US2754911A

US2754911A - Oil production method

Info

Publication number: US2754911A
Application number: US363804A
Authority: US
Inventors: Spearow Ralph
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-06-24
Filing date: 1953-06-24
Publication date: 1956-07-17
Anticipated expiration: 1973-07-17

Description

y 7, 1956 R. SPEAROW 2,754,911

OIL PRODUCTION METHOD Filed June 24. 1955 2 sneets'sheet l INVENTOR. flqafi fi uea/vw A 30mm mw F I? V v v$- m W 4 a o w 1 v H .0 O 0 0 n 0W =i o a w u I "u c "0M" w July 17, 1956 R. SPEAROW OIL PRODUCTION METHOD Filed June 24. 1955 2 Sheets-Sheet 2 IN V EN TOR. flack 5 04/0W BY 5.

ORA/5K United States Patent OIL PRODUCTION METHOD Ralph Spearow, Paola, Kans. Application June 24, 1953, Serial No. 363,804

17 Claims. (Cl. 166=-35) This invention relates to improvements in methods for individually producing oil wells and refers more particularly to such methods wherein gaseous pressure is applied to the upper portion of an oil horizon and oil is removed from the lower portion of the horizon.

This application is a continuation-in-part of my application Serial No. 262,568, filed December 20, 1951, which in turn is a continuation-in-part of my application Serial No. 750,396, filed May 26, 1947, and issued into United States Patent No. 2,593,497 on April 22, 1952.

Methods of removing oil from oil producing areas have been provided in my above-mentioned applications where gaseous pressure is applied to the upper portion of an oil horizon thereby causing the oil to migrate downwardly within the horizon and oil is removed from the lower portion of the horizon. These methods have proved and are proving highly successful in actual secondary recovery operations in eastern Kansas as noted in application Serial No. 262,568.

When gaseous pressure is applied to the top portion of an oil horizon, the oil in the area of the horizon which is affected by the pressurization migrates downwardly through the oil sand. Gradually, as the pressurization continues and a greater quantity of gas is forced into the horizon, a definite separation of the gas loaded top of the formation and the oil bearing portion of the formation occurs. Corings of pressurized formations have clearly indicated that the major portion of the oil in the oil horizon is moved downwardly in the course of pressurization. This segregation results in the formation of a pressure-fluid or gas-oil interface within the horizon, which, as the gas cap expands and increases in volume, moves gradually downward through the horizon. While the pressurization is going on at the top of the formation, oil is withdrawn from the lower portion of the formation. When it is desired to obtain the maximum production of oil per day or when permeability conditions dictate, it is necessary to use as large a cross-sectional portion of the oil horizon as possible as an oil withdrawal zone. The most eificient way to accomplish this would be to utilize the entire cross-sectional area of the oil horizon below the gas-oil interface as the oil withdrawal zone. since the interface is constantly, though gradually, moving downwardly through the horizon, heretofore it has been impossible to accomplish this and the oil has been withdrawn only from a limited area in the lowest portion of the horizon. Furthermore, in certain situations, a single well is often employed both as a pressurization well and a production well and in such a case it would be desirable as well to increase the available cross-sectional input area for pressurization as the gas-oil interface moves downwardly within the formation.

A tremendous advantage of the applicants gas pressurization methods as set forth in the above mentioned applications, which involve sealing the casings of the pressure and production wells to the surrounding formations to permit no escape of pressure from the horizon However, a

except at the oil withdrawal points, is the fact that when the pressurization of the horizon has proceeded to a certain level it is possible to shut down the compressors and continue to produce oil by the agency of sealed-in" pressures for a long time interval without further input of pressure into the horizon. This result is greatly in contrast to such a pressurization method such as water drive, wherein shutting down of the water pumps causes the production of oil, by water drive means, to cease immediately. As an example of this performance of the applicants method, in an actual operation in eastern Kansas the compressors to a field of over twenty wells producing under gaseous pressurization were shut off for a period of over five weeks and at the end of this time (the present writing) the wells were still flowing and pumping oil without any diminishing in volume. This is additional proof of the necessity of sealing the space between the casing and the formations and the effectiveness of that sealing.

Therefore, an object of the present invention is to provide a method of employing substantially all of the cross sectional area below the gas-oil interface as an oil production area despite the fact that the interface is constantly moving downwardly through the formation due to gaseous pressurization at the top of the formation, which produces constant shrinkage in size of non-liquified and non-liquifiable (at operational sand depths and feasible operational pressures and sand temperatures) gas bubbles adsorbed in the oil below the gas-oil interface.

