,w, 190' '"' 2001y"", Yo>'
llB-9
'00• o e,02'; O_~7:::• 000,0"',0"'j 0"'
!OM0,01
1998 2000
"OM
Fig, 4,2 b Yearly average specific water production from Habiganj gas field
• • • "Table 4.7 is constructed with the shut-in pressure dllt.!1of different wells of the
Habiganj gIIs field 111different time interval. All lhe pressure points were at datum
level of 4815 feet TVD. The initial pfC5sure point is 2149.64 psia.. which was
recorded on 18.6.63 from 1iB-1. After this reading. there is a long gIIP between the
year of 1963 10 1984 due to lhe unavailability of pressure data from BGFCL.
From these raw datil., it is seen that there IIrc some pressure points very close to dille
and some are on the same dale. The pressure dala of 17.1.90. 21.1.90, 23.1.90 are
vcl)' close \0 their dille. Depending on the fluid deposition and tightness of the sand
around the wc:llbore (CiIpil1aryelTect), the pressure wried lit dilTcrclI! wells. in thaI
case, lIvcrBge JlTe5sure value was I!.ken. In the case of different pressure
measurement during IIvery short period in the same "~ll, the highest pressure Vllluc
was taken lISII stabilized pressure.
So, for the cccurncy and clarity of the calculations, !onother t!oblc (hble 4,8) is
constructed by screening out the obvious !onoll1lllicsin the pressure points for funher
study of the Habiganj 88s field. This processing, analyzing, synthesizing, and
screening of prC:S5Urcdata is a standard procedure followed by all oil companies in
the world. This reduces thc uncertainty in datil and increases the accuracy of the
results
•
Tablr 4.7 Shut.in pressure data ofHmbiganj ga~ field (HCUINPD 2001)
"
Jm.l 18.6,63lill-424.1.85HB.) 23.5.85HB-129.5.86IID.2) 1.5.86HB-S 31.1.89HB-617.1.90HB.221.1.90HB.) 23.1.90HB-917.7.98HB.S 11.2.99HB-119.7.99HB-S 31.7.991iB.13,8.99HB-63.8.99
. Im.IO 26,8.99
Well No Dllte
"2149.642127.982119.752108.952114.'2089.982096,522094.582090.052026.512019.242014.942011.572015.352009.982012.94
T.l:tlr 4.8 Selected pressure points ortlle Habiganj gas field
Dau No. OllIe """'"'@4815 TVO nsia0 18.6,63 2149,64I 24.1.85 2127,982 23.5,85 2119.75l 29.5.86 2108.954 21.1.90 2094.585 17.7.98 2026.516 11.2.99 2019,24
1 19.7.99 2014,94, 3.8.99 2012.66
•
Chapter 5
STUDY OF MATERIAL BALANCE EQUATION
5.1 Introduction
Schilthuis first developed the general material balance equation in 1941. This
chapter will derive the Schilthuis general material balance equation and will be
applied by adopting the illustrative technique of Havlena and Odeh (1968) for the
understanding of reservoir drive mechani~ms. Generally, material balance equation
counts no geological aspects and can be used to calculate the hydrocarbons in place
and identify the drive mechanisms,
5.2 General Material Balance Equation (MBE) for a Hydrocarbon Reservoir
Here, the derivation of the general material balance equation is shown as a
volumetric balance, A picture in Fig 5,1 (a) shows the hydrocarbon volumes at the
initial pressure p. in a reservoir that possesses a finite gas cap, The total hydrocarbon
volume in this diagram is the hydrocarbon pore volume of the reservoir (Hep\')
Fig, 5.1 (b) shows the effect of pressure reduction by an amount 8{Jdue to
production, which makes a false expansion of hydrocarbon volumes in the reservoir.
The increment of volume X is due to the expansion of oil and the dissolved gas
Volume increment of Y occurs due to expansion of the initial gas cap. The third
volume increment Z occurs due to reduction of HCPV because of the effects of
expansion of connate water and decrement of reservoir pore volume The (X+ Y+Z)
i~ total volume change of the original HCPV. Thus the volumetric balance can be
shown in reservoir barrels as the summation on components
Underground = I, (Expansion of oil + originally dissolved gas, rb) + 2. (ExpansionRemoval of gas cap gas, rb) + 3. (Reduction of HCPV due to connate water
expansion and decrease in the pore volume, rb),. . .. (5.1)
Gp, Np, WI'
Gas cap
Oil zone
Ca)
Gas cap
Oil lOne
Aquifer water -inflll~
Rock andconnate waterexpanslOIl
0)
Fig 5.1 Volumetric balance in the reservoir associated with a finite pressuredrop; (a) volumes at initial pressure, (b) at the reduced pressure
• First component is composed of liquid expansion and liberated gas expansion,
which are N (Bo-H.,J rb and N(R,,- RJBg rb respectively,
• Second component is the expansion of the gas cap gas, The tolal volume of gas
cap gas is mNB", rb, which in scCcan be expressed as
~ mNB""" (sct),B"
This quantity of gas, at the reduced pressure p, will occupy a reservoir volume
B,mNB" B., (rb),
Therefore, the expansion of the gas cap is
mNBrn(;:, -I) (rb)
• Third component is the change in the HCPV due to the connate water expansion
and pore volume reduction, The total volume change due to these effects can be
expressed as
d(HCPV)= -dV,. +dV[
Where, VI is the toW pore volume"
'"or as II reduction in the hyd~n"pore volume, ud(HCPV)= -{cwV. +cIV,}Op
HCPV andl-S.
V•. is the connate water volume. ", 'J( S '" (HCI'V)S ••- {l-S_>
As the tot!l HCPV, including the gas cap. is
(l+m)N8 •• (rb)
then the HCI'V reduction can be expresr.ed 113
(" +<)--d(HCPV)=(I +m)NB.. ••• II-S-
The reduction in the volume which CIInbe ocx:opied by the hydrocalbons III the lo\VC:r
pressure, p must correspond to an equivalcnlllInount of fluid production expelled from
the reservoir, and hence should be added 10 the fluid expansion terms.
The underground removal becomes
N,ln.+(R,-R.",) (rn)
where N~B.is the volume of oil plus dissolved gas at reduced pressure In the
reservoir condition. N,(R, - R.) scfis the remaining produced gas.
Therefore, equBl.ing the underground removal to lhe sum of the volume changes
(Eqn,5.1) in the reservoir, the gCl1Cf1l1 expl'C$Sion for material balance 11$
N,(B. +(<<,-8,ln.)' NBo[(B.-Bo)::R. -8,)n. +~~-I)+(I +m('~:;:'}-]+V': - W,)n•.................._ _ _ - -: - _ _ _(5.1)
5.2.1 Addition rif Water Influx Term
For connectivity to an adjacent active aquifer, the pressure drop in the reservoir due to
production, the water in the aquifer expands that results an influx of W, stb. or W,Bw
rb, The cumulative amount of aquifer water produced is W,. So, the net water influx
term of (w, -W,)B. has been added as an extra term to the right hand side of the
balance, which push out an equivalent amount of production from the reservoir Bwis
generally close to one because the solubility of gas in water is very small and this
assumption will be made throughout this study.
5.2.2 Conditions/or the AppficlItion 0/ Material Balance
There are two conditions that should be satisfied
• Sufficient data collection (production! pressure! PVT), both in quantity and
quality. In practical situation, of course, there will always be some errors and
uncertainties in all measured values In this case, the industry practice is to count
those values, which are within accuracy range of HO%, The most anomalous
points are discarded for the sake of good quality, accuracy and refinement of the
data,
• It must be possible to define an average pressure decline trend for the system
under study,
Many reservoirs show a phenomenon of "tank-like" behavior. That means, the
pressure show a consistency in decline when referred to a common datum level. In this
study, the datum pressures are used in the production history data, The rapidity of
transmission of pressure disturbances all over the reservoir for attaining equilibrium
kdepends upon the degree of the hydraulic dilfusivity, (-) The larger the value of
IfX'hydraulic diffuslvity, the quicker would be the equilibrium of pressure. For the
Habiganj gas field, that equilibrium or stability of pressure can be reached very
quickly due to very high permeability value of Upper Gas Sands. This was also
supported by the Beicip Franlab-RSC study (2000),
"•5.1.3 Nt!C~s(lr}' Data/or the Ctrfcu/ation 01Material lJa/onct! Equation
The following production, re5elVOirand laboratory datll are needed for the ealaJlations
• The inilial reservoir pressure and the IVCl1Igc reservoir pressure al successive
intervals lifter start ofpro<luction,
• TIle tot.e.! sl.andard cubic feet of gas produced. When gas is injected into the
reservoir, Ihis will be the difference between the total gas produced and that
returned 10the reservoir.
• The nllia Ortlle initial gas ClIpvolume 10the initial oil volume (m)
m - Initial re$elVoir rr~ gas volume Ilnitilll reservoir oil volume
If this value of m can be detC!Tllined with reasonable precision. there is only one
unknown Ct.? in the lTllIu:rill!blllnrn:c 011volumetric gas Cllpreservoirs, lind two (N IllId
IV.) in Wllter-drive reservoirs. The value afm is determined from [og and core dlltllllnd
from well completion dRIll.,which frequently helps to 10000lethe gas-oiland Wlltcr-oil
conlaaS,
• The gas lind oil volume factors and the solution gas-Qil ratios. These are obtained
as functions of pressure by laboratory measurements on bottom-hole 5ampl~ by
lhe different and flash liberation methods.
• The quantity ofwtlter that has been produced.
• The quantity ofWlller that ~ been encmac:hed into the rcsc:rvoir from the aquifer.
5.3 ~bterinl B.lance for Gu Resen'olrs
For gas rC!erVOirs.Eqn.S.2 can be customized by recognizing that N,R, = G, and
N.H•• = GH". Substituting t~ terms into Eqn.S.7
N,B, + (G, -N ,R.)s •• N(B, - B••)+G(B, - B,.)+Vm••+GIl ••{( t~~'s:eIrJ+(w. _ w,)n.. H •••• ' ••••••••••••••••••••••••••••••••• _ ••••••••••• , ••••• , •••••••••••••••••••••••••••••• _ •••••••••••••••••••••••••••••• ( $.J)
"For the ga~ reservoirs. there is no initial oilllmounl; therefore N Il~ N, lire equal to
=0.
The general mllterial balance equation for IIgas reservoir can then be obtained
G,H~+ fl."', '" G(B. - n..)+GB••[_'.s~-~,_,~{l..n+ W,B•............................ (5.4)I-S•• j' -
5.1.1 App/ictttion of HQ~'1mfllind Od~hMdhod (1968) for MlJIi (lIGm R~oWJin
Here HllvklUl lind Odeh method (1968) is applied to the gCllCflIl material balance
equation of8" reservoir (Eqn.5.'$).
