Method and System for Electrically Heating an Earth Formation

Abstract

A core of subterranean oil sand formation is delineated by a unit of three wellbores extending horizontally through the formation. The wellbores are spaced apart, parallel and coextensive. They are arranged in a triangular pattern. Multi-phase AC power is applied to electrode assemblies positioned in the wellbores to cause current to flow through the core, thereby heating it with the objective of reducing the viscosity of contained oil to render it mobile.

Description

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims benefit under 35 U.S.C. 119(e) of pending U.S. Provisional Application No. 61/929,055 filed 18 Jan. 2014 by the applicant; the complete disclosure of which—including its Example/Appendix A—is incorporated herein by reference, to the extent the disclosure of pending U.S. Provisional Application No. 61/929,055 provides support and further edification of this technical disclosure.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for electrically heating an earth formation.

BACKGROUND OF THE INVENTION

The present invention was conceived with the intention of applying it to a subterranean earth formation containing viscous heavy oil, with the objective of heating the oil to reduce its viscosity and thereby enhance its mobility and produceability. More particularly it was aimed at assisting in the recovery of bitumen (a specie of heavy oil) from the Athabasca oil sands located in the northern region of Alberta.

However it is also contemplated that it may have application in connection with remediating surface earth formations contaminated with undesirable contaminants, such as hydrocarbons and toxic chemical compounds. In this application, the objective is usually to vaporize the contaminants, so that they may be recovered through suction wells.

In both cases there is a need for a feasible technique for raising the temperature of the earth formation, to thereby enable recovery of the contained hydrocarbons or contaminants. One potential technique for this purpose is electrical heating of the resistive earth formation.

Electrical heating has previously been applied to the Athabasca oil sands. See Canadian Patent No. 2,341,937. The '937 patent and a pilot project based thereon taught penetrating subterranean oil sands with vertical, spaced apart electrode wells and applying multi-phase alternating voltage to the well electrodes. As well, water was injected into the formation. As a consequence, steam was formed adjacent the wells and current moved through the span of formation material between the electrodes of adjacent wells. The electrically resistive oil sand was thereby heated and the steam further advanced heat in the formation. Contained bitumen was thereby mobilized and produced through vertical pumping wells.

The present invention provides a different approach to electrically heating a subterranean formation.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a system and method designed to raise the temperature of a portion of formation.

In accordance with one embodiment, “units” of several electrode wells are utilized for electrically heating a portion of subterranean earth formation containing heavy oil, such as bitumen. Each electrode well comprises a wellbore having an upper leg extending downwardly from ground surface and a lower leg further extending horizontally through the formation. In this embodiment, the upper leg may be cased with electrically insulated pipe and the lower leg is completed ‘open hole’. The lower leg of the wellbore contains an elongate electrode assembly having at least one electrode electrically coupled to the formation for the transmission of electric current thereto. Preferably, the electrode assembly is ‘segmental’—that is, it comprises a plurality of electrode segments separated by non-conductive connectors.

The number of electrode wells in a unit is equal to the number of phases of alternating current supplied to the unit. For example, if three phase AC power is used, the unit will consist of three electrode wells. If six phase AC power is used, the unit will involve six electrode wells.

Electric power is provided by a source at ground surface. Power is transmitted to each electrode well within the unit by means such as a cable, so that each electrode of the unit has a different phase of AC current. For example, if three phase power is used, one phase is connected to the electrode(s) of one electrode well of the unit and the other two phases are connected to the electrodes of the other two electrode wells of the unit.

The electrode wells of each unit are substantially parallel and equally spaced apart. Preferably, they are coextensive in length. They delineate a central core of formation material between them. In the case of a three well unit, the wells are positioned at the apices of a triangle. Otherwise stated, the wells of the unit are evenly distributed in a triangular pattern. In the case of a six well unit supplied with six phase power, the wells are evenly distributed in a hexagonal pattern. The wells are preferably positioned quite close together and extend a significant distance through the formation. For example, the wells may be spaced twenty meters apart and extend for a hundred meters or more.

From the foregoing, it will be appreciated that the invention is directed to delineate a long, narrow, horizontally extending core of oil sand with a pattern of electrode wells and to thereby focus the heating action on this core so that it rises in temperature and creates a ‘heated finger’ that further emanates heat by conductance to surrounding oil sand. The system can be replicated and stacked within the formation.

