Note: Descriptions are shown in the official language in which they were submitted.
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1 Field of the Invention
2 This invention relates to an improvement of a steam
3 injection process for the recovery of heavy oil. More
4 particularly, it relates to injecting non-condensable gas into
a depleted portion of a reservoir to pressure it up and prevent
6 the escape of steam thereinto, which steam is being injected into
7 an adjacent portion of the reservoir.
8 BACKGROUND OF THE INV~NTION
9 It is conventional practice to inject steam into a
heavy oil reservoir to heat the formation and reduce the
11 vlscosity of the oil, thereafter producing the oil onoe it~
12 mobility has been improved. Such an operation is commonly
13 referred to as a "thermal projeat". -
14 A problem can arise with respect to a thermal project
if a "thief zone" is in communioation with the oil reservoir into
16 which the steam i6 being injected. If this is the case, the
17 in~ected steam will preferentially move into the thief zone.
18 Heating of the oil-saturated portion of the reservoir i~ then
~19 reduo-d.
Prequently the thief zone is a laterally extending
21 section of that portion of the oil-containing reservoir that is
22 to be heated. The sectlon typically will have a relatively high
23 ga~ or water saturat1on. Often it is located at the top of or
24 high in the reservoir.
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1 A thief zone can al~o occur ln another manner. In
2 heavy oil thermal projects lt i6 common procedure to practice
3 steam injection and oil productlon in a fir6t area and, when the
4 reservoir underlying the area iB 6ignificantly depleted, to then
expand the project by commencing operations in an adjacent second
6 area. In some cases, the depleted first portion of the reservoir
7 i9 in fluid communication with the non-depleted second portion
8 of the reservoir. In this situation, steam injected into the
~ non-depleted portion of the reservoir may migrat~ into the
depleted portion. As a re6ult, the depleted first portion of the
11 re6ervoir constitutes a thief zone for steam being injected into
12 the second portion.
13 When steam escapes into 6uch a thief zone, it i6 found
14 that injection pre66ure diminishe6 and the temperature in the
producing portion of the re~ervoir i6 relatively low. As a
16 result, the oil production rate also drop6 off.
17 There i8 therefore a need for a process that will
18 inhlbit 10~6e6 of injected steam through or into a thief zone.
19 SUMNa~Y OF THE INVENTIO~
Thi6 embodiment of the invention i6 concerned with a
21 situation where there are two adjacent 6team injection and fluid
22 production patterns, both completed in the same re6ervoir. The
23 reservolr portion underlying the fir~t pattern ha6 already
24 experienaed some steam injection and oll produation. Thus it is
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1 partially depleted. The reservoir portion underlying the second
2 pattern has experlenced less depletion. There i6 fluid
3 communication between the patterns - stated otherwise, steam
4 injected ~hrough the wells of the second pattern will enter the
more depleted reservoir portion.
6 In accordance with the invention, non-condensable gas
7 is injected through wells of the first pattern into the more
8 depleted reservoir portion at the same time that steam is
9 injected through wells of the second pattern. Preferably the
non-condensable gas is injected at a rate and in an amount
11 sufficient to substantially equalize the pressure in the more
12 depleted reservoir portion with the pressure in the steam zone
13 in the second reservoir portion. When this is done, steam loss
14 into the more depleted portion of the reservoir is inhibited with
a concomitant improvement in oil productlon and steam/oil ratio
16 at the second pattern. The gas injected into the first pattern
17 may also contribute to improved performance in the production
18 wells within the first pattern.
19 DESCRIP~ION OF ~HE DRAWINGS
Figure 1 is a schematic showing the patterns and the
21 gas injection wells which were used in demonstrating the
22 invention at a pilot project;
23 Figure 2 illustrates with logs the nature of the
24 reservoir in the pilot test area;
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1 Figure 3 is a plot showing steam injection and bitumen
2 production rates for the B pattern of the pilot test. Arrows on
3 the plot indicate when the injection well BI1 was started up,
4 when BI1 injection was switched from hot water to steam, when the
middle zone was completed, when injection wells BI8 and BI9 were
6 started up, when the high steam rate test was conducted, and when
7 outside gas injection began; and
8 Figure 4 is a plot of gas injection rate through the
9 wells identified on the plot.
DESCRIPTION OF THE P~EFERRED EM~O~IMEN~
11 The invention is exemplified by the following example
12 based on a pilot test conducted in the Rearl Lake region of
13 Alberta.
14 The reservoir at the pilot site, depicted in Figure 2,
has two oil producing pay zones, a lower zone 1 and a middle zone
16 2. The middle pay zone 1 i8 approximately 35 m thick and has a
17 sand region 3 at its upper end. This region 3 is approximately
18 10 m thick and has signifiaantly higher water saturation than the
19 pay zone 2. The region 3 constitutes a thief zone for steam
in~ected through well perforations in the pay zones 1,2.
21 The bitumen in the pay zone 2 is effectively immobile
22 at initlal reservoir conditions.
