-
T. P.8038
Improving Miscible Displacement by Gas-Water Injection
ABSTRACT
B. H. CAUDLE
A. B. DYES
MEM6ERS AIME
Miscible displacement recovers all oil in the area con-tacted by
the injected fluid, whereas water or immiscible gas drives usually
leave substantial amounts oj oil as residual. However, the !Joor
mobility ratios associated with a gas-driven miscible displacement
cause the sweep pattern efficiency to be much lower than that
obtained with water flooding. One way in which the swee_v
effi-ciency in a miscible displacement process can be in-creased is
by decreasing the mobility behind the flooding front. This can be
achieved by injecting water along with the gas which drives the
miscible slug. This water reduces the relative permeability to gas
in this area and thus lowers the total mobility. The main operating
con-ditions for the simultaneous injection _vrocess are that a zone
of gas exists between the miscible slug and the leading edge of the
water and that a sufficient amount of gas be injected with the
water to form the gas volume which is being left in the water zone.
Laboratory model studies have shown that the ultimate swee.T)
pattern efficiency can be as high as 90 per cent for a fivespot
flooding system. If gas alone is used as the driving me-dium an
ultimate sweep-out efficiency of about 60 per cent would be
obtained in the same system.
INTRODUCTION
The miscible displacement processes are a step towards total oil
recovery. Conventional gas or water drives usually leave 25 to 50
per cent of the oil as residual in the swept portion of the
reservoir. This residual can be eliminated if the oil is driven by
a fluid with which it is miscible. At some reservoir conditions
natural gas will become miscible with the oil. This is the "high
pres-sure gas process".' More often, the oil does not contain
enough light hydrocarbons to cause the gas to become miscible with
the oil at reasonable pressures. In these cases a small band of
fluid which is miscible both with the oil and gas must be kept
between them'. Less than 2 per cent of the reservoir volume of the
slug material is needed to keep the displacement miscible.
Both processes work in the same manner, recovering all of the
oil in the portion of the reservoir contacted by the injected
fluids. The only difference is the manner in which the miscibility
between the oil and the injected
Original manuscript received in Society of Petroleum Engineers
office July 16, ;1957. Revised manuscript received Sept. 17, 1958.
Paper presented at 32nd Annual FalI Meeting of Society of
Petro-leum Engineers in Dallas, Tex., Oct. 6-9, 1957.
lReferences given at end of paper. SPE 911-0
VOL. 213, 1938
THE ATLANTIC REFINING CO. DALLAS, TEX.
gas is obtained. Previous publications have contained detailed
descriptions of these processes,""'"
However, total displacement of the oil in the swept region does
not guarantee an efficient recovery process. The amount of oil to
be recovered is also determined by the fraction of the reservoir
contacted by the flood. This fraction is largely determined by the
mobilities of the fluids. (The fluid mobility is the permeability
of the rock to that fluid divided by the fluid's viscosity, k / fL)
. This dependence of the fraction swept on the mobility ratio has
been shown in previous studies.',,1 Fig. 1 shows the ultimate
fraction swept in a five-spot system as a function of the mobility
ratio. The small drawings show the location of the areas left
unswept for two dif-ferent mobility ratios. The ultimate fraction
of the reser-voir swept is here considered to be attained when the
producing stream contains less than 5 per cent oil at reservoir
conditions.
THE GAS-DRIVEN MISCIBLE DISPLACEMENT
Since there is no oil left in the swept region after miscible
displacement the mobility in this region is very high. It is often
50 times the mobility in the unswept regions. This means that the
fraction of the reservoir contacted by the injected fluid will be
less for a gas-driven miscible displacement than for a conventional
water or gas drive. For a five-spot injection system, water would
contact the entire reservoir volume, and the low pressure gas would
contact about 90 per cent of this volume, while a gas-driven
miscible displacement would only contact about 65 per cent of the
reservoir. This poor sweep efficiency often offsets the benefits
ob-tained through miscible displacement. Fig. 2 shows what the
recovery curves for the three processes might look like for a
five-spot system. The curves show the frac-tion of the in-place oil
recovered as a function of reser-
FIG.