Using nomenclature ofHllvieTUlllrtd Odch (1968),
F '" G,8. +W,B. '" Totlll gas and ",.•ter productiolB (ref)
£. = B. - B•• =Underground gu apansion (rcOset)
(c S •• +c)EJ. =B... / /'yJ '"Expansion of the connate ",'lllcr 8I1dreduction of the pore
I-S_
space (rc17scl)
This reduces Eqn.5.4 10the simple form
F = c{E. +E,..)+ W,B•............................................................... (5.S)
In most rcalistic situations, E,..« £. and may be omitted but not before checking the
validity of this IlSsumption fOTtm: entire range of pressure depletion, The material
balanee then becomes
F=GE~+W.B.
Finally, dividing both sides of the equation by E~gives
F WB_ = G + -!-!- ............•...•...•••....•...............................••..........•(5.6)E~ E~
H
When expansion of the connate water and reduction otthe pore space are needed toinclude in the calculation, then the Eqn.5 6 will be
__(5 7)
The left-hand side of/his expression should be ploued as a function of the cumulative
gas production, G p using the production, pressure and PVT data. Fig. 5.2 shows a
Srrongaquircr
FIE,Moderateaquif.r
', Volumetricr depletion
G=GIlP
G,Fig. 5.2 Diagnostic gas material balance plot to determine the GlP and define the drivemechanism (After Dake 1994)
diagnostic diagram where the variation of the curve is demonstrated during depletion.
The above diagram can also be plotted as £ versus time or pressure decline (!;p)E,
which are equally illustrative. The plot may be one of the three trends shown in Fig.
5.2.
• For the volumetric depletion type reservoir, W. ,,0, the plot should be as a straight
line parallel to the x-axis whose ordinate value becomes GIP,
• For a reservoir affected by nalUral water influx the plot of F will usually show aE,
concave downward shaped curve whose shape is dependent upon the aquifer size
and strength and the gas off-take rate.
]
"TIle mBia IIdVllntage in the :" versus G ,plOI is tlull it is much more feJlpornive than
"
other methods in establishing llquifer support in the reservoir,
5.J.1 pIZ_lntupretatiOfl Mdhod
This is II well-accepted method where the gas material b111.nceequation is shown III
standa"! conditions (scf) liS
Cumulative gas production" GIP-GlIs remaining in the re!el'VOir
where f:,lInd E arc the gas e:>;pansiOllfaaors II the initial and reduced IIVeT8ge
pressure. 2.. is the original HCPV and the secornl par1 within the brackets takes careE,
the e.'l:parnion of the connate waler lind reduction of pore volume resulting from
compaction. The term W,represents lhe cumulative net ••.•.tlter influx. Usually. the
water and pore compressibilities are negligible in comparison to thlll of gas and the
second part within the brllckets can be omitted after checking its validity.
This reduces the equlliion 10
lind if the reservoir temperature remains constant, the gas expansion factors can be
replaced by their corresponding values ofplZ to give,
The'term /V.Bw) represents the fraction of the HCPV invaded by water.
GIE,
,
As ft result, t~ larger the influx, the higher the pressure for II given off-take of gas.
Whcn the reservoir is of the volumetric depletion type, then the equation may be
reduced to the form
P;:P'(I-G'Jz .~, G
which is a simple lineaT relationship bctwemplZ and the fmctional gas recovery. ~11
il becomes a popular ficld'le<:hniquc ofplotling the reservoir averaged values of ptz in
which the pressures life referred to some common datum level as II function 'of the
cumulative gas production, G,
When a reservoir is of tile volumetric depletion type, then the plol must be linear as
sltoWIl,
p,l,
P1
Fig_ S.3 Gas matenal balance plot for depletion water drive reservoirs
in Fig. 5.3 and its exlfllpollllioll to the abscissa (p/Z" 0) gives the cffectivt: GlP to be
determined as G,::: G. Presence of Illy natural WllteT influx from lin adjoining
llquifcr, makes theplZ plot be non-linear as shown in Fig. 5.3.
•
Though, this method is simple in application, it possesses the following potential
dangers.
• Mislead whether the line is straight or not
• In many situations, the plot for a water drive field shows to be linear until a very
matured stage of depletion.
• The extrapolation of apparent linear trend shown in Fig 5.4 (a) gives a value of
the GIP at the intersection point of abscissa. which is too large (G '>G) Here the
value used taken from the present study that is described in the following chapter.
This figure is a real example using Habiganj data.
(a) across the IOtal range ofp/Z (O-3000).(b) Over a reduced range of
p/Z (2400-2600).
•
"The arc two types of error (lui! may be occurred in thcpfl plot
• Jnsp~rlion ElTOr
Inspection of the pIZ plot ami marking ila apparent linearity leads the practicing
engineer 10 assume the reservoir to be of the volumetric depletion type .. Actually,
erroneous extnlpolll\ion ofthcpfl. trend 10the absdsSlI determines a too large value
orthe GIP.
• Sea/inK BlTOr
SCll.lingerror tui!Cll simply from plotting the daU! acro~ the full 11Ingt:of
pI Z(O _ p, Il,) to demonstrate the full extfllpolltion (Fig, 5A a). The other plot
(Fig,SAb) shows an enlarged pll 5ale, over the range of depletion, where slight
curves appear. This figure shows only lioCllr portion al very early stage of the
production, before the willa- influx is significant llIld extrapolation of this early trerKI
will give IImore reliable value ofthc G1P.
"ESTABlISIIMJ<:l\'T or DRIVE M'ECHANISM IN HABIGANJ GAS
FJELD
6.11ntroduction
This chapter will discuss about the presence of water.drive in the Hnbiganj gas field.
Many reservoirs lire bounded on II ponion or all of thc1r peripheries by WIller bearing
rocks etIlled aquifers. The word "aquifer" comes from LIllin language. WAqUll"melIns
water and "(crre" means to bear. The aquifers may be $0 large compared with the
reservoirs they adjoin as 10 appear infinite fOf all practical purpose!. Ilnd they may
range down 10 those so small 11$ to be negligible in their effe<:1 on reservoir
perfonrumce. The aquifer itself mllYbe entirely bounded by impermeable rock so that
the reservoir and aquifer together form a closed or volumetric unit. On the olhtt hand,
the reservoir may produce Ilt onc or mort places where it rll4y be replenished by
surface water. An aquifer may be horiwntal Wilh the reservoir it adjoins, or it may
rise, as al the edge of suuccUl'al lwins, considerably above the reservoir 10 provide
somt artesian kind of flow ofWllter to the reservoir. In response: to n pressure drop in
the reservoir, the aquifer relicts to offset, or n:urd. pressure deeline by providing Il
source ofWllter inf1u){or encroachment by (a) ClCpIlnsionofthc WIIter, (b) expansion of
other known or unknown hydrocarbon aca.mlUlatcs in the aquifer rock; (c)
compressibility ofaquifer rock;
6.1 T«hniques for F..rtllbli,hing Driv~ M«hllnism
To establish the pre!ellte of W11ler-drive in the Upper Gas Sand of Habiganj the
follo .•••1ng techniques have been IlJlPliodhere.
• Havlenalnd Odeh approach (1968)
• ptZ interpretation technique
6.1.1I1aI1~nn and Od~hApproach (1968)
-This approach has been 'elaborately described In Chapler S. So, here only its
appliClllion is shown for the case ofHabiganj gas field. Table 6.1 is conSlruaea using
the previously described screened dalll of the Habiganj gas field. A graph of ~ vs,f"
G, i~plotted by laking tM values from this table.
T.hlr 6.1 Malerial balance prediaion of the Habiganj UGS using Havlena and Odeh
Equation
D", Pr@. P5i~ G,~
w•• " p:';' ,IF.
4875'TVl) "'" bof Z - ""od '"18,661 21(11.&1 , , , 0.8)4~1 0,006)246 , 1.79E+1l1 ,2.U.8S 21l1l.7S lOS 8J 6123 J,112E-OS 0.83H6S4 O.OO6408S 8.39W.-oS 8.(1I9SE+1l3 130('.2&11]I.U6 2IM.IIS 141.28 .,,, S,04SE-OS 0,lIn120S 0.(064)9S O,OOO1l49 I,07,SOE-+m 7918,610721,],90 20'94.S8 261.16 180IS 0,0001011 0.8)27948 0,0064811 OooolS6S 1.97SIIE+(l4 IOS2USS2].1.90 209I,Q.I 261.16 """ 0.0001015 0.8)27IH 0.006491S 0,0001669 1.9:!Il6E->04 10166,8l!17.7.9lI 2026.S1 MS.76 48]89 00002715 0,8312516 '''''''''' 0,0001618 5,OO74E->04 1211893611.2.99 2019.14 6112.27
"""0.0002361 0.8310368 0.0067091 0,0003~6 5.2677£-+m 1207UlO3
H,99 2016.21 '" S]J84 O.OOO299S 0.8310181 0.0067181 0,00031141 5.5OfoOE-+{I( 12326,6S13,899 20lUS 723.18 5]]9lI '''"'''' 0,8309986 0,0067214 0,000)%11 5.S07]E+<l4 1225Cl,S0226,899 2012.94 727.35 S3727 0.000]014 0.II)m4 0,006729 0,0004(»( H(OOE-+m 12103.12(
"..".."..~
,'0000
""" .z::-•••~
,..•••,..•••,.. •,.. ,,..,, '" '00 .00
~
Fig. 6,1 Plot to determine the drive mechanism and the apPllfef1tGIP ofHlibiganj UGS
,
- ,
"From this plot, it strongly indicates that the reservoir is affected by natural water drive,because the plot of - produce a concave downward shaped arc whose exaCI fonn is
E,
dependent upon the aquifer size and strength and the gas off-lake rate. From the
sequence and direction of the plotted points, it appears that the aquifer oflhe Habiganj
gal; field is i strong. The backward extrapolation of the FIf'g trend intersects the
ordinate to the point of 5072 bscf or 5,0 TCF, which provide an estimate uflhe GIP,
So, the apparent GIP of the Habiganj gas field according 10the Havlena and Odeh plot
is approximately 5.0 TCf without calculation afwater influx.
6.2.2plZ Interpretation Technique
This approach has been elaborately described in Chap.3 So, here only its application
is shown for the case ofHabiganj gas field, By taking all the pressure points available
of the Habiganj gas field the following Table 6.2 is constructed Then some figures of
plZ vs, Gp are dra\Vl1with the values taken from this table,
Table 6.2 Material balance prediction of the Habiganj gas field usmg plZ
interpretation method.
