Method and apparatus for electrically heating a subsurface formation

Abstract

One or more wellbores extending from the surface into a subterranean electrically conductive formation are each provided with a deviated bottom section extending laterally for a predetermined distance with respect to the vertical axis of the wellbore and in a predetermined direction. The deviated section is filled with electrolyte adapted to electrically contact the formation along its interface with the deviated section. Insulated conductive means are introduced within the electrolyte from the surface so that the electrolyte effectively constitutes an electrode to which a suitable source of potential may be connected at the surface through the conductive means. Passage of current to and from this effective electrode through the formation as part of a closed circuit establishes a heated zone, which extends laterally from the wellbore in the direction of the deviated section.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to an apparatus and method for electrical heating of a subterranean electrically conductive formation and more particularly to an apparatus and method for accomplishing such heating, utilizing electrodes positioned within one or more wellbores extending within the formation.

2. Description of the Prior Art

Prior art methods and apparatus for electrically heating a subterranean hydrocarbonaceous formation involving the passage of a current therethrough are well-known. One such apparatus and method employs a single wellbore extending within the formation provided with separate electrode means such as an upper section of conductive casing and the lower end of a central conductor insulated therefrom, each in electrical communication with the formation at a different height. Passage of current between these two electrode means from a suitable voltage source at the surface causes a current to flow between them through the formation to establish a heated zone in the vicinity of the wellbore. The extent of this zone depends upon the strength of the current and resistivity of the formation, but in any event the current flow will be greater close to the wellbore in order to minimize the distance between the electrode means. In order to increase the size of the zone in the formation which may be effectively heated in this manner, the current flow paths to the formation may be distorted or deflected radially outward from the borehole. For example, as exemplified in the patent to Crowson, U.S. Pat. No. 3,620,300, this may be done by introducing an insulating barrier extending radially into the formation from the wellbore at a point intermediate the two electrodes so that the current paths must circumvent such barrier. A further technique for accomplishing a similar result is to introduce annular cavities or fractures extending into the formation from the wellbore and filling them with conductive particles adapted to contact a conductive tube within the wellbore so that these conductive particles constitute an electrode of considerable contact area with the formation. However, practically speaking, the exact lateral extent of these elements is usually unknown, and their precise dimensions and direction cannot be easily controlled.

The prior art of electrical heating of a formation also comprehends placement of two electrodes respectively within a pair of wellbores spaced some distance apart within the formation of interest. When these electrodes are formed as part of an electrical circuit, current passes between them through the formation. However, as in the case of the one wellbore method described above, the zone of heating of the formation tends to be confined fairly narrowly to a zone about a line joining the two electrodes. As the current conducting zone expands transversely from the most direct path between the electrodes, the current follows paths of progressively greater curvature, and consequently the cross-sectional current density through the formation decreases in such transverse direction.

It is, therefore, a general object of this invention to provide an improved method and apparatus for electrical heating of a subsurface electrically conductive formation.

It is another object of this invention to provide an improved method and apparatus for electrical heating of a subsurface hydrocarbonaceous formation.

It is a further object of this invention to provide such a method and apparatus wherein the zone of effective electrical heating within the formation is expanded to a controllable extent and in a desired direction.

It is yet another object of this invention to provide a method and apparatus of the type described wherein the effectively heated zone within the formation may be laterally expanded with the use of a single wellbore.

It is a further object of this invention to provide a method and apparatus for the type described wherein the effectively heated zone within the formation between a pair of spaced apart wellbores may be extended transversely to the direction between such wellbores.

It is still another object of this invention to provide a method and apparatus of the type described wherein the uniformity of electrical heating of a formation may be effectively enhanced.

Other objects and advantages of this invention will become apparent from a consideration of the detailed description to follow in conjunction with the drawings provided herewith.

SUMMARY OF THE INVENTION

In a preferred embodiment, the apparatus of this invention comprises one or more spaced apart wellbores extending vertically from the surface within an electrically conductive formation of interest, the bottom of the wellbore being provided with a deviated section which extends laterally for a predetermined distance from the vertical axis of the wellbore and in a predetermined direction. The deviated section is preferably uncased and is completely filled with electrolyte. An exposed electrode is suspended within the electrolyte from a centralized insulated conductor extending from the surface within the wellbore. The vertical section of the wellbore is provided with an upper section of conductive casing and a lower section of nonconductive casing. The fill line of the electrolyte remains below the junction between the conductive and nonconductive casing so as to eliminate a conductive path from the surface to the electrode. The space in the wellbore above the electrolyte is filled with an insulating oil. Current may be applied to the electrode from a suitable source at the surface through the central conductor. As a result, the electrolyte constitutes an effective electrode in communication with the formation over the entire interface with the deviated section.

