Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MULTIPLE ZONE OIL RECOVERY PROCESS
EMPLGYING COUNTERCURRENT STEM FLOOD
Field of the Invention
This invention relates to a method for recovering oil
from a subterranean, viscous oil-containing formation
containing multiple, overlying oil-bearing permeable strata
separated by impervious shale layers. The method employs a
countercurrent steam flood in adjacent strata.
Background of the Invention
Steam has been used in many different methods for the
recovery of oil from subterranean, viscous oil-containing
formations. The two most basic processes using steam for the
recovery of oil include the "steam drive" process and the
single well or "huff and puff" processes. Steam drive involves
injecting steam through an injection well into a formation.
Upon entering the formation, the heat transferred to the
formation by the steam lowers the viscosity of the formation
oil, improving its mobility. In addition, the continued
injection of the steam provides the drive to displace the oil
toward a production well from which it is produced. The single
well process is operated by injecting steam into a formation
through a well, stopping the injection of steam, permitting the
formation to soak and then producing oil through the original
well.
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One of the problems associated with a steam drive oil
recovery process is the loss of heat by conduction into
non-production zones such as overlying and underlying strata.
In some oil-containing formations, the formation
contains a plurality of substantially parallel oil-bearing
strata which may be separated by an impervious shale layer that
restricts vertical fluid flow. U.S. Patent No. 3,180,413
describes a cross flow (countercurrent flow) steam drive oil
recovery process for vertically-separated strata of this kind.
The process is stated to result in increased oil recovery and
an increase in the efficiency with which the injected steam
displaces the oil from the formation.
My co-p~n~ Cain Application Serial No. 475,140, filed
Febn~ry 26, 1985, describes a method for recwering oil from a sub~ranean,
viscous oil-conta~ng formation with multiple o~-conta~ng pa~`ble
strata separated by a thick, impervious shale layer. In the
process described in that application, steam is injected
countercurrently through adjacent strata to maximize efficiency
of heat utilization in the formation due to a more uniform
heating and lower heat losses to overlying and underlying
strata thereby enhancing oil recovery. The recovery is
terminated from both zones upon breakthrough in the top zone.
Surprisingly, breakthrough occurs first in the bottom zone and
only later in the top zone where the heat transferred from the
bottom zone has produced a more vertical steam front than in
the bottom zone.
Summary of the Invention
According to the present invention, a process for
recovering oil from two vertically-separated permeable strata
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containing viscous oil and separated by an impervious layer
such as a æhale layer, employs countercurrent steam flooding,
as gaily descry in ~dian Application No. 475,140, but in the
present process, the injection wells in both the upper and
lower strata are shut in when steam breakthrough occurs in the
lower zone or stratum. Production, however, is allowed to
continue using pressure drawdown and this results in notable
economies, indicated by a higher oil:steam ratio. It i5
particularly notable that production from the upper zone may
continue for a significant time until steam breakthrough occurs
in this zone or the produced fluids include an unfavorable
proportion of steam or water. At this time, the recovery
process may be terminated in both zones.
Thus, the present invention relates to a method for
the recovery of oil from a subterranean, viscous oil-containing
formation having at least two upper and lower oil-bearing
permeable zones separated by an impervious layer such as a
shale layer. The upper and lower oil-bearing zones are
penetrated by first and second wells which are provided with
two separate flow paths. The first flow path establishes fluid
communication between the surface of the earth and the upper
oil-bearing strata and the second establishes fluid
communication between the surface of the earth and the lower
oil-bearing zone. Steam is injected into the upper oil-bearing
zone through the second well by means of the first flow path
and fluids including oil are recovered from the upper zone
through the first well by means of the first flow path. Steam
is injected into the vower oil-bearing zone through the first
well by means of the second flow path and fluids including oil
are recovered from the lower oil-bearing zone through the
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second well by means of the second flow path. The injection of
steam into both the upper and lower zones is continued until
breakthrough of vapor phase steam occurs in the lower zone.
