Method For Recovery Of Petroleum Deposits

Abstract

A method for recovery of petroleum deposits, particularly those deposits which are underlain by a water zone, which method includes the steps of introducing a dissolving gas product into an upper region of a petroleum deposit to enable gravity head build-up of more soluble petroleum products which then flow downward toward a recovery zone for delivery to a surface recovery unit. The method is particularly adaptable for utilization with a single well-bore whereupon a dissolving injection gas can be introduced down an outer annulus of the pipe string while recovered petroleum can be withdrawn from a lower recovery zone for conduction up a tubing string or such to the earth's surface. An inert gas introduced through another bore into upper regions of the deposit serves to maintain proper pressure balance throughout the system.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to secondary recovery processes for earth deposits of petroleum product and, more particularly, but not by way of limitation, it relates to an improved secondary recovery process for withdrawing high viscosity petroleum from an earth bound pool or reservoir which is underlain by a water zone.

2. Description of the Prior Art

The prior art includes various methods and types of apparatus for employ in withdrawing additional petroleum product from earth reservoirs after initial pressures have reduced, or for use when the petroleum viscosity versus bottom hole pressure relationship is such that natural flow is stopped or at least greatly reduced. There are various forms of such secondary processes such as steam injection, water flooding, alcohol injection, in situ combustion and various forms of gas injection.

One well-known form of gas injection process utilizes an input gas for dissolution of higher viscosity oil within a permeable strata whereupon constant application of nitrogen under pressure will cause migration of lowered viscosity petroleum product towards a recovery zone at an adjacent well bore. Nitrogen gas is introduced for the purpose of maintaining a relatively constant driving pressure for any gas cap produced by the injected gas, e.g. carbon dioxide. While the various recovery processes form migrating heads of petroleum product in many different compositions of strata with varying efficiency and degrees of success, none of the prior art methods has been directed to the utilization of a gravity driven cap or head which is particularly adaptable for use in a single well-bore recovery process.

SUMMARY OF THE INVENTION

The present invention contemplates a secondary recovery process for withdrawing certain forms of petroleum product from an earth reservoir. In a more limited aspect, the invention consists of a method for injection of a gas into the upper portion of a petroleum reservoir, the gas dissolving in the petroleum liquid and thereby lowering the liquid's viscosity, and the lowered viscosity petroleum liquid then moved downward under force of gravity towards a lower recovery zone in the petroleum reservoir. The lower viscosity petroleum is withdrawn from the recovery zone for processing and storage at a surface unit and, if desired or even necessitated, pressure stabilizing inert gas can also be injected from a surface storage facility in order to occupy the space voided by the lowered viscosity petroleum liquid which moved toward the recovery zone.

Therefore, it is an object of the present invention to provide a recovery process which is more economical and more efficient than existing recovery processes as employed in certain recovery situations.

It is also an object of the present invention to provide a secondary recovery process which can be employed with petroleum deposits which are underlain by water zones.

Finally, it is an object of the present invention to provide a secondary recovery process which may be variously employed in well pools having any of one or more well bores in communication therewith.

Other objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying drawings which illustrate the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional, schematic representation of earth including a petroleum deposit and utilizing the recovery method of the present invention; and

FIG. 2 is a block diagram of one form of surface processing equipment which may be utilized in carrying out the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a section of earth 10, having a surface 12 is underlain by a permeable rock formation 14 in which the upper portion 16 is occupied by a petroleum reservoir and the lower portion 18 is occupied by water. The permeable rock formation 14 may be overlain and thereby confined by such as an impermeable formation 20 while the underside of the water bearing portion 18 is also limited by a low permeability stratum 22.

The exact materials, permeabilities and configurations of oil-bearing strata and surrounds are variable within wide limits as the present method is adjusted for whatever the applicable exigencies. The recovery method is particularly applicable to those petroleum reservoirs containing high viscosity oils present within oil-bearing strata having high rock porosities and high permeability. It is then intended that lowering of the oil viscosity by a predetermined amount followed by production removal of the oil from the reservoir without creating large pressure differentials will enable a more efficient oil recovery.

Assuming then that oil-bearing stratum 16 contains a quantity of high viscosity petroleum product which it is desirable to recover, extraction can be effected through any one of a plurality of well-bores 24, 26 and 28. The well-bores 24, 26 and 28 merely illustrate a plurality of well-bores in an established oil field or producing area, any one of which may be utilized for carrying out a secondary recovery process. Still another bore hole 30 may also be employed for purposes of pressurizing the oil-bearing stratum 16, as will be further described below.