The invention also comprehends apparatus wherein a wellbore having a bottom deviated section of the type described also includes second electrode means supported within the wellbore at a predetermined distance above the deviated section by means of a second insulated conductor introduced from the surface. The second electrode may be surrounded by a second quantity of electrolyte adapted to communicate with the formation through perforations extending through the wall of the vertical csing of the wellbore. The two electrodes are insulated from each other within the wellbore, and a suitable voltage gradient may be established between them by connecting the two conductors to a source of alternating current at the surface.

The invention also comprehends a method for electrically heating a subterranean formation comprising the steps of providing a wellbore extending vertically from the surface within the formation, extending the bottom of the vertical wellbore laterally for a predetermined distance and in a predetermined direction, establishing electrode means within the laterally extending wellbore section communicating with the formation along the interface between the formation and such laterally extending wellbore section, connecting the electrode means to a source of potential at the surface through conductive means extending within the wellbore, and establishing a current path through the formation including said electrode means, said conductive means, and said potential source, whereby a heated zone is established within the formation which expands over the length of and in the direction of the laterally extending wellbore section.

Finally, the invention comprehends a method of electrically heating a subterranean formation as described above additionally including the steps of establishing additional electrode means within the wellbore supported by additional conductive means extending therewithin from the surface, said additional electrode means being vertically spaced above the laterally extending wellbore section, providing electrical communication between the additional electrode means and the formation, interconnecting the additional electrode means with the source of potential through the additional conductive means to cause current to flow through the formation between the two electrode means, whereby a heated zone in the formation is created in the vicinity of the wellbore expanded laterally from the vertical wellbore axis in relation to the length and direction of the laterally extending wellbore section.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view partly schematic and partly in section illustrating one simplified embodiment of this invention.

FIG. 2 is a side elevational view partly schematic and partly in section illustrating a modified form of this invention.

FIG. 3a is a diagrammatic representation of typical current paths established between two laterally spaced electrode means within an electrically conductive formation in accordance with the prior art.

FIG. 3b is a diagrammatic representation of typical current paths established through an electrically conductive formation between a pair of vertically spaced electrode means within a single wellbore in accordance with the prior art.

FIG. 4a is a diagrammatic representation of typical current paths established through an electrically conductive formation between two laterally spaced electrode means in accordance with this invention.

FIG. 4b is a diagrammatic representation of typical current paths established through an electrically conductive formation between two electrodes within a single wellbore in accordance with this invention.

DETAILED DESCRIPTION OF THE INVENTION

With particular reference now to the embodiment of FIG. 1, a wellbore 10 extends from the surface of the earth into an electrically conductive formation of interest 11. The substantially vertical section 17 is lined with an upper conductive casing 12 and a lower nonconductive casing 13. At a suitable depth, the wellbore 10 may be directionally drilled along a controlled lateral path (obviously not shown here to scale) to form a deviated section 18 of preselected cross-section extending for a predetermined distance, perhaps several hundreds of feet, from the vertical axis of the wellbore 10 and in a predetermined direction therefrom. The section 18 is preferably uncased as shown, but may optionally be provided with perforated casing to establish selected fluid communication paths from the interior of section 18 into the formation 11. An electrode 19 such as a steel bar may be suspended from the surface within the wellbore 10 at any desired depth by insulated conductive means 20 such as a wire connected at the surface to an alternating current source 21. The deviated section 18, and, if necessary, a portion of the vertical section 17, is filled with an electrolyte 22, such as a sodium chloride solution, to a level sufficient to cover the electrode 19 but in any event terminating at a level below the junction between casings 12 and 13, the space above the electrolyte 22 being filled with an insulating oil 23. Thus, the effective contact area between the electrode 19 and the formation 11 is determined by the length and the dimensions of the section 18.

The apparatus described may be incorporated as part of a complete electrical circuit adapted to pass an electric current through the formation in several ways. For example, the current source 21 at the surface may be connected to a suitably located electrical ground (not shown) from which multiple current paths are established passing through the formation 11 and terminating with uniform spacing along the deviated section 18. The length and direction of this section 18 governs the effective extent of the uniform heated zone so created within the formation 11.

