System and Method of Liquefying a Heavy Oil Formation for Enhanced Hydrocarbon Production

Abstract

A system and method of liquefying a heavy oil formation to enhance oil production in a well, by inducing shear stress reversal within the formation by a plurality of expanding/contracting bladders in contact with the formation. The induced liquefaction enables formation materials and fluids to flow into the well and thus initiate and propagate the CHOPS (Cold Heavy Oil Production System) process, and thereby enhancing the hydrocarbon production of the well.

Description

FIELD OF THE INVENTION

[0001] This invention relates to an in situ method and system for enhancing hydrocarbon production from a well. The heavy oil formation is subjected to induced liquefaction to initiate and propagate the cold heavy oil production system (CHOPS) process and thus enhance the well's productivity and recovery.

BACKGROUND OF THE INVENTION

[0002] Heavy oil and bitumen oil sands are abundant in reservoirs in many parts of the world such as those in Alberta and Saskatchewan in Canada, Utah and California in the United States, the Orinoco Belt of Venezuela, Indonesia, China and Russia. The hydrocarbon reserves of the oil sand deposit is extremely large in the trillions of barrels, with recoverable reserves estimated by current technology in the 300 billion barrels for Alberta, Canada and a similar recoverable reserve for Venezuela. These vast heavy oil (defined as the liquid petroleum resource of less than 20.degree. API gravity) deposits are found largely in unconsolidated sandstones, being high porosity permeable cohensionless sands with minimal grain to grain cementation. The hydrocarbons are extracted from the oils sands either by mining or in situ methods.

[0003] The heavy oil and bitumen in the oil sand deposits have high viscosity at reservoir temperatures and pressures. The oil sands herein will be defined as those deposits that contain a high viscosity bitumen; whereas heavy oil formations while of similar formation type contain hydrocarbons such as heavy oil at much lower viscosity. Such heavy oil formations are located in the Lloydminster area of Alberta and Saskatchewan in Canada. These formations are collectively known as the Lloydminster Group of heavy oil deposits.

[0004] In situ methods of hydrocarbon extraction from the oil sands consist of (1) cold production, in which the less viscous petroleum fluids are extracted from vertical and horizontal wells with sand exclusion screens, (2) CHOPS (cold heavy oil production system) cold production with sand extraction from vertical and inclined wells with large diameter perforations thus encouraging sand to flow into the well bore, (3) CSS (cyclic steam stimulation) a huff and puff cyclic steam injection system with gravity drainage of heated petroleum fluids using vertical and horizontal wells, (4) streamflood using injector wells for steam injection and producer wells on 5 and 9 point layout for vertical wells and combinations of vertical and horizontal wells, (5) SAGD (steam assisted gravity drainage) steam injection and gravity production of heated hydrocarbons using two horizontal wells, (6) VAPEX (vapor assisted petroleum extraction) solvent vapor injection and gravity production of diluted hydrocarbons using horizontal wells, and (7) combinations of these methods.

[0005] CHOPS is a primary production method involving the intentional production of formation sand and fines to enhance heavy oil production rates. The behavior of CHOPS wells is well known (CHOPS.SPE Petroleum Engineers Handbook, Volume VI-Emerging and Peripheral Technologies (EMPT), Chapter 5, 40 pages, Dusseault, M. B., 2007). The CHOPS process is a process of foamy oil drive with the intentional production of formation materials and fluids. CHOPS depends on solution gas, with the Canadian heavy oil deposits having gas in solution; the bubble point being usually at or near the initial reservoir pressure. Wells are subject to aggressive drawdown and gas exsolves as bubbles that expand in response to the pressure decline during flow to the well. The bubbles act as an additional drive mechanism, driving the mixture of formation sand and heavy oil towards the well. Unfortunately, many wells installed for CHOPS production are poor producers, either due to a lack of initial sand production or to the premature stopping of sand and heavy oil production. Borehole logs of the formation typically can not distinguish between those areas of the reservoir that will produce by CHOPS from those that will not. Generally it is a small increase in the fine fraction of the formation material that can result in poor or minimal CHOPS production.