A further object of the invention is to provide a method of producing oil from oil wells which are both pressuriza-' tion and production wells wherein the cross-sectional input area is increased as the gas-oil interface within the horizon moves downwardly while using substantially all of the cross-sectional area below the gas-oil interface as an oil production area.

Another object of the present invention is to provide a method of individually producing oil wells wherein.

gaseous pressure is applied to the upper portion of an oil horizon and oil is' removed from a plurality of points in the oil horizon below the pressure application zone or pressure-fluid interface.

Another object of the invention is to provide a method of producing oil wells by applying gaseous pressure to the top portions of the horizons and withdrawing oil from the lower portions of the horizons wherein the pressurization may be applied intermittently to the horizon with the production remaining relatively constant independent of the pressurization periods.

Other objects and features will appear in the course of the following description of the invention.

In the accompanying drawings which form a part of the specification and are to be read in conjunction therewith, there are shown embodiments of the invention and like numerals are used to refer to like parts.

Fig. 1 is a cross-sectional view of an earth formation in an oil field showing a single well which is used both for pressurization and production in which applicants inventive method may be practiced;

Fig. 2 is a cross-sectional view of an earth formation in an oil field showing three wells, two of which are production wells and one of which is a pressurization well;

Fig. 3 is a cross-sectional view of an earth formation in an oil field showing two wells, one of which is a production well and the other a pressurization well.

In the figures are shown several embodiments of the a preferred form Patented July 17, 1956 first, the descripand like numerals being usedj In Fig. l, the numeral refers to an oil horizon which is surrounded by the earth formations 11 and has an impervious cap 12. The ground level is indicated at 13.

Borehole 14 of an oil well is shown extending through the oil horizon 1.0. The borehole is shown extending; through: the horizon for the reason that the currently available oil well perforating guns are not able to operate at the base of an oil zone without the necessity for sending portions of the gun below the shooting spot. When such guns are available which will operate at the base of an oil zone without parts extending below the shooting spot, then the borehole will need to extend only to the vicinity of the base of the oil saturated zone. A primary casing 15 having a brad'enhead 15a is shown extending to the bottom of the borehole 14. Annular seal 16 is a cementing material by which the outside of primary casing 15 is united with the inside wall face of the borehole 14. The annular seal 16 extends completely across the face of the oil horizon from the bottom of casing 15 to any desired level above the top of the oil horizon. Indicated at 17 are a series of perforations in the primary casing 15 and annular seal 16 at selected points across the face of the oil horizon and extending from the vicinity of the top to the vicinity of the bottom of the oil horizon.

Secondary casing or tubing 18 having perforations 19 near its base is suspended within primary casing 15. An anchoring nipple 20 may be placed at any desired point in secondary casing 18 and employed to hold a pumping string which may be inserted in the secondary casing and used in pumping the well if desired. Nipple 20 may be placed at any point within secondary casing 18 or not installed at all if it is determined that the well will flow without pumping.

Between primary casing 15 and secondary casing 18 lies annulus 21. Into this annulus the air or gas is introduced from either surface equipment or from other suitable sources, such as a high pressure gas zone available to the well from a source other than the oil stratum from which oil is being produced within the earth. Av conventional type packer 22 is mounted on secondary casing 18 and, in the figure, is set to the inside of primary casing 15 at a point just below the top set of perforations 17. Pipe 23 is the delivery channel for pressure introduced into annulus 21 between the primary and secondary caslngs.

Referring now to the well shown at the left in Fig. 2, oil horizon 10 is surrounded by earth formations 11 and has a relatively impervious cap 12. The ground. level is indicated at 13. At 10a is indicated the gas-oil interface within the formation at an early stage of pressurization and at 10b is indicated the interface at a much later stage of pressurization of the horizon.

The borehole 14 of an oil well is shown extending to the lower portion of oil horizon 10. Primary casing 15 having bradenhead 15a, extends to top of horizon 10. Annular seal 16 unites the outside of primary casing 15 with the inside wall face of the borehole 14 and extends from the bottom of casing 15 to any desired level above the top of the oil horizon. Secondary casing or tubing 18 having perforations 19 near its base is positionedwithin primary casing 15 and extends to the lower portion of horizon 10. Ananchoring nipple 20 may be placed at any desired point in secondary casing 18- and employed to hold a pumping string. Between primary casing 15 and secondary casing 18 lies annulus 21 which is sealed at the top by bradenhead 15a at all times. Packer 22- is mounted on secondary casing 18 so as to form a pressure-tight seal between tubing 18 and the wall of the oil horizon. In the figure the packer 22 is shown positioned below the gas-oil interface 10a. Pipe 18a, attached to tubing 18 above ground level is employed to drain ott oil from the tubing.

