Method Of Producing Bitumen From A Subterranean Tar Sand Formation

Abstract

A method of recovering bitumen from a subterranean tar sand formation characterized by a plurality of steps. First, a plurality of wells comprising at least one injection well and at least one production well are drilled from the surface and completed in the tar sand formation. Next, the bitumen and the tar sand formation are heated between the respective injection and production wells sufficiently to render the bitumen feasibly mobile in the tar sand formation. The heating is effected by passing a predetermined electrical current between the wells for a predetermined time. Thereafter, a fluid is injected into the injection well and the bitumen is produced from the production well. The fluid is preferably hot, for example an aqueous fluid such as steam or hot water. Also disclosed are specific details of the respective operational steps, including determining the predetermined time interval of electrical heating; and the use of alternate and/or simultaneous steps of heating with electrical energy and injection and production.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of producing bitumen from a subterranean tar sand formation in which the bitumen is in a nonflowable, highly viscous state. More particularly, this invention relates to a method of recovering the bitumen from the subterranean tar sand formation by thermal means for increasing the mobility of the bitumen in the tar sand formation and, thereafter, driving the mobilized bitumen to one or more production wells.

2. Description of the Prior Art

Large deposits of bitumen in surface tar sands, and subterranean tar sand formations have long been known to exist in several nations of the world. These tar sands are discussed in detail in Kirk-Othmer ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Second Edition, Anthony Standen, Editor, Interscience Publishers, New York, 1969, Vol. 19, Pages 682-732. That discussion points out that the bitumen in the tar sand formations has been carelessly referred to by a variety of names; such as, "tar," "hydrocarbons" and "crude oil;" but that this is a misnomer, since the bitumen contains nitrogenous compounds, as well as other constituents not usually found in the named products. In fact, that discussion goes on to emphasize that the recovery of "tar" from the tar sand has proved a fertile field for inventors, since almost all of the methods of recovering conventional hydrocarbons are inoperable in the tar sands because of the problem of achieving mobility of the highly viscous bitumen. All of the bitumen has a viscosity greater than at least 5,000 centipoises and the majority of the bitumen has a viscosity in the range of 500,000 -5,000,000 centipoises at 50.degree. Fahrenheit (.degree.F). The bitumen has a density greater than water at 60.degree.F, with a specific gravity equivalent to about 6.degree.-10.degree. API.

The bitumens are so different from crude oil that, not only are the production problems different, but also the upgrading and refining of the bitumen after being produced presents problems that are unique. For example, the bitumen has to be upgraded by partial coking, by delayed coking with catalytic hydrodesulfurization of the coke or distillate, or by direct catalytic hydrovisbreaking to be salable.

The geology of the subterranean tar sand formations is described in detail at page 688 of the above referenced Kirk-Othmer Encyclopedia. This discussion indicates that about only 10 percent of the known bitumen is recoverable by conventional mining techniques. The subterranean tar sand formation comprises about 99 percent quartz sand and clays. The sand particles are coated with a connate water envelope. The bitumen exists in the interstices intermediate the water enveloped sand grains. Ordinarily, the tar sand formation 15 is underlaid and overlaid by impermeable shales having different physical properties.

A large number of different techniques have been tried in attempting to feasibly recover the bitumen from the tar sand reservoirs. A large number of these earlier attempts and patent references and the like are catalogued in a comprehesive bibliography entitled "Preliminary Report 65--3, Athabasca Oil Sands Bibliography (1789-1964)," M. A. Carrigy, comp., Research Council of Alberta, Alberta, Canada 1965. The large number of recovery processes have included a variety of flooding methods, such as fire floods; exotic recovery schemes, such as emulsion steam drives; and even atomic explosions and the like. Despite the large number of processes tried, the only commercial processes are those employing surface mining. Yet, surface mining of tar sand having a 10 percent saturation requires handling about 2 tons of tar sand per barrel of bitumen recovered. In the commercial processes employing surface mining, the bitumen is recovered by steam or hot water extraction and upgraded by processes comprising: (1) thermal cracking and hydrotreating or (2) coking and hydrotreating. Since the price of crude oil has been driven upwardly by the energy storage in some of the more industralized nations, such as the United States, there are pilot operations being conducted at present to see if some recovery scheme effecting in situ separation of the bitumen from the tar sand can be made economically feasible. The pilot operations have employed a wide variety of different techniques to try to pre-heat the bitumen to attain a mobility sufficient to produce it from the tar sand formation. For example, it has been known to inject hot water or steam to heat soak around a given well and thereafter try to produce the heated and "molten" bitumen, or bitumen of reduced viscosity, from the same well.