The system may optionally incorporate one or more of the following features into the combination:

• • utilizing removable electricity supply connections, to enable selective activation of electrodes and replacing of corroded connections;
  • forming a large diameter horizontal wellbore using underground ‘river crossing’ technique to form the wellbore and pulling or pushing the electrode assembly into place;
  • incorporating a plurality of axially spaced apart electrode segments separated by non-conductive connectors, into the electrode assembly;
  • enhancing the electrical conductivity of the annular space between the electrodes and the formation by packing it with an annular column of electrically conductive fill;
  • utilizing an electrode assembly comprising a bundle of tubular members adapted to enable production of draining heated oil, placement of the annulus fill and supply of power to the electrodes; and
  • operating the method on a ‘dry’ basis—that is, without water injection.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top plan view of a facility having a layer of electrode wells;

FIG. 1B is a sectional side view showing top and bottom wells of a unit and a production well in a subterranean formation in the context of the facility of FIG. 1A;

FIG. 1C is an end sectional view of a layer of three phase electrode well units in a subterranean formation in the context of FIG. 1A, said electrode wells being arranged in triangular patterns;

FIG. 1D is a perspective view showing a single three phase electrode well unit in a subterranean formation in the context of FIG. 1A, with the wells arranged in a triangular pattern;

FIG. 2 shows a double pleat arrangement of three phase units;

FIG. 3 is an end view of an alternate embodiment of the present invention, showing the horizontal well lay out for a six phase installation;

FIG. 4 is a vertical cross-sectional view of a formation having two units of the six phase installation of FIG. 3;

FIG. 5 is a perspective view of the six phase installation of FIG. 3, wherein each electrode well has pipe electrodes 12 meters in length separated by 3 meter insulator pipe lengths or ‘connectors’;

FIG. 6 is a graph of the results of tests done to determine the viscosity of bitumen at various temperatures;

FIG. 7 is a vertical cross-section view of a formation having three arrays or units of the six phase installation of FIG. 3 and two sets of hot sweep assist wells located between the units;

FIG. 8 is a cross-sectional view of one embodiment of a composite production/electrode well comprising a conductive fill delivery pipe, a pipe electrode, a production pipe, a conductor bundle and centralizers;

FIG. 8A is a sectional side view of a three phase unit;

FIG. 9 is a side view of the conductive and low conductivity segments of a segmental well electrode;

FIG. 9A is a end view of the electrode well of FIG. 9;

FIG. 10 is a side view of a slotted pipe used to introduce conductive and non-conductive materials into the annulus of the electrode well;

FIG. 11 is a sectional end view of the slotted pipe of FIG. 10;

FIG. 12 is a fanciful side view showing a horizontal electrode design for use in the system and the use of an upper pipe to inject conductive and nonconductive fill into the annular space of the wellbore;

FIG. 13A is a cross-sectional view of an electrode assembly ‘bundle’ in a wellbore after conductive fill has been placed in the annular space around the conductive electrode segment;

FIG. 13B is a cross-sectional view of the electrode assembly ‘bundle’ in a wellbore if non-conductive fill has been placed in the annular space around the non-conductive connector;

FIG. 14 is a schematic side view of a retractable electrode connection tool for use in an electrode well, shown in a collapsed position;

FIG. 15 shows the tool of FIG. 14 in the expanded position;

FIG. 16 is a fanciful end view of the tool of FIG. 15 expanded within an electrode;

FIG. 17 shows a proposed six phase electrode well unit in a hypothetical formation with proposed parameter values;

FIG. 18 is a side view showing two electrode wells of a three phase unit, the wells extending down from ground surface, through formation and back up to ground surface;

FIG. 19 is a sectional end view such as that taken along XIX-XIX of FIG. 18 showing multiple three phase well units in a formation;

FIG. 20 is a sectional end view such as that taken along XX-XX of FIG. 18 showing a bundle or multiple tube combination in a wellbore;

FIG. 21 is a fanciful end view showing actions expected when drive gas is injected into heated core through the upper electrode wells of three phase units;

FIG. 22 is a fanciful end view showing actions expected when hot water is injected into the core;

FIG. 23 is a projected core temperature profile over time compared to a profile of temperature expected exterior of the core due to conductive heating;

FIG. 24 is also a projected core temperature profile over time compared to a profile expected exterior of the core.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In General

The assembly of components and method of operation, as described herein, are adapted to apply electric current to a subterranean, electrically-resistive, viscous heavy oil-containing earth formation 1, to thereby heat the oil and reduce its viscosity with the aim of rendering it producible. One such formation is the Athabasca oil sands. One such heavy oil is bitumen contained in the oil sands.