[0006] Thermal recovery methods, CSS and SAGD, utilize steam to heat the heavy oil and bitumen thus reducing its viscosity, so it can flow much easier to the well. Solvents applied to heavy oil and bitumen soften the heavy oil and bitumen and reduce its viscosity and provide a non-thermal mechanism to improve mobility. Hydrocarbon solvents consist of vaporized light hydrocarbons such as ethane, propane or butane or liquid solvents such as pipeline diluents, natural condensate streams or fractions of synthetic crudes.

[0007] Therefore, there is a need for a method of enhancing the CHOPS well completion system to ensure the well initiates and continuously produces hydrocarbons.

SUMMARY OF THE INVENTION

[0008] The present invention is a system and method of inducing liquefaction in the formation around the well, the formation liquefied zone will ensure that the CHOPS process initiates, a disturbed zone is created, and the well produces hydrocarbons at economical rates.

[0009] Formation liquefaction results from an increase in formation pore pressure induced by transient or repeated ground motions or shocks. Pore pressure increases may be induced by earthquakes, explosions, impacts, and ocean waves. Soil liquefaction occurs in water saturated, cohesionless soils and causes a loss of soil strength that may result in the settlement and/or failure of buildings, dams, earthworks, embankments, slopes and pipelines. Liquefaction of sands and silts has been reported in almost all of the major earthquakes around the world. The imposed ground stress waves from earthquakes or other transient or repeated loading induces shaking or vibratory shearing of saturated loose fine sand or silts, causing a phenomenon known as liquefaction. When loose sands and silts are subjected to repeated shear strain reversals, the volume of the soil contracts and results in an immediate rise in the pore pressure within the soil. If the pore pressure rises sufficiently high, then the soil grain to grain contact pressure drops to zero, and the soil mass will lose all shear strength and temporarily act like a fluid, i.e. liquefaction occurs. Such temporary loss of shear strength can have a catastrophic effect on earthworks or structures founded on these deposits. Major landslides, settling or tilting of buildings and bridges and instability of dams or tailings ponds and failure of pipelines have all been observed in recent years and efforts have been directed to prevent or reduce such damage.

[0010] The factors that affect the occurrence of liquefaction are formation type, grain size distribution, consolidation of the formation, formation permeability, effective stress state, pore pressure gradient, magnitude and number of the shear strain reversals. Fine cohesionless soils, fine sand or fine cohesionless soils containing moderate amounts of silt are most susceptible to liquefaction. Uniformly graded soils are more susceptible to liquefaction than well graded soils, and fine sands tend to liquefy more easily than coarse sands or gravelly soils. Moderate amounts of silt appear to increase the liquefaction susceptibility of fine sands; however, fine sands with large amounts of silt are less susceptible, although liquefaction is still possible. Recent evidence indicates that sands containing moderate amounts of clay may also be liquefiable.

[0011] The induced liquefaction of the formation around the well is generated by placing a portion of the formation around the well under cyclic shear stress reversals, under zero volume change and undrained pore fluid conditions. The cyclic shear stress reversal is imposed on the formation by a cyclic shear stress reversal device, such as a bladder assembly comprising a plurality of expanding and contracting bladders imposing a cyclic stress reversal on a body of the formation in situ. The simultaneous expansion/contraction of the bladders under a zero volume change condition is achieved by cyclic upward/downward vertical movement of a piston inside of a fluid pressure cylinder connected to the plurality of bladders. The fluid system ensures the bladders are simultaneously expanded/contracted under zero volume change. The formation stress state varies from a horizontal maximum principal stress during the expansion phase of the bladder, and changes to a vertical maximum principal stress state during the contraction phase of the bladder. Thus the formation immediately in the zone of influence of the bladders undergoes shear stress reversals, much like that imposed in a cyclic triaxial laboratory test, except that the process is conducted in situ. Upon liquefaction onset, formation materials and fluids are extracted from the formation to initiate and maintain propagation of the CHOPS process. Once the CHOPS process has been established, the bladders are removed from the well, and the well is completed with a perforated liner, pump and production tubing. If after time, the well needs to be further stimulated by induced liquefaction then the bladders can be lowered into the well and further enhance its production potential.

