US3547193A - Method and apparatus for recovery of minerals from sub-surface formations using electricity - Google Patents

Description

MINERALS FROM SUB-SURF ACE FORMATIONS USING ELECTRICITY 36 Claims, 7 Drawing Figs.

[52] U.S. Cl 166/248, 166/60, 166/302, 166/308 [51] Int.Cl E2lb43/24 [50] Field of Search 166/248, 308, 57, 6O

[56] References Cited UNITED STATES PATENTS 2,758,653 8/1956 Desbrow 166/308X 2,801,090 7/1957 Hoyer et a1. l66/248X 2,802,531 8/1957 Cardwell et al. l66/308X 2,818,118 12/1957 Dixon 166/248 3,149,672 9/1964 Orkiszewski et al 166/248 3,236,304 2/1966 Sarapuu 166/248 3,417,823 12/1968 Faris 166/248 3,428,125 2/1969 Parker 166/248 Primary Examiner-Stephen .l. Novosad Att0rneys-Giles C. Clegg, Jr. and Peter J. Murphy ABSTRACT: There is disclosed a method and apparatus in which electrical current is caused to flow through water in a mineral bearing, subsurface formation to produce heating of the mineral and promote the recovery thereof. An electrode is established in the formation which extends into the formation from the bore hole, and which has a greater effective diameter than the diameter of the bore hold. Electrical current is caused to flow from the electrode through the formation to heat the mineral to be recovered. One specific electrode disclosed is formed of shot or pellets packed into the formation and another electrode disclosed is a collapsible member which extends into the formation.

PATENTEDDEBISIQYU 35471193 SHEET 1 UP 6 AC SOURCE IIIII INVENTOR WILLIAM G. GILL ATTORNEY PATENTEUUEC1 51970 SHEET 3 BF 6 AC SOURCE MENU MENU! INVENTOR WILLIAM G. GILL ATTORNEY PATENTED DECI 5 I970 SHEET I [1F 6 .I 0 0 0 n ,5 ooow 0o AC SOURCE on 00 a &0"

FIG. 5

INVENTOR WILLIAM G. GILL ATTORNEY F AC I SOURCE \Jmo ii I24 o I I 112 FIC36 //VVE/V7'0/? WILLIAM G. GILL ATTORNEY PATENTEDDEBIBISYG 3.5471193 SHEET 8 OF 6 AC SOURCE L/ II II II/ IV IVTY 1 IE II [I ll H .II/

lNVE/VTOR WILLIAM- G. GILL ATTORNEY METHOD AND APPARATUS FOR RECOVERY OF MINERALS FROM SUB-SURFACE FORMATIONS USING ELECTRICITY REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of copending Application Ser. No. 752,112, filed Jul. 10, 1968, now abandoned entitled Method And Apparatus For Recovery of Minerals From Sub-Surface Formations Using electricity".

BACKGROUND OF THE INVENTION In the copending Pat. application Ser. No. 767,917, filed Sept. 30, 1968, now US. Pat. No. 3,507,330, and assigned to the assignee of the present application, there is disclosed a method and apparatus for secondary recovery of oil in which electrical current is caused to flow through water at the bottom of the bore hole to produce heating of the water and the oil in an oil-bearing formation. The method and apparatus disclosed in the copending application have been found to produce significant results in the secondary recovery of oil. However, the rate at which heat can be produced in the'formation is limited in view of the fact that the area heated by the flow of electrical current is a function of the diameter of the electrode from which the current flows. Experimental data in dicates that the area heated by the flow of current has a radius of approximately times the radius of the electrode. Approximately 50 percent of the heat is produced within a radius twice the radius of the electrode and approximately 85 percent of the heat is produced Within four times the radius of the electrodes. Thus, if the casing in the well bore is used as the electrode and has a diameter of 6 inches, most of the heat would be dissipated within a diameter of approximately 54 inches. The concentration of heat within a relatively limited area effectively restricts the maximum amount of heat which can be generated in the formation as excessive temperatures will produce coking of the oil and resultant blocking of the flow of the oil.

The method and apparatus of the present invention are especially adapted to the secondary recovery of oil, but also has application to production of other minerals, particularly sulphur. The present invention renders it feasible to provide an electrode of any desired diameter in mineral-bearing formation, such that any desired area can be heated. For example, if an electrode is installed in the mineral-bearing formation and the effective diameter of the electrode is 25 feet, and assuming favorable boundary conditions, an area of approximately 225 feet in diameter would be heated with approximately 50 percent of the heat being produced in an area 75 feet in diameter. Accordingly, it is feasible to introduce into the formation a much greater amount of heat in a much shorter period oftime.