It is also contemplated that the system may be applied in an environmental context to heat an earth layer or formation containing contaminants, such as coal tar, which are to be recovered.

When power is supplied in different phases to a set or unit 2 of closely positioned electrode wells 3, a voltage difference between the electrodes 4 of the wells is created and this causes current to pass between them.

In the present system, the electrodes 4 are positioned in horizontal wellbores 5 extending through the subterranean formation 1. The electrodes communicate electrically with the formation. Since the formation resists the current passing through it, the resistance manifests as heat. As the formation heats up, the viscosity of contained heavy oil diminishes, thereby improving its mobility and enhancing the potential for recovering it.

Alternating current (AC) electricity can exist in a number of different phases. Typically, a power plant produces three different phases of AC power simultaneously, with the three phases being offset by 120 degrees from each other (meaning each phase involves a sinusoidal voltage with a phase angle of either 0, 120, or 240 degrees). AC power can also be provided in two phase, four phase, five phase or six phase forms. For example, as known in the art, six phase power can be obtained from standard industrial three phase power by using transformers.

In the present system, a number of electrode wells 3 are provided. Each well has an electrode assembly 6 positioned in an open hole or cased wellbore 5. The number of electrode wells 3 in the “unit” 2 is equal to the number of phases of electric current supplied. So, if three phase AC power is supplied, a unit 2 will have three electrode wells 3. Each of these electrode wells is individually supplied with a phase that is different from that of the other two wells. If six phase AC power is supplied, the unit will have six electrode wells and six different phases are individually and simultaneously applied to the electrodes of each of the six wells.

The electrode wells 3 are equally spaced apart, substantially parallel, coextensive and positioned in a triangular or hexagonal arrangement in the cases of the three and six well embodiments.

The wells 3 will normally extend horizontally some significant distance through the formation 1, for example up to 100 meters.

A means is provided for producing to ground surface the oil that has been heated and mobilized. For example, separate horizontal production wells 6 may be provided adjacent to and at the base of the electrode wells 3 or within the borehole 5 adjacent to the electrodes 4. The production wells are equipped with conventional production piping including slotted liners and downhole pumping equipment. The heated oil may drain into these production wells and be pumped up to ground surface. Alternatively, as described below, a slotted production pipe or tube 27 may be incorporated into the electrode well 3 itself.

Multi-Well Units

FIGS. 1A, 1B and 1C illustrate an embodiment having units 2 of three electrode wells 3 arranged in triangular patterns 8 for use with three phase power. Each electrode well 3 has a downwardly extending leg 9 connecting with a generally horizontal leg 10 positioned in the oil-containing formation 1.

The units 2 are positioned in side-by-side relation so as to provide one or more stacked layers 12.

Their location and use of phased electricity allows the electrode wells to be positioned apart a distance up to about 20 meters. The closer the spacing the faster the heating process; however, this results in a higher cost of installation per unit volume of oil heated. Therefore a central oilsand core 13 of considerable volume is delineated by the three electrode wells 3 of each unit 2.

The provision of three electrode well units 2 arranged in a triangular pattern can be replicated over a large area. The units 2 are stackable and can be adapted to the shape of the formation 1. Where the well spacing approximates the thickness of the formation 1, a single layer 12 can be used. In a thicker formation, stacked layers 12 can be used, as illustrated in FIG. 2.

FIG. 3 shows an alternate embodiment wherein six phase electricity is used with a unit 14 of six electrode wells 2 arranged in a hexagonal pattern 15. As indicated in the Figure, a seventh well 16 may be provided in the center of the pattern 15 for use as a production well. This production well 16 is electrically neutral.

Forming the Wellbore

The electrode well wellbores 5 may be directionally rotary drilled from ground surface 18.

However, as a large diameter (e.g. 30 inch) wellbore 5 may be desired (as explained below), the wellbores 5 may be formed using underground “river crossing” borehole-forming procedures involving rotary drilling a small diameter pilot hole and then increasing its diameter by reaming. The resulting wellbores 5 are illustrated in FIGS. 18, 20. These large diameter wellbores 5, extending from ground surface 18, down and through the formation and back up to ground surface, have the advantage that the electrode assembly 6 can be pulled into place. These wells have the further advantage of being able to pull electrode connecting devices and oil producing pumps into place rather than pushing the system into place or using downhole ‘tractors’ to pull them into a dead-end borehole or casing.

Casing the Wellbore

The upper leg 9 of the wellbore 5 will normally be cased with non-conductive pipe 19, such as plastic-coated steel pipe or fiberglass reinforced plastic. The horizontal leg 10 may be cased or uncased as described below.

