A review of Geopressure Evaluation from Well Logs.pdf

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A Review of Geopressured EvaluationFrom Well Logs Louisiana Gulf CoastR. A. Lane, SPE-AIME, Shell Oil CO.L, A. McPherson,* SPE-AIME, Shell Development Co.

IntroductionSince the beginning of geopressured drilling in theLouisiana Gulf Coast, : empts have been made toquantify log parameters as an aid in pressure prediction.In 1965, Hottman and Johnson presented an empiri-cal correlation relating Am.mal formation pressures todepartures from normal shale velocity and resistivitytrends observed in Gulf Coast formations. These rela-tionships have been used widely for predicting youngerTertiary abnormal pressures, although both sets of datawere obtained from Miocene-Oligocene sediments.In recent years, other empirical but large] y un-documented resistivity relationships, based chiefly onmud-weight observations, have been established and arecommonly used offshore. These account for local trendanomalies wherein the Hottman and Johnson resistivityrelationship is not suitabi y accurate. Some of these er-ratic trends have been found to be systematic either inkind or areal extent and, once recognized, can be inter-preted. In 1972, while this study was in progress,Eaton2 suggested that variations in overburden gradientmight be responsible for irregularities in departure

trends.Since 1965, drilling activity has moved farther off-shore into younger Pleisto-Pliocer.e sediments. With theonset of production in these newer fields, some 50 ad-ditional pressure measurements in virgin geopre~-sured reservoirs have become available. The density 1C6has become the primary porosity log offshore, and the*No w w ith She ll 011 C o.. Ne w O rl ea na , La .

prevalence of density data provides a meand to calculateoverburden gradients in these fields. It is consideredtimely to include the new data with those of Hottmanand Johnson. Resistivit y data are emphasized becausethe resistivity device often is the only log run over suf-ficient intervals of borehole.Pressure estimationTo estimate formation pressures from logs in the GulfCoast, the following information is necessary: (1) anestablished normal log response trend in hydropressuredshales, (2) an observed departure from the normaltrend, and (3) an empirical relationship between thistrend departure and formation pressure gradient.Hydropressured TrendsThe first trends of sonic and. resistivity dztti for theoffshore Miocene-Oligocene were presented byHottman and Johnson. These trends are averages ofearly observed data in the Louisiana Gulf Coast. How-ever, since compaction trends probably depend not onlyon depth but also on rate of compaction, cementation,and overburden, these Miocene-O1igoccne data shouldnot necessarilyy apply to the Pleisto-Pliocene sedimentspresently being explored. Fig. 1 shows the observednormal pressure resistivity trends superimposed on anage-correlation dip section from Atchafalaya Baythrough Vermilion Block 321. Because of sedimentage, the Hottman and Johnson trends apply to Atcha-falaya Bay and Eugene Island Block 100. However,

* Recent Gulf Coast drilling experience and log data reveal irregularities in resistivity trends.Anomalies caused by age boundaries, younger sediments, and other phenomena muy makelog relationdtips di~cult to apply. The geographic distribution and interpretation techniquesfor some of these anomalies are presented. Resistivity-trend departurelpressure relationshipsare examined.SEPTEMBER, 1976 %3


