Electrical Method And Apparatus For The Recovery Of Oil

Abstract

Two well bores extend from the surface into the oil bearing formation defining a producing well and an electrode well. Electrodes in each well, contacting the formation, are connected to a unidirectional current voltage source at the surface through conductive tubing or pipe in the respective well bores to produce a unidirectional voltage gradient between the electrodes, with the producing well poled to be the cathode. Additionally, an alternating current voltage source is connected between the producing well electrode and another conductive path extending from the surface to the formation, to effect the flow of alternating current through the formation adjacent to the producing well to heat the formation.

Description

BACKGROUND OF THE INVENTION

This invention relates to an apparatus and method for electrically stimulating the production of oil from a subsurface formation, and more particularly to such apparatus and method utilizing the effect of electro-osmotic pressure. This invention is concerned with the movement of oil through a reservoir formation including rock or sand, where flow of the oil under the extant driving forces to a well bore has reduced to the point where it is no longer economically producible.

It is known that the movement of oil through a formation is adversely effected by the presence of water in the formation, and it is also known that the effective permeability of the formation to the flow of oil varies somewhat inversely with the percentage of water saturation in the formation. Accordingly, if the percentage of water saturation in the formation can be reduced, or if the percentage of oil saturation can be increased, the flow of oil within the formation may be increased to a significant degree. It is particularly desirable to improve the percentage of oil-to-water saturation in the area of the formation immediately adjacent to the producing well, since the greatest hydraulic pressure gradient involved in moving fluids from the formation into the well occurs within this area.

It is an object of this invention to provide an improved apparatus and method employing electrical means for stimulating the flow of oil from a formation into a producing well.

It is another object of this invention to provide such an improved apparatus and method employing electro-osmotic means.

It is a further object of this invention to provide such an improved apparatus and method employing a combination of electro-osmotic and electric heating means.

The apparatus and method according to the invention include the provision of adjacent well bores extending from the surface to the oil producing formation, defining a producing well bore and an electrode well bore. Electrodes are placed in each well bore in electrical contact with the formation. A source of unidirectional supply voltage is connected between the electrodes through suitable conductive pipes or rods placed in the well bores to cause the flow of direct current through the formation originating and terminating within the formation. The conductive pipes are effectively insulated from the walls of the boreholes above the electrodes to assure maximum potential difference and current flow within the producing formation and to reduce electrolytic corrosion of the pipe. The insulation may be effected by strings of insulating casing extending fro a substantial length above the electrodes. The producing well is poled to be the cathode of the unidirectional current circuit. To further stimulate oil flow, the formation in the area of the producing well may be heated by means of alternating current carried through the producing well electrode and an additional conductive path which may be provided in the producing well bore or in an adjacent well bore. A source of alternating current supply voltage connected between the electrode and the additional conductive path directs alternating current through the formation adjacent to the borehole, the current being carried through connate water in the formation to heat the formation and the oil in order to reduce its viscosity.

DRAWINGS

The novel features of the invention, as well as additional objects and advantages thereof, will be understood more fully from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of an earth formation including a producing well and an electrode well embodying one form of the invention;

FIG. 2 is a diagrammatic illustration of an earth formation including a producing well and an electrode well embodying another form of the invention; and

FIG. 2A is a fragmentary illustration of a modification of the form of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a producing well bore 10 and an electrode well bore 40 extend from the surface into an oil bearing formation 12 lying between the overburden 11 and an underburden 13.

The well bore is cased from a point adjacent to the top of the formation 12 to the surface, with the portion of the bore within the formation defining an open hole completion. The casing includes a lower portion, consisting of an insulating casing 15 fabricated of fiberglass for example, which extends upwardly from the formation for a substantial distance into the overburden 11. The remaining upper casing portion 16 may be a conventional metal casing, fabricated preferably of a good conductive metal. A metallic screen 17 is provided in the lower open portion of the well bore below the casing and is secured in the borehole and mechanically coupled to the insulating casing 15 by a packer 18. The screen 17 is preferably seated on a bottom plug 19 of insulating cement or epoxy, for reasons to be described.

