CA1237069A - Multiple zone oil recovery process employing countercurrent steam flood - Google Patents
Multiple zone oil recovery process employing countercurrent steam floodInfo
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- CA1237069A CA1237069A CA000475139A CA475139A CA1237069A CA 1237069 A CA1237069 A CA 1237069A CA 000475139 A CA000475139 A CA 000475139A CA 475139 A CA475139 A CA 475139A CA 1237069 A CA1237069 A CA 1237069A
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Abstract
Abstract A method for the recovery of viscous oil from a subterranean, viscous oil-containing formation having a number of overlying permeable oil-bearing strata separated by impervious layers. Steam is injected countercurrently through adjacent oil-bearing strata and oil is produced utilizing spaced-apart injection and production wells provided with separate flow paths. The injection of steam is terminated when steam breakthrough occurs in the next lower layer or when the fluids recovered from that layer reach a predetermined oil:water ratio. Production is continued using pressure drawdown for improved efficiency.
Description
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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 i9 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 ~3'i'~ S9 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 ~2~
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 '`7~ 3 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 06~
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 ~.~3~ 3 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 :~l2~'7~
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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 of 320~
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 `¦ 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.
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 Oil Characteristics API Gravity 11.4 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 'I.X3t7~69 3~08 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 TV, ~7~
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.
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 i9 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 ~3'i'~ S9 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 ~2~
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 '`7~ 3 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 06~
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 ~.~3~ 3 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 :~l2~'7~
320~
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 of 320~
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 `¦ 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.
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 Oil Characteristics API Gravity 11.4 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 'I.X3t7~69 3~08 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 TV, ~7~
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.
Claims (20)
1. A method for recovering oil from a subterranean formation having upper and lower permeable zones containing viscous oil which are each penetrated by an injection well and a production well, the method comprising:
(i) injecting steam into the upper zone through the injection well in the upper zone and passing steam through the upper zone and recovering fluids including oil from the production well in the upper zone, (ii) injecting steam into the lower zone through the injection well in the lower zone and passing steam through the lower zone in countercurrent to the steam in the upper zone and recovering fluids including oil from the production well in the lower zone, (iii) terminating injection of steam into both zones when vapor phase steam production occurs at the production well in the lower zone or the fluids produced at this well comprise a predetermined amount of steam or water.
(i) injecting steam into the upper zone through the injection well in the upper zone and passing steam through the upper zone and recovering fluids including oil from the production well in the upper zone, (ii) injecting steam into the lower zone through the injection well in the lower zone and passing steam through the lower zone in countercurrent to the steam in the upper zone and recovering fluids including oil from the production well in the lower zone, (iii) terminating injection of steam into both zones when vapor phase steam production occurs at the production well in the lower zone or the fluids produced at this well comprise a predetermined amount of steam or water.
2. A method according to claim 1 in which production in the upper zone is continued after the injection of steam is terminated.
3. A method according to claim 2 in which production in the upper zone is continued until vapor phase steam production occurs in the upper zone.
4. A method according to claim 2 in which production in the upper zone is continued until the fluids produced in the upper zone comprise a predetermined amount of steam or water.
5. A method according to claim 1 in which production in both zones is continued after the injection of steam is terminated.
6. A method according to claim 5 in which production in both zones is continued until vapor phase steam production occurs in the upper zone.
7. A method according to claim 5 in which production in both zones is continued until the fluids produced in the upper zone comprise a predetermined amount of steam or water.
8. A method according to claim 1 in which the temperature of the steam injected into the upper and lower zones is within the range of 250° to 650°F and steam quality is within the range of 50 to 90 percent.
9. A method according to claim 1 in a dual completion of a single well provides the injection well in the upper zone and the production well of the lower zone.
10. A method according to claim 9 in which a dual completion of a single well provides the injection well in the lower zone and the production well of the upper zone.
11. A method for the recovery of oil from a subterranean, viscous oil-containing formation having upper and lower permeable, viscous oil-bearing zones separated by an impervious layer, the upper and lower zones being penetrated by first and second wells, comprising:
(i) providing the first and second wells with two separate flow paths, the first path establishing fluid communication between the surface of the earth and the upper oil-bearing zone and the second establishing fluid communication between the surface of the earth and the lower oil-bearing zone;
(ii) injecting steam into the upper zone through the second well by means of the first flow path and recovering fluids including oil from the upper zone through the first well by means of the first flow path;
(iii) injecting steam into the lower zone through the first well by means of the second flow path and recovering fluids including oil from the lower zone through the second well by means of the second flow path;
(iv) continuing injection of steam into both zones until the fluid recovered from the lower zone comprises a predetermined amount of steam or vapor phase steam production occurs at the second well in the lower zone, upon which the injection of steam is terminated; and (v) continuing the recovery of fluids from at least the upper zone through the first well.