When this occurs, steam injection in both zones is terminated
and production is csntinued in both zones but more particularly
in the top zone until either vapor phase steam breakthrough
takes place in the top zone or the recovery becomes uneconomic,
as indicated by a predetermined oil:water ratio in the produced
fluids. because production continues by pressure drawdown
after steam injection has been terminated, the recovery process
will operate more favorably in relatively thick oil-bearing
formations of this kind, erg. up to 15m (about 50 feet) in
order that sufficient reservoir energy can be accumulated
- during the injection phase, but because heat transfer betweenthe two zones is required in orcler to increase the efficiency
of the process, the impermeable layer between the two
production zones is preferably not more than 5m (about 16 feet)
- thick.
The Drawings
Figure 1 illustrates in vertical section, a formation
with two oil-bearing zones separated by an impervious
shale layer and a double well completion scheme for
recovering oil from them;
Figure 2 illustrates in vertical section, a formation
with four oil-bearing zones separated by impervious
shale layers and a well combination recovering oil
irom them
Figure 3 illustrates in vertical section, a formation
with three oil-bearing zones separated by impervious
shale layers and a well combination for recovering oil
from them;
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Figure 4 shows a reseevoir model configuration used in
a simulation described below;
Figure 5 is a graph showing the effect of
countercurrent steam flooding on oil recovery;
Figure 6 is a plot of reservoir temperature
distribution for the concurrent steam flood;
Figure 7 is a plot of reservoir temperature
distribution for the countercurrent steam flood;
Figure 8 is a graph showing the effect of temperature
on oil viscosity; and
Figure 9 is a graph showing the improvement in
oil/steam ratio obtained with the present method.
Detailed Description
Figure 1 shows a subterranean, viscous oil-containing
formation 10 comprising an upper and lower oil-bearing
permeable strata or zones 12 and 14, respectively, separated by
an impervious shale layer 16 that restricts vertical fluid
flow. An overburden 18 resides above the strata 12.
The upper and lower zones 12 and 14 are penetrated by
spaced-apart wells 20 and 22 provided with suitable means for
dividing each well into separate fluid flow paths for the upper
and lower oil-bearing zones. Each well is similarly
completed. Well 20 includes a tubing string 24 that extends
through the zones 12 and 14 and is in fluid communication with
strata 14 by means of perforations 26. (For reference
components on well 22 corresponding to those on well 2Q are
designated with a prime (') mark. This discussion focuses on
well 20, but applies to well 22 also Tubing 24 is disposed
within a surrounding larger diameter outer tubing 28 that
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extends into upper zone 12 and terminates in the lower portion
of it above impervious shale layer 16~ Casing 28 is in fluid
communication with zone 12 by means of perforations 30. Inner
tubing 24 cooperates with the outer casing 28 to form an
annular space 32 closed off near the bottom of zone 12 by a
member 33 connecting inner tubing 24 and GUter casing 28. The
above-described well completion provides each well with two
separate flow paths, one between the surface of the earth and
the lower zone 14, and a separate flow path, in the same well,
which establishes fluid communication between the surface of
the earth and the upper zone 12, enabling each well to serve as
both an injection and production well for the two adjacent
oil-bearing strata.
It should be pointed out that while each well 20 and
22 shown in Fig. 1 is provided with two separate fluid paths,
one between the surface of the earth and the lower oil-bearing
zone, and a separate flow path which establishes fluid
communication between the surface of the earth and the upper
oil-bearing zone, which is the preferred method, parallel
separate injection and production wells in fluid communication
with each oil-bearing zone by means of perforations may be used
for the injection of steam and production of oil from the
respective oil-bearing zones. In addition, another effective
means for accomplishing multiple zone completion involves using
a well equipped with a casing provided with upper perforations
within the upper oil-bearing zone 12 and lower perforations
within the lower oil-bearing zone 14. The wells are equipped
with a tubing to establish communication with the bottom
perforations, the tubing being packed off from the casing at a
point intermediate between the two sets of perforations. The
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annular space between the tubing and casing is employed as the
second flow path which establishes communication hetween the
surface of the earth and the upper oil-bearing zone. Multiple
zone completion is used in the field for selective steam
injection and concentric tubing for dual zone completion is
commercially available. Further details on multiple zone
completion techniques may be found in the following
references: Carlos, J.B., "Steam Soak on the Bolivar Coast,"
CIM Special Volume 17 (1977), pp. 561-583; Burkill, G.C.C.,
"Thermal Well Completion Design with Openhole Gravel Packed
Liners and Methods for Selective Steam Injection," CIM Special
Volume 17 (1977), pp. 595-608; and Burkill, G.C.C., "How Steam
is Selectively Injected in Openhole Gravel Packs," World Oil
(January 1982), pp. 127-136, to which reference is made for
such details.