The Electrode Assembly

Having reference to FIG. 8, 8A, the electrode assembly 6 comprises a multiple tube combination or ‘bundle’ 20 of components, as shown in FIG. 8. The bundle 20 comprises an outer slotted electrode tube 21 connected to and extending horizontally from the non-conductive casing 19 in the first leg 9. The electrode tube 21 incorporates a plurality of linearly arranged, conductive electrode segments 22, separated and connected together by non-conductive connectors 23. The outer electrode tube 21 further contains a smaller diameter tube 24 containing insulated conductor cables 25 extending up to ground surface 18 through the casing 19. The upper ends of the conductor cables 25 are connected at ground surface to a source 26 of electric power. They are also individually connected at their lower ends to the electrode segments 22. The outer electrode tube 21 may also contain a slotted production tube 27 within the horizontal portion of the electrode well and extending up hole cased to ground surface 18. Heated oil draining into the bore of the outer electrode tube 21 can pass into the production tube 27 and be pumped to surface by a downhole pump (not shown). A slotted fill tube 29 is attached to the outer tube 21 and extends through the upper leg 9 to ground surface. It can be used to pump a slurry of conductive fill particles 17, such as steel grit, into the annular space 30 defined between the wellbore 5 and outer electrode tube 21.

It is contemplated that either a single electrode or several axially spaced electrode segments will be used, depending on the circumstances. The multiple electrode segments have the advantages of enabling heating to be carried out sequentially or focussed on particular sections of the formation to compensate for differences in heterogeneities of electrical properties of the formation.

Various electrode designs are available. One might: use a metal casing extending from ground surface and extending horizontally through the formation; or use a metal tube in the horizontal wellbore, connected by insulated cable to the power source; or use a fixed casing in the horizontal wellbore, said casing having conductive and non-conductive segments with which a retrievable cable and connecting device (as shown in FIGS. 14-16) can be used.

Operating Control and Model Test Observations

A controller for data acquisition and operating control may be used. Such controllers are conventional and do not form part of the present invention. The controller will incorporate a control logic program for modulating operations to meet desired voltage, power or temperature conditions.

Watts per meter of electrodes will be monitored in order to maintain low enough settings to protect the electrodes, which ideally should be below 1000 w/m but can go higher to heat larger areas more rapidly.

Duration of heating of the formation and oil will involve a function of the power delivered to the formation. The power application can be specifically controlled by power supply units (PSU's) which can consist of one or more independent three phase silicon controlled rectifiers rated, as an example, at 350 amps at 480 VAC for a 750 Kw power supply. The power controller permits one or more zones of operation, with each zone operated at constant voltage, constant power and constant current or constant subsurface temperature. The output of each rectifier is connected to a field located three phase isolation transformer. The transformers can be placed in the field to minimize line losses as they distribute power in lower voltages and higher amperage to the individual electrodes.

Power is increased slowly to the formation. By way of example the power may start at about 25% of the normal power and increased slowly over a few weeks at startup and over several hours upon restart up. A common starting voltage for many formations is 50-90 volts. As the formation heats up it is often desirable to keep the power factor below about 0.7 for long term heating.

A power density of 80 watts per m³ will heat the formation relatively quickly (1 C/week) with a relatively short spacing (i.e., 5 m). However, it is preferred to start out slowly with a low power density and see how the formation responds in terms of amperage and power factor. As the formation load changes during heating, the target power factor determines how often the transformer taps need to be adjusted to match the load. Typically, the lower the power density the longer it will take the formation to increase in temperature. However, a lower power density will also extend the life of the electrode.

When using three phase power, alternating current is applied to three separate electrode wells with three separate phases of electricity (standard industrial power, with phase A to one well, phase B to another and phase C to the third well—as shown in FIG. 2) at a voltage that is related to the spacing of the wells, speed of heating and electrical conductivity of the formation. Preferably a variable voltage regulator is used, providing a voltage in the range of 60-200 volts. However, the voltage used is dependent on formation resistivity. As outlined above, as current is passed to the electrode wells, it will also pass between the wells, using the formation as a large resistor, which causes the formation, including the oil and water within the formation, to heat up.