(i) providing the first and second wells with two separate flow paths, the first path establishing fluid communication between the surface of the earth and the upper oil-bearing zone and the second establishing fluid communication between the surface of the earth and the lower oil-bearing zone;
(ii) injecting steam into the upper zone through the second well by means of the first flow path and recovering fluids including oil from the upper zone through the first well by means of the first flow path;
(iii) injecting steam into the lower zone through the first well by means of the second flow path and recovering fluids including oil from the lower zone through the second well by means of the second flow path;
(iv) continuing injection of steam into both zones until the fluid recovered from the lower zone comprises a predetermined amount of steam or vapor phase steam production occurs at the second well in the lower zone, upon which the injection of steam is terminated; and (v) continuing the recovery of fluids from at least the upper zone through the first well.
12. A method according to claim 11 in which the recovery of fluids from the upper zone is continued until vapor phase steam production occurs in the upper zone.
13. A method according to claim 11 in which the recovery of fluids from the upper zone is continued until the fluids produced in the upper zone comprise a predetermined amount of steam or water.
14. A method according to claim 11 in which recovery of fluids from both the upper and the lower zone is continued until vapor phase steam production occurs in the upper zone.
15. A method according to claim 11 in which recovery of fluids from both the upper and the lower zone is continued until the fluids produced in the upper zone comprise a predetermined amount of steam or water.
16. method according to claim 11 in which the temperature of the steam injected into the upper and lower zones is within the range of 250° to 650°F and steam quality is within the range of 50 to 90 percent.
17. A method for the recovery of oil from a subterranean, viscous oil-containing formation having upper, lower and riddle permeable, viscous, oil-bearing zones, the middle zone being separated from the upper and lower zones by impervious layers, the upper, middle, and lower zones being also penetrated by at least two spaced-apart first and second wells, the method comprising:
(i) providing the first and second wells with three separate flow paths, the first path establishing fluid communication between the surface of the earth and the upper zone, the second establishing fluid communication between the surface of the earth and the middle zone, and the third establishing fluid communication between the surface of the earth and the lower zone;
(ii) injecting steam into the upper zone through the first well by means of the first flow path and recovering fluids including oil from the upper zone through the second well by means of the first flow path;
(iii) injecting steam into the middle zone through the second well by means of the second flow path and recovering fluids including oil from the middle zone through the first well by means of the second flow path;
(iv) injecting steam into the lower zone through the first well by means of the third flow path and recovering fluids including oil from the lower zone through the second well by means of the third flow path;
(v) terminating the injection of steam into a zone upon steam breakthrough at the next lower adjacent zone;
(vi) continuing recovery of fluids from each zone following the termination of steam injection into that zone.
(i) providing the first and second wells with three separate flow paths, the first path establishing fluid communication between the surface of the earth and the upper zone, the second establishing fluid communication between the surface of the earth and the middle zone, and the third establishing fluid communication between the surface of the earth and the lower zone;
(ii) injecting steam into the upper zone through the first well by means of the first flow path and recovering fluids including oil from the upper zone through the second well by means of the first flow path;
(iii) injecting steam into the middle zone through the second well by means of the second flow path and recovering fluids including oil from the middle zone through the first well by means of the second flow path;
(iv) injecting steam into the lower zone through the first well by means of the third flow path and recovering fluids including oil from the lower zone through the second well by means of the third flow path;
(v) terminating the injection of steam into a zone upon steam breakthrough at the next lower adjacent zone;
(vi) continuing recovery of fluids from each zone following the termination of steam injection into that zone.
18. A method according to claim 17 in which recovery of fluids from each zone is continued until steam breakthrough occurs in that zone.
19. A method according to claim 17 in which recovery of fluids from each zone is continued until the fluids recovered from that zone comprise a predetermined amount of steam or water.
20. A method according to claim 17 in which recovery of fluids from each zone is continued until the fluids recovered from the upper zone comprise a predetermined amount of steam or water.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US58356984A | 1984-02-27 | 1984-02-27 | |
| US583,569 | 1984-02-27 | ||
| US68140284A | 1984-12-13 | 1984-12-13 | |
| US681,402 | 1984-12-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1237069A true CA1237069A (en) | 1988-05-24 |
Family
ID=27078842
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000475139A Expired CA1237069A (en) | 1984-02-27 | 1985-02-26 | Multiple zone oil recovery process employing countercurrent steam flood |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1237069A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2015049125A3 (en) * | 2013-10-01 | 2015-10-29 | Wintershall Holding GmbH | Method for extracting crude oil from an underground oil deposit using a borehole that acts simultaneously as an injection and production borehole |
-
1985
- 1985-02-26 CA CA000475139A patent/CA1237069A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2015049125A3 (en) * | 2013-10-01 | 2015-10-29 | Wintershall Holding GmbH | Method for extracting crude oil from an underground oil deposit using a borehole that acts simultaneously as an injection and production borehole |
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