With respect to the well-bore 24, an extraction well as employed in the method, a suitable form of petroleum soluble gas is injected down the well-bore 24 through the annular volume 32 as may be defined between casing 34 and a tubing string 36. The casing 34 will be existing structure previously set down, cemented and finished upon initial completion of the well-bore 24, and the tubing string 36 may also be the existing or prior-used oil delivery tube.

The upper reaches of oil-bearing stratum 16 along well-bore 24 are then determined so that the injected gas from annulus 32 can be directed into the oil-bearing stratum 16 at that level. Thus, the casing 34 is suitably perforated in the area 38 to allow passage of the injection gas in annulus 32 into the upper region of oil-bearing stratum 16 in the immediate surrounds of well-bore 24. Any of various dissolving gases may be employed as the injection material; however, from the standpoint of availability and economy, it is contemplated that carbon dioxide and/or a mixture of carbon dioxide and liquid petroleum gas (LPG) may be utilized to form the dissolving zone in the oil-bearing stratum 16. Where necessary, a suitable form of packer assembly 40 may be installed between tubing 36 and casing 34 immediately below the perforation area 38. Such a blocking device or equivalent is generally existant within a completed well-bore.

The dissolving gas entering through perforation area 38 into the oil-bearing stratum 16, as shown by arrows 42 near the top of the producing formation, enters the reservoir and contacts the under-saturated petroleum products to effect dissolution thereof. As the amount of gas going into solution increases, the viscosity of the petroleum will decrease, and such decreased petroleum viscosity tends to set up a gravity flow head in the area 44 which migrates downward, as shown generally by arrows 46, to a petroleum recovery zone 48 at some lower point of well-bore 24.

The action of dissolution in situ tends to create a condition of increasing volume of lowered viscosity petroleum product per unit time, such volumetric increase being exponential in nature. That is, upon introduction of the dissolving gas and initial decrease in petroleum viscosity, the oil tends to flow by gravity head toward the recovery zone 48 at continually increasing rate. This re-exposes additional volume of undersaturated petroleum to the injected gas so that the process continues. Also, as the injection gases go into solution, there is a release of heat energy causing a temperature rise in the oil which, in turn, causes further reduction in viscosity and increased flow rate.

It is apparent then that an attendant step in the method may include heating of the dissolving gas prior to injection so that it will reach the oil-bearing stratum 16 at a temperature which is appreciably higher than the original formation temperature. This will cause the gas to heat the formation adjacent to the well bore 24, in the area of perforations 38, to further aid the viscosity reductions and gravity flow functions.

The flowing petroleum product available at recovery zone 48 may then be withdrawn up tubing string 36 in conventional manner by such as an oil pump 50 or other existing pumping equipment, e.g. hydraulic, pneumatic or electrical submergible pumps.

The above-described recovery method is especially useful when the oil-bearing stratum 16 is underlain by an active water zone such as water-bearing stratum 18. The injection gas can be introduced into the producing well or well-bore 24 at a pressure which is sufficiently high to maintain the bottom hole pressure essentially constant at original pressure levels. This then prevents water from flowing into the recovery zone. In the case where the oil is not underlain by water, the pressure need only be high enough to give the desired oil production rate. In either case, the injected volume of dissolving gas will be dependent upon the injection pressure, the solubility of the injection gas in the oil, and whatever the desired oil production rate. Care must be taken that the injection rate of the dissolving gas is not so high as to cause a pressure buildup above desired levels at the producing wells.

The injection gas, e.g. a carbon dioxide and/or LPG combination as previously described, may be obtained from a suitable form of injection gas generator 52 for conduction via conduit 54 for injection in annulus 32 of well-bore 24. Recovered petroleum product from tubing string 36 is conducted via a suitable conduit 56 to a conventional mode of output processing 58. While the oil entering the lower portion of tubing string 36 is pumped from the well or tubing string by the pumping unit 50, a back pressure must be maintained on the tubing string to prevent the oil from flowing naturally from the well. If the well were allowed to flow, a pressure differential may be created in the reservoir which could result in flow of water from the water-bearing stratum 18 upward to the recovery zone 48 of the well bore 24. After the oil product reaches the surface, the pressure may be partially reduced for flow through gathering lines or conduit 56 to output processing 58. Gas products may be recovered in output processing 58 for recycling in the method, as will be further described below.