A string of conventional tubing 21 defines the production tubing for the well bore 10, extending from the bottom of the bore hole to the surface; and this tubing is preferably of a good electrically conductive metal to define a low resistance conductive path from the surface to the bottom of the borehole.

An electrode 22 for providing good electrical contact with the formation 12 is defined, for example, by a mass of conductive particles 23 carried to the bottom of the well bore and urged into an annular cavity 24 formed in the walls of the well bore below the casing 15. The conductive particles 23 may consist, for example, of metallic or carbon pellets or metallic pellets coated with carbon. The conductive particles are packed or urged into the cavity by conventional techniques before the placement of the screen 17; and the screen is then placed to retain the particles within the cavity and within the annulus between the screen and the bore, the screen then defining a part of the electrode 22. The electrode is connected to the lower end of the conductive tubing 21 by means of a metallic centralizer 25 which is fixed to the lower end of the string of tubing and has bands which are bowed outwardly into engagement with the inner surface of the metallic screen. The bands of the centralizer may be coated with carbon to improve electrical contact between the centralizer and the screen.

For reasons which will be described, it is important that the conductive tubing 21 be well insulated from the formation in the area of the electrode 22, and from the conductive casing 16 which extends to the surface. For this purpose, the insulating casing 15 is provided; and a further insulating effect may be provided by cementing the well bore throughout the extent of the insulating casing with an insulating cement or epoxy 26.

To insulate the tubing from the conductive casing 16, a string of insulating casing 27 is provided extending from the surface to the screen 17; and the annulus between the insulating tubing and the insulating casing at the lower end is preferably sealed by the packer 18 which secures the sleeve in position.

The electrode well includes the well bore 40 which extends from the surface into the formation 12, the well bore 40 also being cased with a casing which extends from the surface downward to a point short of the bottom of the well bore. An electrode 42 is defined in the bottom of the electrode well bore 40; and the casing includes a lower insulating portion 43 which extends upward from the electrode for a substantial distance, into the overburden 11 as viewed in the drawing, while the upper portion 44 of the casing may be of any conventional material including an electrically conductive material.

A low resistance conductive path is provided in the electrode well bore by a string of conductive tubing or rod 45 which extends from the surface to the bottom of the well bore. This tubing or rod is preferably fabricated of a metal having good conductive characteristics and is sealed relative to the lower end of the insulating casing 43 by a packer 46.

The portion of the conductive tubing or rod, which extends below the packer 46, may consist of a carbon rod 47 joined to the tubing 45 in any suitable manner. The lower end of the well bore 40 may be filled with a mass of conductive particles, such as metallic or carbon pellets or metallic pellets coated with carbon, which surround and engage the carbon rod 47. To insure good electrical contact with the formation 12 and to increase the diameter of the electrode, the particles are urged into an annular cavity or notch 49 extending laterally from the walls of the well bore. The electrode 42 then is defined by the conductive particles 48 which are in electrical contact with the carbon rod 47.

As with the producing well bore 10, it is desirable to effectively insulate the conductive path to the electrode 42 for a substantial distance above the electrode. This is accomplished in part by the insulating casing 43; and additionally such casing may be cemented in the borehole by an insulating cement or epoxy 50. The conductive tubing 45 can be insulated from the walls of the well bore 40 remote from the electrode 42 by providing insulating spacers 51 in the annulus between the tubing 45 and the conductive casing 44. If desired, the entire string of casing, that is both the portions 43 and 44, may be fabricated of an insulating material such as fiberglass.