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,

because of tie younger sediments encountered, somedeviation might be expected from Eugene Island Block188 gulfward.Note that the trend of Eugene Island Block 276 has aresistivity shift at the paleo-marker at about 7,500 ft.Note also that the normal-trend section at Ship ShoalBlock 274 is very short and that no obvious trend existsat Vermilion Block 321. Although other localanomalies in other areas of the Gulf arc likely, the trendsituations found along the dip section are analyzed bydescribing five general cases.Case 1 Normal Miocene-Type Trends. These ap-pear to be applicable near shore in Miocene sectionsand in long Pliocene sections. Although some shallowPleistocene is present in these areas, geopressured occurmuch deeper in the section. Discrepancies owing toPleistocene sediments are unimportant, as illustrated inFig. 2, and these trends are zimilar to the Hottman-Johnson data.case 2 Long Pleistocene Sections With Geopres-sured Within Pliocene. Fig. 3 shows resistivity, acous-tic, and density data with percent sand from Eugene k.-land Block 276 field. Note that one line could be drawnthrough the ac{ustic data to yield a normal-trend line.However, the density and resistivity trends exhibit ob-vious departures at Paleo-Marker B (base Pleistocene)and pressure data confirm that two normal trends areactual] y present.There appears to be a shale compaction or composi-tion change at PaIeo-Marker B not associated with geo-pressured. This trend shift has been observed in tdl thewells at Eugene Island Block 276 field. It rdso has beenobserved at South Marsh Island Block 73 field (Fig. 4)at a paleo-marker and in East Cameron Block 185 (Fig.5, not as obvious), again at a paleo-marker. These three

examples indicate that this shift usually occurs at apaleo-msrker and in proximate areas in the Gulf. Theyalso illustrate why paleo-markers should be included assupplementary data on all pressure plots.Pressure detection can be difficult when geopressuredinitiate in the interval between normal trends, as in EastCameron 185 (Fig. 5). However, this shift thicknessappears to be a fairly uniform 600 to 800 ft and theresistivity ratio (shift) is almost constani at about 0.75.Once recognized, transparent overlays can be con-structed that define the onset of geopressured within thetransition zone between normal trends.

Case 3 Long Pleistocene Sections With PressuresInitiating at Pliocene Contact. An example of this isShip Shoal Block 230 field with the interpretation asshown in Fig. 6. Although the normal section is veryshort, the trend appears to be valid.Case 4 Long Pleistocene Section With ObservableTrend Line and Pressures Initiating Within Pleisto-cene. An example of this is Ship Shoal Block 274 withthe interpretation as shown in Fig. 7.Case 5 Long Pleistocene Section With Abnor-mally High Resistivities in Long, Normally Pte s-sured Shale Sections. Several fields, including SouthMarsh Island Block 115, Eugene Island Block 331, andVermilion Block 321, appear to have this type of anom-aly that makes normal-trend definition difficult to as-sess. Fig. 8 presents a data set from Vermilion Block321. An example of this case has been investigatedmore fully.Investigatio~~of Case 5. An apparently normal straight-line acoustic trend is evident through the high-resis-tivity shale sections. These high-resistivity shaie anom-

PALEO CONTROL EX TRA PO LA TE

AT CHAFALAYA El. El. El. $.$. VERBAY BLK. ICQ BLK. 1 88 9LK. 276 BLK. 274 8LK. 3 21

2,000 -

4,000 -

6/ 200 -

8/ 300 -

10,000

12,000 -

14,000 -

16000 -

lls/Joo~

---- ------ ----- -------. + ---

Fig. 1 Louisiana Gulf Coast approximate age-correlation dip section.%4 JOURNAL OF PETROLEUM TECHNOLOGY


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03PWALE0 WADn RKISTVITY (Q - m) fLT I p S C/ fl) P b (! #CC) MUDW. [PPCI

Fig, 2 Trend interpretations, OCS-G 0798 lease,Eugene island Piock 100

10 12 14 Is 18

field.

x 1000

t.

4-

6-

a-

10-

1?-

14-

16-

2 .4 1 10 15 20 70 100 150 200 2.0 2 I 22 23 24 25

OfPTH/ PALIO SAN D% R S S lV llY ( it -m ) h~ [p s[c / fll ~b (g/ CC) HODVI IPPCIx 1040 100 0

2-

4-

s- A-08--c

10-

12-

14- I

16-

2 .4 7 10 15 20 70 Ico 150 2W

1,,, ITOP ?, ~: I

?0 21 22 ?3 24 25 10 12 14 16 18

MEABHP

Fig. 3 Trend interpretations, OCS-G 0985 lease, Eugene Island Block 278 fieid.