Pr.@Well No Date 4875'TVD G ,bcf , ,I'
1 18.6,63 2149.64 0 0,834(14'l 2577.3732
424.1.85 2127.98 10293 0,8335524 2552.9049
3 23 5.85 2119.75 1O~,83 0.8333654 2543.6023
231.5.a6 21J4.1 141.47 0.a.n2373 2537.2125
531.1.89 2089,98 229.86 0.~326905 2509,9122
617,1.90 2096,52 261.06 0.832!1387 2517,3181
2 n1.90 2094.58 26l.l6 0.8327948 2515,1215
3 Hr.90 20\1'0,05 261.52 08326921 2509,9915
917.7,98 2026.51 65576 0,8312516 2437 \1'02
811.299 2019.24 692,27 08310868 24296379
7 19,7,~~ 2014,94 720,45 0,8309893 2424,7483,31.7,99 2011,57 722.64 0.8309129 2420.9155
0 3,8.99 200998 723.18 0.830a769 2419.1069
"Fig. 6,2 shows that the extrapolation of the apparent linear trend 10 the abscissa will
yield a value oflhe GlP, which is too large i.e. about 10,5 TCF. In this case the error
occurs in two way. Following inspection ofthc plZ and noting its apparent linearity,
one can assume the reservoir to be of the volumetric depletion type. This is followed
by the erroneous extrapolation of the trend to the abscissa, which determines too
large a value orlhe GLP
3000
2500 .
2000
11500':[
1000
500
oo 2000 4000 6000 8000 10000 12000 14000
C41,bscf
Fig. 6.2 plZ plot for a water drive reservoir across the full range ofplZ (0-3000)
In this cases, the error arises simply from plotting the data across the full range of
p/Z (0-3000) to demonstrate the full extrapolation (Fig.6 2); whereas, if the plot is
made with an enlarged p/Z scale, over the range of depletion, then subtle curves
appear in the plot as shown in Fig.6 3 which is far from linear and, in fact, displays
"the typical shape of Fig,S,) (b) of Chapter 5 expected from a fairly strong water
drive gas reservoir.
If'....
"~.~'",
, '",,,, "-,,, , "f..-.,,
-"-, I "~, -,-,,,
1600
25~O
2560
2540
2520.);!':" 2500N
" 24MO
2460
2420
2400
" 100 ;00 500
Fig, 6.3 Enlarged pI Z plot for a water drive reservoir over a reduced range ofplZ (2400-2600)
Fig, 6.3 is actually drawn over reduced range of p/Z from 2400 psia to 2600 psia.
From this, it can be seen that only linear portion of the plot occurs very early in the
lifetime of the field, i,e. on 23.5.85, before the water influx is sigmficant and
extrapolation of this early trend will give a more reliable value of the GIP
Referring to the Fig. 6.4, it can be shown that the extrapolation of the apparent linear
trend to the abscissa (pIZ=O) would give an estimate of the GIP of 10500 bscf (i, e,
10j reF). This value is about 47.6% in excess of the correct value of apparent G1P
"of 5160 bscf (i.c. 5.16 TeF) which is found by joining the initial plZ point and the
point 0[23.5,85 as indicated by the dashed line in the same figure
1400012()()Owooo2000
/' IFig 6.3
~I. ,
\ '"..'. '-'. ""\
\ '-. "-'. I'-.\
. "'-. D..-'."," o
1000
2(l(lO
25111)
3000
~ 1500
Gp,bscf
Fig 6.4 plZ plot for the Habiganj gas field indicating the correct value ofGIP
A study of Bashirul Haq and E. Gomes (December 200\) estimated the G1P of
Habiganj gas field about 8,022 TeF on 7 wens using the flowing material balance
method. They also referred, that figure mentioned was perhaps overestimated due to
the presence of water drive, as flowing wellbore method of materia! balance can not
be used for water driven reservoir.
Chapter 7
AQUIFER MODELS FOR WATER INFLUX CALCULATION
7.1lnrrodudion
In this chapter, the different aquifer models are discussed for the calculation of water
influx from aquifer. Petroleum reservoir is often in contact with an aquifer that
provides pressure support through water influx. Thus, the prediction of reservoir
behavior usually requires an accurate model of the aquifer. Reservoir/aquifer systems
are commonly classified on the basis of flow geometry as either edge-water or bottom-
water drive. These models can be generally categorized by a time dependence i e.
steady-state or unsteady-state. Choosing an appropriate model for waler influx
involves many uneenainties Some of these include the size and shape of the aquifer
and aqUIferproperties such as porosity, permeability. Nonnally, little is known about
these parameters largely because the cost to drin into the aquifer to obtain the
necessary data is not often justified.
7.2 Carter-Tracy Water Influx Calculations (1960)
Carter-Tracy water influx calculation is a simplistic model where no aquifer/reservoir
geometry or flow geometry is counted The equation is basically the constant tenninal
rate (CTR) solution of the diffusivity equation as opposed to the constant terminal
pressure used in the Hurst-van Everdingen approach. The final form of the equation of
Carter-Tracy water influx calculation is
in which the subscript "I' refers to the present time step and ";-1" the previous Theparameters in the equation are as follows:
"U" aquifer cormant • 1.119/fhc,r; (bbllpsl)
for radial geometry.
f'" Fl1IClionalencroachment angle.
'II = Reservoir radius (ft.)
Op= Tot!l pres5Uredrop, p, - p, (psi)
W. ",'Cumulative water influx (bbl)
tp = Dimensionless time" O.l)0634( ~ ,J, t in days.• ' - 9/r,'"
1'(10) = Dimensionless erR solution oCthe diffusivity equItion.
P'(fD)= Time derivative, i.e. dpD('D).
'",
•
"
The Ii/D) is presented in tllbulllf form by van Everdingen and Hurst but an easie~
WIlyof evaluating them luis been presented by Fanelli (1985) who matched the
functions with the regression equation.
P(ID) " Q. +Q,'o -+- (l,ln /D +o,(ln 'DY " (7.2)
in which the regression coefficients are as listed in TlIblc-7.1 fOf different values of,
the ratio, rdJ "'...!... when. '. is the oula- radius oCtile aquifer for radial geometry.'.
Tllble-7.1 Values of regressions co-dIicienls
R sions coefficients
'. •• " " "L5 0.10371 1.66657 .(l,04579 .(l.01023, 0.3021 0,68178 .(l.o1599 .(l.01356
l 0.51243 0.29317 0,01534 .(l.06732, 0,63656 0.16101 0.15812 .(l,09104
S 0,65106 0.10414 0.30953 .(l.112586 0.63367 0.0694 0.4175 .(l.111378 0.40132 0,04104 0,69592 .(l.1435
10 0.14386 0.02649 0.89646 .(l.15502ro 0,82092 .(l.OOO37 0.28908 0.02SS2
"Application of Eqn.7.1 for water influx calculations with P(ID) funetlOns and their
time derivatives determined using Eqn.7.2 gives results that are very close to those as
determined using the method of Hurst and van Everdingen (1949).
7.3 Steady-state Model
This is the simplest model of time-independent nature. In this model the rate of water
influx, dW,,/dI, is directly proponionaJ to pressure change between initial pressure and
pres~ure at the original gas-water contact. This model assumes that the pressure at the
external boundary of the aquifer is maintained at the initial value, p" and that !low LO
the reservoir is proportional to the pressure differential, assuming the water viscosity,
average permeability and aquifer geometry remain constant. A drop in the reservoir
pressure, due to the production of fluids, causes aquifer water to expand and flow into
the reservoir. Applying the compressibility definition to the aquifer, then
Water influx =Aquifer compressibility X Initial volume of water x Pressure drop
W, = lcw+cfp,¥ = c,w,(P, -p)
in which the total aquifer compressibility is the direct sum of the water and pore
compressibilities since the pore space is entirely saturated with water The sum of c".
and cf is usually very small, say IO.'lpsi, therefore unless the volume of water W, is
very large the influx into the reservoir will be relatively small and its influence as a
drive mechanism will be negligible.
The above steady-state equation assumes that the pressure drop tJp at the reservoir
boundary is instantaneously propagated throughout the aquifer This will be a
reasonable assumption only if the dimensions of the aquifer are of the same order of
magnitude as the reservoir itself. This steady-state equation is only applicable to very
small size aquifers.
7.4 Unsteady-state model
For large aquifers a mathematical model is required, which includes time dependence.
That means, it takes a finite time for the aquifer to respond fully to a pressure change
•
, " '''I''."in the reservoir. The transient nature of the aquifers suggests that a time-depcndcnl
term be included in the alculations of "Iv,. In the next two sections, unsteady-state
models for both edge-WIller and bottom-waler drives are discussed.
7.4.1Edge-Waf" nriw Mmit'l
For edge-water drive, the most severe aquifer influx model developed to date is llull of
\IlIn Evc:rdingen and Hurst. which is a solution orthe llIdial dilTusivity equation. The
assumptions made in deriving this model 11ft wlid (or redial flow systems. In edge.
waler drive, .•••'IIler moves into the flanks l?f II gas reservoir as gas is produced. The
edge-WIller drive flow model treated by '1M Everdingen and Hurst 'is shown in Fig,7.!.
The aquifer thickness h is small in relation 10 ~oir adius, r/l, water invades or •
recedes from the field al the laneT's edges. and only hori7.ont41 radial flow is
considered as shown in Fig.7.].
"'-,Aquifer / '"
Fig. 7.1 Idea.lized flow model for Edge-Waler.Drive system
• Derivation of Equationfor the Model
Consider a circular reservoir of radius rEI as shown in Fig,7.2 in horiwntal circular
aquifer of radius r., which is uniform in thickness, permeability, porosity and in rock
and watcr compressibilities, The radial diffusivity Eqn.7.3 expresses the
Fig, 7,2 Circular reservoir inside a circular aquifer
relationship between pressure, radius and time for a radial system such as Fig.72,
where the driving potential of the system is the water expandability and the rock
compressibility
a' p I ap if;;.u:, 8p--+--~-~~~_.~ar' r Dr O,0002637k at " ..... (73)
This diffusivity equation is applied to the aquifer where the inner boundary is defined
as the interface between the reservoir and the aquifer With the interface as the inner
boundary, it would be more useful to require the pressure at the inner boundary to
remain constant and observe the flow rate as it crosses the boundary or as it enters the
reservoir from the aquifer,
'"MathemntiCll.lIy.this condition is staled lIS
P •• conStant. pr IJ.JJftl r • 'II
Where.'11 is a consu\J1t!ondis eql,llli10 the outer nidi,,! of the reservoir (Le. the
original gas-water contact)
The pressure p must be determined lit this original gll5-'wlltercontact. VllnEverdingen
and Hu~t solved the difTusivityequation for Ihis condition, which is referred 10as the
constant terminal pressure case and the following initial and outer boundary
condilioll!l:
Initial condition:
P" p, for all Vl1luesof r
Outer boundary condition:
For an infinite aquifer:
p.p, al '"'<:(}
For Illinile .e.quifer:
at ''"''.
At this point, We rewrite the difTusivity equation In terms of the following
dimensionless p!.llIml.'tcD.
Dimensionless time, 11)"0,0002637 tel.;1£,',
Dimensionless Illdius,'D ••.!....'.
Dimensionless pressure, PD: p, - pp, - P.,.
