Note: Descriptions are shown in the official language in which they were submitted.
MULTIPLE ZONE OIL RECOVERY PROCESS
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This invention relates to the recovery of oil from
subterranean, oil-bearing formatîons and more particularly, to the
recovery of oil from subterranean formations containing heavy oil in
two or more zones which are separated by an impermeable barrier.
A number of different thermal recovery processes have been
used or proposed for producing heavy or viscous oils from
subterranean formations such as tar sands. Of these processes,
those employing steam injection form one readily definable type;
steam injection processes in turn, are basically of two types, being
either of the single well injection or "huff and puff" type or of
the steam drive type. The steam drive processes operate by
injecting steam through one or more injection wells into the
formation and producing the formation fluids from one or more
production wells which are situated at a horizontal distance or
offset from the injection well or wells. As the steam enters the
formation through the injection wells, 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 a driving force for displacing the oil towards the
production well. Other effects such as visbreaking may occur,
depending upon the temperature of the steam.
One of the problems associated with steam drive recovery
processes is the loss of heat by conduction into non-produc-tive
zones including the strata above and below the producing interval
and various measures have been proposed for minimizing thermal
losses.
In certain oil bearing -Formations, there may be a number of
vertically separated oil bearing strata separated by layers of
impervious material such as shale. These shale layers may prevent
or at least, substantially restrict, vertical fluid flow between the
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layers and therefore in formations of this kind, it may be necessary
to inject steam into each of the layers if complete recovery is to
be ensured. It is possible to reduce the thermal losses by
employing a countercurrent flow of the steam in adjacent zones, as
described in U.S. Patent No. 3,180,413. In the recovery process
described in this patent, two multiply completed wells are sunk into
the two adjacent strata separated by the shale layer and steam is
injected at one well into one of the layers with production being
taken out at the other well in the same layer. In the adjacent
layer, the direction of fluid flow is reversed so that the steam
flow is countercurrent in the two layers, to permit the utilization
of thermal energy which would otherwise be lost without contributing
to the recovery process.
Although the recovery process described in U.S. 3,180,413
has a number of advantages, there are also disadvantages. First,
the wells require to be multiply completed so that each well may be
used for both injection and production purposes, albeit in
different, vertically separated zones of the same reservoir and
second, the use of a single, multiply completed well for injection
and production purposes may have an undesirable effect upon the
produced fluids as they are brought to the surface. For example, if
particularly high temperature steam is being used as the injection
fluid, the produced fluids may be vaporized or pyrolyzed in the
production flow passages and this may give rise to handling problems
at the surface. Other problems may arise from the pressure
differentials between the injection and production streams in
multiply completed wellso leaks around the packers and the cement
may occur to the detriment of process efficiency. It would,
therefore, be desirable to avoid the use of multiply completed wells
without altogether losing the advantages of countercurrent steam
flow recovery.
According to the present invention a method has been
devised for the recovery of oil from subterranean reservoirs which
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have two or more vertically separated oil bearing, permeable zones
separated by impervious layers such as shale layers, which method
avoids the use oF multiple well completions but minimizes the
thermal losses which occur with concurrent steam flow techniques.
According to the present invention, ~urthermore, the method
for recovering the viscous oil from a subterranean reservoir having
upper and lower oil bearing permeable zones separated by an
impervious layer comprises (l) injecting steam through a first
injection well into the upper zone and producing fluids including
oil from a first production well in the upper zone which is situated
at a horizontal distance from the first injection well; and (2)
injecting steam into the lower zone through a second injection well
which is situated at a horizontal distance from the First injection
well and producing fluids, including oil from a second production
well in the lower zone which is situated at a horizontal distance
from the second injection well.
The method of the present invention is, of course,
applicable to reservoirs which have more than two vertically
separated production intervals and in such cases, each adjacent pair
of zones is produced by the same technique.