In accordance with the present invention, there is provided a method and apparatus for producing minerals from a subsurface formation through a bore hole extending from the surface into the formation wherein an electrode is established in the formation which extends into the formation from the bore hole and thereafter electrical current is caused to flow from the electrode through the formation to heat the mineral to be recovered. In accordance with one specific embodiment of the invention, the electrode comprises conductive shot or pellets which extend into the formation and which is electrically connected to a source of supply voltage. In accordance with still another preferred embodiment of the invention, a collapsible member is utilized as the electrode, such that when the electrode is properly positioned in the bore hole it can be collapsible, causing portions thereof to be extended into the formation providing an electrode of increased diameter.

DRAWINGS Many objects and advantages of the invention will become apparent to those skilled in the art as the detailed description of the preferred embodiments thereof unfolds when taken in conjunction with the appended drawings wherein like reference numerals denote like parts and in which:

HO. 1 is a view diagrammatically illustrating a well bore penetrating a mineral-bearing formation in accordance with one embodiment of the invention;

FIG. 2 is a side elevation view of a tool for providing an electrode in accordance with a second embodiment of the invention;

FIG. 3 is a view diagrammatically illustrating a second embodiment of the invention in which the tool of FIG. 2 is used, the tool being shown in the operative position;

FIG. 4 is a view diagrammatically illustrating a typical oil field layout;

FIG. 5 is a view diagrammatically illustrating a third embodiment of the invention;

FIG. 6 is a view diagrammatically illustrating a fourth embodiment of the invention; and

FIG. 7 is a view diagrammatically illustrating a fifth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1 of the drawings, there is shown a well bore 10 which penetrates the surface of the earth into a mineral bearing formation l2. As mentioned before, the present invention is especially adapted for the secondary recovery of oil and the preferred embodiments thereof will be described with reference to the secondary recovery of oil, but also can find great utility in production of sulphur and other liquifiable minerals. Apparatus in accordance with one specific example of the invention especially useful in the recovery of oil includes a string of casing 14- which extends from the surface into the formation 12. The string of casing l4 includes an upper conductive portion 18 suitably of conventional steel casing material, an insulating portion 20, and a screen 22 which can be considered as part of the casing string. The insulating portion 20 is suitably of fiberglass material although in some instances ceramic material is preferred.

In accordance with the specific example of the invention shown in FIG. I of the drawings, the lower end 24 of the insulating portion 20 extends to the top of the mineral bearing formation l2 and the screen 22 extends from the bottom of the bore hole into the insulating portion 20. A thermal packer 28, suitably of the type made of asbestos, is positioned at the top of the screen 22 for holding the screen 22 in place and closing the annular space 27 between the screen and the insulating portion 20 of the casing. The screen 22 is suitably of conventional type and comprises a length of pipe having longitudinal slots cut therein. I-ieavy wire is wrapped around the length of pipe and in conjunction with the slots cut in the length of pipe provides aperatures through which oil can flow into the easing. Screen 2.2 is preferably formed of stainless steel material. preferably formed of stainless steel material.

Suspended within the casing from an insulating flange member 26 is a string of conventional tubing 16 which is of steel or other conductive material such as aluminum. Tubing 16 extends below the lower end 24 of the insulating casing 29, preferably to a point near the bottom of the bore hole. Also positioned within the casing 24 is a string of insulating tubing 29 which extends between the tubing 16 and the conductive portion 18 of casing M. The tubing 29 is preferably formed of fiberglass material as it has been found to provide excellent results at reasonable cost in most application. Other types of insulating tubing can be used, such as epoxy-coated tubing of the regular type. However, it has been found that if a string of conductive tubing coated with insulating material is used, in many instances portions of the insulation will be knocked from the tubing in the course of placing it in the casing permitting an electrical short circuit between the tubing 16 and the conductive portion 18 of the casing 14.