~ 100,-______ ==~~------------_-__ __, IL
'" ,. IJ) 80
... '" 0:: ... 60 ...J
~ o 40 ~
~
~ 20
-
L&J 0100 -It: W > 20 0 0 w a: 0
I
0
WATER DRIVE
CONVENTIONAL GAS DRIVE I
.~~--_~ _J I 2 3
RESERVOIR OIL VOLUMES INJECTED
FIG. 2-COMPARISON OF THE GAS-DRIVEN MISCIBLE PROCESS
WITH CONVENTIONAL INJECTION PROGRAMS.
voir volumes of fluids injected_ For these data we have assumed
that the water recovered 67 per cent of the oil in the area swept
and that the low pressure gas re-covered 50 per cent of this oil.
Here, if only the re-coveries are considered, the gas-driven
miscible displace-ment is much better than a conventional gas
drive; but it is not quite as good as a water flood. (Other
factors, however, such as well injectivities or fluid availability
will help determine which process is chosen.)
SIMULTANEOUS INJECTION OF GAS AND WATER BEHIND THE MISCIBLE
DISPLACEMENT
The miscible displacement must invade a larger por-tion of the
reservoir if it is to represent a generally im-proved process.
Reducing the mobility in the swept re-gion is one way in which the
sweep efficiency may be increased. The fluid mobility in a porous
medium may be reduced by (1) reducing the permeability of the
matrix to that fluid, and (2) increasing the viscosities of the
fluids in the region. The reduction of the per-meability in the gas
region is easily accomplished by injecting water along with the
gas. This reduction in relative permeability during multi-phase
flow is a well-defined characteristic of porous media which greatly
re-duces the fluid mobility. The mobility is further reduced
because the water is more viscous than the gas which it replaces.
In this manner the high mobility in the swept region can be greatly
reduced and so increase the sweep efficiency of the process. The
effect of different gas and water saturations on the mobility in
the swept region is illustrated in Fig_ 3. This simultaneous
injection of gas and water behind the miscible displacement will
make the miscible processes much more attractive.
To ensure miscible displacement, a zone of gas should be kept
between the miscible slug and the region of water flow. If water
flows into the miscible zone ahead of the gas, a reduction of the
displacement efficiency occurs. On the other hand, if the gas zone
were al-lowed to become large the operation would approach the poor
sweep-out pattern performance of the gas-driven miscible process.
If too high a ratio of gas-to-water is injected the gas will flow
faster than the water and enlarge the gas zone. If the ratio is too
low the water will flow faster than the gas and invade the
mis-cible zone. The proper injection ratio will keep the gas zone
at a constant volume as the flood progresses. This ratio can be
determined from the relative per-meability relationships, the water
and gas viscosities and the saturations established in the region
of simul-taneous flow. A calculation of this ratio will be
illus-
282
TOTAL FLUID MOBILITY (::+;;) o ~ 0 ~ ~ ro()l o ,_-,--__ ,-_--,
__ .;::o_---=;
INTERSTITIAL WATER
--------- -------RESIDUAL GAS
g'----------___ --.J
FIG. 3-ToTAL FLUID MOBILITY IN THE WATER-GAS ZONE AT VARIOUS
WATER SATURATIONS.
trated in the discussion of the behavior of the sample five-spot
reservoir.
EXPERIMENTAL 4
The effect of mobility ratio on the flood pattern has been
described in the literature: These results, however, are for
systems in which the mobilities in only two re-gions were
concerned. In the simultaneous gas-water in-jection process there
are three regions of mobility which will influence the sweep
pattern. These are: (1) the relatively low mobility oil region, (2)
the high mobility gas zone, and (3) the lower mobility region in
which both gas and water are flowing. The relatively narrow zone of
miscible displacement which lies between the gas and the oil will
not significantly affect the sweep efficiency for the process, no
matter which miscible pro-cess is used. No one mobility ratio can
be ascribed to the process. The relative mobilities of all three
regions must be considered.
If we confine ourselves to the effect of the mobilities on the
sweep efficiency, simple laboratory models with three zones of
miscible fluids can show the benefits to be obtained by the
simultaneous injection process. The models used represented
elements from a large five-spot injection system. They were
0.25-in. thick and the dis-tance between wells was about 10 in.
Miscible oils were used to represent the three mobility regions.
The X-ray shadowgraph technique' was used to determine the
sweep-out pattern efficiencies. The mobilities in the oil region
and in the water-swept region were equal. The mobility in the gas
zone was 17 times higher, instead of 50 times higher as in the
field. This number for the
w 0
100 -It: W 20 > 0 0 w 0 a: 0
20'X. GAS BAND
LEGEND FOR DIAGRAM: !Ii ~ OIL REGION
o GAS BAND ~ WATER+GAS
REGION
2 3
RESERVOIR OIL VOLUMES INJECTED
FIG. 4 - EFFECT OF GAS BAND SIZE ON RECOVERY IN THE SIMULTANEOUS
WATER-GAS INJECTIO:'-I MISCIBLE PROCESS.