With these dimensionless pllramelen;, the diffusivity eqU!lion becomes:
Q'Pp +..!.. iPp "" q,p ,, (1.4)/)'~ TD aD C1to
"van E~-erdi"gen and Hurst converted their SalUlio"s 10 dimensionle~. cumulative
water influl( vlIlues and made tlte results IlVllilablcin II convenient form given in
Tabular form in the literatures for various flIlios ohquifer to reservoir si7.c,clIpresscd
by the nllio of their flIdii, ~. ~ datil are given in temu ofdimcnsionlcn time.'l>and'.
dimensionless water inOux IV"". 50 ,luil one set of values suffices for all aquifers
whose behaviour CllJlbe represented by the I1Idialform of the diffusivity equalion. The
wall!r influx is then (()lind by using Eqn.7.5.
w. '"U4'J.W•••(I,,) "' (7.5)
wh=.
U '" 1.119Rlle,r: ,' (7.6)
•f '"360-
There are differences in the way, in which the dimensionless time and aquifer constllnt
aTC C111culaled,dependent on the geometry. These are summarized referring Fig. 7.2
ftIld 7.3.
For Radial Aquifa Gtomt:IrJ'
DIlICVUllit~
"Ip '" -- (t-sec)9JlC";
U=1.l19/#Ic,r; (cc1atm)
Fjeld Units
k,Ip"'conSf. I
9PC,r~
Com/am" 0.000264 (t.hours)
- 0.00634 (t-days)
- 2.309 (t-years)
U = 1.1 19/(hc,r; (bbVpsi)
"
TI "-'-
---I \-:::'w
Fig. 7.3 Linear aquifer geometry
For Unl!!Draquift'r gt'omdry
Darty Units
"'0:::: (I_sec)
fPC'!:
II = M'Lh~, (cdatm)
7.(.2 Roundl!!d Aquifus
mid Units
"'D=conq, ,9pt:,l.
Com/ani - 0.000264 (I-hours)
.0.00634 (t-days)
.2.309 (I-yean)
II = O.178Iwl.h(hc, (bbllpsi)
lm:spec:tive orllle geometry there is IIvlllue of'o for wIlich the dimensionless WIller
influx reaches I conmnt maximum VlI!ue,This V11lueis, however, dept:ndent upon the
geometry lIS follows:
Radial W", (max) - ~.~.:, -I) :." , (7.7)
Linear W<l)(max)" 1 ; (7.8)
Note llull if W.o in Eqn.7.7 ISused in Eqn.7.5 for II full llldialaquifeT (("'1), the rt$lIlt
will be
"1 I (r} -r:) I_I 1',_W. '" 2Jrf/1c,r• .tJp.-. , eo m{. -r~ f'fr.,.IJp2 '.
This laller expression is also equi'o'l11enllothe total innu", oeeuning, lS$\Iming thai the
Ap i~inslftJlll1ncously transmitted throughoullhe aquifer.
A simiiar resutl can be obtained using Eqn.7.8 for linear geometry. Therefore, on<:e
the plateDu level of IV.,,(10) hM been reached, iI means thai lhe minimum wlue of '0
II which Ihis occurs h1l$been ~meiently large for the imlllflianeous pressure drop
llplO be felt throughout the aquifer. The plateau level of W•••(ID) is then the
maximum dimensionless WIller influx resulting from such a pressure drop.
7.4.J Infinite Aquifer
Natu1'1llly,no maximum value of IV",(1/»15 reached in this = since the WIlier influx
is always governed by tramlienl flow conditioM. For fIIdial geometry, VlIlues of
W,j>(lo)CIlII be obtlincd from the tables for roD::<o lIS presentc:d by van Evcrdingen
and Hurst in a convenient form, There is no plot of W",(to)for an infinite linear
aquifer. llUtead. the cumulative water influx ClIn be Cllieulalcd directl)' using the
following equation,
W. = 2h.w {(tel x tv> (7.9)\ 'P
whieh is expressed in Darcy units,
The corresponding equation in field units, with 1musured in hrs, is
, ~ .W; = 3.26)( 10 h,w'o/--;;j;)( lip (7.10)
• Application ojthc mil EI'trdillgtlJ a1ld HIITst (1949) wa/tT influx thto,>, In his/MY
ma/chillg
In the previous section the cumulative water infJulC into /I reserVOIr, due 10 /In
instant/lneous pressure drop applied /It the outer bound/ll'}', was expressed lIS
W.= U~IW..,(ID) •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (7.11)
In the more prnctic:al= of history mBlching the obser'.ed reservoir pressure, it is
necessary to extend the theory 10 calcuhlle !he cumuhuive ,,"tiler influx corresponding
to IIconlinuous pressure decline ftlthe rescrvoir.ftquifcr boUndllry. In order to perform
sueh c.nleuIBtioM, it is con\'Cntiollll1 to divide the continuous decline into II SCriC5of
discrete p=ure stcps. For the pressure drop between ench step. ~. the
corresponding wnter inflllX can be calcuillted using Eqn.7.11. Superposition of the
scparnte influxC5, with TC5pCCtto time, will giV(lthe cumulatiV(l wnter influx.
The recommended method of approximtlting the continuollS pressure decline, by II
r"'L
,I
•..j, <::::l A
--.:.D, A.• I A
~l u- A
o " " " Time
Fig, 7.4 Matching IIcontinuous pressure decline lit the reservoir aquifer.boundll1)' by II serlC5of discrete preMure st~
series of pressure steps, is that suggested by vnn Everdingen tmd Hurst, Timmermnn
and McMahon which is illustrnted in Fig. 7.4. Suppose that Ihe observed rescl"'oir
pressures. which nre assumed to be equtllto !he pressures at !he originnl hydrocarbon-
v.nter contact,
"are P,. P" P" p),..e1C,Bllirries 0, ',. t" ', •..••...•............ dc. Tllcn the llverage
prcs$Urelevels during the lime intervals should be dl1lwnin such IIway lhat
P- _P,+P,,- ,_ P,+P,p, =--,-
Pi '" PI-, + P J •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (7.12),. .
The pressure drops oa:umng at times O. '" I,. " ... etc. are then
'" "I' -I' =P -(P,-p,)",P.-P,"1'•• 1, 2 2
'" "P- -P- c(P,+p,)_(P,+p,J_P,-Pl•.•••""222
'" -p- -p- -(p,+p,)-(p,+p,)-p,-p,•.•••.•, - I I - -, , ,
_- - _(PI.,+PI)_{PJ+PI")_~P.,-~,_-~p",~-,_'", _ p, - P,., - ---- ----- .. , (7. 13), 2 2 2
Therefore, to calculate the cumulative waler infiul(. W.1l1rome arbitrary time, T,
which correspond 10 the end of the 11mtime step. requires the rupcrposition of
solutions Oflype, &i"_7.ll to give
"""here /vlJis the pressure drop It time 'I given by Eqn.7.13, and W•••(T" - tD
, J is tile
dimensionless cumulative water influx, obtained from table for the dimensionless time
Til -t", during which the effect afme pressure drop is felt.
Summing the terms in the laner equation gives
..'()It:,(r) = lIL:/vJi':O T"-'", (7.14),.10 the s;peciaJ Cllse of M infinite linear aquifer for which lISnoted in previous seclion,
there is no IV.., function, The curnull\tive waler influx at lime T due to 11step-like
pressure decline al the aquifer-reservoir boundary ClIObe calculated using Eqn.7.9 as
which when expressed in field units has the same COnstant, 3,26>: 10-': liSEqn,7.9
The solution of radial dilTusivity equation presented by van Everdingen and Hu~t
(1949) included no term describing vertical flow from the aquifer, Theoretically, this
model should not be used when there is significant movement of W1IltTinto the
reservoir fmm II bonom_wlller drive. To IIccounl for the flow of Willer in II venical
direction, COlI.ls(1962) and later Allard and Chen (1988), added a term to the equation
ofvlln Everdingcn and Hurst model which CIInbe treated as modified VllnEverdingen
and Hurst model. A sketch of the boUOm-W1IU!Tdrive reservoir-aquifer system is
shown in Fig. 7.5, Here the: aquifer thickness h is appreciable in relation to r~,Wllter
flows into and oUI of the reservoir across roughly horizontal reservoir fluid-W1IttT
interferelK:c and flow component in the vcrtical direction exists.
Fig. 7.5 Idealized flow model for bottom-water drive system
For the bottom-water drive model, the reservoir is typically visualized as a
• Right cylinder surrounded by a series of concentric cylinders representing the
aquifer of height h and exterior radius, r, with upper and lower faces impermeable
except for thaI portion (r < rR) of the upper face intersected by the reservoir
• The aquifer formation is considered to have constant, but unequal, permeabilities
in the horiwntal and vertical directions.
• The case of an average vertical permeability equal to a fraction of the average
horizontal permeability is a practical one in aquifers riddled with thin,
discontinuous shale streaks. This fraction may be taken as 1.0, of course, for
applications of this thick sand model to aquifers considered homogeneous
,
"• Math~maJica1 Consideratioll.t
To accoum for the flow afwater in a vertical direction, Coats (1962) lUIdlater Allard
IllIdChen (1988), added a term to yield lilt following:
ffp ..!. if' .. ifp _ P}IC, it'iJr' + , ()- ~f.";r.' - -O-.OOO~2-6~l-7k-~01.............•...••...•...••......•..... (7.15)
Where, ,.~is the ratio ofvenicallo hOrU:ontalpermeability.
There lift an infinite number of solutions to Eqn.7.1S representing all possible
reservoir/aquifer configurations, It is possible, lIowever, to derive a general solution
that is applicable to a vnriety of 5%lems.
Using the definitions of dimensionless time., radius and pressure and introducing a
second dimensionless distance, :0' Eqn.7.IS gives lilt dimensionless form of lhe
difTusivityequation:
,, - .D--''.. h
:D"'--, .,.F.'
6.33kt/0 '" I (7.16)
11~,r~
ifpp +_1 q,p + if~p = <Pp (7.17)a.~ rD erD doD iJlD
CoatS (1962) solved Eqn.7.17 for the terminal nlle CllSC for infinite aquifers. Allard
lind Clten (1988) used 11numerical simulator to solve the problem for tlte terminal
pressure case. In the deriVlltionof an equation for cumulative \WIer influx, it is
convenient to define a dimensionless pressure drop as
kF'"IJpD '" IJp '... ..•.....•.............................•.......................... (7.18)0.282,t'.11
"Eqns 1.16 lind 7.18 can be solved for land I'" respectively, to yield
I "'0 :::ik '"" (7.19)
"'"__6p 'IF," It. .~~ "" (7.:l11)
/:.po 0.282p
The equation for cumulative water influx, W•• wrillen in finite difference fOI111is
gwen as
SubStituting EqnS.7 ..19 and 7.20 into the above expression, we obtllin
"'. "O.5609:r:l,~'llt:JpL2!P.. " " (7.21)"".Finally. 10 conven this apression to a form comparable with tMt of vtln Everdingen
and Hurst. they defined a WIller influx consuml U. and a dimensionless wllter influ)(.