To date, however, insofar as I am aware, none of the processes have been provided economically feasible. Thus, it can be seen that the processes that have achieved success in the recovering of the bitumen from the tar sand formations are vastly different from those employed in recovering conventional crude oil; and none of the conventional oil recovery methods or methods attempting separation of the bitumen from the tar sand in situ has proved commercially successful.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a method of recovering bitumen from a subterranean tar sand formation by separating the bitumen from the sand in situ, without requiring handling of the large mass of tar sand.

It is another object of this invention to provide a method of recovering bitumen from a subterranean tar sand formation by thermally mobilizing the bitumen within the tar sand formation such that it can be separated from the tar sand formation in situ; and then driving the mobilized bitumen toward one or more production wells.

These and other objects will become apparent from the descriptive matter hereinafter, particularly when taken in conjunction with the drawings.

In accordance with this invention, bitumen is produced from a subterranean tar sand formation by the following multi-step process. First, a plurality of wells are drilled from the surface into and completed in the tar sand formation in a predetermined pattern, including at least one injection well and at least one production well. Next, the bitumen and the tar sand formation are pre-heated, to mobilize the bitumen in the tar sand formation, by heating with a predetermined electrical current for a predetermined time interval. Thereafter, a drive fluid is injected into the injection well. The fluid can be at an ambient earth surface temperature or heated above ambient but in any case is preferably immiscible with bitumen and, ordinarily, will comprise steam or hot water or a mixture thereof. An ambient earth surface temperature fluid can be liquid water pumped from an unheated lake or other water source on the earth's surface directly to an injection well. Alternate slugs of heated fluid and ambient earth surface temperature fluid can be injected. The bitumen is produced from the production well.

The steps of pre-heating with the electrical energy and injecting of the fluid may be employed once only, simultaneously, or alternately as desired to attain the desired throughput rate and production rate.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is a profile of averaged temperatures through the formation following the step of preheating with electrical energy, in accordance with FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a plurality of wells 11 and 13 have been drilled into and completed within the subterranean tar sand formation 15. Each of the wells 11 and 13 have been completed so they may be operated as either injection or production wells. Specifically, the wells have a string of casing 17 that is inserted in the drilled bore hole and cemented in place with the usual foot 19. A perforate conduit 21 extends into the subterranean tar sand formation 15 adjacent the periphery of the wellbore that was drilled thereinto. Preferably, the perforate conduit 21 includes a lower electrically insulated conduit for constraining the electrically current flow to the subterranean tar sand formation as much as practical. The perforate conduit 21 may be casing having the same or a different diameter from casing 19, or it may be large diameter tubing inserted through the casing 19. As illustrated, the perforate conduit 21 comprises a separate string of conduit extended from the surface for better preserving the heat content of an injected hot fluid.