Referring tothe well illustrated in the center of Fig. 2, borehole 14 of the well is shown extending into, the lower portion of horizon10. Primary casing 15 having; braden-v head 15a extends below the gas-oil interface 10a. Annular seal 16 unites the outside of primary casing 15 with the inside wall face of the borehole 14. The annular seal 16 extends from the bottom of casing 15 to any desired level above the top of the oil horizon. Pipe 15b is employed to drain off oil from the casing 15 in the early stages of pressurization. Secondary casing or tubing 18, positioned within primary casing 15, extends to a level below gas-oil interface 101). Annular seal 25 is a cementing material by which the outside of secondary casing 18 and the inside faces of horizon 10 and primary casing 15 are united. Seal 25 extends from the bottom of tubing 18 to a level above the lower end of primary casing 15. Pipe 18a is employed to drain off oil from secondary casing 18 in the later stages of pressurization. In some instances, where sand thicknesses seem to justify, tubing 18, as here illustrated, may feasibly be repeated many times with tubing of decreasing diameters as gas-oil interface lowers in sand body. Or tubing 18 may be removed in so far as possible and seal 25 and the unretrievable portion of tubing 18 drilled out or reamed out to permit resetting of a new string of tubing 18 at a new sand level below the then gas-oil interface where it can be resealed in like manner as originally with seal 25 as herein shown.

To the right in Fig. 2 is shown a pressurization well having a borehole 26 which extends into the top portion of horizon 1t). Pressurization casing 27 extends into the top of horizon 10 and has perforations 28 at its lower end. Pressurization casing 27 is sealed to the borehole 26 by an annular column 29 of scaling material which extends from the top of the oil horizon to any desired level above the oil horizon. Pipe 30 is used as the input for gaseous pressure to casing 27.

In Fig. 3 is shown a lenticular oil horizon 10 having an impervious cap 12 and surrounded by earth formations 11. Ground level is indicated at 13 and at 1011 is shown the gas-oil interface at an intermediate stage of the pressurization of the formation. The well at the left of Fig. 3 is a production well having a borehole 14 which extends into the lower portion of horizon 10. Primary casing 15 extends at least to the top of the horizon and is sealed to the well wall by a surrounding annular column of cementing material 16, which extends from the bottom of casing 15 to a level above the top of the oil. horizon. Pipe 15b is used to withdraw oil from casing 15. To the right in Fig. 3 is shown a pressurization well similar to that in Fig. 2 having a casing 27 with perforations 28 in borehole 26 with a sealing column 2.9 and a pressure input pipe 30.

It will be noted at this point that a gas pressurization method such as is set forth here is feasible and workable only when the casings of the wells employed are properly sealed throughout the designated portions of their lengths in, as well as above and below, the horizons to be produced. All casing seals in both pressure and production wells must be of sufficient strength to secure whatever pressures are employed in driving the fluid and gaseous hydrocarbons downward through the sand body and must offer no means of escape for those pressures except at the points designated within the oil sand.

The operation of the method in the various wells which are embodiments of the invention will be described in the same order as the previous description.

Referring to Fig. l, in operation of the method the primary casing 15 which extends into the lower portion of or below the oil horizon 10 is cemented to. form. a seal between itself and the oil sand from the bottom of the primary casing 15 to a level above the top of the oil horizon 10. The primary casing 15 and its annular seal 16 are then perforated at selected points 17 along the vertical face of the oil horizon. Perforations 17 preferably extend from the vicinity of the top to the vicinity of the bottom of the oil horizon 10. Secondary casing or, tubing 18 having perforations 19- near its lower end is run into the primary casing 15 to a level below the upper perforations in the primary casing. The space between the primary and secondary casings is then packed off using a conventional packer 22 at a point below the topmost perforations in the primary casing 15.