PETROLEUM TRANSACTIONS, AIME
-
mobility in the gas zone was as high as could be ob-tained
conveniently in the laboratory. The difference in the recovery
histories between mobility ratios of 17 and 50 should not be
significant in this study since both re-sult in similarly poor
sweep efficiencies.
Fig. 4 shows the results of this study. The per cent of the
in-place oil produced (which for miscible pro-cesses is the same as
the per cent of the reservoir area contacted) is shown as a
function of the number of res-ervoir volumes injected. The result
of two model studies is shown. In one study the gas zone occupied
20 per cent of the reservoir volume and in the other only 5 per
cent. These results show that the simultaneous gas-water injection
process will approach the high areal sweep efficiencies obtained
with a conventional water drive. There is a noticeable difference
between the curves for the 5 per cent and 20 per cent gas zones.
This, how-ever, is not nearly so striking as is the improvement
over the gas-driven miscible displacement.
It should be remembered that these results apply directly only
to uniform formations in which the effects of gravity are
negligible. In cases where these factors are significant, the
presence of the relatively low mo-bility water zone behind the
advancing front will tend to offset their detrimental effects on
the sweep efficiency.
APPLICATION OF THE SIMULTANEOUS INJECTION PROCESS
To illustrate better the mechanics of the simultaneous injection
process and its benefits, a hypothetical case will be discussed. We
will consider a five-spot injection pattern with: (1) a specific
permeability of 1 darcy; (2) an oil viscosity of 1 cp; (3) a gas
viscosity of 0.02 cp; (4) a water viscosity of 0.5 cp; (5) an
interstitial (non-flowable) water saturation of 25 per cent; and
(6) the relative permeability curves shown in Fig. 5. For this
discussion it makes no difference whether the.mis-cible
displacement is obtained by the high pressure gas process or the
miscible slug process.
First, the gas to water injection ratio must be calcu-lated. As
discussed earlier, the desired ratio is that at which the gas and
the water flow at the same velocity. This ratio can be determined
from the relative per-meability curves and the fluid viscosity. In
line with Darcy'S law, the volumetric flow rate of a single fluid
in a two-flowing-phase system is proportional to the mo-bility of
the fluids (effective permeability/fluid viscos-ity). Therefore,
the linear velocity of a fluid is propor-tional to its mobility
divided by the fraction of the porosity which it must fill. Thus,
for gas the relative velocity would be (effective permeability to
gas) -:-- (gas viscosity) (fractional gas saturation); and for
water (ef-
1.0 10
~
'" >- ..... .... . '" ~ -
In 0.1
2 C .... ... ::E C 0:
II:
... >-11.
= 0.01 0.1 ~ ... III > C r- ... c ::E ~ 0: ... ... 0::
11.
0.001 0.01 0.1 0.5 0.6 0.7 0.8
SATURATION FIG. 5-GASWATER RELATIVE PER:\IEABILITY CURVES.
VOL. 213, 1958
fective permeability to water) -:-- (water viscosity)
(fractional water saturations over and above the inter-stitial
water)'. From the proper relative permeability curves (Fig. 5) the
relative velocities for gas and water as a function of gas
saturation can be obtained. These values are then plotted as shown
in Fig. 6. Here we see that the gas and water velocities will be
the same at a fractional gas saturation of 0.31. The permeability
ratio curve (k",/k g shown in Fig. 5) and Darcy's law show that a
gas to water injection ratio of 0.7 (in terms of reservoir bbl) is
necessary to maintain a fractional gas saturation value of 0.31.
This is the desired injec-tion ratio. In practice, a slightly
higher gas-to-water in-jection ratio might be used to ensure the
presence of a gas zone behind the miscible front. A similar type
calculation can be made whenever the gas and water are considered
to be segregated.
In the example five-spot segment a miscible displace-ment zone
between the oil and gas is set up by either the high pressure gas
or the miscible slug process. A quantity of gas equal to 5 per cent
of the reservoir volume at reservoir conditions is injected to form
the gas buffer zone. Then gas and water are injected in a ratio of
0.7 volumes of gas (at reservoir conditions) to one volume of water
(at reservoir conditions). Then the recovery would be as shown in
Fig. 7. Here the per-centage of in-place oil recovered is shown as
a function of total fluid injected (gas plus water). The solid
curve shows the recovery to be obtained by the simultaneous
gas-water injection process. In this case, 53 per cent of the
in-place oil is recovered at breakthrough and 98 per cent is
obtained when two reservoir volumes of fluid have been injected.
