US3605888A

US3605888A - Method and apparatus for secondary recovery of oil

Info

Publication number: US3605888A
Application number: US870006A
Authority: US
Inventors: Fred L Crowson; William G Gill
Original assignee: Electrothermic Co
Current assignee: Electrothermic Co
Priority date: 1969-10-21
Filing date: 1969-10-21
Publication date: 1971-09-20
Anticipated expiration: 1988-09-20

Abstract

THERE IS DISCLOSED IN THE SPECIFICATION AND DRAWINGS A METHOD FOR SECONDARY RECOVERY OF OIL IN WHICH ELECTRICAL CURRENT IS CAUSED TO FLOW THROUGH WATER IN THE BOTTOM OF A BORE HOLE TO PRODUCE HEATING OF THE WATER. SUFFICIENT PRESSURE IS MAINTAINED ON THE WATER AS THE WATER IS HEATED TO PREVENT BOILING OF THE WATER AT TEMPERATURES SUBSTANTIALLY IN EXCESS OF THE BOILING POINT OF WATER AT THE SURFACE OF THE BORE HOLE. IN ACCORDANCE WITH ONE EMBODIMENT OF THE INVENTION, WATER IS INJECTED INTO THE STRATA TO PRODUCE A WATER FLOOD. THE WATER WHICH IS INJECTED IS HEATED BY THE FLOW OF ELECTRICAL CURRENT SUCH THAT A HOT WATER FLOOD IS PROVIDED. IN ACCORDANCE WITH THE SECOND EMBODIMENT OF THE INVENTION THE PRESSURE ON THE WATER IS PERIODICALLY RELIEVED WHEN THE WATER IS HEATED TO A TEMPERATURE AT LEAST AS GREAT AS THE BOILING POINT OF THE WATER AT THE SURFACE TO PERMIT CONVERSION OF THE HEATED WATER INTO STEAM. IN ACCORDANCE WITH THE THIRD EMBODIMENT OF THE INVENTION, THE WATER IS CONTAINED WITHIN A RESERVOIR UNTIL THE TEMPERATURE OF TEH WATER IS SUFFICIENTLY HIGH TO PRODUCE STEAM AT THE PRESSURE PRESENT IN THE OIL BEARING STRATA THE WATER AND STEAM IS THEN RELEASED FROM THE RESERVOIR INTO THE STRATA. THE WATER WHICH IS RELEASED IS CONVERTED INTO STEAM. THERE IS ALSO DISCLOSED APPARATUS FOR PRACTIVING THE METHOD OF THE INVENTION.

Description

Sept. 20, 1971 F. CROWSON ETAL 3,605,888

METHOD AND APPARATUS FOR SECONDARY RECOVERY OIL Original Filed Sept. 30, 1968 4 Sheets-Sheet 1 Ill" 20 2e i a J 5E 24 36 Q3 g 3 45 26 E 5 22 38 .r s a W F i i i v z {a INVENTOR 611d 0C. Cmwson wldi'am 0.61%

BY (TTORNEY Sept. 20, 1971 CRQWSQN EIAL 3,605,888

METHOD AND APPARATUS FOR SECONDARY RECOVERY OIL Original Filed Sept.

4 Sheets-Sheet 5 R Y E Q A H A P 2 w u w m. 4 W m s l I M@HA/II///./// 1 4w .5 U M M H2 14 3 0 F m W P 0 w A 70 WATER SUPPLY Elm & INVENTOR Jud'fi. awm