DEPIH/PALIO SABO% l l[ S ST Vi lY ( .Q - m ) AI (# SC/ ill Pb (dCC) lluOWT (PPC)x 1000 100 0

2-

4--As - YB

8-

10--c

12. d-D

14-

14~

2 4 1 10 15 20 10 100 150 2@ 20 ?1 22 t3 24 2$ [0 12 14 16 10

MEABHP

Fig. 4 Trend interpretations, OCS-G 1194lease, South Marsh Island Block 73 field.


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.- +

O fPI HI ML[ O si ze % MS , W?? (Q- m) A T (IL S C/ fl) # b ( @ WO W. IW6)IIMO 100 0

2

4

6

I

1A

10

t

B12

14

16

~ FRoM PREV1OUS WELLS -

I 1111111

?0 100 150 2M 20 21 22 23 24 25 10 12 14 16 N

Fig. 5 Trend interpretations, OCS-G2035 ieaae,East Cameron Biock 185fieid.

OfPTHIPALfO MWD% Rf$lSWll (0 -m) AT (p StC/fT) P b ( 91 CC)I 1040 100 0

2

4

\

A3:

I

10-

12-

14-

la-

~ . 4 7 10 Is 20 70 100 1s0 Ml

. Fig. 6 Trend interpretations, OCS-G 1026iease, Ship Shoal Biock 230

O fPTH/ PA lfO Wlo mI 1000 100 0

?-

$-

6-

1-

1 0- - A

12-

14-

I1-

R151S11V11YQ -m) AT ( P :f C/1112 .4 7 10 Is 20 70 100 150 204

P&9/cc )?0 ?1 ?2 23 ?4 23

U lo w , ( PPC I

field.

WI w [PPCI10 12 14 16 18

. Fig. 7Wend interpretations, OCS-G 1043lease, Ship Shoal Biock 274 fieid.

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..q .

O fPTH/ M Lf O $A ll o% RUIS IWTV ( ~ - m ) Al (p S C/ F) Ph ( dd 12uam (PP6)x 10(

2

4

s

I

m

12

14

16

100 0 2. 0 2I 2; 23 24 25

Fig. 8 Trend interpretations, OCS-G 2088 lease,Vermilion Block 321.

alies, therefore, could be due to either salinity or to cl~yconductivityy changes in the section. Clean shales arerare in the Gulf Coast. Mineralogical analyses indicatetypical shales contain 30- to 50-percent quard fines.These observations provide a basis for considering theshales to be quite similar to very shaly sands and fordescribing their conductivities by the Waxman-Smits3equation:Co=-&e QD+C w), ................ .(1)

whe~Co= specified conductance of sand, 100-percentsaturated with aqueous salt solutionFR* = formation resistivity factcr for shaly sandsB,= equivalent conductance of clay-exchangecationsQ= effective concentration of clay-exchangecationsCW= specific conductance of aqueous electrolytesolution.Analyses were made of shale sidewall samples froma well (smtth Marsh Island Block 115, Well A) similarto the one shown in Fig. 8. Laboratory measurementsof QO and soluble chloride contents coupled ,vithtemperature-corrected B values (as described byWaxman and Thomas4) indicate that the anomalous re-sistivities are caused by changes in salinity and notporosity or lithology. A comparison of observed logvalues with laboratory-calculated resistivities (Eq. 1)over the sampled intervals using laboratory data (sup-

tl plied by Waxman) is shown in Fig. 9. They are in ex-cellent agreement.The Fig. 9 data also indicate that, in practice, thenormal-trend line should be drawn through the lower-1 resistivity (higher sand-shale ratio) sections as shown inI Fig. 8. - -

Departures From Hydropreasured Trenda Related toGeoprewuresTwo techniques have been used in the past for compr-ing departures from nonmal trends. These are the ver-SWTEMBER, 1976