At the surface, a source of unidirectional current voltage 55, preferably a pulsating direct current voltage, is connected between the producing well and the electrode well, one terminal of the source being connected through a conductor 56 to the conductive tubing 21 in the producing well bore, and another terminal of this source being connected through a conductor 57 to the conductive tubing or rod 45 of the electrode well bore. As indicated in the drawing, the negative side of the direct current voltage source is connected to the producing well; and accordingly the electrode 22 of the electrode well is poled to be the cathode, and the electrode 42 of the producing well is poled to be the anode. It will be seen, between the electrodes 22 and 42, a potential difference or gradient is established through the formation causing a unidirectional flow of current. It has been found from tests that oil in the formation moves toward the cathode and that the water in the formation moves toward the anode. This results in a reduction of the water saturation and accompanying increase in oil saturation in the area of the cathode electrode 22 at the producing well, and a corresponding increase in water saturation and decrease in oil saturation in the area of the anode electrode 42. This increase of oil saturation in the area of the producing well, coupled with the increased permeability of the formation to the flow of oil resulting from the reduction of the water saturation, results in the increased flow of oil into the producing well. This phenomenon of movement of fluids through a porous solid under the influence of an electrical potential difference is referred to as electro-osmosis.

Where the producing formation 12 is a generally horizontal stratum, as in the diagram of FIG. 1, it may be advantageous to place the producing well electrode 22 at a higher elevation than the electrode 42 in order to have the additional benefit of the gravity flow of the connate water relative to the oil. The flow under electro-osmotic pressure will occur, however, independently of the relative elevation of the electrodes.

To further stimulate the flow of oil from the formation adjacent to the producing well electrode into the well, the formation is heated by a second electric circuit. As illustrated in FIG. 1, the electric circuit for heating the formation includes an alternating current voltage source 60 provided at the surface and having one terminal connected by a conductor 61 to the upper end of the conductive tubing 21, and another terminal connected by a conductor 62 to the upper end of the conductive casing 16 of the producing well. This provides an alternating current path through the conductor 61, the tubing 21, the electrode 22, and upward through a portion of the formation 12 and of the overburden 11 to the conductive casing 16, and through the conductor 62 back to the source.

The alternating current flowing through the formation is conducted principally by the connate or other water in the formation, thereby heating the formation and the oil within the formation. The principal effect of the heating is to reduce the viscosity of the oil within the formation to further stimulate the flow of the oil into the producing well.

While some of the elements of the above described direct current circuit and alternating current circuit are common to both circuits, such as the electrode 22, the circuits function independently of each other. One circuit defines a closed loop system for unidirectional current and the other circuit defines a closed loop system for alternating current, so that there is no interference of the circuits with each other or their intended functions.

With the flow of unidirectional current through the formation between the electrodes 22 and 42, there will be dissipation or consumption of the electrodes due to electrolysis; and accordingly, it is desirable that the electrodes which are in contact with the formation 12 be formed in a manner that they are replacable or replenishable. Electrodes formed of the described conductive particles are well suited to this purpose. The rate of electrode dissipation is a function of he current density; and accordingly it is desirable to establish electrodes in contact with the formation which are sufficiently large to minimize, to the extent possible, the effect of electrolysis. The described electrodes, which are radially enlarged by extension into the annular cavities, are again well suited to this purpose.

It is desirable to prevent dissipation of the screen 21, by the action of electrolysis; and for this reason the screen is seated on the insulating bottom plug 19, and the annulus between the screen and the walls of the well bore are completely filled with the electrode particles 23. The screen 17 then is electrically isolated from the formation so that the electrode dissipation is confined to the conductive particles 23. For the same reason, the carbon rod 47 of the electrode well terminates short of the bottom of the well bore and is completely surrounded by the conductive particles 48. Similarly, to prevent any dissipation of the conductive casing 16 of the producing well, the insulation between the electrode 22 and the casing 16 provided by the insulated casing 15 and the insulating cement 26 extends for a substantial distance above the electrode and, preferably, into the overburden 11. This minimizes the possibility of any direct current flow through the formation other than through the electrode 22. Similarly, the insulation of the electrode well, defined by the insulating casing 43 and the cement 50, extends for a substantial distance between the electrode and the conductive casing 44, preferably into the overburden formation 11. The insulation of the producing well bore defined by the insulating casing 15 and cement 26 provides the additional function of assuring a flow path of substantial length in the formation 12, between the electrode 22 and the casing 16 to produce the desired resistance heating in the formation due to the flow of alternating current.