The string of tubing 16 is electrically connected to the screen 22 by a centralizer 3% or by a decentralizer, both of which are well known in the art. For providing good electrical contact, a preferred form of centralizer may include longitudinal bands or springs 31 mounted in a manner to be forcibly bowed outwardly from the tubing 16 into engagement with the internal cylindrical surface of the screen 22, with the bands being coated with carbon in the area of engagement with the screen surface. For the same reason, the inner surface of the screen which is engaged by the centralizer may also be coated ing 16 by conductor 34 and connected by conductor 36 to the conductive portion 18 of the string of casing. Electrodes 40 extend from the screen 22 into the formation from the bore hole 10. It will be noted that the effective diameter of the electrodes 40 can be substantially greater than the diameter of the screen 22. The flow of electrical current will be from the electrodes 40 through the conductive portion lb of the casing 14 to the source of alternating current supply voltage and through the tubing 16, the centralizer 3b and the screen 22 to the electrode.

in accordance with the specific example shown in H6. 1 of the drawings, electrodes an comprise a mass of conductive particles which are packed into cavities which extend into the formation from the bore hole but which remain porous and permeable to the flow of fluid. The conductive particles preferably consist of conductive metal shot, or pellets coated with carbon, but may consist of uncoated metallic pellets or carbon pellets or granules. These particles preferably directly engage the exterior cylindrical surface of the screen 22; and

this surface may be carbon coated for the purpose of providing good electrical contact with the conductive particles.

The present invention also provides a method for establish- .ing the apparatus shown in FIG. 1 of the drawings. In accordance with the method of the present invention, conventional drilling methods are used to drill a bore hole which exrtends from the surface into the formation 12. The portion of the bore hole within the formation can also be formed by conrventional drilling methods, but it is preferred that the portion of the well bore extending into the formation be cored in order that information can be obtained as to the character of the mineral-bearing formation. The casing is then set in the 'bore hole with the lower insulating portion preferably extending a short distance into the formation. The casing 14 is set in place by pumping concrete under pressure into the earth surrounding the lowermost portion of the casing to fill the annulus between the casing and the well bore. A cement basket (not shown) of the type well known in the art can be used to prevent the cement used to set the casing from flowing down into the portion of the well bore that extends into the mineralbearing formation. 112.

After setting of the casing, cavities 42 are formed in the formation 12 with the cavities 42 extending outwardly into the formation 12 from the well bore litl. Cavities 42 can be formed at one or more elevations depending upon the thickness of the mineral-bearing formation. The notches or cavities 4 2 are preferably formed by using an abrasive jet or hydrojet of the type well known in the oil industry. Thus, in the use of such a tool a stream of sand or other abrasive material carried in either water or light oil is pumped through a nozzle at high pressure against the face of the producing formation with the jet nozzle rotating in a circular path to cut a disc from the formation to any desired depth. The vertical dimension of the notch or cavity can be controlled by raising or lowering the jet nozzle. Subsequent to formation of the cavity or notch, the formation can be treated with a suitabe acid of the type well known in the art if the reservoir or formation is oflimestone or other calcareous structure, such that the chemical reaction would be helpful. It will be noted in this connection that the notch or cavity can be formed with a mechanical underreamer or, if the formation is of a character permitting such, by a treatment with acid alone.

The portion of the bore hole extending into the formation and the cavity or notches 26 formed as described above are then filled with conductive particles such as metallic shot or carbon granules. This is suitably accomplished by pumping the particles into the bore hole with the particles being carried in suspension in a liquid such as saltwater or diesel oil to which a jell-forming organic material has been added. Conventional drilling mud can be used, but in general the salt water jell or diesel oil jell is preferred as there is greater tendency for the shot to fall out of suspension in the bottom of the bore hole and there is less chance of formation damage. After the bottom of the bore hole and the cavities are filled with the con ductive particles, the size of the electrode can be increased further by applying pressure to the bore hole in a manner conventional in the art to produce fracturing of the formation and further increase the extent to which the electrode extends into the formation away from the bore hole.

After the conductive particles are positioned as described above, the screen 22 is washed into position using conventional techniques. After the screen is washed into place, the packer 28 is set to mechanically connect the screen 22 into the string M of casing. it will be noted that not only does the packer hold the screen in place but it also seals the joint therebetween holding the conductive particles forming the electrode in place but permitting the conductive shot which remains within the screen to be washed out of the string of casing providing a space into which mineral to be produced can flow. it will also be noted in this regard that in the event the electrode should become dissipated, it can be replaced by removing the packer and washing additional conductive material into place in the formation.

it is important that the conductive path not be provided between the string of tubing 15 and the conductive portion 18 of the string of casing l4, except through the formation. Accordingly, depending upon the characteristics of the well, the amount of water present, and other factors, it may be desirable to set an additional packer 46 in the space between the tubing 28 and the insulating portion 20 the string of casing 14 to prevent liquid rising in the annulus 48 therebetween. If desired, the annulus can be filled with oil or nitrogen gas to further restrict the possibility of flow of current directly between the tubing 15 and the casing 18 and also to serve as heat insulation. However, the packer 46 will not be required in all installations.