When six phase electricity is used each phase of electricity is considered numbered one through six where phase 1 is zero phase shift, phase 2 is 60 degrees phase shift, phase 3 is 120 degrees phase shift, and so on. Phase 1 electricity would be connected to the top electrode well, phase 2 electricity would be connected to the adjacent well in one direction from the first, phase 3 electricity would be connected to the next well around the hexagon in the same direction and so on (as shown in FIG. 4). In this way, any adjacent electrode has current supplied at 60 degrees of phase shift plus and minus in each opposing directions and every opposing electrode in the hexagonal array is 180 degrees phase shifted. This arrangement provides for a relatively uniform distribution of current throughout the formation.

Once the formation is heated to a point where the oil viscosity has dropped substantially (for example to less than 3000 mPa-s) the oil can be produced. FIG. 5 shows the typical relationship between oil viscosity from the Athabasca oil sands region and temperature. For an incremental heating of 30° C., the viscosity drops by more than an order of magnitude. However, the exact relationship between temperature of formation heating, oil production and further energy input will be site specific. Furthermore, it may be advantageous to continue heating to yield lower viscosities to facilitate faster recovery of oil. For example, the mobility of oil raised to 150° C. would be expected to be an order of magnitude better than oil raised to 90° C., based on the data shown in FIG. 5.

The duration of heating required to get to the target temperature will depend upon the spacing of electrodes and current supplied to the formation, as discussed above. The larger the spacing of electrodes the longer it will take to heat the formation. Similarly, the lower the voltage and current flow, the more time it will take to heat up the formation. The target temperature depends upon the target oil viscosity.

By way of example, at a spacing of about four meters between electrode wells and an energy input of 1000 watts/m it may take about six weeks to heat up the oil to a viscosity low enough to pump it by conventional means. Alternatively at an eight meter spacing of electrode wells, it would be expected to take about twenty-four weeks to heat the core or zone between the electrode wells and an area beyond. However, it may be desirable to heat the formation for a longer period to further reduce the viscosity of the oil and facilitate faster recovery of heavy oil. In order to achieve optimal heating of the formation, temperature sensors set in the formation may be used to control the power.

For conditions substantially similar to the Athabasca Oil Sands (i.e., conductivity, water content, oil saturation, depth, etc.), a proprietary mathematical model has been run to estimate the time to heat up the formation and oil to temperatures where production of oil may be considered economic. This model has been shown to be reasonably accurate for numerous environmental applications of the 3 Phase power application option. Test data shows the heating time for various depths within the formation as it is heated with electrodes spaced 9 m apart heated at a rate of about 1000 w/m. This shows that the formation oil may take about 200 days to heat to about 100° C., which is about where the viscosity would drop to a level where it could readily flow to a production well. After about 300 days of heating, the formation would be expected to heat up to about 140° C., where the oil would flow more easily. At this point, if more energy was put into the system, steam would be generated in-situ and could further assist in the recovery of oil with the induced pressure gradient that the steam would provide.

By comparison, a similar model, but at larger spacing between electrodes (22 m) and higher power input (about 3900 w/m), the heatup would take longer (about 350 days to 100° C. and about 450 days to 140° C.; see FIG. 23). Depending up the cost of electrode/well installation and cost of electricity, it may be more economical to take longer to initially heat the formation at a higher power input into fewer wells to a higher temperature so the oil flows more readily from a large volume.

In addition to heating the core 13 encompassed within the unit, this method also results in some heating of the formation outside of the unit. It is contemplated that this may occur to a distance of about half of the spacing of the well, while the temperature is raised to only about 25-75% of the temperature of the internally heated core. There is a decline in temperature the further the distance from the electrode wells; however, this will vary with specific formation characteristics of formation, including resistivity, fluid flow and heterogeneities. For the examples tested in the models, a fringe temperature effect was seen with the temperature responses showing for the edge of the heated zone (i.e., along the outer edge of the electrodes), which climbed to about 80-100° C. and for a point considered about 8 ft beyond the edge of the heated zone the temperature was expected to rise only about 30° C.

Once the formation around the electrode wells is heated sufficiently, all of the electrode wells adjacent to or within the heated zone can be disconnected and one or more of the production wells may be used to pump the heavy oil by conventional means (for example using a downhole pump or pumpjack). In the six phase array, the central well may be electrically neutral and can be a normal heavy oil production well with some electrical isolation at the heel of the well.

Once the formation has been heated and oil has begun to flow to a production well the formation pressure will drop. It may be desirable to maintain formation pressures near static pressures. As oil is produced, formation water will be produced as well. The formation water may be separated, preferably heated and re-injected in one or more wells to balance the pressure of the formation. In addition, formation water from other operations that require disposal can be heated and injected as well in order to balance the pressure of the formation to be nearly neutral.