BY Z'ITORNEY Sept. 20, 1971 CROWsON EI'AL 3,605,888

METHOD AND APPARATUS FOR SECONDARY RECOVERY OIL Original Filed Sept. 30, 1968 4 Sheets-Sheet 4 AC VOZJZIGE 70 OTHER WELL J \IIIIIIIIIIIIIIIHI' Ill INV Em OR WA. Crowun o ATTORNEY United States Patent 01 fice U.S. Cl. 166-248 21 Claims ABSTRACT OF THE DISCLOSURE There is disclosed in the specification and drawings a method for secondary recovery of oil in which electrical current is caused to flow through Water in the bottom of a bore hole to produce heating of the water. Sufiicient pressure is maintained on the water as the water is heated to prevent boiling of the water at temperatures substantially in excess of the boiling point of water at the surface of the bore hole. In accordance with one embodiment of the invention, water is injected into the strata to produce a water flood. The Water which is injected is heated by the flow of electrical current such that a hot water flood is provided. In accordance with the second embodiment of the invention, the pressure on the water is periodically relieved when the water is heated to a temperature at least as great as the boiling point of the water at the surface to permit conversion of the heated water into steam. In accordance with the third embodiment of the invention, the water is contained within a reservoir until the temperature of the water is sufliciently high to produce steam at the pressure present in the oil bearing strata the water and steam is then released from the reservoir into the strata. The water which is released is converted into steam. There is also disclosed apparatus for practicing the method of the invention.

CROSS REFERENCE This application is a division of co-pending application Ser. No. 767,917, filed Sept. 30, 1968, now US Pat. No. 3,507,330 which is a continuation-in-part of co-pending application Ser. No. 677,836, filed Oct. 16, 1967, now abandoned which is a continuation-in-part of application Ser. No. 449,077, filed Apr. 19, 1965, also now abandoned.

During the early years of the oil industry, it was relatively easy to find new reserves and as soon as the amount of oil being pumped from a well fell below that required to maintain the operation of the well profitable, the well would be shut down and abandoned. At the present time, however, the major share of existing onshore oil in the United States has probably been discovered. The conclusion is inescapable that the future of the domestic oil industry is closely tied to the eflicient, economic production of known petroleum reserves.

It has been estimated that at least 50% of the known petroleum reserves of the United States cannot be recovered using conventional pumping methods. Accordingly, a substantial amount of effort of the petroleum industry will fall under the general category of secondary recovery. Thus, a system of secondary recovery known as water flooding has attracted a substantial amount of attention in recent years and several flood projects have 3,605,888 Patented Sept. 20, 1971 therewith, the pumping of water into the formations tend to cause the oil to flow ahead of the water toward wells which are pumped to recover the oil.

Very recently the attention of petroleum engineers has been attracted to secondary recovery techniques referred to by the general term thermal flooding. While conventional water flood techniques are aimed at stimulating existing production by increasing the natural reservoir pressure hydraulically, thermal flooding, as described, is a technique whose aim is the alteration of certain physical properties of the reservoir by means of the addition of heat. Reservoir pressure may or may not be increased in the process.

The three general methods of application of these recovery techniques are in situ burning (fire flood), steam injection, and hot water flood. In the first of these, controlling burning of some of the petroleum in place generates heat that lowers the viscosity of the remaining oil and pressure that improves reservoir drive. Steam injection is aimed at utilizing the heat of vaporization of water to lower the viscosity of the oil for a limited distance around the well bore. Generally, steam injection does not materially change the effective reservoir pressure. In the third method, the effect of normal water flood on a high viscosity reservoir is enhanced by preheating the flood water. All of these methods require extensive and quite expensive surface installations for their implementation.

The present invention provides an improved method of secondary recovery in which electrical current is used for heating of water in the formation. In accordance with one embodiment of the invention, substantially the same effects are obtained as in a hot water flood. In accordance with the second embodiment of the invention, substantially the same results are obtained as in a steam injection system. One great advantage of the present invention is that although the cost of energy is generally higher than conventional methods, the installation cost is much le's'si In general, the method of the present invention consists of converting electrical power into heat by utilizing water in the formation as a resistance heating element. The gen erally high cost of electricity as a source of energy is overcome by the increased thermodynamic eificiency of the method due to reduced heat loss, the accurate manner in which the heat input may be controlled and by the vastly reduced investment requirements of the initial installation. The method of the present invention can be used at greater depths than normal hot water flood or steam injection systems due to a substantially increased thermodynamic etficiencv. Also. the small installation costs involved make it feasible to use the present method in small investigative operations for obtaining recovery data on which to plan lar e'scale conventional projects.