It should be noted that the extension of secondary casing 18 to any distance below the base of the packer 22 is entirely optional and the casing need not be so extended unless the operator desires to employ the extension as an anchor to support the secondary casing 18 rather than hang it from the bradenhead 15a. In shallow wells it will probably be found desirable to employ the anchor type of installation. In very deep wells, to obviate the necessity for pulling long strings of pipe when changing packer settings, it may be more feasible to suspend the secondary casing from the bradenhead 15a.

Gaseous pressure is then applied through pipe 23 into the annulus 21 between the casings. This pressurizing agent is forced through the perforations 17 above the packer into the upper portion of the oil horizon. As described in my previous applications, which are mentioned above, the oil within the horizon migrates downwardly under the action of this pressure. Oil is then accumulated within the primary casing below the packer 22 through the perforations 17 which are below the packer 22. If the pressure within the oil horizon is great enough, the oil, which is received within the secondary casing 18 through perforations 19 may be flowed to the surface without pumping. If necessary or desired, a pumping string may be run within the secondary casing 18 and the oil pumped to the surface. As the oil migrates progressively downward within the horizon, the perforations 17 immediately below the packer 22 become less and less useful in the intake of oil into the primary casing. Furthermore, it becomes desirable to have a greater vertical area within the oil horizon for application of pressure. Therefore, as the gas-oil interface moves downwardly, packer 22 may likewise be shifted downwardly within the annulus 21 whereby a successively greater number of perforations 17 are exposed above the packer for use in pressurization and a successively lesser number of perforations 17 are available below the packer for withdrawal of oil from the horizon. In this way the greatest efficiency in the use of this vertical drive method may be achieved.

The method as described above may, of course, be employed in a plurality of such wells.

In some oil fields it will be found that pressure applied in the above described manner is all that is needed to deliver satisfactory daily oil volumes without the added instrumentality of fracturing the reservoir. As the form of perforating the primary casing 15 is optional, so also is the fracturing of the oil horizon. If it is desired to fracture the oil sand, after perforations 17 are made, the oil horizon may be oil-cracked or otherwise fractured with pressures that will crack the sand to a measurable distance away from the borehole 14 at each perforation or at selected perforation spots. This is done at the top and bottom perforation points with the use of but one packer. Any intervening fractures at intervening perforation points will be made with the instrumentality of two or more packers, the fracturing medium being pressed into the sand through the perforations set between the packers thus employed.

Referring to the well at the left in Fig. 2, in operation of the method primary casing 15 is cemented to form a seal between itself and the well wall from the bottom of the casing to a level above the top of the oil horizon. Secondary casing or tubing 18 is then run into the lower portion of the horizon and packer 22 set below the existing gas-oil interface within the horizon. Pressure may then be applied to the top portion of the oil horizon from a pressurization well such as is shown to the right in Fig. 2 and oil Withdrawn from the horizon below the packer through tubing 18. Then, as the gas-oil interface gradually, in the years or months of production, moves downwardly in the oil sand body, the packer 22 may be lowered or replaced with one of sufficient length to continue to reach down to a point below the migrating gas-oil interface.

Thus, at all times a maximum of cross-sectional production area is provided in the production well.

Referring to the operation of the center well in Fig. 2, primary casing 15 which extends below interface 10a is cemented to form a seal between itself and the oil sand and the surrounding horizons 11 from the bottom of casing 15 to a level above the top of the horizon 10. Pressure may then be applied to the top portion of the horizon and oil withdrawn from casing 15 through pipe 15b until the interface passes below the bottom of casing 15 and its seal. At this point secondary casing or tubing 18 is run below the then existing interface and sealed to the horizon wall and the inside of casing 15 as shown at 25. Pressurization may then be continued until once again the interface passes below the end of the producing casing and its seal. The process of running extra casings within the original casing and sealing them to the producing horizon and concentric casing may be continued until the bottom of the horizon is reached. If an extra large original casing is used and an extra large original borehole drilled, quite a number of casings may be employed successfully. In some instances, where sand thicknesses seem to justify, tubing 18, as here illustrated, may feasibly be repeated many times with tubing of decreasing diameters as gas-oil interface lowers in sand body. Or tubing 18 may be removed in so far as possible and seal 25 and the unretrievable portion of tubing 18 drilled out or reamed out to permit resetting of a new string of tubing 18 at a new sand level below the then gas-oil interface where it can be resealed in like manner as originally with seal 25 as herein shown. Thus, in this manner, substantially the whole area below the gas-oil interface may be employed at any given time.