[0012] Formation sand withdrawal through liquefaction and transport to the well creates increased porosity and permeability within the formation, as a channeled and remolded zone. The zone increases the permeability around the well, with the well behaving as if it has an increasing radius of influence with time. Thus the highest pressure gradient is removed from the well, further enhancing destabilizing of the formation and outward growth of the disturbed zone. Wormholes develop by liquefaction in the formation and can extend considerable distances from the well. Thus operating the well below the bubble point in a CHOPS well actually dramatically increases production rate and recovery. Also the CHOPS process removes wellbore skin by continuously shearing the sand grains, thus preventing pore throat plugging. Due to the growth of the disturbed zone around the well, the wellbore skin becomes increasingly negative with time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1A is a cross sectional view of a well showing a bladder assembly in accordance with the present invention for in situ inducing a cyclic shear stress reversal on the formation, and thus inducing liquefaction of the formation in the vicinity of the well.

[0014] FIG. 1B is an enlarged cross sectional view of the bladder assembly in the well of FIG. 1A.

[0015] FIGS. 2A, 2B, and 2C are cross sectional views of the bladder assembly in accordance with the present invention showing the bladder assembly in the equilibrium state (FIG. 2A) and the two extreme expansion/contraction phases (FIGS. 2B and 2C).

[0016] FIG. 3 is a cross sectional view of the bladder assembly in accordance with the present invention showing the equilibrium state with a mechanism for the cyclic loading of the bladders through the expansion/contraction phases.

[0017] FIG. 4 is a cross sectional view of the formation liquefied around the well and the bladder assembly and showing the initiation and formation of the CHOPS process due to extraction of formation materials and fluids from the depth horizon.

[0018] FIG. 5 is a cross sectional view of the formation liquefied around the well showing the final completion of the well with a perforated liner and a down hole progressive cavity pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present invention is a method of liquefying in situ a heavy oil formation by placing a portion of the formation under cyclic shear stress reversals, under zero volume change and undrained pore pressure conditions, to initiate the flow of formation fluids and fines into a cyclic shear stress reversal device, such as a bladder assembly. One form of the invention is illustrated in cross section in FIGS. 1A and 1B, with a well 1 drilled from the surface 2 to the top 3 of a heavy oil formation 4 and completed with a steel liner 5 grouted in place by cement 6. The well 1 is then further drilled to total depth 7 to the formation depth horizon, beneath the bottom 8 of the heavy oil formation 4. A cyclic shear stress reversal device, such as a bladder assembly 9 is lowered into the well 1, and locates onto a latch 10 to position bladders 11 and 12 within the heavy oil formation 4. The bladder assembly 9 may be self-boring down to the required formation depth horizon. Alternatively, the bladder assembly 9 may be driven down to the required formation depth horizon. In addition, the bladder assembly 9 may be inserted into an existing well, in which the existing casing has been removed across the formation depth horizon.

[0020] The bladder assembly 9 consists of expanding/contracting bladders 11 and 12 that are cyclically alternated from expansion to contraction to place the formation under cyclic stress reversal thus inducing formation liquefaction under a zero volume change condition. The bladders 11 and 12 are constructed similar to conventional fixed or sliding end packers, being mounted on a central anvil and containing an expanding/contracting reinforced rubber element. The middle section 13 of the bladder assembly 9 between the two bladders 11 and 12 comprises an inlet having perforations 14 of sufficient size to allow easy passage of formation materials and fluids to enter the bladder assembly 9. An extraction device, such as a piston sampler 15 (FIG. 1B) is connected to the perforations 14 and extracts formation materials and fluid from the formation 4 during and/or after the formation is subjected to liquefying shear stress reversals in the vicinity of the bladder assembly 9. Alternatively, the extraction device may also comprise a progressive cavity pump. Further, the extraction device may additionally include a pump that injects a foaming fluid into the formation.

[0021] In order to facilitate extraction of formation materials and fluid from the formation 4, steam may be injected into the formation upon the onset of induced liquefaction of the formation and prior to extraction of formation materials and fluids. Alternatively, vaporized hydrocarbon solvent may be injected into the formation upon the onset of induced liquefaction of the formation and prior to extraction of formation materials and fluids. The vaporized hydrocarbon solvent is maintained saturated at or near its dew point. The vaporized hydrocarbon solvent may be selected from the group of ethane, propane, butane or a mixture thereof.