After wells 20 and 22 are completed, steam is injected
into lower zone 14 via tubing 24 and perforations 20 in well
20. The injected steam passes through zone 14 effecting a hot
drive of the oil through the strata toward well 22 and fluids
including oil are recovered from zone 14 via perforations 26'
and tubing 24' in well 22. Injection of steam into zone 14 is
continued until the water cut of the fluid being produced from
the strata 14 by means of well 22 increases to a predetermined
value, preferably at least 95 percent, or until vapor phase
steam production occurs at well 22. At this time, steam
injection is terminated in both zones.
Simultaneously with the injection of steam into lower
zone 14 via well 20, steam is injected into the upper zone 12
via annular space 32' and perforations 30' in well 22 and
fluids including oil are recovered from zone 12 via
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perforations and annular space 32 in well 20. Injection of
steam into the upper zone 12 is continued and fluids incLuding
oil aee recovered through well 20.
Once steam injection is terminated, production may be
continued by pressure drawdown, using the accumulated formation
energy to provide a drive for continued production. because,
at this time, steam breakthrough will have occurred in the
lower zone (or the ratio of produced oil to water become
unattractively low), the fluids produced from the lower zone
will contain large amounts of steam or water However, because
the heat transfer from the lower zone to the upper zone
produces a sharper, more vertical steam front in the upper
zone, the produced fluids from this zone may still contain
satisfactory quantities of oil. Production may therefore be
continued in both zones, but particularly the top zone, until
either steam breakthrough occurs in the top zone or the
produced fluids are found to contain a predetermined amount of
water, usually and preferably at least 95 percent.
Steam injection temperatures will typically range from
120 to 350C (from about 250 to about 650F) and the quality
of the injected steam is typically within the range of 50 to 95
percent.
The steam injection rate will vary depending upon the
size of each oil-bearing strata and is preferably within the
range of 0.5 to 2.5 barrels of steam (CWE) per day per
acre-foot of oil-bearing strata.
Thus, the oil is recovered from both oil-bearing
strata by means of a steam drive in which the steam flows in
countercurrent directions through the strata as illustrated by
the arrows in Fig. 1. This arrangement (1) reduces net heat
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losses to the overburden and understrata for
multi-thin-oil-bearing zones thereby becoming equivalent to one
thick zone, (2) effects indirect heat exchange or the produced
oil with the injected steam so as to reduce its viscosity and
pumping efficiency, (3) enables the steam to be insulated by
warm oil when flowing downhole, and (4) by the same token,
indirectly increases steam quality all the way down the wells.
The recovery process may also be applied to thicker
oil-containing formations having multiple substantially
parallel oil-bearing permeable strata separated by impervious
shale layers. For example, Fig. 2 illustrates a well
completion scheme for a subterranean, viscous oil-containing
formation 34 having four oil-bearing permeable zones or strata
36, 38, 40, and 42, respectively, separated by impervious shale
lS layers 44, 46, and 48. Wells 50 and 52 penetrate strata 36 and
38 extending from the earth's surface to the bottom of
oil-bearing strata 38 and wells 54 and 56 penetrate all of the
strata 36, 38, 40, and 42 extending to the bottom of
oil-bearing strata 42. Each well is similarly completed as
wells 20 and 22 described above and illustrated in Fig. 1 to
provide separate flow paths for each portion of the well in
fluid communication with adjacent upper and lower oil-bearing
strata.
Steam is injected countercurrently through the
adjacent oil-bearing permeable strata 36, 38, 40, and 42
through wells 52, 50, 56, and 54, respectively, and oil is
produced from wells 50, 52, 54, and 56, respectively.