Each of the wells 11 and 13 has an electrode 23. The respective electrodes 23 are connected via electrical conductors 25 and 27 with surface equipment 28 and a source of electrical current, illustrated as alternating current (A.C.) source 29. The electrical conductors 25 and 27 are insulated between the electrodes 23 and the surface equipment. The surface equipment 28 includes suitable controls that are employed to effect the predetermined current flow. For example, a switch (SW) 31 and voltage control means, such as rheostat 33, are illustrated for controlling the duration and magnitude of the current flow between the electrodes 23 in the wells 11 and 13 by way of the subterranean tar sand formation 15. It is preferred that the alternating current source 29 be adjusted to provide the correct voltage for effecting the currentl flow through the subterranean tar sand formation 15 without requiring much power loss in surface control equipment, exemplified by rheostat 33. The respective electrical conductors 25 and 27 are emplaced in their respective wells 11 and 13 with conventional means. As illustrated, they are run through lubricators 35 in order to allow alternate or simultaneous heating, and injection and production; without having to alter the surface accessories; such as, changing the configuration of the well head 37, with its valves and the like.

As illustrated, the well 11 is connected with a hot fluid injection system by way of suitable insulated surface conduit 39. The illustrated hot fluid injection system comprises a boiler system 41 for injecting steam of slightly less than 100 percent quality. More specifically, the boiler system 41 comprises one or more field boilers with feed pumps and inlet water surge tanks. A minimal water treating facility may be employed if necessary for a particular water source. The boiler system 41 is constructed and operated in accordance with conventional engineering technology that does not, per se, form part of this invention and is well known and is not described in detail herein. The conventional boiler system technology is contained in a number of printed publications which are incorporated herein by reference for details.

The perforate conduit 21 in well 13 is connected to surface production facilities by way of a second surface conduit 45. The production facilities are those normally employed for handling normally viscous crude oils and are not shown, since they are well known in the art. The production facilities include such conventional apparatus as heater treaters, separators, and heated storage tanks, as well as the requisite pumping and flow facilities for handling the bitumen. The production facilities also are connected with suitable conventional bitumen processing facilities (also not shown); such as are employed in the conventional processing of the bitumen after it is recovered from the tar sand formation by surface mining techniques, or otherwise. Since these production and processing facilities do not, per se, form a part of this invention, they are not described in detail herein.

Operation: In operation, the wells 11 and 13 are completed in the tar sand formation 15 in accordance with conventional technology. Specifically, bore holes are drilled, at the desired distance and patterning, from the surface into the subterranean tar sand formation 15. Thereafter, the casing 17 is set into the formation to the desired depth. As illustrated, the casing 17 may comprise a surface string that is cemented into place immediately above the tar sand formation. Thereafter, a second string of casing, including an insulated perforate conduit 21 is emplaced in the respective bore holes and completed in accordance with the desired construction. For example, a perforate conduit 21 may have its foot cemented in place, or it may be installed with a gravel pack or the like to allow for expansion and contraction and still secure the desired injectivity and productivity.

In any event, the electrodes are thereafter placed in respective wells. For example, the tar sand formation may be from 100 to 300 feet thick and the respective electrodes 23 may be from 50 to 100 feet or more in length. The electrodes 23 are continuously conductive along their length and are connected with the respective electrical conductors 25 and 27 by conventional techniques. For example, the electrodes 23 may be of copper based alloy and may be connected with copper based conductors 25 and 27 by suitable copper based electrical connectors. Thereafter, the alternating current source 29 is connected with the conductors 25 and 27 by way of the surface control equipment, illustrated simply as switch 31 and rheostat 33. If the desired current densities are obtainable without the use of the rheostat, it is set on the zero resistance position to obtain the desired current flow between the wells. Since there will be a high current density immediately adjacent each of the electrodes 23, the temperature will tend to increase more rapidly in this area. Accordingly, it may be desirable to periodically interrupt the flow of current and inject a small amount of electrolyte around each of the wells in order to keep the conductivity high in this region. The current flow through the tar sand formation to heat the tar sand formation 15 and the bitumen therewithin depends on the connate water envelopes surrounding the sand grains. Accordingly, the temperature in the regions of highest current densities; for example, in the regions immediately about and adjoining the wells; must not be so high as to cause evaporation of the water envelopes at the pressure that is sustainable by the over burden. Expressed otherwise, the predetermined electrical current is maintained low enough to prevent drying of the tar sand formation 15 around the wells 11 and 13.