In another feature, it is contemplated that recovered water could be re-injected into the upper well(s) and extracted from the lower well(s). As shown in FIG. 7, after a zone has been produced, a nearby unit may be heated and a hot water drive may be used to produce oil between the two units. FIG. 7 specifically illustrates this objective with three units in a dual hot water sweep. This method may be pulsed or extended to higher oil recovery percentages by injecting steam or hot gases. This approach may be further enhanced by adding material to the injected fluid to reduce the viscosity, thereby increasing the relative mobility of oil and reduce the volume of water handled in the process.

Another alternative arrangement for heating and producing oil is shown in FIG. 22 where 5 horizontal electrodes are spaced equilaterally in what may be called a 5-Pleat pattern, where the formation is heated electrically, as described above and then hot water is injected in the upper wells (assuming the oil density at temperature is greater than that of water) and oil is recovered from the lower two wells. The injection pressure would be expected to be relatively low so as not to blow out the shallow wells or fracture the overburden into the fresh water aquifer, yet high enough to provide a driving force for the water to help push the heated oil to the lower wells.

An alternative drive mechanism with gravity drainage of the oil is shown in FIG. 21. In this arrangement hot gases (CO2, flue gases, steam, etc.) may be injected at relatively low pressure (i.e. 4 bar or about 60 psi), so that the oil flows by gravity downward and heating of the top and sides of the electrically heated zone continues to expand as the hot gasses mobilize the oil at temperature and provides an additional driving force to move the oil into the production wells at the bottom of the arrangement.

The near wellbore formation commonly contains sand, oil and salt water, of which, only the saltwater is substantially conductive. To further enhance electrical conductivity between the electrode and the deeper portion of the formation, material can be injected into the formation to displace the formation fluids with more electrically conductive material such as ferrous sulphate or nano particle graphite, iron or other conductive materials. Electrical conductively in the formation around the well can also be important to reduce the current density around the well as energy is supplied to heat up the formation, especially when wellbores are small (less than 30 cm) or the distance between electrodes is greater than 7 meters. To enhance the near wellbore conductivity, electrically conductive material may be precipitated by oxidation reduction reactions, acid base reactions, electroplating or physically moving conductive material into the formation. By increasing the conductivity of material near the borehole the current density can be increased and the formation can be heated at an increased rate and over larger distances in reasonable amounts of time.

APPENDIX A

Further Details Outlining Two Operational Examples of the Instant Design

Three Phase Application

• 1. Install a horizontal borehole into oil laden formation using a suitable directional drilling rig. It is assumed that the formation depth, thickness and orientation are known from previous investigations using conventional techniques of drilling, coring, seismic and/or geophysical logging. The oil laden formation should be sufficient thickness and extent to accommodate at least 3 horizontal wells spaced equilaterally apart by at least 4 m.
• 2. Within the borehole at least one casing is installed with an electrically conductive portion (electrode) within the horizontal leg of the borehole and a substantially less conductive portion (i.e., plastic, ceramic, insulated or coated steel) extending from the heel of the well (the part that begins to turn toward the vertical direction from horizontal) to ground surface. The annular space between the lowest portion of the nonconductive portion of the casing should be filled with less electrically conductive material (i.e., rubber packer, low electrical conductive sealant) to reduce current traveling up the annulus directly.
• 3. Install 2 other wells substantially parallel to the first well and spaced an equilateral distance from the other wells and installed with similar casing characteristics.
• 4. Alternating current is applied to the 3 separate wellheads with 3 separate phases of electricity (standard industrial power, with phase A to one well, phase B to another and phase C to the third well) at a voltage that is related to the spacing of the wells, speed of heating and electrical conductivity of the formation, or preferably with a variable voltage regulator (i.e., using a SCR [silica control rectifier]), typically in the range of 60-200 v, but could be more or less, depending on formation resistivity. As current is passed to the electrode wells, it will also pass between the wells, using the formation as a large resistor, which causes the formation, oil and water within the formation to heat up.
• 5. The duration of heating required to get to the target temperature will depend upon the spacing of electrodes and current supplied to the formation. The larger the spacing of electrodes the longer it will take to heat up. The lower the voltage and current flow, the slower the heat up. The target temperature depends upon the viscosity of the oil, which based on Athabasca oil sand samples are typically represented in the attached graph, FIG. 5.
• 6. At a spacing of about 4 m between wells and an energy input of 1000 watts/m it should take about 6 weeks to heat up the oil to a viscosity low enough to pump by conventional means. However, it may be worthwhile heating a longer period to reduce the viscosity of the oil further to facilitate faster recovery of oil. At an 8 m spacing of wells, it would be expected to take about 24 weeks to heat the zone between the electrode wells and an area beyond. The area beyond the area encompassed by the wells is expected to typically heat to a distance of about ½ spacing of the well and to about 25-75% of the temperature of the internally heated zone, and decline in temperature with additional distance from the electrode wells; however, this will vary with specific formation characteristics, including resistivity, fluid flow and heterogeneities.
• 7. Once the formation around the electrode wells is heated sufficiently, all of the electrode wells can be disconnected and one or more of the wells can be used to pump oil by conventional means (downhole pump, pumpjack, etc.). Typically, for the very heavy oils such as the Athabasca, gravity drainage to the lower most wells is advantageous.
• 8. As oil is produced, formation water will be produced as well, which can be separated, preferably reheated, and reinjected in one or more wells. One embodiment of the invention involves reinjecting recovered water into the upper well(s) and producing oil through the lower well(s) using gravity to assist in the drainage process. In addition, formation water from other operations that require disposal can be heated and injected as well in order to balance the pressure of the formation to be nearly neutral, which will aid in order to replace the voidage created by the extracted oil. This will provide revenues for disposal operations and aid in recovery of oil, especially if the water is preheated to the target temperature for optimal oil viscosity. Additional substances can be added to the injected water to raise the viscosity of the water phase and thereby enhance the relative mobility of the oil.