In practice, of course, more than one injection well and
more than one production well will invariably be used in order to
cover the entire producing field and to maximize recovery. The
wells may be arranged in any convenient pattern such as in straight
lines, for a line drive flooding process, or in conventional
patterns such as five spot, inverted five spot, seven spot or
inverted seven spot. Regardless of the nature of the pattern, the
injection and production wells in any one layer are arranged in a
pattern which is staggered with respect to that oF the next adjacent
layer. This has the result that the steam flow in each two adjacent
layers proceeds partly by concurrent flow and partly by
countercurrent flow.
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The present invention, therefore, in one aspect,
resides in a method for the recovery of oil from a
subterranean reservoir having upper and lower oil
bearing, permeable zones separated by an impervious
layer, comprising injecting steam into thP upper and
lower zones through patterns of injection wells which
extend respectively into the upper and lower zones and
producing fluids including oil from patterns of
production wells which extend respectively into the upper
and lower zones, the injection and production wells in
each pattern in the upper and lower zones being
horizontally separated from one another, with the
patterns of injection wells being staggered with respect
to one another in the two zones and the patterns of
production wells being staggered with respect to one
another in ths two zones, and causir,g the steam flow in
said zones to proceed partly in concurrent fashion and
partly in countercurrent ashion.
In another aspect, the present invention resides in
a method for recovering ~iscous oil from a subterranean
reser~oir having an upper permeable zone and a lower
permeable zone, each of said permeable zones being oil
bearing and separated by ~n impermeable layer, which
comprises (1) in~ecting steam through a first pattern of
injection wells axtending from the surface of the earth
into the upper zone and producing fluids including oil
from a first pattern of production wells extending from
the surface o the earth into the llpper zone, each of
which is at a horizontal distance from the injection
wells in the first pattern; (2) in~ecting steam into the
lower zone through a second pattern of injection wells
each of which is situated at a horizontal distance from
the wells of the first pattern of injection wells; (3)
controlling the steam flow in said zones 80 that said
steam flow proceeds partly in concurrent fashion and
partly in countercurrent fashion; and (4) producing
fluids including oil from a second pattern of production
wells in the lower zone, each of which is at a
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horizontal distance from the injection wells in said
second pattern of injection wells.
The invention is better understood by refe~ence ~o
the accompanying dr~wings, wherein:
Fiyure 1 is a vertical section in a simplified
schematic form of a subterranean oil reservoir
with uppex and lower oil bearing formations
with staggered steam drive well patterns;
Figure 2 is a simplified vertical section of a
three layer subterranean reservoir using
staggered well patterns; and
Figure 3 is a graph showiny the respective oil
recoveries obtained with a staggered pattern of
wells as in Figure 1 and a regular pattern of
wells.
Figure 1 shows a simplified vertical section of a
subterranean oil reservoir having an upper, oil bearing,
permeable zone 10 and a lower, oil bearing, permeable
zone 20 separated by a
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substantially impermeable shale layer S, underlying overburden 0. A
number of injection wells I-ll and I-12 extend into upper layer 10
from the surface of the earth and a number of separate injection
wells I-21 and I-22 extend into lower zone 20 from the surface of
the earth. Production wells P-ll, P-21 and P-22 extend into the
upper and lower zones at horizontal distances or offsets from the
injection wells in their respective zones. In addition, the
injection wells of the two zones are situated at horizontal
distances or offsets from each other so that the steam flow in the
two layers proceeds partly in concurrent fashion and partly in
countercurrent fashion. For example, the steam flow in the upper
zone proceeds countercurrent to the steam flow in the lower zone
between wells I-ll and I-21 and concurrent between I-21 and P-ll.
Similarly, in the lower zone, the steam flow is concurrent between
wells I-21 and P-ll and countercurrent between wells P-ll and P-22.