W.p u:
U=111'~,'. ~"~IIt
"d
where h is the aquifer thiekne:ils, This reduces Eqn.7.21 to
W. = UlJplYdJ •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• (7.22)
wllicll is Bnalogous to tile toimulBlion of van Everdingen and Hurst except tllat the
actual values of JY<II \'$. 'D for tile bottom water drive system will, of course, be
different from tlloS<!of the radial system because W<II for the bottom-WIller drive is a
function oftlle vertical permeability.
r~' _.-. 60
BCClIuseof this functionality, the solution presented by Allard and Chen (1988), found
in Table A.2 (Appendix), are functions of two dimen~innless panlmeters, r~and :~.
. "ro = - (7.23)'.
. h:0 = --, (7.24)
f"r~.•
F, =~'.where, r. i~ the aquifer radius, For fixed values of these two panuneters, W~ 's a
function of only '0'To apply the results of the col\StllTlt-terminal pressure CIlSCto the gerte:rtIJcasc: in which
pressure lit the gas!wllter contact V!ries with time Eqn.7.22 will be modified by use of
the principle of superposition,
w~= (Jt(npl~Y.•••.)") (7.25),..The use of this equation is described in several reservoir engineering texts.
7.5 Suitllbilit). of llquifrr model for the Rllbiganj gu fieJd
From the comprehensive descriptions of aquifer models, the suitahility of its
application to the H!biganj gas field is discussed here.
The Cllrter-TTIICYWllter infiul!; CIllcullltion model (1960) is simple in application
because it does not account the proper reservoir/aquifer geometry in it's calculation,
The final equation of this mode! is derived from the diffusivity equation for constllnt
terminal rate and also for nldial geometry. The equation incorporates no term
describing vertical flow from the aquifer. So, this model should not be used when
there is signifiCllnt movement of Wllter into the reservoir from a bottom water drive.
From log and core datil, it is clear that the Ilquifer suppan in the HllbigtlTljgas field is
"..", .through botlOm-Wllter drive: So, the applicaticln" of Carter-Tracy water influx
calcuilltioll model in this case 'Niilg; •..!:wrong estimate of wtItcr influx.
The VIlli Everdingen and Hurst model (1949) discussed previously is based on the
radial ditTus;"ity equlItion for COIIS!lIIIIterminal pressure without including the term
describing vertical flow from lite aquifer. This model was !Clulllly developed for
ClIkulating water influx from the edge water drive. So, it should not be used for II
reservoir having bottom water drive like Habiganj,
Another model developed by Allard and Cllen (1988) included the proper term to the
equation ofVlln Everdingen and Hurst (1949) model which accounts for the movement
of water into the reservoir from II.bollam water drive. This model Cln be applied for
both finite llnd infinite aquifer and also be treated as modified WII Everdingcn and
Hum model. Actually, this is the most appropriate model that can be Bpplied for wtlltt
infhn{ calculBlion for the HBbiganj gBSfield.
"Chnpltr 8
F.ST1MATE OF ACTUAL GAS-iN.PLACE IN TIlE IIAB1GAN.1
GAS J?IElD WlllJ AQUIFER FIITING
8.1 Introduction
Actual gas-in-place (GlP) in the Upper Gas SlllIds or Hllbiganj will be estimllled in
Ihis chapter with necessary calculations of the Wiler influx by applying suitable
aquifer model discussed in tile previous chapter. For the case of II singlc-phase gas
reservoir affected by natullll water influx, the most appropriate oprt'Ssion of the
malerial balance 10 history match the performance is Havlena lind Odell (1968) liS
described by Eqn.S.7 in Chapter 5. In principle, the left-hllnd side of the equlltion
should be determined from observed data but the righI-hand side conlllins two major
unknowns: the G1P (G) aM the cumulative water influx (W.), With
geological/petrophysical datil the reservoir engineer cormruCls 1\ suitable physical
model of the aquifer as described in the literature based on the geology and
estimated rock properties and applying a theoretical model, such as described in
Chapter 7, calculates the W!tCfinflux to t114tchthe reservoir offiake.
8.2 Aquifer Fitting Using Ihe Method of lIavlenll And Odeh (1968)
To estimate the GiP, the following $Iepsarc applied:
• Plouing of F versus G,.(backed-up by the plZ plot) as described in lheE,
Chap.2,
• Calculation ofWllter influx using the data provided by explof1ltionldevdopment
geologists, The data life the physical properties of the aquifer, such as, shape.
siu and rock properties, In abSCIl<:Cof such datil, values from a range of datil .
relevant to that geological area are used.
6l• Application of full Havlena and Odeh equation (&;n.5.7), by means of which a
Fplot of (E. + £/0)
IVB., is dI1lWIl.A correct aquifer model will
(E. + Efw>
simply provide a S'Irl1ightline ofunh slope whose inle«:epl on the ordil1lltc gives
tile GIP. If the selected .quifer model is ill fitting, however, the trend will
deviate !bove or below this line dcpc:ndent upon who:thcr the modeled aquifer is
too weak or too strong in providing WIller. In this ease the influJI calculation
muSl be modified until the required slope is obtained,
6.1./ Calculation anrl Graphical Rrprc;n,tation a/Hal'fena and Odeh f."quaJiaQ
In this section, there aTe twu parts of calculations for estimating the GIP of the
Habiganj gas field:
(a) Calculation ofw:tter influ," from .e.quifer.
(b) Application of full Havlena and Odeh equalion for gnlphical
representation
• WaItT Influr ('Aiclllotion Using Allord QndC~/1 (/988) Method
For the eulculation of water influx, Allard and Chen method ofBOtlom.water drive
model is used here because of the nature of water drive in the Habiganj gas field.
From log /IJ1dcore d!ta. it is cleM tlmt the aquifer support is through bottom-water
drive.
Here a list ofbue run data for WIIter-influx calculation is shown, basis of which is
described in the following section.
aUt run dalll
h, fl..
Aquifer k,mD
F,
10,387
"0180
1.0
"; 0.30
s.. 0.20
S. 0.212
Jl.,cP 0.63
-, 0.00003Cf,p.tl
-, 0.000003c.o pSI
-, 0.000033C" pSI
Now, tl description of cstimating the .e.bovephysical propenies of reservoir/aquifer
geometry orllle Habiganj Upper GIS Sand is given on the N.sis of study repon and
trial-error technique.
• Estimation of porosity (;) . initial connate waler satUTlllion (S••) and residual
gas saluretion (S•• )
In both lKM (1991) lUId Bcicip Franlllb-RSC (2000), the porosity value was
estimated to be 0,30. On the basis of their estimation, the present study assumes
that Vlliuc lIS0.30.
In IKM (1991) study, the Ilver1l8CS••wille was estimated as 0.28. IKM
cona:ntl1l.ted mostly on log eVlllual!ons thaI were highly questionable. For
Habiganj gas field, the Bvt:l"lIge wlues of S••for the Upper Gas Sand should be
0.20 as deduced by the study of the Bcicip Fnmlab-RSC (2000) which is lower
than that arthe IKM study report (1991). Beicip C:lIplainedthatlKM valu~ were
not based on the special core aJIlllysis (SCAL) data or previous ~tritnce with
this type of reservoir. Beicip also explained that irthe avcnlgc S., becomes 0.28,
the gas saturation will be 0,72 that WlISnot acceptllble by looking at the logs.
Resarding residual gas saturation (SO')' a section of Bangladcshi gtolOSim
claim that this value would be in the lange of 40"/•. This argument is solely
based on log analysis. The interpretation from log data frequently be wrong and
Beicip Franlab-RSC (2000) indicated, it should be on the ba,i, of performance
of Upper Gas sands.
The Density-Neutron Log, ,eparation below the current GWC indicate that the
"momentarily residual gas" saturation is higher than 20% as GWCn is
approached from below but much lower than 20% below the lowest depth where
the separation of density and neutron traces still indicating gas, to the extent that
these log, can no longer pick-up the "residual" gas pressure,
Actually, two method, (Craft and Hawkins 1959) are recommended for
estimating S.,. One from special core analysi, (SCAL) data and other one is
direct method from the reservoir performance. A, there is no SCAL data 'of
Habiganj ga, field, Beicip used the second method This method of estimating
the Sg, is generally carried out on the water-invaded zone So, in this case a
reliable estimate of the invaded volume is an important factor.
Water-invaded
•
Fig, 8 1Typical po,ition ofGWCi and GWCn
With clearly defined clean anticline structures and data from 10 wells, Habiganj
UGS qualifies for such estimation method,
"Beicip adopted thi, method in the following wtIy. The equation used to calculate
residual gas SlltUJ1uion(Sr) 'HaS:
.'I,. - (Gas left in the water-inVllded zone) + (Water-invaded zone pore volume)
GM len in the WIIler-invaded zone" aGJP - G,- PGJP
Where, OGIP '" Original gas-in.place
PGIP" Present ga.s-in-place
Gp - Cumulative 11Mproduced
Fig. 8.1 clarly illustrates tllis well.established engineering method, [n Beicip
Franlab-RSC (2000)report the 'above pal"ometen alongwith pore volume of ~
invaded zone were: estimated and later following the above method they
determilll:d the value of S,. liS0.212 i,e. 21.2%.
• Estimation of reservoir radius (r~) •
A amen! publication of Habigllnj Study Interim Report by Beicip Franlab-RSC
(2000) mentioned that the net reservoir bulk volume (V.) is about 1.2375" 1011
ell.1i and reservoir thickness (h,) is about 365.1 fl.
On the basis of these 1v.'O Vlllue!I,the equivalent reservoir radius is calculated as
follows
V ••trr,'1.. ."Then, r.-IO,387It
• Estimation ofBquifcr C'Xtemlll radius (r,>
In tbe present study the aquifer elItemal llldius is IlSsumedas infinite in
comparison to the rescrvo~r so that the dimensionless innux VlIlues (IV..,) for
oottollrwater drive iU desailK:d by Allard and Chen can be used. As hinted by
all the f1udies Ilbout the aquifer size, this seems to be 1\valid assumption.
• Estimation of aquifer thickness (h)
The estimation of aquifer thickness is mainly based on the trial and error
technique because of unavailability of any physical data of aquifer By applying
several values, as such 200, 250, 300, 350 ft alongwith the combination of k of
aquifer, Fig. S.3 shows a belter straight line of unit slope for the value of 250 ft
remaining other parameters same So, this value is taken for the present study
• Estimation of pore compressibility (cf) and water compressibility (c")
For the Upper Gas Sands and their cap rocks, no special core analysis work has
been done for the measurement of pore compressibility,c, Based upon routine
core data, porosity and permeability, grain size descriptions and data from
analogous reservoirs, Beicip Franlab-RSC (2000) mentioned in their study report
that one could expect the pore compressibility would be somewhere between 30
and up to even SOx10'" l/psi. which is 10 to 26 times more than water
compressibility The present study assumes that value as 30 X 10,6 Ilpsi. The
water compressibility assumes here as 3 Ox 10-6 llpsi which was considered in
all the previous studies.
• Estimation of aquifer permeability (k) and water viscosity ()lw)
Actually the value of k was estimated on trial and error basis alongwith the
combination of aquifer thickness, h within a reasonable limit for that geological
area. Present study assumes permeabiEty of the aquifer is j 80 mD estimated
from trial and error technique.