After completion of the above, electrical current is caused to flow from the alternating current supply source 32 through conductor 34, tubing to, centralizer 30, screen 22 to the electrodes 40. The flow of current from the electrode will be through the formation 12 and the strata overlying it to the conductive portion 18 of the casing 14 and thence through conductor 36 to the source of supply voltage. it will be noted that in most instances a path for flow for electrical current through the formation will be provided by connate water present in the formation. However, if sufficient connate water is not available, additional water can be pumped into the formation, suitably using an installation as described in the copending application mentioned previously. it will be noted in this regard that in many reservoir systems adequate drive is present and it is only necessary that the temperature of oil be increased sufficiently to lower the viscosity to the extent that it will flow into the screen. Oil within the casing can be pumped to the surface or, if sufficient drive is present, it will flow through the tubing 16 without necessity of pumping. It is im portant to note that in accordance with the present invention a very large area of the mineral producing formation is heated with the temperature of this area slowly rising to the desired temperature. Accordingly, it is not necessary to pump water or other material through the heating zone in order to obtain heating of a large area. The large area which is heated provides a substantial volume in which the mineral to be recovered has high mobility, and if the mineral is oil, even though the oil outside the heated area remains very viscous, the area surrounding the volume which is heated to the extent that the oil has a high mobility due to its reduced viscosity is very large. Accordingly, even though oil outside the heated area is very viscous, a very large effective area is provided into which the oil can flow and, assuming adequate drive is present in the field, substantial quantities of oil can be recovered as a result of the flow of the oil into the area of heat. Further, it is important to note that the heating process is continuous and is not in any way effected by the flow of oil into the screen, assuming that sufficient connate water is present to maintain the electrical circuit. in virtually all formations, this amount of connatc water will be present.

in accordance with a second embodiment of the invention, the screen is formed to have a center section as shown in FIG. 2 of the drawing with the remainder of the installations as respect to the tubing and easing, suitably being as shown in FIG. l. in accordance with this second embodiment of the invention, the screen includes an upper portion 60 and a lower portion 62, each of which has apertures formed therein as described previously with respect to screen 22 of FIG. l of the drawings. Upper portion 64) includes a cylindrical portion 64 of reduced diameter which is adapted to slide within the upper end 66 of the lower portion 62. The portions 64 and 66 are suitably connected by a pin 68. Positioned on and carried by the screen is a collapsing metal basket '70 which is connected at one end '72 to the upper portion 6t) and a lower end '74 to the lower portion 62. When the tool is in the extended position as shown in FIG. 2, it will be noted that the basket diameter is very little more than the diameter of the screen, accordingly the assembly can be moved up and down through the casing 14. However, when the lower end 76 of the screen is in position and force is applied to the upper end 66, the pin will be sheared permitting cylindrical portion 64 to slide further into the lower portion 62 into a position as shown in FIG. 3 of the drawings. When in such position, the metal basket will be expanded outwardly such that the circumference of the electrode provided by the metal basket is substantially greater than the circumference of the bore hole. if the formation from which the mineral is to be produced is of soft material it may be possible for the basket to punch into the formation as it expands. However, if the formation is of hard material, it is preferable to form notches or cavities as described with reference to H6. i into which the basket will protrude as it is expanded.

H6. 3 also illustrates an alternative means for insulating the tubing 56 from the string of casing 14. It will be noted that in the embodiment of the invention shown in FIG. 3 of the drawings, a string of insulating tubing is not provided. Rather insulating packer 78 is set between the tubing 16 and the insulating portion 20 of the string of easing. Positioned along the length of tubing above the tubing is a plurality of insulating spacers 8b which are formed of an insulating material. The insulating spacers are preferably of a spider configuration and formed of Teflon or other insulating material which can withstand the temperature present in the well bore. The insulating spacers maintain the tubing 16 centrally positioned within the bore of the casing, preventing electrical or mechanical contact between the tubing l6 and the conductive portion of the casing 14-. The annulus between the tubing 16 and the casing 14 is preferably filled with insulating fluid 79, which suitably can be a heavy crude oil. Thus, the tubing is insulated from the casing 14 except through a conductive path comprising the electrode defined by the bmket 70.