10 12 14 H 10

tical comparison technique, where an observedabnormal-trend value is compared with an equalnormal-trend value and the overburden between them isused to calculate formation pressure, and the horizontalcomparison technique, where observed abnormal-trendand extrapolated normal-trend values are compared atthe same depth and related empirically to formationpressures.All the data presented in this paper are derived fromthe horizontal method, since the two parameters pre-sented are ai the same depth and, hence, at appr@xi-matel y the same temperature.Pressure Gradient RelationshipsShale Resiativity Relationships. The resistivity pres-sure data of Hottrnan-Johnson can be replcmed on a co-ordinate scale as shown in Fig, 10. The relationship maybe expressed approximately by the equation( )p=0.465+m 1-* , ......... . .(2)8.

q 10G VAt Ut$x T x lk l to . m s? 1

1) @rDf - w~t; SAMPLE C0t47AhNNA7E0WIT nRr,,l NG MUDILOG VAIUE

-_ __x

1-~L 1 1 1 104 0s 06 07 08 09 10 11

RfSl$TIVIT V,lOMM -M)(After w. -)

Fig. 9 Shale resistivities vs depth, 0C8-G 2094No. A,South Marsh Island Block 115.%7


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+.+++.~+-l

,,-+1+.. ,. ;7T?H-H-iJi-L!iQ

Fig. 10 Pressure-gradient/resistivity relationship ofHottman and Johnson.

Lo

.9

.8

.1

,6

m??? ~n~

I--i-l!l -1 ,8M-b*~s 9m. lns 95a L-u

14,,I I I I I

.4 $ .6 .7 .a .9 Ln# ,(Pw n .)

Ftg. 11 Presswe-gre..t&:istivity relationships --%8

wheregp = formation pressure gradientm= slopeR$h~~= observed shale resistivityR,hN= normal shale resistivity.

A least-squares fit of these data forced through (gP =0.465 at R8hoEjR,M= 1.0) yields m = 0.592.To investigate the validity of the resistivity relation-ship and review the more recent offshore Louisianadata all reliable BHP data in virgin, geopressured res-ervoirs were collected. The results (shown in Table 1)am plotted in Fig. 11. An unfmved least-squares fit ofthe data yields m = 0.519. All new data were obtainedfrom short normal electric logs to facilitate comparisonwith the Hottman and Johnson data.An attempt was then made to incorporate overburdengradient into the pressure-msistivity relationships to de-termine whether a correlation existed. Offshore overbur-den gradients wem determined chiefly from compositedensity-log responses using a mean sea-level depthdamm. These are presented in Fig. 12. Because of ascarcity of suitable density logs, onshore Louisianaoverburden gradients were more difficult to establish.Gravimeter data for Cote Blanche Island and Iowafieldss were ultimately used but are confirmed by avail-able derisity-log data. These overburden stresses wereapplied to Johnsons pressure points (Table 2). Next, alldata points were grouped in three categories:0,85< go s 0.900.90< go s 0,950.95 c go = 1.00,

where gO= overburden gradient.Fig. 13 presents the dat~ and the least-squares-fitlines through the du se groups. The relationships areforced through the point where gP = 0.465 and resistiv-ity ratio = 1.0. The standard deviations and coefficientsof determinations presented are for the unforced fits thatare very similar (see Table 3).

-t--

?

\

i.-j-1-..-.-+1=

Y ,9~mm I ~mlffl I b ( ~1~1)Fig. 12 Overburden gradients.