In Beicip Franlah-RSC (2000) study report, water viscosity was taken as 0.63 cp
at reservoir temperature of 116°Fwhich is used in the present study.
••• Determination of dimensionless time, ID
The expression for dimensionlen time.
'D" O.00634( b.). I in days;P.c,r,All the properties III 1m: right_hand side for the aquiferlrescrvoir geometry of thet1l1biganj gas field IIrc described !bove lISsystem pllJ1lmder. The following Table isconstructed on the basis of those data.
T.hle 8.1 Determination of 't>WIlles
~" • Av. l}l • " • "Dm. ~2d1)~"'" • i" s.. c••.•••• '''' '''' '''' " "18.66J 21~9.64 0 ,., ,., 0.6) ,., 0.00000(I(, 3.00Il.os 3.00E-<l6 3.]00-05 ]0]87 ,2J.s.ss 2119,75 .., ,., ,., 0.6) ,., 0""""" 3,OOI!-OS 3.00E-06 3.30F.-os 10357 0,6
31.51\6 21011,95 8J74 ,., ,., 0.63 " 0""""" 3,OOO'{)S 1.OOE-06 3.301:-05 101K'l 14,1021.1.90 Itl94.S8 "" ,., ,., 0.63 " '''"'''''' J,OOE.(ls 1.OOE.(l6 3.]01'..-(15 lOlI7 16.4
23.1.90 2O<JUI4 "" ,., '" 0.63 0.' '''"'''''' J,OOE.os J.OOE.06 J.300..os lalit' 16.417.7.98 2026.51 "'''' ,., " 0,6J " '''"'''''' J OOE-oS J,OOI!-06 3.300.05 10]87 21.7011.2.99 2019.24 1]011 '" " 0.63 ,., '''"'''''' 1.OOE.oS 3,OOE-ll6 3 ..\llE-oS 10)87 11.102.899 2016.21 1)180 '"0 OJ 0.63 " 0.""""" J.ooe-os J.OOE-lI6 J.lOE-oS 10387 22.403.899 201S.3S 13181 ,., ,., 0.63 0.' 0.""""" l.lIQE-oS l.l101!-06 J.JOE-oS 103ft7 22.4026.8,99 2012.94 132IM ,., " 0.63 ,., 0.""""" 3.00F.-oS 3.0010.06 J,JOE-oS 10387 22.40
• Determination of aquifer constant (U)
The equation for determining aquifCl"constant is U '" 1.119(hc.r;
Now, putting the an respective wlues in the above equation,
U. 1.119X 0.30 x 250 x 0.000033 x 10387' - 298803.412 bbl/psi.
• Determination of dimensionless influx values (W",l
For the determination of 11'",wlues the following parameters are determined:
,.• _ Ie,r. __
'.h
"D "'----,r~F;
According to Coats (1962), in practical situation, an average venical permeability
(k.) equal to 1\fraction of the average homontal permeability (kH ) for the aquifers
"riddled with thin, discontinuous shale streaks. this fraction may be taken, as 1.0 for
application of this thick sand model to aquifers considered homogeneous. The
present study assumes the value of k, as the unit fraction of kll _Because, Beicip
study (2000) on the Habiganj Upper Gas Sand (VGS) indicated that the net pay
thicknes:; to gross pay thickness ratio was over 95% and also absence of shale
streaks in the VGS, This is a very good indication of cleanliness of sand and close
to the homogeneity. So, in this study, the F, value IS taken as 1,0. Now, by taking
the values of W.J) from Table A-2 (Appendix) against the values of IDand z~,
Table 8,2 is constructed, It should be mentioned here that Table A-2 is taken from
the paper of Allard and Chen (1988) for infinite-acting aquifer where W.o valLles
start from the minimum value of z;,as 0,05, Any value below 0.05 does not vary
significamly. From the present study, the base run value of z~ is estimated as
o 025. As there is no W,D values for z~= 0.025 in the table, W.J) values for 0 05 are
used as minimum.
The last column of this table estimates the water influx values from the aquifer in
breE.
Table 8.2 Calculation of water influx values using Allard and Chen method (1988)
Pr@,psia ,'0 0025
'0 4875' TVD Step pc. ft;n,Cha1\ W~bbt W"brcf
" 2149.64 " " 0 "l3.(' 2119.75 14.945 8.4672 378tt272 0,2121212t4.20 210M,9; 20345 8,7172 90490414 0,5076512t6.4 2094,58 12585 9.708 128307412 0,719l\O46t6.4 2091.04 8955 9,708 157880778 0,8857112
21.70 202(',51 34035 !l,9517 258382893 1.4495282210 2019.24 359 12.11(,] 368928346 2,069(,882240 2016.21 5.15 12.n9~ 404714435 2.27044822,40 201535 L945 12.2394 427X36073 2.400160422.40 201294 1.(,35 12 2394 457344973 2.5657053
'"Here. II !la/llple aicullltioll for determining wattT influx (W.) III pressure 2094,58
psia (fab. 8,2) is shown ..,The equation for W." u" :~:>\"IJV<I)(TD-to),.fI'" 1.119XO.JO X250 XO.OOOO33x 10387' •• 298803,412 bblfpsi.
W."' 298803.4 12 ,,(14.945:-: 9.708 + 20.345" 8.7372 + 12,585" 8.4672) bbl
"'(l28307412bbl >:5.61 cfl)lIO' bn::f
•• 0.1198046 bTcf
tl.2.1 Appliral;on of Furl lIa,'/ena and Odeh Equation (1949) lor Grophica/
Rtp~~enlntion
AI this Slllllc. the full HlIvletL!1lind Odch equation is applied for estimating the GIP
of the UGS of lIabiganj gas field. Table 8.3 is conmrucled in lille with the
parameters of the equation ofHllvlclllIllnd Odeh.
Tllhle 8.3 Values for the application of full equation of HIIVlclllI and Odeh
w n~'"
£ F n~ W;V(e +/.:•B,d , - ""'" '" "'" "'", 1I.834M] 0.00632455 " , , , ,0.2121212 1I.SJJ36S4 O.(l(l(i..\08SS 8,397E..oS 7.2J08-IE.06 0.69748 7(,(7.7061 231S,~S\I6JI(I,~J6S12 (l8JJl20S (I.OOI.()94~ (1,0001149 9.&4353E..Q6 0.9O'J!2 129l.J339 4(169 6Jll602(1,1198046 08321948 0.OO6..\R1l2 (I,ooolSl>S l.lJl9SE.oS 1.6904(1 9912.9S99 4231,649218(I,SSSJ112 0.83271., 0.()O("'9146 0,0001669 L.41762l!'()5 1,6967 93708668 489l.J479"1.449528 0.8312516 '''''''''' (1,0003618 2.9'1lr7E.()5 U8-49 11197,142 J70L42ros)2.069688 0,11)10863 0,00670915 0,000]&46 3.15.,7E!.()5 4.6448 11162,]68 497].8373]92.270«8 0,8)10181 0,00671868 0000]941 3.nn1E!.oS 4,8"9 I139M89 5324,99972.400160-1 0,8)09986 0,00672138 o oooJ96l\ 3.H86lIE!.o5 4.8611 1132l.44) 5590,9659022.5M1OSJ 0,8)0944 0,00672899 ,."..," 3.]lI698E!.oS 4,8946 111811.231 586031906
By laking the wlues from the above table, I plot of FI(f:.+f:I-) versus
W.8./(E. +EI-) is drawn.
Now, applying least square fitting by regression analysis, Fig,8,2 shows a straight
line of 45Q slope, which is the main criterion of Havlena and Odeh plot. After
backward extrapolation, this straight line intersects the ordinate at the point of
approximately 5453 bscf. i.e, about 5 453 TCF. So, Glr (G) of the Upper Gas Sand
of the HabiganJgas field is estimated to 5.453 TCF with 'aquifer fitting.
1]000
120GO
IIGGO
10GGO
900"
SQOO
i 7MG" "~~E 6MG
.IMO
4000
3MO
,000
1000
, 1.0119:<+5 53,4
R'-0470 •• f-'• •
~
'/
./
,m' 5,453 TCF
1000 3000 ,GOO (,000
Fig 82 Estimation ofGIP by Havlena and Odeh plot with aquifer fitting
n
8.3 Sensitivity of Input Parameters f(lr Reserve Calculation
Fig.8.2 is actually drawn on the basis of several trial and error values of kh
combinatIOn, which mainly governs the flow of water from aquifer to the reservoir.
Other kh combinations would give the same result. Actually, no one can deduce a
definite value of k or h for the aquifer under study. The main reason for not
recording physical data of aquifer properties is that it involves a huge extra
investment of drilling beyond the owe leveL
Here, the sensitivity of F, is tested varying this value from the base run data
(i.e. F.=1,0 Fig.B,2), as 0.1, 0,5 keeping all other parameters same In all sensitivity
analysis cases, slope oflhe tine is always maintained at 45"
• For f: '" 0,1, the manual fitting indicates the value of GIP as 5 44 TCF
(Fig.8,3a)
• For F,= 0,5, the manual fitting indicates the value of GlP as 545 TCF
(fig.S 3b)
With the variation of k in between :t44 44% from the base run data (i.e 180 mD)
remaining all other parameters same, the manual titting (slope at 45°) (Fig,8.4a and
8.4b) indicates the variation of GIP of (-) 17.43% to (+) 37.61%, which is 4 5 TCF
and 7.5 TCF respectively.
With the variation of h in between :1:20"10from the base run data (i.e. 250 ft),
remaining all other parameters same, the manual fitting (slope at 45") (Fig 8.5a and
8.5b) indicates the variation of GIP of(-) 9,92% to (+) 28 44% This implies that
the GIP estimation to be in between 4,90 TCF and 7 TCF respectively.
In the same way, with the variation of pore compressibility (cf) in between :83%
from the base run data of 30 x 10"" llpsi, the manual fitting (slope at 45°) (Fig,8,6a
and 8.6b) indicates the variation ofGTP of(-) 2.57% to (+) 14,67%. The variation of
GTP estimation is in between 5,3 TCF and 6.25 TCF respectively, So, the present
study attempts to estimate an approximate GIP of the Habiganj gas field within
"reasonable limit adopting trial and error method for the aquifer fitting which is the
common technique in the area of reservoir engineering.