FIG. 2 is a diagrammatic illustration of an alternative system according to the invention including a producing well and an electrode well. In this system, the producing well is defined by a well bore 70 extending from the surface into the oil bearing formation 12, and having an enlarged bottom portion 71 within the formation. A bottom plug 72 is formed in the bottom of the bore hole from insulating epoxy or insulating cement, for example. The well bore 70 is cased with a casing 74 which extends from the surface into the enlarged well bore portion 71, the casing in this system being fabricated of a conductive metal and being provided with an external insulating layer or coating 75. An annular cavity 76 is formed in the wall of the enlarged well bore portion 71; and the enlarged bore and the cavity are filled with conductive particles or pellets 77 to define an electrode 78 which has a diameter substantially greater than that of the well bore 70. The bore portion 71 is completely filled with the conductive particles 77 so that these particles surround and engage both the outer and inner surfaces of the casing portion which extends into the bore portion 71. A conductive screen 79 is then placed in the bottom of the borehole resting on the bottom plug 72 and extends upwardly within the casing 74 concentric therewith to define an annular space 80 between the screen and casing. The screen is physically coupled to the casing by means of a packer 81 placed at the upper end of this annular space 80, the packer and screen then confining and retaining the conductive particles in the bore portion 71 outside of the screen.

The interior conductive walls of the casing 74 then are in electrical contact with the conductive particles confined in the annular space 80 to provide the conductive path between the casing and the electrode 78. Since the conductive particles surround the lower end of the casing, the casing is not in direct contact with the formation so that dissipation of the casing due to electrolysis will be inhibited.

A string of conductive tubing 83 extends from the surface to the screen 79, defining the production tubing for the well and also defining a second conductive path to the electrode 78. The lower end of the tubing 83 is electrically connected to the screen 79 by means of a centralizer 84 fixed to the tubing and engaging the inner walls of the screen.

With this described arrangement, the conductive casing 74 and the tubing 83 provide electrically parallel conductive paths through the well bore 70 to the electrode 88. This may be particularly desirable to increase the effective conductor area to obviate losses due to excessive and unnecessary heating of the conductor. Additionally, this arrangement permits the carrying of larger currents to the electrode, which may be particularly desirable in this system where the conductive casing and tubing are employed as conductors in two independent close loop circuits.

While the casing 74 defines a conductive path, the insulating coating 75 also defines a complete insulation of the path from the walls of the well bore. The continuity of the insulating coating throughout the length of the well bore may be assured by applying insulating material to the joints as the casing is set and rapidly curing the material by techniques which are known in the art.

The electrode well is defined by a well bore 90 which also includes an enlarged portion 91 at the bottom of the bore. The well bore is cased with a string of casing 92 which extends from the surface downwardly and partially into the enlarged well bore portion 91, the casing 92 also being fabricated of a conductive metal and provide with an interior insulating coating or layer 93. During the setting of the casing the continuity of the insulating layer may again be assured by coating the joints with additional insulation material.

An annular cavity 94 is provided in the walls of the enlarged well bore portion 91; and an electrode 95 is formed by filling the enlarged well bore portion and the cavity with a mass of conductive metal or carbon particles 96. These particles are packed around the lower end of the casing 92 which extends into the bore 91, engaging both the exterior and interior surfaces of the casing to isolate the conductive interior surface from the walls of the bore. The mass of particles also extends upwardly within the casing for a sufficient distance to assure good electrical contact between the particles and the casing wall. A string of conductive tubing or rod 97 is provided in the bore extending from the surface and into the mass of conductive particles 96, but spaced from the bottom of the well bore, to provide a second conductive path from the surface to the electrode 95. The conductive particles may be retained in place by means of a packer 98 placed to seal the annular space between the casing 92 and the tubing 97. For the electrode well then the casing and tubing define electrically parallel paths from the surface to the electrode, providing the same advantages as the parallel conductive paths for the production well.

To provide the above described electro-osmotic pressure between the producing well 70 and the electrode well 90, a source of unidirectional current voltage 100 is provided at the surface, the negative terminal being connected by means of conductors 101 and 102 to the production well tubing 83 and casing 74 respectively, and the positive terminal being connected by means of conductors 103 and 104 to the electrode well tubing 97 and casing 92 respectively. There is provided then, a closed loop electrical system for providing a unidirectional potential gradient between the producing well electrode 78, which is poled to be the cathode, and the electrode well electrode 95, which is poled to be the anode.