Six Phase Application

• 9. Form a horizontal wellbore extending into an oil laden formation using a suitable directional drilling rig. It is assumed that the formation depth, thickness and orientation are known from previous investigations using conventional techniques of drilling, coring, seismic and/or geophysical logging. The oil laden formation should be sufficient thickness and extent to accommodate at least 6 horizontal wells spaced equilaterally apart by at least 4 m in a hexagon pattern.
• 10. Within the wellbore at least one perforated casing is installed with an electrically conductive portion (electrode) within the horizontal leg of the wellbore and a substantially less conductive portion (i.e., plastic, ceramic, insulated or coated steel) extending from the heel of the well and to the surface. The annular space between the lowest portion of the less conductive portion of the casing should be filled near the conductive portion with less electrically conductive material (i.e., rubber packer, low electrical conductive sealant) to reduce current traveling up the annulus directly.
• 11. Install 5 other wellbores substantially parallel to the first well and spaced an equilateral distance from the other wells in a hexagon pattern and installed with similar casing characteristics. A seventh well can be installed in the middle of the hexagon with the objective of producing oil from this well, as the center of the hexagon will be substantially neutral in its electric capacity and in the center of the heated oil.
• 12. Utility companies generate three-phase electric power, which consists of three sinusoidal voltages with phase angles of 0, 120, and 240 degrees. Six phase power can be obtained from standard industrial three phase power by using transformers that are center tapped. The transformers are center tapped so that the potential difference between the center tap and one end of the secondary is 180 degrees out of phase with the potential difference between the center tap and the other end. In this manner, 6 phases of power is generated.
• 13. Alternating current is applied to the 6 separate wellheads with 6 separate phases of electricity so that each phase is connected to each well in sequence around the hexagon. If each phase of electricity is considered numbered one through six where phase 1 is zero phase shift, phase 2 is 60 degrees phase shift, phase 3 is 120 degrees phase shift, and so on, then phase 1 would be connected to the top electrode well and phase 2 is connected to the adjacent well in one direction from the first, phase 3 is connected to the next well around the hexagon in the same direction and so on. This way any adjacent electrode has current supplied at 60 degrees of phase shift plus and minus in each opposing directions and every opposing electrode in the hexagonal array is 180 degrees phase shifted. This arrangement provides for a relatively uniform distribution of current throughout the formations.
• 14. The voltage that is related to the spacing of the wells, speed of heating and electrical conductivity of the formation, is set or preferably controlled with a variable voltage regulator (i.e., using a SCR [silica control rectifier]), typically in the range of 60-200v, but could be more or less, depending on formation resistivity. As current is passed to the electrode wells, it will also pass between the wells, causing the formation to behave as a large resistor, which causes the formation, oil and water within the formation to heat up.
• 15. The duration of heating required to get to the target temperature will depend upon the spacing of electrodes and current supplied to the formation. The larger the spacing of electrodes the longer it will take to heat up. The lower the voltage and current flow, the slower the heat up. The target temperature depends upon the viscosity of the oil, which based on Athabasca oil sand samples are typically represented in the attached graph, FIG. 5.
• 16. At a spacing of about 4 m between wells and an energy input of 1000 watts/m it should take about 6 weeks to heat up the oil to a viscosity low enough to pump by conventional means. However, it may be worthwhile heating a longer period to reduce the viscosity of the oil further to facilitate faster recovery of oil. At an 8 m spacing of wells, it would be expected to take about 24 weeks to heat the zone between the electrode wells and an area beyond.
• 17. The area beyond the area encompassed by the wells is typically heated to a distance of about ½ spacing of the well and to about 25-75% of the temperature of the internally heated zone, and decline in temperature with additional distance from the electrode wells; however, this will vary with specific formation characteristics of formation, including resistivity, fluid flow and heterogeneities.
• 18. Once the formation around the electrode wells is heated sufficiently, all of the electrode wells can be disconnected and one or more of the wells can be used to pump oil by conventional means (downhole pump, pumpjack, etc.). In the six phase array, the central well is electrically neutral so it convenient to install this as a normal heavy oil production well with some electrical isolation at the heel of the well.
• 19. As oil is produced, formation water will be produced as well, which can be separated and reinjected in one or more wells. One embodiment of the invention would be to reinject recovered water into the upper well(s) and extract from the lower well(s). In addition, formation water from other operations that require disposal can be injected as well in order to balance the pressure of the formation to be nearly neutral, which will aid in order to replace the voidage created by the extracted oil. This will provide revenues for disposal operations and aid in recovery of oil, especially if the water is preheated to the target temperature for optimal oil viscosity.