Referring to Fig. 2, steam is injected into zone 36 through
annular space 58 and perforations 60 and fluids including oil
are produced from well 50 through perforations 60' and annular
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space 58l in fluid communication with zone 36. Steam is
simultaneously injected into zone 38 through tubing 62 and
perorations 64 in well 50 and fluids including oil are
produced from well 52 via perforations 64' and tubing 62' in
fluid communication with zone 38. Steam is also simultaneously
injected into zone 40 through annular space 66 and perforations
68 in well 56 and fluids including oil are produced from well
54 through perforations 68' and annular space 66' in fluid
communication with zone 40. Steam is also simultaneously
injected into zone 42 through tubing 70 and perforations 72 in
well 54 and fluids including oil are produced from well 56
through perforations 72' and tubing 70' in fluid cGmmunication
with zone 42. Injection of steam and production of oil with
any pair of adjacent strata is continued until the water cut of
the produced fluid from any well in the lower zone of the pair
increases to a predetermined value, preferably 95 percent, or
until vapor phase steam production occurs at the well. At that
point, steam injection into both zones of the pair is
terminated and production continued by pressure drawdown.
Production may be taken from both zones but is particularly
taken from the upper zone of each pair where the steam front
lags behind that of the lower zone. Injection of steam at the
next successively higher zone ma next be terminated when steam
breakthrough occurs in the upper zone of the two lowest æones
(or when production from this zone reaches a predetermined
oil:water ratio, typically at least 95 percent water).
Injection and production is then terminated in successively
higher zones in the same manner.
Fig. 3 illustrates a double-~ell completion in a
subterranean, viscous oil containing formation 74 having three
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overlying oil-bearing permeable zones or strata 76, 78, and 80,
respectively, separated by impervious shale layers 82 and 84.
Wells 86 and 88 extend from the earth's surEace Jo the bottom
of lower oil-bearing zone 80. Each well is similarly
completed. Well 86 includes a tubing 89 that penetrates all
the strata 76, 78, and 80 and extends from the surface of the
earth to the bottom of lower zone 80; it is in fluid
communication with lower strata 80 through perforations 90.
(For reference, components on well 88 corresponding to those on
well 86 are designated with a prime (') mark. This discussion
focuses on well 86, but applies to well 88 also). Tubing 89 is
disposed within a larger diameter outer casing 92 that extends
from the earth's surface to the lower portion of middle zone 78
to form an annular space 94 that is in fluid communication with
middle zone 78 through perforations 96. The lower end of
annular space 94 is enclosed near the bottom of zone 78 by a
member 98 connecting casing 89 and tubing 92. Casing 92 is
disposed within a larger diameter outer casing 100 that extends
from the earth's surface to the lower portion of upper zone 76
to form an annular space 102 that is in fluid communication
with zone 76 through perforations 104. The lower end of
annular space 102 is enclosed by member 106 connecting casing
92 and casing 100.
Steam is injected into lower zone 80 through tubing 89
and perforations 90 in well 86 and fluids including oil are
produced from well 88 through perforations 90' and tubing 89'.
Steam is simultaneously injected into middle zone 78 through
annulus 9~' and perforations 96' in well 88 and fluids
including oil are produced from well 86 through perforations 96
and annular space 94. Steam is simultaneously injected into
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`¦ upper zone 76 through annular space 102 and perforations 104 in
! well 86 and fluids including oil are prociuced prom well 88
through perforations 104' and annular space 102'. Injection of
steam and production from each oil bearing strata is continued
until the water cut of the produced fluid from the next lower
zone increases to a predetermined value, preferably at least 95
percent, or until vapor phase steam occurs in the lower zone.
At this point injection is terminated in the upper zone and
production continues from both zones, but particularly the
upper zone, of each adjacent pair of zones. Injection and
production is then terminated in successively higher zones in
the same manner.
To illustrate the invention, the following laboratory
experiments were conducted in a thermal process simulator.
Two 40-ft. (12.2m.) thick oil sands separated by a
3-ft. (0.9m.) impermeable shale barrier was simulated, a
situation commonly found in a Cold Lake heavy oil reservoir in
Alberta, Canada. The reservoir data is summarized in Table 1
below.