The bore hole 30, generally selected as that bore in the field or area in communication with the highest point of the oil-bearing stratum 16, may also be utilized for pressurizing the field. Thus, nitrogen or any other low value gas from a suitable source or generator 60 is conducted via line 62 for input under preselected pressure to the upper terminal of bore hole 30. The bore hole 30 is perforated by suitable means in the area 64, the upper reaches of well bearing strata 16, to build up a pressure head as indicated generally by dash line 66. This pressurizing may be continually increased in accordance with withdrawal of petroleum product to maintain a desired quiescent pressure sufficient to hold the overall reservoir pressure essentially constant.

While the foregoing is directed primarily to the simultaneous injection and production from a single well-bore 24, it should be understood that the similar producing activity can be carried out for each of the additional well-bores 26, 28 and whatever. Thus, with respect to well-bore 26, the solvent gas from injection gas generator 52 is applied via conduit 54 to an annulus 68 formed by casing 70 and tubing string 72. The well-bore 26 is also pre-worked so that it includes a perforated zone 74 to allow introduction of the solvent gas to form its gas cap head for gravity flow downward toward a recovery zone 76. In like manner, well-bore 28 may constitute similar structure arrayed for parallel production function. That is, it includes similar annulus 78 between a casing 80 and tubing 82, and it would be pre-worked to have a perforation zone 84 and lower recovery zone 86.

Various forms of surface support equipment may be employed in carrying out the present method, and the block diagram of FIG. 2 illustrates a generalized form of such supporting installation. As shown in FIG. 2, the respective units of output processing 58, injection gas generator 52, and nitrogen generator 60 are each shown in dish-line outline with the respective input conduits 56 and output lines 54 and 62 in communication therewith. While description proceeds with respect to this particular type of installation, it should be understood that there are various forms of supporting equipment which may be employed in carrying out the respective output processing, injection gas generation and other satellite functions.

The output processing 58 may consist of such as conventional forms of heater unit 90, a separator 92 and heater-treater 94. An oil and gas mixture as recovered from an oil production reservoir, e.g. from well tubings 36, 72 and/or 82, is available in conduit 56 for input to a heater 90. Heater 90 serves to increase the temperature of the oil and gas mixture so that a more efficient separation of the gas and oil will occur in the next following phase. The heater 90 also receives input via conduit 96 of carbon dioxide which is obtained, for example, from nitrogen generator 60 in a manner as will be further described below; and also, LPG may be introducted at input 98 for mixture with the CO.sub.2 as applied in conduit 96 to heater 90.

A heated component of LPG and carbon dioxide is supplied out via line 102 while the heated oil and gas mixture is applied in line 104 to separator 92. The separator 92 serves to divide as between the oil or petroleum product and gas components, the gas components being conducted in line 106 for mixture with the CO.sub.2 -LPG components in line 102, while the liquid components are present through line 108 to the heater-treater 94. Depending upon prevailing pressure conditions, it may be more efficient to employ two or more separators operating in series, each functioning at a different, selected pressure.

Upon leaving the separator 92, the degassed liquid in line 108 flows into heater-treater 94 whereupon any free or entrained water is separated from the oil. The water recovery is then conducted in line 110 for proper disposal and the petroleum products are separately conducted for further disposition. That is, any remaining vapors or volatile end products from the oil product are conducted through a line 112 for further use in such as nitrogen generator 60, as will be described, and the liquid product is conducted in a line 114 for flow to storage or pipe line and subsequent sales disposition.

Referring again to separator 92, the output of gases in line 106 are combined with any LPG and carbon dioxide gases present in line 102, and the combined gases are supplied to a dehydrator 116 within the injection gas generator 52. The dehydrator 116 serves to remove all water vapor to minimize corrosion problems whereupon the mixture of petroleum solvent gases are applied in a line 118 for input to a compressor 120. The compressor 120 serves to repressure the solvent gases, i.e. carbon dioxide and/or LPG mixtures thereof, so that it will flow through line 54 back to the producing wells for re-entry into the annulus spaces as previously described. In some cases, it may be desirable to further heat the injection or solvent gas prior to re-injection, and this may be carried out by passing the output from compressor 120 through a suitable heater 122 with further conduction through input conduit 54 to the selected well sites.

The nitrogen generator 60 may be such as a conventional form of flue gas plant which not only provides the reservoir pressuring nitrogen as applied on line 62, but also provides a source of carbon dioxide as applied in line 96 back to the heater 90 of output processing 58. Thus, fuel gas such as that available on line 122 from heater-treater 94 is supplied along with air on line 124 as input to a flue gas generator 126. After combustion in the flue gas generator 126, there is provided an output on line 128 which includes a mixture of nitrogen and carbon dioxide for application to a compressor 130 wherein partial repressuring takes place. The repressured gas mixture then flows through a line 132 for input to a separation system 134 wherein the nitrogen and carbon dioxide components are separated into their individual streams. While various commercial methods of separation are available, a very economical method consists of the separation system 134 as shown. The input from line 132 is supplied to an absorber unit 136 utilizing production oil such as might be obtained from output oil line 114. The resulting oil and dissolved carbon dioxide may then be routed back through heater 90 and separator 92 for separation.