Claims

1. A system for electrically heating a portion of a subterranean earth formation containing heavy oil, from ground surface, comprising:

a plurality of substantially parallel and equally spaced apart electrode wells forming a unit, each electrode well comprising a wellbore having an upper leg extending downwardly from ground surface and a lower leg further extending horizontally in the earth formation, said horizontal lower legs being positioned in a pattern so as to delineate a central core of formation;

an elongate electrode assembly located in each lower leg of the unit, said electrode assemblies each having at least one conductive electrode electrically coupled to the formation; and

means for supplying multi-phase alternating current power through the wellbores from a source at ground surface to the electrodes of the electrode assemblies so that different phases of power may be simultaneously applied to the electrodes;

whereby the core may be heated by electric current passing through the core between electrodes.

2. The system as set forth in claim 1 wherein the unit consists of three electrode wells whose lower legs are arranged in a triangular pattern.

3. The system as set forth in claim 2 wherein each upper leg is cased with electrically insulated casing and each lower leg combines with its contained electrode assembly to form an annulus; and further comprising:

an annular column of electrically conductive fill in each annulus for enhancing transmission of electric current between the electrode(s) and surrounding formation.

4. The system as set forth in claim 2 wherein each electrode assembly comprises:

a plurality of electrically conductive electrodes separated and connected by electrically insulating connectors.

5. The system as set forth in claim 3 wherein each electrode assembly comprises:

a pipe having electrically conductive electrodes separated and connected by electrically insulating connectors; and

a tubular member positioned within the pipe and containing conductor cable means connected with the power source, extending through the casing and connected to the electrodes.

6. The system as set forth in claim 2 comprising:

means, extending horizontally through the formation, for producing heated oil up through the casing.

7. A method for electrically heating a portion of a subterranean earth formation from ground surface, comprising:

forming a unit of a plurality of substantially parallel and equally spaced apart electrode wells, each well comprising a wellbore having an upper leg extending downwardly from ground surface and a lower leg further extending horizontally in the earth formation, said horizontal lower legs being positioned in a pattern so as to delineate a central core of formation;

providing an elongate electrode assembly in each lower leg, said electrode assemblies each having at least one conductive electrode electrically coupled to the formation; and

supplying multi-phase alternating current power to the electrodes of the electrode assemblies so that different phases of power are simultaneously applied to the electrode wells;

whereby the core may be heated by electric current passing through the core between electrodes.

8. The method as set forth in claim 7 wherein the unit consists of three electrode wells whose lower legs are arranged in a triangular pattern.

9. The method as set forth in claim 8 comprising multiple units arranged in side by side arrays.

10. The method as set forth in claim 8 wherein each upper leg is cased with electrically insulated casing and each lower leg is uncased and combines with its contained electrode assembly to form an annulus; and further comprising:

providing an annular column of electrically conductive fill in each annulus for enhancing transmission of electric current between electrode and surrounding formation.