23 A steam drive pilot was initiated in an A pattern
24 oonsisting of steam ln~ection wells and production wells. The
layout of the A pattern wello i6 shown in Figure 1. Each well
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l is identified as to pattern (A), nature (injection (I),
2 production (P), or observation (O)) and number. The A pattern
3 was an inverted 7-spot with peripheral steam injection to enclose
4 the pattern and make it eguivalent to an inner pattern in a
commercial project. The pattern covered 5.37 acres.
6 At the same time that the A pattern was drilled, an
7 adjacent B pattern was also drilled. The B pattern was
8 originally an inverted 5-spot surrounded by 8 steam injection
9 wells. It was decided to delay start-up of the B pattern to gain
operating experience on the A pattern.
11 A steam drive was initiated in December, 1981, in the
12 A pattern and continued for 5 years. Steam was injected into the
13 AI wells and fluid was produced from the AP wells.
14 It became clear that a large volume of steam was being
lost from the A pattern, as the steam-oil ratio was very high.
16 As a result of the A pattern experience, changes were
17 made to the B pattern prior to its start-up. It was decided not
18 to inject steam into the peripheral wells of the B pattern.
19 Instead the B pattern was converted from a 5-spot to a 9-spot.
Start-up of the BI1 pattern occurred in February, I985,
21 and start-up of the patterns of BI8 and BI9 was initiated in
22 September, 1987. Steam was injected through the 3 injection
23 wells and fluid produced from the 12 production wells in
24 conventional fashion.
Wells BI2, BI3, BI4, BI5, BI6 and BI7 were also
26 completed in the reservoir as observation wells and were used to
27 monltor temperature and pressure outside the B pattern.
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1The chronology of operatlons in the B pattern and the
2ef~ect on bitumen production is shown ln Flgure 3. Hot water
3injection was initiated into the lower zone 1 of the BI1 pattern
4in February, 1985. Steam injeatlon lnto the lower zone 1 of the
5BI1 pattern began in Au~ust, 1985. Middle zone 2 operations
6began in December, 1986. The BI8 and BI9 patterns were added in
7September, 1987.
8A high rate steam test was conducted in the summer of
91988 in which the steam injection rate was approximately doubled
10for a period of about two months.
11The outside gas injection test was begun in April,
121989, with the injection of natural gas into well~ BI2, BI4 and
13BI6 following perforation of tho6e wells in the region 3 of the
14middle zone 2. Gas injection into wells BI7 and AI2 was
15initiated a few months later, as shown in Figure 4.
16As shown in Figure 3, the high rate steam test resulted
17in a significant increase in bitumen production rates, but the
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18steam-oil ratio did not improve. --
19After the high rate steam test, the bitumen production
20rate fell considerably until March, 1989, when the steam- :
21 stimulation of some production wellc began in antioipation of
2~ the outside gas injection test.
23As stated, the outside gaa injection test began in
24April 1989, and i8 still contlnuing. Gas in~ection was conducted
~lmultaneously with steam lnjection. More partlcularly, during
26 the outside gas ln~eation test, the steam injection rate was held
27 ¢on~tant at a rate of only about 60% of that during the high rate
28 ~team test. ~he bltumen produ¢tion rate during the outside gas
29 in~eotlon test
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1 started to increase significantly withln one month, and, over the
2 eight month period since gas in~ectlon began, the bitumen
3 production rate has, on average, been more than 80% higher than
4 that prior to gas in~ection.
The instantaneous steam-oll ratio during the outside
6 gas injection test also improved considerably over that observed
7 prior to outside gas injection.
8 No detrimental effects of outside gas injection have
9 been observed. There has been no noticeable increase in gas
production at the production wells. The injected gas remains
11 near the top of the payzone 2 due to gravity effects, while
12 liquids are produced through perforated intervals near the base
13 of the pay zone.
14 Prior to outside gas injection, the region 3 allowed
fluids to flow out of the B Pattern. In particular, æteam, hot
16 water and hot bitumen flowed out of the B pattern during steam
17 injection within the pattern. This was evidenced by temperature
18 and pressure measurements at the observation wells outside the
19 pattern and by the fact that the pressure within the pattern
remained low. When the steam injection rate was increased in the
21 B pattern, a temperature response could be detected even within
22 the A pattern. Thus the A pattern constituted a thief zone in
23 communication with the B pattern.
24 At the time gas injection began into region 3 through
wells outside the B pattern, the pressure within the B pattern
26 was only about 800 kPa. The native reservoir pressure is about
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~ 300 kPa. Within three months o~ the commencement of outside gas
2 injection, the pressure within the B pattern increased from 800
3 kPa to over 1000 kPa and the pressure within the A pattern
4 increased from about 400 kPa to over 900 kPa. Within the B
pattern, the temperature increased along with the pressure as
6 determined by saturated steam conditions within the B pattern.
7 Prior to and during the outside gas injection
8 operation, wells AP1 and AP3 and AP6 were maintained on
g production even though no steam was injected into any wells in
the A pattern. Prior to the commencement of outside gas
ll injection, the A pattern wells benefitted from heat communication
12 with the B pattern but this heat communication was eliminated
13 when gas injection began. Even though the A pattern wells lost
14 heat communication, the production performance of wells AP1, AP3
and AP6 has increased over that prior to gas injection. This
16 increased production is believed to be related to an improved
17 gravity drainage mechanism due to the increased gas saturation
18 in the A pattern.