,,~""'00'
~"'00',
,"00" -,,.,., ,.<! ,. "'~ I" •••••• ,. j 44 -,,;
••••, I, ,. •• - ,. •• •• ••
w,n"-"l'& hfu
,-,,~.". ,
"'",.•• r ,•• 'I
I' •• I ", I••••
I G' -, m••••,. I-,.,, ". •• •• ,. •• •• ••.,...,...•,
Fig. 8.3 (a) Sensitivity ofGIP estimate due tochange in horizontal to vertical permeability ratio.Fk~O,1
Fig 83 (b) Sensitivity ofGlP estimate due tochange in horizontal to vertical penneability nF. = 0,5
""'.''''"LO"M
"00..,"'..',
~.~,..,,""',,',"'00
""''""",
-'
,
'IP= ,'iT F
'"" "" '00' ".. '""" '"'" "" ••••
,., ,
".,. ,•••• G~-7.5T ,,
t,
"j ••~ •• I
••••••••,, '" •• •• •• •• ••w,~_
Fig, 8.4 (a) Sensitivity ofGIP estimate due tochange in permeability, k = 260 mD
Fig 84 (b) Sensitivity ofGlP estimate due tochange in permeability, k = 100 mD
" "" ,"M >OM '"" '"00 """" ".,,' '""W,'.~<"~
"""'"""'''"'
'''00? j ",00,..""""""CO"
•I •
•
:9 CF
., ,- , , •".,- I.'-,. I•- •,., ••,, •• GO, ,re,.
••-,.,, ,. ,. ,. '. ,.,
""~"'.' !
Fig. 8.5 (a) Sensitivity ofGlP estimate due tochange in aquifer thickness, h = 300 ft
"'""m", • • •"""...,"' •,., "Ii ..,, ,'. ,,.. - -,..,.,, ,.. '" ,. .., '""" .., "j.~"".".
Fig, 8.6 (a) Sensitivity ofGiP estimate due tochange in pore compressibility, cf= 40>< 10.G llpsi
Fig, 8.5 (b) Sensitivity ofGIP estimate due tochange in aquifer thickness, h ~ 200 ft
".". • • •".. ~I". •-- •, •,.-"~, ,-".- , - 6.2'",., ,, >000 - •• '" •• ••
W""""''''''
Fig. 8.6 (b) Sensitivity ofGIP estimate due tochange ill pore compressibility, cf= 20.1 x 10.6Ilpsi
"It is vcry important 'specllo ~imate GIP keeping all input dala within reMOlIlI.ble
limit, which was not followed in IKM (1991) study. From the lellsitivity lnalysis, it
can be seen Ihllt the BIlge of estimation may Vllry from a low VlIlut of 4.5 TCF 10.
high value 00.5 TeF, The average of these two values is 6 TCF lind wilh proper
aquifer filling the base run gives an estimate of 5.45 TeF.
8.4 RHo"try Factor
The recovery factor is a number between zero and unity which repre5enlS the
ffllction of reoovcl'lIble gas. The two main ClItcgOriesof hydrocarbon recovery are
calla! primary and supplementary. Primary recovery is the volume ofhydrocarbons,
which ClInbe produced by utilizing the Datum energy llvailable in the reservoir and
its adjacent aquif~. In contrast, supplementary recovery is the 8M obtained by
adding energy 10 the reservoir-fluid system. New information of the reservoir, new
technology of eX\fllClion and the economy orlhe extraction may change this number
from time 10 timc. Recovery fllctor depends mainly on the reservoir characteristics,
drive mechanism and good reservoir management practia::, A study (HCUINPD
2001) of recovery factor for non-asIDc:ialed gas field of over 1 TCF size indicates
11ullIIvCfllge recovery fllctor is Room 75%. However majority of the fields we{e
ranging between 70 to 85%.
Gu expallSion is IIvery cfficient recovery mechanism and when wnter.influl( koqn
the pressurc up. greater 8M volumes lire lcft in the reservoir lit abandonment. A
strong "''liter drive hclps to maintain delivertlbilily sina:: the restTVOir pressure dOC$
not decline lIS rapidly. Ultimllte recoveries of 80-.4-90"10 are common in depletion
drive gas reservoirs while typical recovery factor in wllter drive gllS reservoirs can
range from 50% to 80'% depending on size of the field and the e~ent of aquifer,
suppa". The recovery rate for WIlter drive reervoirs can be achieved up to 80"10or
even more by optimiled production strategy, good reservoir management and right
mal1llgement dedsion.
"t14.1 Cafculation of R«ol't'1')' Factor from Volumnric Gm Rcsen'Oj,.
In many gas reservoirs. plIrticularly during the development period, the bulk volume
is not known. In this CllSC,it is better to place the reservoir Clliculllliolls on II ulIi!
basis, usually I lIe-ft of bulk reservoir rot\;, Then one unit or I IIC-ft of bulk
reservoir rock contains"
Connate wmer:
Reservoir gas volume:
Reservoir pore volume;
43,560x lib: S. cuft.
0,560".9" (1- S.) cull.
43,560" 9 cuft.
The initial standard cubic feet orgas-in-place in the unit is:
G = 43,S6O(pXl-S.,) !lCf7l1c-f1H.
For a reservoir under volumetric control, there is no change in the interstilial water,
SO the n:sclVoir gas volume remains the same. If 8~ is the gu volume factor IIIthe
abandonment pressure, then the slzmdllfd cubic fecI of_gas remaining III
abandonment is:
G = 43.S6O(pXI- S••) sc17ae-f1• B•
Unil recovery is the difference betwt:Cn the initial gas-in-place lind that remaining III
abandonment p~sure (i.e., that produced at abandonment pressure), or:
Unit recovery" 43,S60P(I-S .••J_' 1_] scOac-ftlBr Bp
The unit recovery i~ also ailed the initial unit reserve, whieh i~ generally lower than
the initial unit in-place gu. 1lte remaining reserve at any stage of depletion is the
differern::e between this initial reserve and the unit production at that stage of
depletion. The fractional recovery or recovery factor expressed in a percentage of
the initial in-place gll5 is
,
n
~
I I"
lOO(G-G) H•• -8 •.•Recovery factor" ----.- • %
G 1
".Recovery factor" 100[1- =:] (8.1)
Experience with volumetric gas reservoirs indicates that tile recoveries will range
from 80 10 90% (Crafts and Hawkins 1991).
8.-1.2Caku/alion of Reco~ FltdOTfrom GtURI'$",'Oin Und~Wot& Ori\~
III reservoirs under water drive, the pressure suffers an initial decline. after which
water tnlers the re5efVOir'III 11rate pmportiDnlllc 10 production. llJ1dthe pressure
stabilizes In this use, the st.e.bilized pressure is the abandonment pressure.
If H••• is the gas volume factor III tiM:abandonment pressure and SO" is the residual
gas SIItUTlIlion,expressed 1$ a fraction of the pore volume, after water invades the
unit, then under abandonment cornlitions a unit (I ae-n) of the reservoir rock
contains:
Waler volume:
R~rvoir gas volume:
Surface units of gas:
43,560><9 ><(l- SF) cuft.
43.560'qhS •• cuft.
43,560)( P )(S•• +B•.• cull.
Unit recovel')' is tile difference between the initial and tile residu~1 wrface unilS ofgas or
Unit recovery in SCFI ~e-ft.,43560)( 11-S••1H,!L]Bga
•
- . .' ,The recovery factor e~resscd in a percentage of the Hiilial gas in place is
Recovery factor"
II-S ••_~
B.. B~l-S~B.
lf thc Wlltcr drivc is very active so that there is essentially no decline in reservoir
pressurc unit recovery and recovery factor become
4356O"9"(I-S ••-S ••) .Unit recovery" --------- SCF/ac.ft
B.
100(1-5 -5 )Recoveryfador~ (1-;.,)" % " : (8.2)
As the residual gas saturation is independent of the pressurc, the recovery will be
grClllcr for thc lower stabilization pressure. The residual gas saturation can be
measured in the h!boratory on representative core samples
For tilt: Habiganj gas field. tilt: aVClllgcvalues of Sol for tilt: upper 8115illlOOis 0.20
and S•• is 21.2Y. as deduced by the study of tilt: Beicip-FnIllJab-RSClPetrobangla
(2000) which is mentioned in Section 8.2.1.
Now. by putting the values of S•• as 0.20 and S•• as 0.212 in the Eqn.8.2.
R fi 100"(1-0.20-0.212).,, 13 S%ecovery actor'" -------- •• '" ,(1-0.20) .
The presenl study takes the recovery factor lIS70"/•.
.- ", " .,
",8.5 Ddrnninotion of Drin lnditt5 (Dr) for the Hnbiganj Gas Field
In the study of the Habiganj gas reservoir using ll1B1enalbalance, it is of prlctiCB-1
interest to drtennine the relative magnitude of each drive mechanism Le. gas
expansion, pore compaction and water influll. Pinon (1958) rearranged the general
material balance (Eqn.5.4) to obtain three fractions, ~hose sum is nne that he called
the deplrtion drive inde.'< (001), the segregation (gas ClIp) inde:'l (501), and the
watcr-drive inde:'l (WDl). The method of Pirson for general material balance
equalion un be applied to the ~ reservoir lU follows:
In Chap.5 the material balllflCCfor gas reservoir WllSexpressed in reservoir VOlumes
of production, expansion and influx lIS:
Underground removal • GilS expall!lion + Water expansinn/pore compaction +
Water influx (ref.)
i.e. G,8, +8.W, = O(H,_H,,)+GB,,[c.s- +c{ 1.-+W.8••1-.'1_ r'
And, adopting the noll'ltnclalure ofHavlena and Odeh.
f' = G ,H, +W,B. =Totll1 gas and Wllter pnxluctions (ref.)
Ef. = 8" k.s_ +cr) tJp =Expansion of the connete water and reduction of the pore1-5•
space (rc17scO
The following expression un be found,
F- GEs+ GEf.,+ W.B•...................................... , (8.3)
Now. by dividing Eqn,8.3 on both side by f:1 - Gf;,'F +GE,./F+W.B•.IF (8.4)
"Eqn 8A is II sirnil11Texpression lIS proposition of PitSon (1958) in general material
balance equation. The first lerm in Ihi~ Eqn.8.4 may be called the expansion drive
index (EDI), the second term is the compaction drive index (CDI) and the third term
is the ••••'lIta--<!rivcindex (WDI). Then this expression will be
EDI+CDJ+WDI- 1
Table 8.4 has been constructed where the sum of above drive indices is shown. This
is lin indication of correctness of the mll!crial bnll1ncc:calculation for eslimnting GIP
of the Habiganj gas field. Fig.S,? shows the relative contribution of aii drive3 lI!
dilTerent pressures.