[0022] The extraction of formation materials and fluids promotes the development of wormholes and thus enhances hydrocarbon production from the formation. The piston sampler 15 can be time or pressure activated, or activated by controls from the surface. The initial pressure in the piston sampler 15 is much lower than the formation pressure, so upon activation of the piston sampling, formation materials and fluids are sucked from the formation into the perforations 14 and then into the piston sampler 15. The pore pressure within the formation undergoing the cyclic loading is measured by a pore pressure gage 16 contained within the bladder assembly 9. A data acquisition system simultaneously records the expansion/contraction of the bladders and the induced pore pressure response. Beneath the lowermost bladder 12 is attached a perforated liner 17, with perforations 18 of sufficient size to allow the passage of formation material and fluids to enter. The perforated liner 17 detaches from the bladder assembly 9 and latches into the depth latch 10 upon removal of the bladder assembly 9 from the well 1. The well 1 is then placed on production by lowering a progressive cavity pump 32 and production string 33 into the well 1 (FIG. 5).

[0023] The cyclic loading of the formation by the bladder assembly 9 is shown further in FIG. 2, illustrating three phases, the neutral or equilibrium state 19 of bladders of 11 and 12 (FIG. 2A), the full contraction/expansion state 20 of bladders 11 and 12 (FIG. 2B), and the full expansion/contraction state 21 of bladders 11 and 12 (FIG. 2C). Once the bladder assembly 9 is located at depth, the bladders 11 and 12 are expanded to contact the formation in the neutral or equilibrium position 19, with the bladders 11 and 12 in pressure equilibrium. The bladders 11 and 12 are of the same size and volume. During the cyclic expansion/contraction of the bladders, initially both bladders are in the neutral or equilibrium position 19 (FIG. 2A), then the lowermost bladder 12 is contracted by a volume change equal to the expansion of the uppermost bladder 11, to achieve the full contraction/expansion state 20 (FIG. 2B). Following the full contraction/expansion state 20 of the bladders, the lowermost bladder 12 is then expanded through the equilibrium position 19 to its full expanded state 21 (FIG. 2C) and simultaneously the uppermost bladder 11 is contracted through the equilibrium state 19 and then further contracted to its full contracted state 21 (FIG. 2C). The formation maximum principal stress state is vertical immediately adjacent to the fully contracted bladder 12, and is horizontal immediately adjacent to the fully expanded bladder 11 in the contraction/expansion state 20. Similarly, the formation maximum principal stress state is horizontal immediately adjacent to the fully expanded bladder 12, and is vertical immediately adjacent to the fully contracted bladder 11 in the expansion/contraction state 21. Thus the formation 4 undergoes shear stress reversals in the zones immediately adjacent to the bladders 11 and 12. The expansion/contraction of the bladders 11 and 12 is cyclically pulsed to the desired number of loading reversals to induce liquefaction of the formation in the vicinity of the bladder assembly 9. The induced pore pressure response is monitored by the pressure gage 16 throughout the cyclic loading of the formation.

[0024] One form of the invention to achieve the simultaneous expansion/contraction of the bladders is shown in FIG. 3 in the neutral or equilibrium state 19. The expansion/contraction of the bladders 11 and 12 are driven by fluid contained in a pressure cylinder 22, connected by tubing 23 to the uppermost bladder 11 and also via tubing 24 to the lowermost bladder 12. The fluid in the pressure cylinder 22 is alternatively extracted and injected into the bladders 11 and 12 by the vertical movement of a piston 25 connected to a driving rod 26. The pressure in each bladder is monitored by pressure gages 27 and 28 and recorded on the data acquisition system. The rate of change of vertical movement of the piston 25, and therefore the rate of change of the expansion/contraction of the bladders, is monitored by a linear variable differential transformer 29 or similar device, and the piston rod position is recorded on the data acquisition system.

[0025] The three phases of the cyclic expansion/contraction of the bladders 11 and 12 is shown on FIG. 2, the neutral or equilibrium state 19, the contraction/expansion state 20 and the expansion/contraction state 21. The piston 25 is in the neutral or equilibrium state 19 within the fluid cylinder 22. Upward movement of the rod 26 moves the piston 25 in the fluid cylinder 22 to its uppermost position. In doing so, fluid is extracted from bladder 12 and injected into bladder 11 in a simultaneously controlled manner imposing no volume change on the formation 4. Downward movement of the rod 26 drives the piston 25 from the contraction/expansion state 20 through the neutral or equilibrium state 19 to the expansion/contraction state 21. At the expansion/contraction state 21 the piston 25 is at its lowermost position in the fluid cylinder 22. By movement of the piston 25 from the contraction/expansion state 20 to the expansion/contraction state 21, fluid is extracted from bladder 11 and injected into bladder 12. The fluid displacement by the piston 25 from the neutral or equilibrium state 19 to the contraction/expansion state 20 is controlled by movement of the rod 26 to be exactly the same as that displaced by the piston 25 from the neutral or equilibrium state 19 to the expansion/contraction state 21.