There is also provided at the surface a source of alternating current voltage 105 having one terminal connected by means of conductors 106 and 107 to the producing well, tubing 83 and casing 74 respectively, and having another terminal connected by means of conductors 108 and 109 to the electrode well tubing 97 and casing 92 respectively. This defines a second closed loop electrical system for effecting the flow of alternating current through the formation 12 between the producing well electrode 78 and the electrode well electrode 95. While the conductive paths from the surface to the electrodes are common for each closed loop electrical system, the systems function independently and without interference from each other.

In the system illustrated in FIG. 2, the producing formation 12 is a strata which is inclined relative to the horizontal; and the producing well electrode 78 is preferably located in a portion of the strata which is at higher elevation to obtain any benefit of the gravity flow of water from the producing well toward the electrode well.

A method for stimulating the recovery of oil from an oil bearing formation, which may be practiced with the above described apparatus, may include steps which will now be described. At least two spaced well bores are provided extending from the surface to the producing formation, at least one well bore defining a producing well and another well bore defining an electrode well. Electrodes are established in each of the bores in contact with the producing formation, and preferably extending laterally from the bore to define laterally enlarged electrodes. A first closed loop electrical system is provided for causing the flow of unidirectional current through the formation between the electrodes; this system including a source of unidirectional supply voltage at the surface and suitable conductor means provided in the well bores which connect the voltage source and the electrodes which preferably include conductive tubing or casing in the well bores. Through this first closed loop system, direct current is caused to flow between the electrodes through the formation to effect the flow of oil in the formation toward the producing well which is poled to be the cathode, and to effect the flow of water toward the electrode well which is poled to be the anode.

A second closed loop electrical system is provided for causing the flow of alternating current through the formation, at least adjacent to the producing well to heat the formation and thereby reduce the viscosity of the oil in the formation. This alternating current closed loop system may include the same electrodes and the same conductive tubing or casing defining the conductors through the well bores, which are employed in the unidirectional system. In this case, both the alternating current and the direct current will flow through the formation between the electrodes of the producing and electrode well bores. However, since the two systems are each closed loop systems, they function independently of each other and without interference from each other.

Alternatively, an alternating current system may be provided to include the electrode and conductors within the producing well, which are common to the unidirectional current system, and a separate electrode and conductor which may be provided in the producing well bore or adjacent thereto for causing the flow of alternating current to be confined to an area surrounding the producing well. FIG. 2A illustrates such an alternative arrangement of the system of FIG. 2 wherein the separate electrode is provided by a surface casing 111 which encloses the casing 74 for a relatively short distance beneath the ground surface. FIG. 2A is a fragmentary illustration of the upper portion of the producing well bore 70 of FIG. 2 including the conductive casing 74 with the insulating coating 75 and the conductive tubing 83, which function in the manner described and define portions of both closed loop electrical systems as described. In this modification, however, one terminal of the alternating current source 105 is connected to both the tubing 83 and the casing 74 by means of the conductors 106 and 107, as already described, while the other terminal of the source 105 is connected to the upper end of the conductive surface casing 111 by means of the conductor 112. With the arrangement the advantages of the parallel conductive paths through the producing well bore are maintained, and additionally the flow of alternating current through the formation is confined to an area adjacent to the producing well bore.

While the above described apparatus and method have been described with particular reference to two well bores defining one producing well and one electrode well, the method and apparatus may be practiced as well with a combination of well bores defining, for example, a single producing well with a multiplicity of electrode wells, a single electrode well with a multiplicity of producing wells, or a multiplicity of both producing wells and electrode wells.

What has been described are apparatus and a method employing different electrical techniques and phenomena acting on an oil bearing formation to stimulate or improve the flow of oil within the formation to a producing well bore. The apparatus required for the practice of these methods is relatively inexpensive as compared with apparatus required for other known techniques of secondary oil recovery. Examples of other suitable apparatus are disclosed in my copending applications Ser. Nos. 752,112 filed July 10, 1968, now abandoned, and 767,917 filed Sept. 30, 1968, now U.S. Pat. No. 3,507,330, assigned to the assignee of this application.