Gas output from absorber unit 136 is conducted via line 138 to a compressor 140. The gas output is a high purity nitrogen gas and it is then sufficiently repressured in compressor 140 for conduction via line 62 for input to the bore hole 30 at some selected pressure to repressure the petroleum reservoir 14. If an absorbtion liquid other than production oil from output oil line 114 is used, that absorbtion liquid is supplied to separator 144 through line 142. In separator 144 the carbon dioxide evolves from the absorbtion liquid and leaves through line 96. The now lean absorbtion liquid is pumped back through line 146 to the absorber 136 to again extract the carbon dioxide from the flue gas.

The foregoing discloses a novel method for effecting secondary recovery of petroleum products from sub-terranean reservoirs. The method is particularly adapted to those situations where an oil-bearing strata is laden with high viscosity oil and, at the same time, is underlain by a water zone of appreciably more mobile fluids. The method of the present invention can then enable the setting up of a gas cap head of reduced viscosity oil which migrates under gravitational forces to a petroleum recovery zone, and such migration is affected without allowing the more mobile water to flow upward into the recovery zone.

Changes may be made in the combination and arrangement of steps as heretofore set forth in the specification and shown in the drawings; it being understood that changes may be made in the embodiment disclosed without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A method for recovery of petroleum products from a sub-terranean reservoir utilizing an existing well-bore having standard casing and tubing installation in communication with the reservoir, comprising the steps of:

effecting perforation of a well-bore proximate the upper region of said reservoir;

introducing a gas which is soluble in petroleum product down said well-bore and through said perforation into contact with petroleum product in the upper regions of said reservoir;

injecting a gas inert to said petroleum product at pre-selected pressure at a point removed from said area proximate to the well-bore perforations into upper regions of said reservoir relative to said perforations; and

withdrawing reduced viscosity petroleum product from a recovery zone disposed generally vertically below said perforation area and moving said recovered petroleum product up through the well-bore installation to the surface for further processing.

2. A method as set forth in claim 1 wherein said solvent gas is heated to a preselected temperature relative to the reservoir temperature before introduction through said well-bore perforation into the upper regions of said reservoir.

3. A method as set forth in claim 1 which is further characterized in that:

said method including introduction of solvent gas through perforations of a well-bore with recovery of dissolved, less viscous petroleum product therebelow, is effected at each one of a plurality of well-bores in communication with said reservoir.

4. A method as set forth in claim 1 which further includes steps of:

processing said recovered petroleum product to extract said soluble gas therefrom; and

reintroducing said soluble gas down said well-bore and through said perforation.

5. A method as set forth in claim 1 wherein said soluble gas is carbon dioxide.

6. A method as set forth in claim 1 wherein said soluble gas is a mixture of carbon dioxide and liquified petroleum gases.

7. A method as set forth in claim 1 wherein said gas inert to the petroleum product is nitrogen or other low value gas.

8. A method as set forth in claim 1 which is further characterized by the step of:

maintaining sufficient back pressure against the petroleum product in the recovery zone such that additional force must be exerted to move said recovered petroleum product to the surface without reducing pressure within the recovery zone.

9. A method as set forth in claim 1 which is further characterized to include the steps of:

maintaining predetermined back pressure against the less viscous petroleum product in the recovery zone; and

pumping said less viscous petroleum product to the surface while maintaining the bottom hole pressure at an optimum level.

10. A method for recovery of petroleum products from a sub-terranean reservoir having an underlying formation containing water, the method utilizing existing well-bores having standard casing and tubing installations in communication with the reservoir, comprising the steps of:

effecting perforation of one of said well-bores proximate the upper region of said reservoir;

introducing a gas, which is soluble in and effective to reduce the viscosity of petroleum product, down said one well-bore and through said perforation into contact with petroleum product in the upper regions of said reservoir adjacent said one well-bore, said gas introduction being effected at a pressure which is generally equivalent to the bottom hole pressure of said well-bore thereby to prevent flow of water into the bottom hole recovery zone; and

drawing reduced viscosity petroleum product from said recovery zone disposed generally vertically below said perforation area and pumping said recovered petroleum product up through said well-bore installation to the surface for further processing.