The electrical current will flow primarily through the tar sand formation, although some of the electrical energy will flow through the bitumen-impermeable shales, as illustrated in the dashed lines 47. The voltage and current flow are adjusted to effect the desired gradual increase in temperature of the tar sand formation 15 and the bitumen therein without over heating locally at the points of greatest current density, as indicated hereinbefore. For example, the current may run from a few hundred to 1,000 or more amperes at the voltage drop between the electrodes 23 in the wells 11 and 13. This voltage drop may run from a few hundred volts to as much as 1,000 or more volts.

In any event, the pre-heating is continued for a predetermined period of time. For example, the predetermined period of time may be between 2 and 4 years. There will be temperature variations throughout the formation. Even in averaged temperatures, such as illustrated in FIG. 2, there is variance because of the distance between wells and the differences in current densities. The larger cross sectional areas near the midplane between the wells have less current density and, hence, less temperature increase. Also, there are variations because of the heterogeneities in the tar sand formation 15. In any given tar sand formation 15, the period of time may be determined empirically, if desired, to check the theoretical, or projected, calculations of the temperature. The empirical determinations resulting from a given test pattern can then be extrapolated to larger production patterns in accordance with conventional technology. Since the tar sand formation and the bitumen therewithin do not behave in conventional manner, the empirical approach is preferred over initiating a commercial venture without a pilot and test pattern in a given tar sand formation. The empirical testing may take the form of testing mobility by the ultimate test of a "throughput" rate that is sustainable between the injection and production wells. The "throughput" is evidenced by the injection rate and the production rate both remaining above an equivalent rate in barrels per day. On the other hand, the empirical approach can be employed directly on a commercial scale venture with assurance of ultimate success because of the flexibility of operation inherent in the various embodiments of this invention.

In any event, the theoretical calculations can be made using conventional computer-based temperature formulae for heating subterranean formations containing the bitumen. As the bitumen is heated, it begins to have a greater mobility in the tar sand formation 15. Once the yield point of the bitumen is reached in the tar sand formation, or at least the temperature at which plastic flow begins at the pressure that can be imposed at the injection well, as described hereinafter, mobility begins to make feasible in situ separation. The mobilities or high viscosities of bitumen in the least heated part of the tar sand formation limits a commercially feasible process. As illustrated in FIG. 2, the temperature profile across the formation may be at some hot temperature T.sub.6 adjacent the wells but will be at a relatively significantly cooler temperature T.sub.1 at the midpoint between the wells. Starting the injection of the hot fluid initiates a complex series of events. These events are affected by the temperatures that have been effected in the tar sand formation and determine what overall throughput rate can be sustained between the injection and production wells. Greatly simplified, the events are about as follows. As the steam is injected at its injection pressure, it will lose heat to the formation and condense. The losing of the heat will tend to increase the mobility of the bitumen, but the condensation of the steam to a liquid will tend to decrease the overall mobility. Moreover, the different temperature bitumen moving into different temperature formation creates complex and interrelated pressure, temperature, mobility relationships that are difficult to predict absolutely. The hotter bitumen from near the injection well; for example, at D.sub.1 ; is forced into the colder formation. It heats up that formation, but the heated bitumen loses heat and becomes less mobile. The less heated bitumen from the colder part of the formation; as from D.sub.2 ; moves, in turn, into the formerly heated tar sand formation near the production well. Consequently, there will be a decrease in the mobility of the bitumen near the production well at D.sub.3 as the bitumen starts to flow toward the production well.

The complicated and interrelated events that determine throughput; and, hence, productivity; require that the predetermined time interval for electrical heating must be long enough to get an overall mobility of the bitumen that is high enough to sustain a minimum flow through from the one or more injection wells to the one or more production wells. If the predicted temperature at the midpoint D.sub.2 intermediate the two wells is inadequate to sustain the minimum flow through, the electrical heating will have to be continued, with or without the simultaneous injection of steam. Thus, empirically, once injection is started, if mobility of the bitumen will sustain a minimum throughput of at least 10 barrels per day, the steam injection is economically preferable over the electrical heating and the injection of steam is continued without the electrical heating.