In either of the production wells shown in Fig. 2, pumping strings may be employed to aid in the production of the oil if it is not desired to flow the oil to the surface.

Referring to the operation of the pressurization well at the right in Fig. 2, the borehole 26 is drilled into the top of the oil horizon and pressurization casing 27 run therein so that its perforated area 28 is positioned within the top of the horizon. The casing 27 is then sealed from the top of the horizon to any desired level above the top of the horizon. Gaseous pressure may be applied to the top of the oil horizon through pipe 30.

Referring to the operation of the method in the wells shown in Fig. 3, borehole 14 is drilled in the lenticular horizon at a point below the upper area of the horizon. Casing 15 is then run to the top of the horizon and sealed to the borehole wall from its bottom to a level above the top of the horizon. Pressure well 26 is drilled into the upper area of the lenticular horizon 10. Casing 27 having perforations 33 is run into the top of the horizon and sealed to the borehole wall from the top of the horizon to a level above the top of the horizon. Gaseous pressure is then applied to the top portion of the lenticular horizon whereby the oil is forced downwardly into the lower portions of the horizons. Oil may then be withdrawn or pumped from casing 15 until the gas cap at the top of the horizon has expanded to where interface 10a goes below the end of casing 15. At this point, one of the sealing-off methods as shown in Fig. 2 must be employed to extend the life of the producing well. Thus, a secondary casing may be run within casing 15 and either packed off or cemented as shown in Fig. 2. In this manner also, utilization of the entire cross-sectional area of the oil horizon below the gas-oil interface for production is provided.

As noted earlier in the specification, once pressurization of a horizon has proceeded to successful production from the horizon, it is feasible to intermittently shut down the compressors and continue to produce oil by the agency of sealed-in pressures for long time intervals. This performance is, of course, feasible only when the pressurize-- tion and production wells are sealed properly as delineated above.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hero inabove set forth together with other advantages which are obvious and which are inherent to the method.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcom'oinations. This contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, 1 claim:

1. A method of producing oil from oil wells comprising the steps of cementing a primary casing which extends into the lower portion of an oil horizon to form a seal between the primary casing and the oil sand from the bottom of the primary casing to a point above the top of the oil horizon, perforating the primary casing and its surrounding seal at selected points across the face of the oil horizon, running a secondary casing perforated near its lower end into the primary casing to a point below the upper perforations in the primary casing, packing off the space between the primary and secondary casings at a point below the topmost perforations in the primary casing and above the perforations on the secondary casing, applying gaseous pressure to the upper portion of the oil horizon through the perforations in the primary casing above the packer, accumulating and producing oil from the primary casing perforations below the packer, and periodically moving the packer downwardly as the gas-oil interface is lowered within the horizon whereby a successively greater number of the primary casing perforations are exposed as pressure inputs and a successively lesser number of perforations are available as production outlets.

2. The method as in claim 1 installed in a plurality of wells.

3. A method as in claim 1 including the additional step of fracturing the oil sand within the pressure input area.

4. A method as in claim 1 wherein the borehole is drilled below the base of the oil horizon.

5. A method as in claim 1 wherein a pumping string is used to recover oil through the secondary casing.

6. A method of producing oil from oil wells comprising the steps of drilling the borehole of a production well through the oil horizon, sealing a primary'casing extending at least to the top of the oil horizon from the top of the oil horizon to a. level above the top of said oil horizon, running a secondary casing within the primary casing to a level below the oil-gas interface in the horizon, sealing said secondary casing to the well wall to form an oil production zone below the oil-gas interface, and progressively moving said secondary casing and its seal downwardly as the interface lowers whereby entry of sand fluid to the secondary casing is provided only below the oil-gas interface.

7. A method as in claim 1 with the additional steps of periodically shutting off pressure application to the top of the horizon and producing oil from the accumulated sealed-in gaseous pressures.

8. A method of producing oil from oil wells having a lens-shaped oil horizon comprising the steps of drilling the borehole of a pressure well into the top of the oil horizon near the topmost area of the lens, cementing a casing positioned within said borehole to the surrounding earth formations from the top of the horizon to any desired level above to form a pressure-tight seal therebetwecn, drilling the borehole of a production well into the lower portion of said horizon in an area of the lens below that of the pressure well, sealing a primary casing extending at least to the top of the oil horizon from the top of the oil horizon to a level above the top of said oil horizon, running a secondary casing within the primary casing to a level below the oil-gas interface in the horizon, sealing said secondary casing to the well wall to form an oil production zone below the oil-gas interface, and progressively moving said secondary casing and its seal downwardly as the oil-gas interface lowers whereby entry to the secondary casing is provided only below the interface.