[0026] The cyclic movement of the rod 26 can be driven at the surface by a conventional hydraulic servo-controlled system or alternatively by electro-mechanical means using a solenoid or purely mechanical means. In another form of the invention the cyclic movement of the rod 26 could be activated and controlled down hole by either a hydraulic or electro-mechanical device contained within the bladder assembly 9 and controlled by instrumentation and power source from the surface. In either form of the invention the stroke of the piston 25 is controlled to achieve the desired expansion/contraction of the bladders 11 and 12 and thus loading on the formation 4, and the frequency of the stroking of the piston 25 is controlled to achieve the desired loading rate on the formation 4.

[0027] Thus the loading state, the frequency of loading, and the pore water pressure response are all simultaneously recorded by the computerized data acquisition system, and from analysis of these data the development of induced liquefaction of the formation can be quantified.

[0028] The induced liquefaction and CHOPS initiation and development in the formation 4 is shown on FIG. 4, with the bladder assembly 9 in the well 1, shown in the expansion/contraction state. Due to the cyclic loading of the bladders 11 and 12, the formation 4 undergoes induced liquefaction in the formation 4 creating a zone 30 of higher permeability in the vicinity of the bladder assembly 9. The extraction of formation materials and fluids through activation of the piston sampler 15 initiates and propagates wormholes 31 within the liquefied formation 4, initiating and thus propagating the CHOPS process and extending the CHOPS disturbed zone within the formation. The CHOPS development enhances the production of the heavy oil, along with formation materials and other fluids. Beneath the lowermost bladder 12 is attached the perforated liner 17, with perforations 18 of sufficient size to allow the passage of formation materials and fluids to enter. The perforated liner 17 detaches from the bladder assembly 9 and latches into the depth latch 10 upon removal of the bladder assembly 9 from the well 1.

[0029] The final well completion is shown in FIG. 5, with the perforated liner 17 latched in place across the producing horizon of the formation 4. The well 1 is placed in production by inserting a progressive cavity pump 32 in the well 1 connected to production tubing 33. Production may also be accomplished by injecting foaming fluids into the well and extracting the formation material, fluids, and foam from the well. The liquefied CHOPS disturbed zone 30 and wormholes 31 propagate further thus enhancing the production of heavy oil from the formation.

[0030] The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned as well as others inherent herein. While presently preferred embodiments of the invention are given for the purpose of disclosure, numerous changes in the details of construction, arrangement of parts, and the steps of the process will readily suggest themselves to those skilled in the art and which are encompassed within the spirit of the invention and the scope of the appended claims.

Claims

1. A system for producing hydrocarbons from a formation at a depth horizon comprising: a. a cyclic shear stress reversal device in contact with the formation for applying cyclical shear stress reversal to the formation to induce liquefaction of the formation; b. an inlet connecting the formation to the cyclic shear stress reversal device; and c. an extraction device for drawing formation materials and fluids from the formation into the inlet.

2. The system of claim 1, wherein the cyclic shear stress reversal device comprises a plurality of cyclically expanding/contracting bladders to load the formation under a near zero volume change condition to induce liquefaction of the formation.

3. The system of claim 1, wherein the cyclic shear stress reversal device is self-boring down to the required formation depth horizon.

4. The system of claim 1, wherein the cyclic shear stress reversal device is driven down to the required formation depth horizon.

5. The system of claim 1, wherein the cyclic shear stress reversal device is inserted into a pre-drilled borehole down to the required formation depth horizon.

6. The system of claim 1, wherein the cyclic shear stress reversal device is inserted into an existing well, in which the existing casing has been removed across the formation depth horizon.