It is preferable to employ a more scientific approach to empirically verifying the degree of heating of the tar sand formation. The preferable approach is to drill a small bore hole from the surface or the earth into the tar sand formation midway between the injection and production wells; for example, at D.sub.2 and measure a temperature profile vertically through the tar sand formation 15 at D.sub.2. Once the temperature in this area has attained the minimum temperature needed; for example, T.sub.1 ; the electrical heating is discontinued and injection of the steam is started. This observed temperature profile then verifies the theoretical calculations; or indicates the nature and degree of erroneous assumptions. This information is then helpful in determining the correct predetermined time interval over which the electrical heating is carried out before the injection of the drive fluid is started.

In any event, injection of the drive fluid is started after the predetermined time interval of electrical heating. The fluid is injected at a pressure below that which is sufficient to lift the overburden, ordinarily referred to as "fracturing pressure."

The fracturing pressure not only limits the injection pressure; but, as indicated hereinbefore, also limits the pressure and temperature for maintaining the water envelopes on the sand grains for conductivity. It is recognized that the pressure that will effect fracturing with a given overburden depth may be determined in accordance with conventional petroleum engineering technology. A safe and over simplified figure may be taken as one-half pound per square inch (psi) for each foot of overburden depth. Thus, an overburden depth of 1,200 feet will safely sustain an injection pressure, or a pressure necessary to retain saturation around a well, of 600 psi. Ordinarily, a somewhat higher pressure may be employed once the geology of a given overburden site is properly investigated. For example, if there is 1,000 feet of overburden on the tar sand formation 15, the injection pressure will probably not exceed 500 pounds per square inch. In order to have retained a saturated condition and prevent drying out of the tar sand formation immediately adjacent the well because of the high current densities and the high temperature, the temperature adjacent the well must not have been allowed to exceed the saturated steam temperature; for example, about 467.degree.F in the exemplified embodiment.

The bitumen is produced from the production well 13 by conventional techniques. For example, if it has been rendered mobile enough to flow readily, the injection pressure will be sufficient to cause production of the heated bitumen out of the production well 13 without requiring pumping facilities. On the other hand, with shallow overburdens, it may be economically feasible to install pumping equipment for pumping the bitumen from the production well 13. As illustrated, the injection pressure is employed to effect flow of the hot bitumen from the production well 13 and to the production facilities through surface conduit 45.

The injectivity and production continues until breakthrough occurs. Breakthrough is defined as the time at which the injected fluid has established a flow path completely between the injection well and the production well. At some point after breakthrough, the amount of drive fluid being flowed through the formation will become economically infeasible as compared with the volume of bitumen being produced. When the flowthrough of the injected fluid becomes economically infeasible compared with the level of production of the bitumen, the patterning of the wells is shifted. For example, the swept out portion of the tar sand formation 15 may be employed as an enlarged effective production area and the two wells 11 and 13 both employed as production wells while a different injection well is employed for effecting flow of the bitumen to the enlarged effective production area.

Thus, by proper patterning and employing the initial pre-heating by use of electrical energy to mobilize the bitumen in the tar sand formation 15, the bitumen in a predetermined pattern can be separated from the sand grains in situ and the bitumen produced to the surface without requiring the handling of the large quantities of sand, as in surface mining technology. The surface mining technology is, as indicated, infeasible for most of subterranean tar sand formations of appreciable depths.