In FlG. 4 of the drawing, there is shown diagrammatically a typical oil field comprising a plurality of oil wells. A source of supply voltage 84 is connected between the tubing of wells 86 and 38 which extend into mineral bearing formations. The source of supply voltage 84 is connected between the tubing of well 86 and the tubing of a well 88 such that the flow of electrical current will be down one length of tubing, through the electrode associated with it thence through the formation to the electrode of the second well with the return path for the flow of current being through the tubing of the second well. it will be appreciated in this regard that the amount of voltage required will be approximately twice that required in an installation such as that shown in H6. 2 if an equivalent amount of heating is to be produced at each well. The principal advantage of installation such as that shown in FlG. 3 is that the insulation requirements between the tubing to and the casing are not as stringent as the purpose of insulation is to insure that the electrical current flows from the electrode in the formation rather than a portion of the casing positioned in the overlying strata acting as an electrode.

in those instances in which the invention is to be used for the secondary recovery of oil, sufficient drive may be present as a result of gravity or the presence of gas or the presence of water to cause flow of the oil into the well bore as the oil is heated by the flow of electrical current. in some reservoirs, however, very little drive is present in the reservoir in which event it may become desirable to generate drive within the reservoir by injection of gas or water at a pusher well. Thus, as shown in H6. 4 of the drawings, alternate wells illfi may be used for the injection of either gas or water to increase the drive of the reservoir with the other wells m2 being used for producing of oil with the formation in the vicinity of the well being heated, suitably as described in respect to the previous embodiments. it will be appreciated in this regard that the pusher wells may also include provision for adding heat to the formation if desired.

H68. 5 and 6 illustrate embodiments of the invention which are particularly suitable for the recovery of minerals from producing formations which are relatively thin.

Referring particularly to H6. 5, there is shown a bore hole which has been drilled from the surface downward through a producing formation 1E2 A string of casing has been set in the bore hole extending to the lower surface of the formation 12; the casing including a lower insulating portion hi3 which extends from the lower surface of the formation to a point somewhat above the surface of the formation, and an upper insulating portion 114 extending from this insulating portion to the surface. The casing is suitably cemented within the bore hole; and a cement bottom plug MS may be provided to define the bottom of the bore hole and prevent direct electrical contact with the underlying strata which may be a low resistivity material such as shale.

An electrode 118, similar to that described in connection with FIG. 1, is provided; however, in this embodiment a portion of the insulating casing 113 is cut away along with a portion of the encasing cement to define an annular slot ii) in the casing wall communicating with the formation Hi2. This slot is provided by techniques which are well known in the art. An adjacent cavity or notch 12% is then provided in the formation M2, again using conventional techniques as described in connection with the embodiment of the FIG. l.

The lower portion of the bore hole including the cavity is filled with conductive particles, of the type described heretofore, to define the expanded electrode lib; and these particles are retained in place by a conductive screen R21 set within the insulated casing 113 and extending from the bottom of the bore hole to a point above the top of the formation 112 but within the insulating portion 113 of the casing, The upper end of the screen is sealed relative to the casing ass by means ofa packer 122 to retain the conductive electrode particles in the cavity R26 and within the annulus defined by the screen and the casing. V

A string of conventional tubing R24, fabricated of steel, aluminum or other conductive material, is supported from the surface and extends downwardly into the screen $21 and is electrically connected to the screen by a centralizer E25. This tubing defines a conductor for the flow of current from the surface to the electrode 118, and is insulated from the conductive casing M4 by a string of insulating tubing 126, as in the embodiment of HO. 2. This insulating tubing 126 extends in the bore hole to a depth sufficient to overlap the upper end of the insulating tubing 126 with the upper end of the insulating casing M3 to prevent a short-circuiting flow ofcurrent, within the casing, from the conductive tubing i134 to the conductive casing lid.

With a source of alternating current supply voltage connected to the conductive tubing .24 and conductive casing ll-.4 by conductors 131 and T32, respectively, current is caused to flow between the electrode 118 and the lower end of the conductive casing lid through the mineral formation lllZ, to heat the formation. The described electrode is porous and permeable to the flow of fluid, which flows into the well through the electrode.