From tests which have been conducted, it appears that the area of the formation which responds to the electro-osmotic effect, is in proportion of the sizes of the electrodes. Accordingly, it is desirable to establish electrodes which have a large effective diameter, that is, much larger than the diameter of the respective bore holes. Electrodes as described herein may be established to have any desired diameter. The area of the formation which is effectively heated by the above described alternating current heating apparatus is also related to the size of the electrode; and accordingly, an electrode of enlarged diameter may be desirable in the heating circuit.

The efficiency with which the electro-osmotic effect or the electric heating effect are provided in the formation is dependent upon producing the electric power at electrodes, either within or adjacent to the formation, with minimum losses between the electrodes and the voltage sources. Accordingly, it is most desirable to provide good conductive paths between the voltage sources and the electrodes to obviate any unnecessary voltage losses; and also to insulate against any extraneous current paths which would carry the flow of current outside of the desired paths within the formation. The described systems are examples of efficient apparatus, according to the invention for practicing the method of the invention.

Claims

What is claimed is:

1. Apparatus for the recovery of oil from a subsurface oil bearing formation penetrated by at least two well bores including at least one producing well and at least one electrode well comprising:

a first closed loop electrical system for causing the flow of unidirectional current through the formation; and a second closed loop electrical system for causing the flow of alternating current through the formation;

said unidirectional current closed loop system comprises: first and second electrode means, each positioned in said producing and electrode wells respectively in electrical contact with the oil-bearing formation, with the electrode means being spaced from each other; first and second conductor means connected respectively to said first and second electrode means and extending to the surface through the respective well bores; and a source of unidirectional voltage connected at the surface between said conductor means to produce a unidirectional potential gradient between said first and second electrode means of said unidirectional system, with the first electrode means poled to be a cathode;

said alternating current closed loop system comprising: said first electrode means positioned in the well bore of the producing well in electrical contact with the oil-bearing formation; a source of alternating current voltage; and means including the first conductor means positioned in the producing well for completing an electrical circuit through the formation between the first electrode means and the source of alternating current voltage.

2. Apparatus as set forth in claim 1

wherein said second electrode means and said second conductor means for said unidirectional current system comprise respectively the second electrode means and conductor means for said alternating current system.

3. Apparatus as set forth in claim 1

wherein each of said electrode means comprises a mass of conductive particles disposed in the bottom of the respective well bores; wherein the conductor means for each of the well bores extends into the respective mass of conductive particles to make electrical contact therewith;

and including insulating means for insulating each of said conductor means from the walls of respective well bores; said insulating means extending into the mass of conductive particles to confine the current flow to a path through said conductive particles.

4. Apparatus as set forth in claim 1

wherein each of said electrode means comprises a mass of conductive particles which maintains contact with the formation and is consumable.

5. Apparatus as set forth in claim 1

wherein each of said electrode means for said unidirectional current system comprises a mass of conductive particles disposed in an annular cavity extending laterally from the respective well bores.

6. Apparatus as set forth in claim 1

wherein said first conductor means of the producing well comprises a string of conductive pipe extending from the surface to said first electrode.

7. Apparatus as set forth in claim 1

wherein said first conductor means of the producing well comprises a string of conductive casing and a string of conductive tubing electrically connected in parallel between said first electrode and the surface; and insulating means comprising an insulating coating provided on the outer wall of said conductive casing.