9. A method as in claim 8 installed in a plurality of pressure and production wells.

10. A method as in claim 8 wherein a pumping string is used to recover oil from the production well.

11. A method as in claim 8 wherein the production well borehole is drilled below the base of the oil horizon.

12. A method of producing oil from oil wells comprising the steps of cementing a primary casing which extends through the earth formations from the surface to the top of an oil horizon to form a seal between the primary casing and the surrounding earth formations from the top of the horizon upwards to any desired level, running a secondary casing perforated near its lower end into the primary casing into the lower portion of the oil horizon, packing off the space between the secondary casing and oil horizon at a point below the gas-oil interface Within the horizon, applying gaseous pressure to the upper portion of the oil horizon, accumulating and producing oil from the secondary casing perforations below the packer and periodically moving the packer downwardly as the gas-oil interface is lowered within the horizon whereby leakage of gas pressure is prevented below the packer.

13. A method as in claim 12 installed in a plurality of wells.

14. A method as in claim 12 wherein a pumping string is used to recover oil through the secondary casing.

15. A method of producing oil from oil wells comprising the steps of cementing a primary casing which extends within the borehole of an oil well into an oil horizon to a level below the gas-oil interface in said horizon to form a seal between the primary casing and the oil sand from the bottom of the primary casing to a point above the top of the oil horizon, applying gaseous pressure to the upper portion of the oil horizon, accumulating and producing oil from the primary casing until the gas-oil interface approaches the lower end of the primary casing and its surrounding seal, then cementing a secondary casing positioned within the said primary casing and extending to a level below the lower end of the primary casing to form a seal between the secondary casing and the oil sand and primary casing from the bottom of the secondary casing to a point above the lower end of the primary casing, applying gaseous pressure to the upper portion of the oil horizon and accumulating and producing oil from the secondary casing.

16. A method as in claim 15 installed in a plurality of wells.

17. A method as in claim 15 wherein a pumping string is used to recover oil through the secondary casing.

References Cited in the file of this patent UNITED STATES PATENTS 1,252,557 Dunn Jan. 8, 1918 1,722,679 Ranney July 30, 1929 1,816,260 Lee July 28, 1931 2,044,657 Young June 16, 1936 2,593,497 Spearow Apr. 22, 1952

Claims

1. A METHOD OF PRODUCING OIL FROM OIL WELLS COMPRISING THE STEPS OF CEMENTING A PRIMARY CASING WHICH EXTENDS INTO THE LOWER PORTION OF AN OIL HORIZON TO FORM A SEAL BETWEEN THE PRIMARY CASING AND THE OIL SAND FROM THE BOTTOM OF THE PRIMARY CASING TO A POINT ABOVE THE TOP OF THE OIL HORIZON, PERFORATING THE PRIMARY CASING AND ITS SURROUNDING SEAL AT SELECTED POINTS ACROSS THE FACE OF THE OIL HORIZON, RUNNING A SECONDARY CASING PERFORATE NEAR ITS LOWER END INTO THE PRIMARY CASING TO A POINT BELOW THE UPPER PERFORATIONS IN THE PRIMARY CASING, PACKING OFF THE SPACE BETWEEN THE PRIMARY AND SECONDARY CASINGS AT A POINT BELOW THE TOPMOST PERFORATIONS IN THE PRIMARY CASING AND ABOVE THE PERFORATIONS ON THE SECONDARY CASING, APPLYING GASEOUS PRESSURE TO THE UPPER PORTION OF THE OIL HORIZON THROUGH THE PERFORATIONS IN THE PRIMARY CASING ABOVE THE PACKER, ACCUMULATING AND PRODUCING OIL FROM THE PRIMARY CASING PERFORATIONS BELOW THE PACKER, AND PERIODICALLY MOVING THE PACKER DOWNWARDLY AS THE GAS-OIL INTERFACE IS LOWERED WITHIN THE HORIZON WHEREBY A SUCCESSIVELY GREATER NUMBER OF THE PRIMARY CASING PERFORATIONS ARE EXPOSED AS PRESSURE INPUTS AND A SUCCESSIVELY LESSER NUMBER OF PERFORATIONS ARE AVILABLE AS PRODUCTION OUTLETS.