FIG. 6 illustrates an alternative embodiment of the invention for use in a shallow producing formation Hi2, where the formation is heated by alternating current flowing through the formation between two wells, as discussed in respect to FIG. 4.

The structure of the well in FIG. 6 is quite similar to that of FIG. with a principal exception that the entire string of casing 135 is fabricated of an insulating material, such as fiberglass or ceramic and the string of insulating tubing 126 is omitted. The structure of the well, in the area of the formation R12, is the same as that of FIG. 5, the electrode E18 being formed from conductive particles packed into a formation cavity 1% and the annular space defined between the conductive screen 121 and the lower end of the insulating casing 135,

the screen being sealed to the casing by means of a packer A second packer 136 may be provided if it is desired to seal the tubing 124 to the casing 135 above the screen 121.

A string of conductive tubing 124% extends from the surface to the screen R21 and is electrically connected thereto by the centralizer 125. The insulating casing 335 insulates the tubing 124 from the bore hole throughout the entire depth of the bore hole except in the area of the electrode lib.

A source of alternating current supply voltage 140 is connected by means of one conductor Hill to the conductive tubing I24; and a conductor 142 may connect the voltage source 140 to the corresponding conductive tubing of an adjacent bore hole to define a second electrode 113 within the formation.

Alternatively, the second conductor 142 may be connected to the conductive casing of an adjacent well, or to an auxiliary electrode or pipe serving as a return for current flowing through the formation.

FIG. 7 illustrates still another alternative embodiment of the invention which is particularly advantageous for use in deeper wells. The structure of the well of FIG. '7 is quite similar to that of FIG. 6 with the well being provided with a string of insulating casing 145 which extends from the surface to the bottom of the well bore defined by the cement bottom plug M5, which is preferably in insulating cement. The insulating casing 145 is not, however, fabricated from an insulating material as is the casing of FIG. 6. Rather, the casing is made up ofa string of conductive metal casing 146 which has been fabricated with a coating or layer M7 of insulating material, such as fiberglass or epoxy. During the setting of the string of casing in the bore hole, additional insulating material may be applied at the joints and cured by known techniques, to assure continuity of the insulating coating along the string.

One of the advantages of this form of casing is that of providing an insulating barrier adjacent to the walls of the bore hole while, at the same time, enabling the use ofa higher strength casing which may be required in deeper wells. An additional advantage is that the casing, while providing the above-mentioned insulating function, may provide an additional low-resistance conductive path from the surface to the bottom of the hole; as illustrated in FIG. 7.

The electrode IE8 is formed in a manner similar to that of FIGS. 5 and 6, being formed of conductive particles packed into a formation cavity 126) at the casing slot ill), and also into the annular space defined between the conductive screen 122 and the lower end of the casing M5, the. screen again being sealed to the casing by means of a packer R22 to retain the conductive particles. in this annular space, the electrode particles conveniently contact the inner wall of the conductive portion 146 ofthe casing to provide the desired electrical connection between the casing and the electrode particles.

A string of conductive tubing 124, which is the production tubing for the well, extends from the surface to the screen Hi. The electrical connection between the tubing llZd and the screen lZll is provided by a plurality of metallic springs or brushes E48 which are mounted in the interior wall of the screen 12.. and extend inwardly to engage the tubing 124. These contact springs M8 are fixed to the interior walls and screen in any suitable manner, such as by welding or riveting, with the lower ends of the springs formed to be urged inwardly. When the tubing 124 is extended downwardly within the screen 121, the tubing engages and spreads the spring whereby an electrical contact is maintained between the screen and the tubing. The portions of the spring which engage the tubing may be coated with carbon to improve the electrical contact.

One terminal of a source of alternating current supply voltage lfi'ili is connected by means of conductors l5l and 152 to the tubing 124 and the casing M5, respectively. Another terminal of the source of supply voltage is connected by means of conductor 153 to another conductive path to the formation which may be defined by conductive pipe in an adjacent bore hole, or other electrode or pipe serving as a return for current flowing through the formation.

By electrically connecting the tubing and the casing in parallel to define one path for the flow of electrical current to the electrode 218, there is effectively provided a conductor of greater cross section to reduce excessive losses due to heating of the conductor between the surface and the electrode.

t will be appreciated that the electrode in accordance with the present invention can be used advantageously in installations other than those described as preferred, specific example of the invention when the mineral to be produced is oil. The installation will, of course, be substantially different if minerals other than oil, such as sulphur are to be produced.