7. The system of claim 2, wherein the cyclic shear stress reversal device comprises a piston in a fluid cylinder connected to the plurality of bladders, and movement of the piston extracts fluid from contracting bladders and injects fluid into expanding bladders to ensure the formation is cyclically loaded under near zero volume change condition.

8. The system of claim 2, wherein a pore water pressure sensor obtains data measurements for formation pore pressure response to cyclic loading, and other sensors measure the rate of change of expansion/contraction of the bladders, and the fluid pressures inside of the bladders.

9. The system of claim 2, wherein the cyclic shear stress reversal device consists of two expansion/contraction bladders.

10. The system of claim 1, wherein the extraction device extracts formation material and formation fluids from the formation during or immediately after cyclic loading of the formation by the cyclic shear stress reversal device.

11. A system of claim 10, wherein the extraction device is a piston sampling device.

12. A system of claim 10, wherein the extraction device is a progressive cavity pump.

13. A system of claim 10, wherein the extraction device further includes a pump that injects a foaming fluid into the formation.

14. The system of claim 1, wherein the cyclic shear stress reversal device is removed from the well, and a perforated liner is placed across the formation horizon.

15. The system of claim 14, wherein a pump is inserted in the well and formation materials and fluids are extracted from the well via production tubing.

16. The system of claim 15, wherein the pump is of the progressive cavity type.

17. A method for producing hydrocarbons from a formation at a depth horizon comprising: a. Cyclically applying shear stress reversal loading to the formation under a near zero volume change condition to induce liquefaction of the formation; and b. extracting formation materials and fluids from the formation.

18. The method of claim 17, wherein the method includes placing a cyclic shear stress reversal device within the formation for cyclically applying shear stress reversal loading to the formation.

19. The method of claim 18, wherein the cyclic shear stress reversal device comprises a plurality of cyclically expanding/contracting bladders to load the formation under a near zero volume change condition to induce liquefaction of the formation.

20. The method of claim 18, wherein placing the cyclic shear stress reversal device includes the cyclic shear stress reversal device being self-boring and boring down to the required formation depth horizon.

21. The method of claim 18, wherein placing the cyclic shear stress reversal device includes driving the cyclical shear stress reversal device down to the required formation depth horizon.

22. The method of claim 18, wherein placing the cyclic shear stress reversal device includes inserting the cyclical shear stress reversal device into a pre-drilled borehole down to the required formation depth horizon.

23. The method of claim 18, wherein placing the cyclic shear stress reversal device includes inserting the cyclical shear stress reversal device into an existing well, in which the existing casing has been removed across the formation depth horizon.

24. The method of claim 19, wherein loading the formation includes alternatively extracting fluid from contracting bladders and injecting fluid into expanding bladders to ensure the formation is cyclically loaded under near zero volume change condition.

25. The method of claim 19, wherein the method further includes measuring a pore water pressure of the formation in response to cyclic loading, measuring the rate of change of expansion/contraction of the bladders, and measuring the fluid pressures inside of the bladders.

26. The method of claim 19, wherein the device consists of two expansion/contraction bladders.

27. The method of claim 17, wherein formation material and formation fluids are extracted from the formation during or immediately after cyclic loading of the formation.

28. A method of claim 27, wherein the method of extracting formation material and fluids is by a piston sampling device.

29. A method of claim 27, wherein the method of extracting formation material and fluids is by a pump, being of the progressive cavity type.

30. A method of claim 27, wherein the method of extracting formation material and fluids is by a foaming fluid injected into the well, with formation material, fluids and foam extracted from the well.

31. The method of claim 19, wherein the bladders are removed from the formation and a perforated liner is placed across the formation horizon.

32. The method of claim 31, wherein a pump is inserted in the well and formation materials and fluids are extracted from the well via production tubing.

33. The method of claim 32, wherein the pump is of the progressive cavity type.

34. The method of claim 17, wherein steam is injected into the formation upon the onset of induced liquefaction of the formation and prior to extraction of formation materials and fluids.

35. The method of claim 17, wherein a vaporized hydrocarbon solvent is injected into the formation upon the onset of induced liquefaction of the formation and prior to extraction of formation materials and fluids.

36. The method of claim 35, wherein the vaporized hydrocarbon solvent is one of a group of ethane, propane, butane or a mixture thereof.

37. The method of claim 35, wherein the injected vaporized hydrocarbon solvent is maintained saturated at or near its dew point.