Other Embodiments: As implied hereinbefore, if the desired throughput rate, as indicated by the obtainable injectivity and productivity, is too low; the injection of the steam through injection well 11 is stopped and the electrical heating resumed. In the event that this course is followed, it may be desirable to inject an electrolyte with the latter portions of the steam to again increase the conductivity around the injection well 11. An electrolyte in aqueous solutions, per se, can be injected into the injection and production wells for increased conductivity if desired. Suitable electrolytes include sodium chloride as the most economically feasible, although other beneficial electrolytes may be employed. Such electrolytes include caustics like sodium hydroxide, sodium carbonate, or the like for improving injectivity around the injection well 11. In any event, the electrical heating is continued for another predetermined time interval to obtain a predicted temperature increase and increase in the mobility of the bitumen in the tar sand formation 15. Thereafter, steam is again injected and the bitumen is produced from the production well 13.

If the desired throughput rate is established the second time, injection is continued as described hereinbefore. If, on the other hand, the desired throughput rate is not attainable, injection of the steam is stopped and electrical heating again carried out for a predetermined time interval. In any event, ultimately, the desired throughput rate, with the desired injectivity and productivity, will be attained and the bitumen from the pattern will be produced as described hereinbefore.

On the other hand, it may be possible to inject the steam simultaneously with a sufficient amount of electrolyte to allow concurrent heating with the electrical energy. It is realized that there may be some hazard if the heating and the injection of steam plus an electrolyte is carried out concurrently. The hazard of electrical shock is not insurmountable, however. Careful insulation and operation can prevent hazard to operating personnel and allow concurrent and simultaneous electrical heating and steam flooding to attain the desired throughput rate.

EXAMPLE

The following example is given to demonstrate a typical process carried out as described hereinbefore with respect to FIGS. 1 and 2. The exemplified tar sand formation had an averaged thickness of 100 feet with an overburden of 1,000 feet in the pattern area. The tar sand formation had an averaged permeability of 700 millidarcies with the overburden and underburden being impermeable shales. The tar sand formation had an averaged porosity of 0.33 with an initial bitumen saturation of 12 percent by weight, when averaged. The averaged electrical resistivity, in ohm-meters at 50.degree.F, were, respectively:

Tar Sand Overburden Underburden ______________________________________ horizontally 30 10 50 vertically 90 ______________________________________

The geological formations adjacent the tar sand, as well as the tar sand formation 15, had an initial temperature of 50.degree.F, an averaged thermal conductivity, in British Thermal Units per foot per hour per .degree.F (BTU/ft-hr-.degree.F) of 0.6; and an averaged thermal volumetric heat capacity in BTU per cubic foot per .degree.F (BTU/ft.sup.3 -.degree.F) of 44.

The exemplified pattern was a 5-spot over 10 acres. The electrical pre-heating time required was three years and the average electrical power input level for the pattern was 3,100 kilowatts. Thus, the total electrical input per pattern 82 .times. 10.sup.6 kilowatt hours (kwh).

Referring to FIG. 2, the temperature adjacent the respective injection and production wells, equivalent to wells 11 and 13 in FIG. 1, was as follows:

T.sub.6 adjacent the respective wells, 466.degree.F. The temperatures T.sub.2 -T.sub.5 represent equal spacing on the ordinate and are 200, 300 and 400, respectively. The minimum temperature T.sub.1 at the distance D.sub.2 between the injection and production wells was 160.degree.F to attain the desired mobility of the bitumen before beginning injection of the steam.

The bitumen had measured viscosities at different temperatures as follows:

50.degree.F - 2 .times. 10.sup.6 centipoises (cp)

160.degree.F - 1,500 cp

466.degree.F - 5.4 cp

The total pattern productivity after the preheating period was 190 barrels per day. The productivity and injectivity increased to attain a peak pattern productivity of 510 barrels per day. The bitumen produced during the steam drive was 657,000 barrels and required a total water injection, as steam, over the six year period of 1.16 .times. 10.sup.6 barrels.