TABLE 1
MAJOR RESERVOIR CHARACTERISTICS
Rock Properties
Temperature 120
Depth, ft. 1300
Porosity, % 35
Average Permeability, md. 5000
kV/kh . 1
Compressibility, psi-l .0001
Heat Capacity, Btu/lb. rock-F .248
Heat Conductivity, Btu/F-ft.-hr. 1.25
Net Pay, ft. 80
Sw 0.35
SO 0.54
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Oil Characteristics
API Gravity 11.4
MW 545
j Density, lb/ft.3 61.8
Compressibility, psi-l 3.2 x 10-6
Heat Capacity, Btu/lb.-F 0.46
The reservoir oil has a high viscosity of 61,900 cp at the
original reservoir temperature of 55F (13C). The reservoir
model configuration is shown in Figure 4. A 2.5 acre (1 ha.)
well-spacing was assumed in this 2-D vertical model. The oil
sands have a permeability of 5 darcies and an oil saturation of
64%. The shale barrier had no permeabilitys To facilitate the
simulation, the reservoir temperature was set at 120F (49C).
This may be justified since one would normally steam stimulate
this type reservoir for a couple of years to establish
communication before attempting a steamflood. As a result, the
reservoir would be at a higher temperature.
The relative permeabilities were taken to be
consistent with other simulation studies on the Cold Lake
reservoir. Steam of 70% quality at saturation temperature of
about 400F (about 200C), was injected at 50 barrels/day into
each zone. This rate is roughly equivalent to 1.5
barrels/day/ac.-ft.
Two runs were made for comparison. Equal amounts of
steam were injected into both zones concurrently (Run 1) and
countercurrently (Run 2). Figure 5 compares the cumulative oil
production of the runs. It is seen that Run 2 consistently
out-performed Run 1. At 600 days, countercurrent steam
flooding produced 28,370 STB, while concurrent flooding
produced only 19, 540 STB. This is an improvement of 45% .
The improved recovery in the countercurrent injection
case may be explained by comparing the temperature
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distributions. Figure 6 shows the temperature contours at 700
days for Run 1, concurrent flooding. As seen, temperatures
! were unevenly distributed, with a large 400F region near the
injection end and cold spots near the producers. The
temperatures in the producing blocks were about 130F, not much
different from the reservoir temperature. Figure 7 shows the
same contours for Run 2. It is seen that the temperature was
more uniformly distributed. Almost all parts of the reservoir
were above 200F, with over 50% in the 300F region. There was
no localized hot zone and the highest temperature was 327F.
The profile of the steam front in the upper zone is sharper and
more vertical than that in the lower zone, indicating that
steam breakthrough in the upper zone will occur later than in
the lower zone, with improved sweep efficiency in the upper
zone.
The primary recovery mechanism of steam is the
reduction in oil viscosity by heating. The oil viscosity is a
non-linear function of temperature. As seen in Figure 8, it
increases rapidly with decreasing temperature n Therefore, a
small different in temperature can cause a much greater
difference in oil viscosity. In Run 1, the oil viscosity in
the producing block was about 3000 cp. Although the high
viscosity region was small, it caused a large pressure gradient
across the entire reservoir approximately 60 psi), whereas in
Hun I, the oil viscosity was about 20-30 cp in the 300F region
and about 200 cp in the 200F region. The average pressure
drop ~ClOSS the reservoir was only about 20 psi.
The effect of terminating steam injection when
breakthrough occurs in the next lower adjacent zone and o
continuing production by pressure drawdown i5 shown in Figure
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9. Steam breakthrough in the lower zone occurred at about Day
600. Both the upper and lower zone steam injectors were then
shut in and production was continued using pressure drawdown.
The oil:steam ratio, the measure of process efficiency and
economics, becomes more favorable after injection is
terminated, as compared to the case when steam injection i9
continued, as shown in Figure 9. Thus, the use of the
accumulated formation pressure and energy for continued
, production permits greater efficiency and economy of operation
to be achieved.
While the invention has been described in terms of a
single injection well and a single production well spaced at a
horizontal distance or offset from the injection well in each
oil-bearing zone, the method may be practiced using a variety
of well patterns, as illustrated, for example, in U.S. Patent
No. 3,927,716.