An important and unexpected advantage obtained from the use of the present invention in the secondary recovery of oil in a substantial upgrading of the oil. Thus, in one field the oil in place in a well was operated with the casing and tested, and the A.P.I. gravity of the well was 12. Oil produced from a well using an installation as shown in FIG. 1 was upgraded to the extent that its A.P.l. gravity was 1%. When the size of the electrode in the well was reduced as a result ofa workover operation, then the A.P.l. gravity of the oil produced decreased to M.

The exact cause of the upgrading of the quality of oil produced is not known. However, it is theorized that the shot and the formation form an underground cracking unit whose efficiency is a function of the surface area of the shot forming the electrode. it is also theorized that the sand in formation further contributes to the surface area. It will be noted, in this connection, that if the field is one having mobility that pressure will be maintained at the bottom of the hole and this, in conjunction with the surface area of the shot in the sand and the heat provided as a result of a flow of electrical current, produces a cracking effect in the formation. it is also theorized that the steel shot may act as a catalyst to some extent. In any event, it has been found that there is no evidence of residue in the formation, but that the gravity of the oil produced is increased substantially, evidencing the fact that the heavier substances are broken down into less-complex molecules. The higher gravity oil is more valuable and easier to handle at the surface providing important economic benefits.

Although the invention has been described with reference to particular embodiments thereof, many changes and modifications will become apparent to those skilled in the art in view of the foregoing description which is intended to be illustrative and not limiting of the invention defined in the appended claims.

lclairn:

l. A method for producing minerals from a subsurface formation through a bore hole extending from a surface into the formation comprising the steps:

forming a cavity extending laterally from said bore hole into said formation;

urging an electrode of conductive material into said cavity in electrical contact with said formation;

positioning a string of conductive pipe in said bore hole extending from the surface to contact said electrode;

insulating said conductive pipe from the strata overlying the formation;

providing a second relatively low-resistance conductive path extending from the surface to said formation; and

producing a flow of electrical current through said pipe, said electrode, said formation, and said conductive path to heat the mineral to be produced.

2. A method as set forth in claim 1 wherein said electrode is established by collapsing a collapsible member within said bore hole, to cause portions of said collapsible member to extend laterally into said cavity from said bore hole.

3. A method as set forth in claim 1 wherein said electrode is established by filling said cavity with a porous mass of conductive particles which remains permeable to the flow of fluid.

4. A method as set forth in claim 1:

including inserting a string of conductive casing in said bore hole extending from the surface to a depth above the electrode to define said second conductive path; and

insulating said conductive pipe and the electrode from said conductive casing to restrict the flow of current between the casing and pipe to a current path through the formation between said electrode and said conductive casing of substantial length.

5. A method as set forth in claim l including positioning a conductive screen in said bore hole in electrical contact with said electrode.

6. A method as set forth in claim 5 including establishing electrical contact between said pipe and said screen by means of a centralizer fixed to said pipe and having carbon-coated members contracting said screen.

'7. A method as set forth in claim l including positioning a conductive screen in said bore hole in electrical contact with said electrode and spaced apart from said conductive casing a substantial distance.

8. A method as set forth in claim ll:

including providing a string of casing in said bore hole extending from the surface into said formation, with at least a portion of said casing positioned in said formation being of insulating material;

establishing said electrode by forming a slot in said insulating casing and forming said cavity adjacent to said slot; and

filling said cavity and said slot with electrically conductive particles defining said electrode.

9. A method as set forth in claim 8:

including providing a conductive portion of said casing, ex-

tending from the surface to said insulating portion, to define said second conductive path; and

insulating said conductive pipe from said conductive casing.

til. A method as set forth in claim 1. including providing a string of insulating casing extending from the surface to said formation, to insulate said conductive pipe from the walls of said bore hole.

ii. A method as set forth in claim 1 wherein the mineral to be produced is one which flows in the presence of sufficient heat.

l2. Apparatus for producing minerals from a subsurface formation through a bore hole extending from the surface into the formation comprising:

means defining an electrode extending laterally into said formation from said bore hole, said electrode having a greater effective area than the cross section area of said bore hole;

a string of conductive pipe extending from the surface to said electrode; means electrically connecting said conductive pipe to said electrode; said electrode, said connecting means and said conductive pipe defining a first relatively low-resistance electrically conductive path to said formation;

insulating means for insulating said conductive pipe from walls of said bore hole above said electrode;

lltll means defining a second relatively low-resistance conductive path from the surface to said formation; and

a source of supply voltage connected at the surface to said conductive pipe and to said second conductive path means to produce a flow of electric current through said formation to heat the mineral to be produced.