General: While the injection of steam has been described hereinbefore, any other fluid that will have the desirable flooding characteristics and convey the heat to the tar sand formation 15 may be employed. As a practical matter, steam or hot water will be the fluids most commonly available in the field and have greatest feasibility because of their economy. Moreover, the hot aqueous fluids have a greater macroscopic sweep efficiency for conveying heat to a greater overall portion of the tar sand formation. Hot fluids that are miscible with the bitumen in the tar sand formation are not employed if banking that comes with such miscible fluids requires intolerably high differential pressures to effect flow to the one or more production wells. Ordinarily, also, the miscible fluids have a lower heat capacity and are not as readily available as are the aqueous fluids. Hence, even though the miscible fluids will effect substantially 100 percent recovery on a microscopic sweep efficiency basis in the areas where they flood, they are, ordinarily, less feasible in recovering the bitumen from the tar sand formation.

From the foregoing descriptive matter, it can be seen that this invention provides a novel and unobvious way of producing bitumen from a tar sand formation that is feasible. This invention overcomes the disadvantages of the prior art processes which have been demonstrated to be inapplicable in recovering bitumen from the tar sand formations heretofore.

Having thus described the invention, it will be understood that such description has been given by way of illustration and example and not by way of limitation, reference for the latter purpose being had to the appended claims.

Claims

What is claimed is:

1. A method of producing bitumen from a subterranean tar sand formation containing viscous bitumen comprising the steps of:

a. drilling and completing a plurality of at least two wells extending from the surface into said tar sand formation for production of said bitumen therefrom; at least one said well being completed as an injection well and at least one said well being completed as a production well; said wells being spaced apart and arranged in a predetermined pattern;

b. pre-heating said tar sand formation and sand bitumen intermediate said wells sufficiently to render said bitumen mobile within said tar sand formation; said pre-heating being effected by passing a predetermined electrical current from one of said wells to the other of said wells for a predetermined time interval;

c. thereafter injecting a fluid that is immiscible with said bitumen through said injection well and into said tar sand formation; and

d. producing said bitumen from said production well.

2. The method of claim 1 wherein said predetermined time interval is sufficient to heat said formation and said bitumen to a temperature that will allow a throughput rate of at least 10 barrels per day that is sustained by injection of said fluid in the one or more injection wells adjacent said production well.

3. The method of claim 1 wherein said predetermined electrical current is maintained low enough to prevent drying said tar sand formation around said wells and said tar sand formation around said wells is maintained electrically conductive.

4. The method of claim 1 wherein said fluid is steam.

5. The method of claim 1 wherein said fluid is hot water.

6. The method of claim 1 wherein said fluid comprises a mixture of steam and hot water.

7. The method of claim 1 wherein said fluid is injected for a time interval and stopped; and electrical heating is again begun and continued for a predetermined time interval; and said injection of said fluid is again commenced; and wherein the steps of said heating and of said injection of said fluid are effected alternately until a desired rate of flow through said formation is effected.

8. The method of claim 1 wherein said electrical pre-heating is continued after said fluid is begun to be injected such that said steps of electrical pre-heating and of injecting of said fluid are carried out simultaneously until a desired throughput rate is effected.

9. The method of claim 1 wherein said plurality of wells are completed so as to be operable as either injection or production wells and wherein the injection and production patterning is altered after predetermined production intervals to effect a more nearly complete sweep of said bitumen from said tar sand formation.

10. The method of claim 1 wherein said fluid is liquid water at ambient earth surface temperature.

11. The method of claim 1 wherein electrolyte is employed in at least one well to improve the electrical conductivity in and around said well.

12. The method of claim 11 wherein electrolyte is employed in at least one injection well and in at least one production well.

13. The method of claim 1 wherein electrolyte is injected into said tar sand formation for increased electrical conductivity between injection and production wells.

14. The method of claim 7 wherein electrolyte is injected into said tar sand formation for increased electrical conductivity during said alternate electrical heating steps.

15. The method of claim 8 wherein electrolyte is injected into said tar sand formation for increased electrical conductivity during said simultaneous steps of electrical pre-heating and injecting of fluid.

16. The method of claim 1 wherein alternate slugs of hot fluid and ambient earth surface temperature fluid are injected into said tar sand formation.