Alternatively, a pair of such boreholes 10 (not shown) may be drilled into the formation at some preselected distance apart. In that event both deviated sections 18 may conveniently be directed in the same sense and at right angles to a line between the vertical axes of the two boreholes 10. When current flows between the effective electrodes in two such boreholes 10 formed by electrolyte 22 in the manner provided in this invention, substantially linear current paths, viewed from above, will be established over the full length of the deviated sections 18. In consequence, uniform electrical heating of the formation 11 can be extended transversely to the direction between the boreholes 10 over an area limited only by the length of sections 18. Obviously, this effect can be still further enhanced by providing each wellbore 10 with additional laterally deviated sections (not shown) directed in an opposite sense or at predetermining angles to the deviated sections 18. In that event each of said additional deviated sections may be filled with electrolyte in contact with the formation 22 and having electrical continuity with electrolyte 22. Furthermore, it should be understood that the deviated sections 18 need not extend horizontally as shown but may curve gradually from the vertical to any desired obtuse angle.

The significance of the improvement of this invention can be better appreciated first by considering FIG. 3a which diagrammatically shows current flow paths such as 24, 25, and 26 between a pair of laterally spaced electrodes 27 and 28 of negligible transverse dimensions in electrical contact with a formation of interest. Here it will be observed that the current paths are characterized by an increasing degree of curvature with departure from the most direct line 29 between the electrodes 27 and 28. In consequence, in such a prior art arrangement, considerable lack of uniformity of heating may be experienced in a direction transverse to the direction between the electrodes; and in consequence, the overall lateral dimensions of the effectively heated zone of the formation are severely restricted.

The previously described current paths may be contrasted with those which will be generated with the aid of the present invention. With reference to FIG. 3b, there is represented diagrammatically a plurality of substantially linear current paths such as 31, 32, and 33 extending between a pair of deviated wellbore sections 18 associated with a similar pair of wellbores 10 and proceeding laterally in the same direction and sense. It is apparent that a high degree of uniformity of electrical heating of a formation of interest can thus be extended in a direction transverse to the direction between the boreholes 10. The limit of the heated zone is determined by the lateral extent of the deviated sections 18.

A modified form of the apparatus of this invention can be observed by reference to FIG. 2. In this embodiment a wellbore 40 drilled vertically from the surface within the formation 11 is provided with a preferably uncased deviated section 42 extending laterally from the vertical axis of the wellbore 40 for a predetermined distance and in a predetermined direction. The upper conductive casing 43 joins a lower nonconductive casing portion 44 which extends to the bottom of the vertical portion of the borehole 40 as shown. A hollow tube 45 centralized within the bore 40 by means of the insulating packers 46 and 47 provides a channel for an electrode 50 such as a steel bar supported by conductive means 51 below the packer 47 and within the electrolyte 52 which fills the deviated section 42. The bore of the tube 45 above the electrode 50 is filled with a nonconductive oil 53. A second electrode 55 is supported between the centralized tube 45 and the nonconductive casing 44 by conductive means 56 interconnected at the surface along with conductive means 51 to the previously described alternating current source 21. The electrode 55 is positioned at a predetermined height above the deviated section 42 determined by the vertical extent of the desired zone to be heated within the formation 11. The electrode 55 is surrounded by a suitable electrolyte 57 while the space thereabove is filled with an insulating oil 58. Perforations 59 are provided through the casing 44 extending into the formation 11 to establish good electrical contact therewith. If an alternating current potential difference is established between the electrodes 50 and 55, current will flow through the formation in accordance with the teachings of this invention between electrode 55 and the effective electrode constituted by the interface between the formation 11 and the deviated section 42. Multiple current paths will thereby be established between these electrode means which will be substantially linear for a distance from the vertical axis of the wellbore 30 determined by the lateral extent of the deviated section 32 and in a direction also determined thereby. Thus, as in the embodiment of FIG. 1, the area of uniform electrical heating of the formation which may be achieved with the use of the apparatus of this invention is considerably enhanced.

Again, the contrast with the prior art may be better understood by reference to the diagrammatic representation of FIG. 4a which illustrates current paths 60, 61, and 62 of increasing curvature between two vertically spaced electrodes 63 and 64 within a formation. This increasing curvature, of course, results from the fact that current density will be greatest where the distance between the electrodes is at a minimum. FIG. 4b, on the other hand, shows the improved linearity of the current paths such as 66 and 67 extending within the formation of interest in accordance with the present invention between an electrode 55 and the effective electrode defined by the deviated wellbore section 42. The important point to be made is that in this manner the lateral extent of the heated zone around the wellbore 10 is no longer confined largely to its immediate vicinity but may extend for a considerable distance therefrom limited only by the length of the deviated section 42. It is reasonable to assume that over much of this heated zone the current density remains substantially constant.

Although the method and apparatus of this invention has been described with a certain degree of particularity, it will be understood that the foregoing is illustrative only. It is clear that many other modifications and alternate embodiments of this invention will occur to those skilled in the art without departing from the scope and spirit thereof as set forth in the claims appended hereto.