Tlible 8.4 De!erminlliion of Drive Indices showing the correctness of lIlllterial
balance calculation
-....D". ,m " "" GE/F G,F. G/o.""'" W.B. WJi,..'1-' EOI+COITV<> "" em wm 'WO'..,
"'" "'" ""0.00 0 0 0 0 0 05072.70.4259363 0,6107 0.0366799 021212 O,lG-l1 0,96745072.7 0.S828347 0,6406 0,05059621 0.50765 0.5579 1.2S425072.70.79.(0782 0,4687 0.06879507 0.71980 O.4U9 G,9HJ~71.' 0.U6S6J 0,4'189 0,07J69163 0,88371 0,3220 1.06443072.7 1.83S4H6 0,4186 0.1330876S 1.449S 0.3303 0.78433072.7 1.9~2 0.419\1 0.1(91768) 2,0697 O.44SS 0.\10203072.7 1.999IJ3J 0,4113 0.17J6%66 2.270-1 0.4673 .91463072.7 2.0128769 0,414J 0.17X16444 2.4002 0.4937 .'}4)S3071,72.0SI4337 0,4191 0.17817&O~ 2.SliS7 0.SH2 9797
From thistablc, it can be seen that there are two bad points where the summation of
the drive indices is otT by more than 20"A. Apart from these two points, all other
points check within 10"10. It is seen that at the elIny !>Iageof production the gas
expansion Wll$the dominllnt drive mechanism (llbout 62"/.) but quickly the Wllter
drive became equally
"• EDl •• <DI .••WDl "ID1~ff-WDl
1A1.2
••0.'
is 0.' • ••"OA •
0.2
0 • • -21S0 2120 2090 2060• Pr=utc. pm
Fig. 8.7 Relative contribution of each drive
•
2030 2000
dominant. It is also sten that water drive supplies about 45 10 SS% of the toul
energy in the reservoir lind the gas compressibility about 40 to 50%, The
combination of port CQrnpaclionis 4 to S%oCtile total energy.
8.6 RtliuliS Rnd Discussions
Now, the results of the study can be presented as follows:
• There is a fairly strong bottom water-drive in the Hllbiganj gas field
• Estimate of gas-ill-place from the Havlellll IlJIdOdeb plot without aquifer fitting
is about 5 TCF.
• Estimate of gas-in-place from the pfZ interpretation method is 5.16 TCF
• &Iimalc of gas-in-place from the Hllvlena and Odeh plot with &(juiCer fitting is
about 5.45 TCF
"
The lalest'volumetric support for the HabigJInj gil.! field is 4.69 TCF as estimated by
HCUn-TD (2001) study. The closet IIllItch to this estimate is 5 reF found by
Havlena ftJ1dOdeh plot. According to reserve categOlY guideline elM (1994),
Habigtmj hils enough pressure-production da~ to cn.llthis S reF liSproven resc:rve,
deriving from the fact that volumetric estimation SUPPO" is resonable. The
difference between Ibis S reF and 5,45 TCF found from Havlena and Odeh plot
with necessary Iquifer Iilling may put imo prohable categOI)' which is 0,45 rCF
(i,e. 5.45-5 Tel'). NOW, by t.e.king70% reoovef)' factor. the recovemble proven
plus prohable reserve (2P) ofthc Habiganj gas field is estimated as 5.45 reF"0.70" 3.815reF i.e. 3.82reF,
Table 8.6 is constructed with the necessary comparison of earlier reserve estimates
induding recovery factor by different authors for the Habiganj gas field.
Table 8.5 Comparison of2P GIP and Recovery Factor for the Habiganj gas field
Estimated by Year Reoove,)' " RecoverableFactor GIP. TCI' Rsrv" TCF
Petro-.c<lnsult 1979 0,70---0.8 '" U8
Petnlbang1a I~S2 Il.S5 2,) l.96
PetrobanglJ "" 0.80
GGAG 1986 '"' 2.42 1.81
HH" 1986 2.98
Gasum. 1989 0,69...(),88 3.71 ''''Tekmca 1989
Well drill 1991 0.75-0.84 U 2.78
H<M 1991 0,41-0.71 3,67 L8~
BAPEX t992 U
Clyde Petroteum t995 0.80-0.90
Petrobangla-Be,clp FrnnJab ''''' 0.74(0,70) 4,62 3,42
HCUINPD 200t 0.75 5,t4 H'
Present Study 21l1l2 0.10 5.45 3.5
In estimating the recoverable reserves for the Upper Sands three main factors are
apparent,
• The high quality and thickness of the sand itself
• The extensive underlying aquifer
• The relatively shallow depth with correspondlfig low initial reservoir pressure
The total gas recovery will be very sensitive to the careful management of the
reservoir development and the final reservoir abandonment pressure, Using IKM
volumetric GIP 0[3.63 TCF, the Well drill Limited (1991) deduced the recovery at
1100 psia surface abandonment pressure was about 1,9 TCF i.e 52% recoverable.
But il was not based on pressure depletion of the reservoir The actual case is that
the abandonment pressure of the Habiganj gas field can never be so small because,
it is a shallow depth reservoir in comparison to 8hakrabad gas field The initial
pressure of Habiganj reservoir is too high and as a water-driven reservoir the
abandonment pressure would not show so small value like 1100 psia. So, in this
case the recovery factor estimated by Eqn,8.1 using fig, and Bgo would give wrong
estimate of rewvery factor. Rather the recovery factor mLlstbe calcLllaled by Llsing
the Eqn,8.2 using the tenns Sw. and Ss"
lKM (1991) also carried out a numerical simulation study in the Habiganj gas field
They deduced the recovery factor at 52% on the basis of numerical simulation
technique that used a smaller volume and a high water influx co-efficient. The
actual case is that the Habiganj gas field has higher volume and the micro-recovery
is 73% found from actual performance data as studied by Beicip (2000).
8.7 Suggested Production Strategies for the Habiganj Gas Field
From all the previous sludies, it was shown that the Upper Gas Sand of the Habiganj
is so clean and unconsolidatcd, well sorted with quartz as the dominant constituent
The a'verageporosity is in the range of30% and permeability commonly in the range
of2 to 4 Darcy. Thc Upper Gas Sand in Habiganj represents the single best reservoir
in Bangladesh, being a relatively continuous stacked beach and barrier bar sand
sequences, reaching over 750 feet in thickness'and extending seven by three miles
aerially at the GWe. This reservoir is supported by a very active bottom-water
aquifer,
In this situation the following suggestions can be made for future production
planning of the Habiganj gas field,
• To produce gas from the reservoir at high rate within practical limit, Due to this
policy, a significant gain in recovery will be achieved for the rapid evacuation of
the gas before Jess mobile water can catch-up and trap significant quantities of
gas behind the advancing flood front, Actually, it is quite feasible due to
abnormally low value of mobility ratio of water-gas displacement. Typically,
this figure is 0,0]0. That mcans, under an imposed pressurc differential, the gas
can travcl 100 times faster than water by which it is being displaced and can
therefore be removed before the water has the opportunity to advancesignificantly.
• There are many cases in the literature (Brinkman 1981 and Chesney, Lewis and
Trice 1982) where significant gains (20-30%) in recovery by the accelerated
withdrawal of gas from water-drive field have been reported If there is not the
opportunity to increase the rate, at lcast every effort should be made to maintain
it at as high a constant level as possible,
• For the water-drive fields like Habiganj, it should always be maintained as a
high base production continuous irrespective of any seasonal or daily variation
of demand. For the case of off-demand any depletion type fields, which are not
rate sensitive, should have their production reduced,
"'• Another way of enhancing the recovery from water-driven gas fields is
"Pressure blow down technique" in the flooded-oul regions. That means, the gas
producing wells flooded by water encroachment should be completed wIth gas
lift strings and converted \0 high-rate water producers_ Due to this program, the
pressure in the water-invaded zone will be reduced and allow the trapped
residual gas to expand so that some portion of it can percolate updip into the gas
column where it can be produced. As depletion occurs, the trapped gas volume
(saturation) remains unaltered but the quantity of trapped gas (n, lb-moles) is
reduced. A case history in the North Alazan Field in Texas, four high-rate water
producers were drilled and completed in the aquifer of the field while a funh.er
th.ree wells in Ih.e water invaded zone were convened as water producers, By
with.drawing water at a rate of 30,000 bid the abandonment pressure was
expected to be reduced from its natural water-drive level of2200 psia to 500 psia
releasing 22 bscf of trapped gas, which raised the recover)' factor by almost
30%, Though this technique is etTeetive in tight reservoirs, but it can also be a
good stratC!,'Yfor the Habiganj gas field at the later stage of its production when
the wells at the edge of the reservoir get flooded,
Chapter 9
CONCLUSION
9.1 Conclusion
Based on the study presented in the preceding chapters, the following conclusions
can be drawn:
• For establishing the drive mechanism in a gas reservoir, the Havlena and Odeh
method (1968) is more scnsitive and practical than plZ plot though the later one
is the most popular method in the industry for applying material balancc. The
main drawback of the plZ plot is that, it could be ill5ensitive to water influx. As a
result, may misjudgc the drive mechanism that could produce serious
overestimatIon of the GIP. So. Havlena and Odch method is recommended as a
means of checking the validity of the p/Z plot
• The latest volumetric recoverable reserve estimated by HCUINPO is 4.69 TCF.
So, the most conservative estimate of the present study which is very close to
this figure is taken as proven category (i e 5 TeF) and 0.45 rCF is the probable
category that was additional estimation from material balance calculation with
necessary aquifer fitting. From the drive indices analysis, it is found that the
calculation by material balance equation has a ,,10% uncertainty,
• From all the sensitivity analysis of input parameters, it is found that the variation
of GIP is in between 4.5 TeF to 7.5 TCF, The average of the two numbers is 6
TCF. The 2P estimate of5.45 TCF found from the base run is a very close match
to this number.
• Extraction rate from the Upper Gas sand of Habiganj should be as high as
practically possible due to the presence of strong bottom-water drive,
9.2 Recommendation
Upon the conclusions of this study, the following areas are recommended for further
study:
• Water coning estimation of the Habiganj gas fic!d may be a future study .
• For establishing the reliability of the material balance study, the numerical
simulation technique can be applied to the Habiganj gas field for verification of
the aquiter filling and future performance prediction and recovery calculation.
REFERENCES
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Reservoir, JPI 2475 (December)
Chesney, T. P" Lewis, RC. and Trice, M.L (1982) : Secondary Gas Recovery
from a Moderately Strong Waterdrive Reservoir: A Case History, JPT 2149
(September)
CIM (1994): "Estimation of Oil and Gas Reserv~" Petroleum Society of Canada
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"Vega, L. and Wattenbarger, R. A. (2000): "New Approach for Simultaneous
Dctennination of the OGfP and Aquifer Performance with No Prior Knowledge of
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Walsh, "MarkP (1999): "Effect of Pressure Uncertainty on Milterial"Balance
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Petrobangla", Petrobangla.
•ArrENDlX- AI: Location orH~biganj Gas Field •
"
, '
"
.' ,~.'", : -
BANGLADESH INDIA
I~
APPENDIX A-2: Dimensionless influx, W,D for infinite aquifer for bottom-water
drive (Allard and Chen, 1988)
"
"0.05 0.1 0.3 05
12 7,742 7.718 7495 7.104
13 8.]96 8172 7.943 7539
<4 8,648 8.623 8385 7.967
15 9094 9,068 8.821 8,389
16 9,534 9507 9253 ~.806
17 9,969 9,942 9.679 9,218
" '10399 10.371 10.100 9.626
19 10.823 10.794 10516 10.029
20 11241 11 211 10.929 10430
21 11.664 11.633 11339 10,826
22 12,075 12.045 11.744 11 219
2J 12.486 12454 12, \47 11 609