13. Apparatus as set forth in claim 12 wherein said electrode comprises a mass of conductive particles urged into a cavity extending laterally from the bore hole.

l4. Apparatus as set forth in claim 13 wherein said conduc tive particles are conductive metal pellets.

l5. Apparatus as set forth in claim 13 wherein said conductive particles are conductive metal pellets coated with carbon.

16. Apparatus as set forth in claim l3 including a length of conductive casing in said formation contacting said conductive particles; and connecting means electrically connecting said conductive pipe and said length of conductive casing.

l7. Apparatus as set forth in claim 116 wherein said length of conductive casing defines a screen.

i3. Apparatus as set forth in claim 16 wherein the interior and exterior surfaces of said length of conductive casing are coated with carbon.

l9. Apparatus as set forth in claim l6 wherein connecting means comprise contact members coated with carbon.

20. Apparatus as set forth in claim l6 including a string of conductive casing in said bore hole extending from the surface to a point above said electrode, defining said second conductive path; and means insulating said string of conductive casing from said length of conductive casing in said formation to effect the flow of current through a path of substantial length in said formation between said electrode and said conductive casing.

21. Apparatus as set forth in claim 16 including a string of casing in the bore hole extending from the surface to said formation; said string of casing including an upper conductive portion extending from the surface to a point above said electrode defining said second conductive path, and a lower insulating portion of substantial length extending between said upper conductive portion and said length of conductive casing in said formation to effect a flow of current through a path of substantial length in said formation between said electrode and said conductive portion.

22. Apparatus as set forth in claim l2 wherein said electrode comprises a collapsible member received in said bore hole, and collapsible to urge portions thereof into a cavity extending laterally from the bore hole.

23. Apparatus as set forth in claim 22 wherein said collapsible member includes a first tubular member having an end portion of reduced diameter, a second tubular member having an end portion adapted to slidably receive the end portion of reduced diameter, means releasably connecting said first and second tubular members in extending relation. and a collapsible basket member connected between said first and second tubular members and adapted to extend. laterally outward in response to movement of said first portion into said second portion from said extended position.

24. Apparatus as set forth in claim if. wherein said means defining a second conductive path comprises a string of conductive pipe positioned in a second bore hole spaced apart from the first mentioned bore hole.

25. Apparatus as set forth in claim 12 including a string of casing in said bore hole extending from the surface to said formation.

26. Apparatus as set forth in claim 25 wherein at least a portion of said casing is fabricated of a fiberglass insulating material comprising said insulating means.

27. Apparatus as set forth in claim 25 wherein at least a portion of said casing is fabricated of a ceramic insulating material comprising said insulating means.

28. Apparatus as set forth in claim 25:

wherein said casing includes an upper conductive portion extending from the surface to a point above said electrode; said portion said second conductive path; and

3 ill said casing including a lower insulating portion of substantial length, insulating said conductive portion from said electrode, to effect the flow of current through a path of substantial length in said formation between said electrode and said conductive casing.

29. Apparatus as set forth in claim 2? including means for insulating said conductive pipe from said upper conductive portion of said casing.

30 Apparatus as set forth in claim 29 wherein said insulating means comprises a string of insulating tubing disposed in the annulus between said pipe and said casing.

31. Apparatus as set forth in claim 29 wherein said insulating means comprises a plurality of insulating spacers disposed between said pipe and said casing.

32 Apparatus as set forth in claim 29 wherein said insulating means comprises an insulating fluid filling the space between said pipe and said casing.

33. Apparatus as set forth in claim 25:

wherein said casing includes a lower insulating portion extending into said formation; and

means defining an annular slot in said casing opening to said formation; and said electrode extending through said slot into said formation.

34. Apparatus as set forth in claim 25 wherein said string of casing is fabricated of an insulating material defining said insulating means.

35. Apparatus as set forth in claim 25 wherein said string of casing is fabricated of a conductive metal having an exterior insulating coating; said insulating coating defining said insulating means.

36. Apparatus as set forth in claim 35 including means electrically connecting said string of casing between said electrode and said supply source to define a portion of said first relatively low-resistance electrically conductive path.

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