Claims

What is claimed is:

1. The method of electrically heating a subterranean electrically conductive formation comprising the steps of:

a. drilling a first and a second wellbore downwardly from the surface in substantially vertically spaced apart relation to within said formation,

b. further drilling each of said first and second wellbores to form respective first and second deviated sections thereof extending laterally in a direction transverse to the direction between the vertical axes of said first and second wellbores,

c. establishing respective first and second electrode means within said first and second deviated sections adapted to communicate with said formation along its interface with said first and second deviated sections,

d. providing first and second conductive means extending from the surface within said first and second wellbores to contact said first and second electrode means respectively,

and

e. applying a voltage potential between said first and second electrode means through said first and second conductive means, whereby a current is adapted to pass through the formation between said first and second electrode means.

2. The method of claim 1 wherein said first and second deviated sections of wellbore are uncased.

3. The method of claim 1 wherein said first and second deviated sections are at least partially filled with electrolyte defining said first and second electrode means.

4. The method of electrically heating a subterranean electrically conductive formation comprising the steps of providing a wellbore extending from the surface within said formation, said wellbore having an upper vertical section and a lower deviated section extending from the bottom of said upper vertical section laterally for a predetermined distance from the vertical axis of said vertical section and in a predetermined direction, establishing first electrode means within said deviated section in electrical communication with said formation along the interface between said formation and said deviated section, providing first conductive means extending from the surface within said wellbore in contact with said first electrode means, establishing second electrode means within said wellbore and insulated therein from said first electrode means, providing second conductive means extending from the surface within said wellbore and adapted to support said second electrode means at a predetermined height above said first electrode means, providing a communication path between said second electrode means through said wellbore into said formation, interconnecting said first and second conductive means at the surface to a source of voltage potential, whereby a current is adapted to flow in a substantially vertical direction through the formation between said first and second electrode means.

5. In the art of electrically heating an electrically conductive formation by positioning a pair of spaced apart electrodes therein and passing a current therebetween through the formation, the method of increasing the linearity of the current paths between said electrodes comprising the steps of positioning at least one of said electrodes within a deviated section of wellbore extending laterally within said formation in a direction transverse to the direction between said pair of electrodes and establishing electrical continuity between said at least one electrode and the surrounding formation substantially over the interface between said formation and said deviated wellbore section.

6. In the art of electrical heating of an electrically conductive formation by passage of an electric current therethrough, the improvement comprising:

a. a wellbore extending from the surface downwardly into the formation consisting of an upper vertical section and a lower deviated section extending from the bottom of said upper vertical section laterally for a predetermined distance from said upper vertical section and in a predetermined direction,

b. an electrolyte adapted to at least partially fill said lower deviated section and to thereby communicate with said formation along the interface thereof with said lower deviated section,

c. an insulating oil filling said wellbore above the level of said electrolyte,

d. conductive means extending within the wellbore from the surface and adapted to contact said electrolyte, and

e. means for connecting a source of electrical potential to said electrolyte through said conductive means.

7. In the art of electrical heating of an electrically conductive formation by passage of an electric current therethrough, the improvement comprising:

a. a wellbore extending from the surface downwardly into the formation consisting of an upper vertical section and a lower deviated section extending from the bottom of said vertical section laterally for a predetermined distance from said vertical section and in a predetermined direction,

b. electrode means within said deviated section adapted to communicate with said formation along the interface with said deviated section,

c. conductive means extending within the wellbore from the surface and adapted to contact said electrode means,

d. additional electrode means positioned within the vertical section of said wellbore and situated at a predetermined height above said deviated section,

e. additional conductive means extending within the wellbore from the surface and adapted to support said additional electrode means,

f. means extending laterally through the wellbore into the formation adjacent said additional electrode means to establish electrical communication between said formation and said additional electrode means,

g. means for insulating said electrode means and additional electrode means from each other within said wellbore,

and

h. means for connecting a source of electric potential serially with said electrode means and said additional electrode means through said conductive means and additional conductive means respectively whereby a current is adapted to flow through the formation between said electrode means and additional electrode means responsive to the application of said electrical potential.

8. The apparatus of claim 7 wherein said additional electrode means is a steel bar, and said additional conductive means is a wire.

9. The apparatus of claim 8 wherein said means establishing communication between said additional electrode means and said formation is a quantity of electrolyte adapted to surround said steel bar.

10. The apparatus of claim 9 wherein said wellbore is provided with a centralized tube coextensive with the vertical section thereof, said centralized tube being insulated from said wellbore, said conductive means being adapted to extend through said centralized tube to the bottom of said vertical wellbore section, and said additional conductive means being adapted to extend externally of said centralized tube and partially above said vertical wellbore section.