Many objects and advantages of the invention will become obvious to those skilled in the art as a detailed description of preferred embodiments of the invention unfold with reference to the drawings in which:

FIG. 1 is a view diagrammatically illustrating a well bore penetrating the formation adapted for producing steam in the formation in accordance with one embodiment of the present invention;

FIG. 2 schematically illustrates the path for flow of current in accordance with the one embodiment of the invention;

FIG. 3 is a view diagrammatically illustrating two spaced apart well bores penetrating an oil bearing formation and adapted for practicing a second embodiment of the invention;

FIG. 4 is a diagrammatic view of a well bore penetrating an oil bearing formation and especially equipped for practicing the second embodiment of the present invention;

FIG. 5 is a schematic diagram illustrating electrical circuitry suitable for controlling the injection of makeup water into a well;

FIG. 6 schematically illustrates the manner in which the principles of the present invention can be applied to produce a steam flood of a conventionally arranged oil field; and

FIG. 7 schematically illustrates an arrangement for treating fields in which oil water emulsions are present.

Among the basic parameters determining petroleum reservoir performance, three primary forces must be considered. These are the forces of capillary action, the forces of gravity, and the forcesof viscous resistance. The action of these basic forces determine certain important reservoir factors. These factors are the original distribution of fluids within the reservoir, the simultaneous movement of fluids through the reservoir and the displacement of one fluid by another within the reservoir. The forces themselves and phenomena resulting from their interplay are dependent upon a number of reservoir fluid parameters including formation viscosity and permeability, viscosity of the fluid, pressure differentials within the reservoir, composition of the fluid, the temperatures of the reservoir, and a number of variables connected with the rock matrix itself.

The action of the force of gravity in reservoir mechanics is somewhat obvious. Its operation is apparent in the segregation of fluids within a reservoir and in down structure drainage.

The forces of viscous resistance are retarding forces against which work must be performed in producing fluid from a reservoir. They are active in direct proportion to the pressure differentials within the reservoir, and depend on reservoir type and the characteristics of the fluid in place.

The most subtle of the forces involved in reservoir mechanics are those involving capillary action. It is through these forces that connate water is retained within an oil zone in spite of the action of gravity tending to remove it to the bottom of the reservoir. It is also in large measure responsible for residual oil remaining within the reservoir pore space, in opposition to the producing differential. All three of the forces mentioned are of surface forces and act in direct proportion to the surface area involved in the reservoir production.

Two general conditions are required for a reservoir to be productive. First, a driving force between the reservoir and the well bore must be present. Energy for the driving force may be supplied naturally in the form of gas, either free or in solution, evolved under a reduction in pressure. The energy may involve a hydrostatic head of water behind the oil, or oil or water under compression. In cases where the natural energy sources are not sufficient to overcome the retarding forces within the reservoir, external energy must be added. Secondly, the produced fluid must be displaced by another fluid, either gas or water.

The type of driving energy available determines the nature of the displacing substance.

Reservoirs are ordinarily classified according to the type of reservoir energy that is available. The four types are solution gas drive reservoirs, gas expansion reservoirs, water driving reservoirs, and gravitational drainage reservoirs. A particular reservoir may, of course, involve more than one of these producing mechanisms.

In those cases where the natural energy of the reservoir is insuflicient to overcome the resistive forces such as the forces of viscous resistance and the forces of capillary action, external energy must be applied. The particular manner in which external energy is supplied will be dependentto a substantial degree upon the characteristics of the formation which the forces predominate.

Turning now to FIG. 1 of the drawings, there is shown a well bore 10 which penetrates the surface of the earth into an oil bearing formation 12. A string of casing 14 extends:from the surface into the formation 12. A string of tubing 16 is supported within the casing 14. The casing 4 14 and tubing 16 are perforated at 15 and 17 respectively to permit oil to enter the tubing from the formation. However, for reasons that will become apparent, the tubing is preferably not perforated below a packer 19, which may be provided in some instances.

A plurality of insulating members 18 encircle the tubing 16 at spaced apart locations for centering the tubing within the casing and electrically insulating the tubing from the casing. If it is desired to produce oil from the well, an insulating packer member 19 can suitably be positioned between the casing 14 and tubing at a level approximately the same as the interface 21 between the oil 23 and connate water 25 found within the formation 12.