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The dependence of the trend departure data on over-burden is obvious. The data for the younger (less dense)sediments farther offshore show lower pressure gra-dients (for the same resistivity ratio) than those pre-dicted by the Johnson trend. The results are in agree-ment with observed mud- weight requirements and otherdrilling and production histories.ShaIe Acoustic Travel-Time Relationships. To re-evaluate Hottmans acoustic pressure relationship, theacoustic data available offshore were collected (Table4), but were found to be meager because (1) few acous-

tic logs are available ovsr entire sections, so normaltrends are diftlcult to establish, and (2) few bottom-holepressure analyses are available from wells with acousticlogs since density logs are the primary porosity tools indevelopment programs.Fig. 14 presents the acoustic relationships from re-cent offshore data together with Hottmans. They showgood agreement and augment considerably the softgeopressuredregion of Hottrnans empirical trend.Conclusions1, Abnoimal pressure-resistivity trend departure rela-

1

FieldEugene IslandBlock 18Eugene IslandBlock 100Eugene IslandBlock 276

South Marsh IslandBlock 73

Ship ShoalBlock 230Ship ShoalBlock 274

Ba: MarchandBlock 2

West CameronBlock 192

WellABcDEFGHIJ

QRsTuJxYAZADBccC)D

NN00Pp

TABLE 1 NEW OFRHORE PRESSURE-RESISTMTV DATADepth- (ft)19,60512,50013,020944610,2699,36110,02310,92910,23010,48310,51710,51610,85710,75811,18610,23011,39511,91310,25010,2979,5319,5719,46910,1611241712,34513,02312,


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TABLE 2 PRESSURE-RESISTIWfY DATA FROMHOllMAN MD JO~SONField

Belle RiverChalkley

:0

9

3 I

1

6

REV ~%

1

3

?

I

Eugene IslandBlock 100Iowa

Kings BayouNorth JeaneretteNorth Oesun

North Sabine LakeSouth ChauvinVlterboWest Lake VerretCameron Ph.

Well

DE:HIiLMNo:RsTuvw

Depth-__@_12,40010,07010,15013,1009,37012,30012,50014,00010,94810,30010,75012,90013,84415,.35312,60012,80011,75014,55011,07011,90013,60010,00010,60012,70013,50013,950

Pressura(psi)10,2407,5008,00011,6005,0006,3506,44011,5007,9707,8007,60011,0007,20012,1009,0009,0006,70010,8009,4008,10010,9008,7507,68011,15011,60012,500

(P%)0.8260.7450.7880.8850.5340.5160.5150.8210.7260.7040.7070.6530.5200.7880.7140.6980.7400.7420.8490.6810.8010.8750.7110.8780.8590.896

RhoBIRdw0.3850.5880,5130.2380.8700.8700.7690.4170.5620.5210.5650.3030.9080.4350.6250.5680.6259.5400.2560.5880.4260.3120.6250.3570.4000.364

(P%t)0.9730,9410.9420.9600.9380.9510.9530.8670.9460.9450.9450.9590.9820.8910.9750.9760.9700.9860.9460.9700.96U0.9410.9640.9580.9630.966

1-H--H+Y-PRIH-H

AgeOligoceneOligoceneOligoceneOligoceneMioceneMioceneMioceneOligoceneOligoceneOligoceneOligoceneMioceneMioceneOligoceneOligoceneOligoceneOligoceneOligoceneMioceneMioceneOligoceneMioceneOligocene?Oligocene?Oligocene?

0.4

05

:.g 07* RELATIONSHIPOF HOTT. ,~

88

q

09

,. I, ! . m a sI 5 6 ,7 I .9 10*(PWfl ,

Fig. 13 Pressure-gradient/resistivity relationships all points.970

. ,. q-.@lNKc / fT)Fig. 14 Pressure- radient/interval-transit-time7e ationship.

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TAELE 3 STANDARD DEVfAT~NS AND COEFFICtENTS OF DETERMINATIONSFOR UNFORCED FtTSStandard Deviation Coefficient ;!Group (psi/ft) (ppg) Determination Number of Points

Johnsons data 0.0344 0.661 0.9174 26Reeent offshore data 0.0281 0.560 0.8367 53Combined data 0.0323 0.621 0.9253 790.65< g. s 0.90 0.0303 0.583 0.5224 240.90< go== 0.95 0.0299 0.575 0.9126 35o.85

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A review of Geopressure Evaluation from Well Logs.pdf