A similar staggered well pattern for a three layer
reservoir is shown in Figure 2. The permeable, oil bearing zones
10, 20 and 30 are separated by impermeable shale layers S-l and S-2,
all underlying overburden 0. Injection wells I-ll and I-12 are
completed into the uppermost layer 10, injection wells I-21 and I-22
in-to the middle layer 20 and injection wells I-31 and I-32 into the
lowest layer 30. Production well P-ll is completed into uppermost
layer 10, production well P-21 into middle layer 20 and production
well P-31 into the lowest layer 30. Injection and production is
carried out in the same manner as described above so that the steam
flow has between any two adjacent layers proceeds partly in
concurrent and partly in countercurrent fashion. For example, the
steam flow in uppermost layer 10 proceeds countercurrent to the
steam flow in middle layer 20 between wells I-ll and I-21 and then
proceeds concurrently between wells I-21 and P-ll.
The injection and production wells may be of conventional
type since no multiple completions are required. Steam injection
temperatures may suitably vary from about 120 to 350 C and the
,
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quality of the injec-ted steam is suitably within the range of
50-100%, more usually 50-90%. The steam injection rate will
generally vary depending upon the thickness of the individual oil
bearing zones but is preferably within the range of 65 to 325
litres/days/1000 m3 (about 0.5 to about 2.5 barrels of steam per
day per acre foot of oil bearing strata).
It has been found that -the use of staggered production
patterns as described above not only avoids the problem associated
with multiple well completions but also offers a definite advantage
in oil recovery as more oil is obtained at an earlier time, as
compared to a process using completely concurrent steam flow. A
computer simulation was made for a two layer reservoir with an
impermeable shale layer separating the two permeable, oil bearing
zones. The major reservoir characteristics are shown in Table 1
below.
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Table 1
Major Reservoir Characteristics
Rock Properties
Temperature, F 80F
Depth, ft 1300
Porosity, % 35
Average Permeability, md 5000
kv/kh . 1
Compressibility, psi .0001
Heat Capacity, Btu/lb rock-F .248
Heat Conductivity, Btu/F-ft-hr 1.25
Net Pay, ft 80
Sw 0.35
SO 0.64
Oil Characteristics
API Gravity, 11.4
MW 545
Density, lb/ft 61.8
Compressibility, psi~l 3.2 x 10-6
Heat Capacity, Btu/lb-F 0.46
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The reservoir was then subjected to a simulated steam flood
oil recovery using a staggered pattern of wells as shown in Figure 1
and second, a regular pattern of wells in which the steam flow in
the two layers is wholly concurrent (see Figure 3). The results are
shown in Figure 3 which shows that a greater amount of oil is
recovered at an earlier time, making for a more economically
favorable process. The improvement in recovery may be attributed to
a more favorable heat transfer between the reservoirs.
Generally, it will be preferred that in any two layer
system, the staggering between the well patterns in the two layers
will be uniform although conditions may dictate a departure from
uniform staggering in particular instances. Thus, it is normally
preferred that an injection well for the lower layer should lie
midway between a pair of injection and production wells for the
upper layer and a production well for the lower layer should be
midway between a pair of injection and production wells for the
upper layer, as shown in Figure 1. Similarly, an injection well for
the upper layer should lie midway between a pair of injection and
production wells for the lower layer and a production well for the
upper layer should lie midway be-tween a pair of injection and
production wells for the lower layer. However, in arrangements
where the wells are disposed other than in a straight line drive
arrangement, the spacings may be somewhat different and more
complicated.
Generally, the thickness of the oil bearing, permeable
layers will not exceed about 15m (about 49 feet) and, in order to
maximize thermal conduction between adjacent permeable layers, the
thickness of the shale layer should not be more than about 3m (about
10 feet). However, greater or less thicknesses may be encountered
with corresponding changes in the efficiency oF the process. It
should be remembered, of course, that when the shale layer becomes
much thicker than about 5m, the heat flow beween adjacent oil
bearing layers will be greatly reduced and the advantages of the
present staggered well patterns will be reduced correspondingly.