The performance of the gas-driven miscible process for the same
reservoir conditions is shown by the dashed line. Here, 42 per cent
of the oil is recovered at breakthrough and 62 per cent is
ob-tained when two reservoir volumes of gas have been injected.
As pointed out earlier, the gas-driven miscible slug process is
generally competitive with water flooding. Therefore, the
simultaneous injection process will proba-bly result in greater
recovery than will water flood-ing. The simultaneous injection
process, however, has one limitation in common with water flooding.
The in-jection wells must be able to take the more viscous water in
sufficient amounts. If they can not, then the gas-driv-en miscible
process should be considered.
CONCLUSIONS
The miscible displacement processes, as originally
100 ~--.---------_---=>
RELATIVE GAS VELOCITY
It 0 ~
"- 10
e :::> ~ "-
w > ;:: .. ~
RELATIVE WATER VELOCITY
W 0:
.1 L-----::':-----::'=----:-~--:-L__L__~---! 0.1 0.2 0.3 0.4 0.5
0.6 0.7 0.8
FRACTIONAL GAS SATURATION
FIG. 6 - RELATIVE VELOCITIES OF GAS AND WATER AT VARIOUS GAS
SATURATIO:,(S.
283
-
~ 100
it ::: ..J
o 50 .. >-a:: .... > 8 .... a:: 0.5
SIMULTANEOUS INJECTION MISCIBLE PROCESS
1.5
RESERVOIR OIL VOLUMES INJECTED
FIG. 7-PRODUCTION HISTORY FOR THE GAS DRIVEN AND THE
GAS WATER INJECTION MISCIBLE PROCESSES.
presented, consist of injecting gas to sweep the oil mis-cibly
toward the producing wells. Because of the low viscosity of gas and
the high displacement efficiency of the processes, the sweep-out
pattern efficiency is usually poor-about 60 per cent of the
reservoir area in a five-spot system at abandonment. Laboratory
model studies have shown that the sweep-out pattern efficiency can
be greatly increased by injecting a fluid of low mo-bility to
follow the miscible displacement front. A sim-ultaneous injection
of water and gas in the proper ratio will create the desired low
mobility zone and increase the sweep-out pattern efficiency while
maintaining a miscible displacement of the oil. This improvement in
recovery can be obtained at little risk, even though the
gas-to-water injection ratio cannot be determined ex-actly. This is
because (1) if too much gas is injected
28
the process can only approach the mechanism of the gas-driven
miscible displacement, and (2) if too little gas is injected the
worst that can happen is that the reservoir will be subjected to a
water drive. In any case the recovery of oil will be good.
We have not tried to cover all reservoir engineering
possibilities in this paper. Each reservoir must be con-sidered
separately from an engineering point of view. Instead, we have
tried to establish guide posts leading to a greater and more
economical recovery of oil through miscible displacement.
REFERENCES
1. Slobod, R. L. and Koch, H. A., Jr.: "High Pressure Gas
Injection - Mechanism of Recovery Increase", Oil and Gas Jour.
(1953) 51,84.
2. Koch, H. A., Jr. and Slobod, R. L.: "Miscible Slug Pro cess",
Trans., AIME (1957) 210, 40.
3. Hall, H. N. and Geffen, T. M.: "A Laboratory Study of Solvent
Flooding", Trans., AIME (1957) 210, 48.
4. Jenks, L. H., Campbell, J. B. and Binder, G. G., Jr.: "A
Field Test of the GasDriven Liquid Propane Method of Oil Recovery",
Trans., AIME (1957) 210, 34.
5. Dyes, A. B., Caudle, B. H., and Erickson, R. A.: "Oil
Production After Breakthrough - As Influenced by Mo bility Ratio",
Trans., AIME (1954) 201, 81.
6. Craig, F. F., Jr., Geffen, T. M. and Morse, R. A.: "Oil
Recovery of Pattern Gas or Water Injection Operations from Model
Tests", Trans., AIME (1955) 204, 7.
7. Aronofsky, J. S. and Ramey, H. J., Jr.: "Mobility Ratio-Its
Influence on Injection or Production Histories in Five Spot Water
Flood", Trans. AIME (1956) 207, 205. ***
PETROLEUM TRANSACTIONS, AIME