8. Apparatus for the recovery of oil from a subsurface oil-bearing formation penetrated by two well bores comprising:

first electrode means positioned in a producing well bore in electrical contact with the oil-bearing formation; first conductor means in said producing well bore extending from said first electrode means to the surface; insulating means for insulating said first conductor means from the walls of said producing well bore above said first electrode means;

second electrode means positioned in an electrode well bore in electrical contact with the formation; second conductor means in said electrode well bore extending from said second electrode means to the surface; insulating means for insulating said second conductor means from the walls of the well bore above said second electrode means;

third electrode means positioned in spaced relation to said first electrode means; third conductor means extending from said third electrode means to the surface;

a source of unidirectional voltage connected at the surface between said first and second conductor means to provide a unidirectional potential gradient through the formation between said first and second electrode means, with said first electrode means poled to be a cathode;

a source of alternating current supply voltage connected between said first conductor means and said third conductor means to cause the flow of alternating current between said electrodes and through said formation adjacent to said producing well.

9. Apparatus as set forth in claim 8

wherein said first conductor means comprises a string of conductive tubing; wherein said third electrode means and third conductor means comprise a string of conductive casing placed in said producing well bore extending from the surface toward said formation; insulating means insulating said string of conductive casing from said electrode to provide a conductive path of substantial length through said formation between said first and third electrode means; and wherein said first named insulating means comprises a string of insulating casing disposed between said conductive tubing and said conductive casing.

10. Apparatus as set forth in claim 8

wherein said first conductor means comprises a string of conductive casing extending from the first electrode means to the surface; wherein said insulating means for said conductive casing comprises a coating of insulating material provided on the exterior surface of said conductive casing; and wherein said third electrode means and said third conductor means comprise a string of conductive surface casing enclosing the upper portion of said coated conductive casing.

11. Apparatus for the recovery of oil from a subsurface oil-bearing formation penetrated by two well bores comprising:

first consumable electrode means positioned in a producing well bore comprising a mass of conductive particles urged into a cavity in the formation extending laterally from the well bore;

a first string of electrically conductive pipe in the producing well bore having its lower end connected to said first electrode and having its upper end extending to the surface;

a first string of conductive casing positioned in said producing well bore and provided with an insulating coating for insulating said first string of conductive pipe and said first string of casing from the walls of the well bore above the first electrode means;

means electrically connecting said first string of conductive pipe and said first string of conductive casing to define parallel electrical conductors in said producing well bore between said electrode and the surface;

second consumable electrode means positioned in an electrode well bore comprising a mass of conductive particles urged into a cavity in the formation extending laterally from the well bore;

a second string of electrically conductive pipe in the electrode well bore, having its lower end contacting said second electrode and having its upper end extending to the surface;

a second string of conductive casing positioned in the electrode well bore and provided with an insulating coating for insulating said second string of conductive pipe and said second string of casing from the walls of the electrode well bore;

means electrically connecting said second conductive pipe and said second string of conductive casing to define parallel electrical conductors in the said electrode well bore between said second electrode and the surface; and

a source of unidirectional voltage connected at the surface between said first and second string of conductive pipe to provide a unidirectional potential gradient between said first and second electrodes, with the first electrode in said producing well bore poled to be a cathode.

12. A method for recovering oil from a subsurface oil-bearing formation comprising the steps:

providing at least two spaced-apart well bores defining a producing well and an electrode well;

establishing first electrode means in the formation at the producing well;

establishing second electrode means in the formation at the electrode well;

placing first conductor means in said producing well bore contacting said first electrode and extending to the surface;

placing second conductor means in said electrode well bore contacting said second electrode and extending to the surface;

connecting a source of unidirectional supply voltage between said first and second conductor means, with said first conductor means poled to be a cathode;

insulating said first and second conductor means from the walls of the well bores to cause the flow of unidirectional current between the first and second electrode means through the earth to originate and terminate in the oil-bearing formation;

and connecting a source of alternating current supply voltage between said first conductor means at the surface and another electrode means spaced apart from said first electrode means to cause the flow of alternating current between said first and said another electrode means through said formation in the area of said producing well to heat said formation.

13. A method as set forth in claim 12

including placing surface casing in said producing well bore to define said another electrode means.

14. A method as set forth in claim 13

wherein the source of alternating current supply voltage is connected between said first conductor means and said second conductor means, and said another electrode means is said second electrode means.

15. A method as set forth in claim 12

including establishing said first and second electrode means to extend laterally from the respective well bores to define electrodes having effective diameters substantially greater than those of the respective well bores.