The tubing 16 is suitably supported within the casing 14 by reducing flange 20 which comprises a first flange member 22 threadedly connected to the upper end of the casing 14 and a second flange member 24 threadedly connected to the tubing 16 near its upper end. Annular member 26 of insulating material separates flange member 22 from flange member 24. The two flange members can be held together if necessary either by utilizing bolts 28 of insulating material, suitably Teflon or Nylon, or passing the bolts 28 through tubes 30 of insulating material and providing insulating washers 32 beneath the head of the bolt and the nut of the bolt. Tubing 16 is terminated in a cap 34 to which a gage 35 is suitably mounted.

There is also provided a sealed tank 26 which is connected through flow control choke 38 and valve 40 to the casing 14. When valve 40 is open, the pressure within the tank 36 will become equal to the pressure within the annulus 42 between casing 14- and tubing 16. Thereafter water will flow from the tank 36 into the annulus 42 at a rate controlled by choke 38.

A conductor member 44 is electrically connected to tubing string 16 at point 46 and a second conductor member 48 is connected to the casing string at 50.

Conductors

44 and 48 are connected to a source 52 of alternating current supply voltage. The connate Water 25 normally present will pass through the perforations 15, filling the lowermost portions of the annulus 42. Since connate water normally contains several salts in solution, it is an eflective conductor of electricity. When a source 52 of alternating current supply voltage is connected between

conductors

44 and 48, current will flow through a series path comprising conductor 44, the resistance of the tubing, designated R,,, the resistance of the water 25, designated R the resistance of the casing, designated R and conductor 48, as shown in FIG. 2. The current flowing in this circuit can be expressed mathematically as:

and the power dissipated in the water, or utilized for heating the water to convert it to steam will, of course, be equal to 12R It will, therefore, be apparent that it is very desirable that the resistance of the Water providing a conductive path between the casing 12 and the tubing 14 have a high resistance as compared to the total series resistance of the casing and tubing, R +R In fact, to achieve this relationship in some instances it may be desirable to utilize tubing formed of aluminum or similar material characterized by a lower resistivity than steel, which is conventionally used for tubing. The current flowing can be controlled by varying the supply voltage potential or by varying the resistivity of the water.

As a result of the flow of current through the water, the water will become heated and converted into steam. As the water is converted into steam, the steam 'will expand and move into the oil bearing formation, heating the oil in front of the steam in a manner characteristic of steam floods presently used. However, in this instance, rather than generate the steam at the surface and pump it into the well, a portion of the annulus of the well and the oil bearing formation itself is utilized as a boiler for generating steam immediately adjacent the point at which it is to be used. Substantially improved results are achieved in that a substantial amount of the thermal energy available in a quantity of steam is not dissipated in the course of pumping the steam down the well bore. Steam of the maximum temperature is, accordingly, available for treatment of the formation.

As current continues to flow, all of the water 25 within the annulus 42 will be converted to steam, which is not an electrical conductor. The resistance shown schematically in FIG. 2 as R will, therefore, become extremely high and essentially an open circuit. Current will cease to flow through the electrical circuit and additional steam will not be generatedlt is, therefore, necessary to maintain a supply of water in contact with both the casing 14 and tubing 16. It will be noted that the well could be fitted with a conventional pump for purposes of producing oil or for removing undesired oil if the well is to be used only for producing steam.

In some instances wherein it is desired to produce oil from the well, best results can be obtained by setting the packer -19. Water flowing down annulus 42 from tank 36 will flow out perforations 15 above packer 19 and in perforations 15 below the packer 19. With such an arrangement, there will be less tendency for the steam to push the oil away from the well bore, but rather the steam will be generated below the oil-water interface 21 and ejected outward to heat the surrounding formation and leach the oil from the sands.

Turning now to FIG. 3 of the drawings, there is shown a second embodiment of the invention wherein current is caused to flow through oil bearing formation 12 between two spaced apart well bores 10a and 10b. In practicing this second embodiment of the invention, advantage is taken of the fact that formation 60 overlying the oil bearing formation 12 and formation 62 underlying the oil bearing formation 12 are normally relatively solid and do not contain fluids, while a substantial amount of connate water is normally present within the oil bearing foundation 12.

Current flows between the tubing 16:: and tubing 16b through a conductive path comprising the water 25 held in the formation 12, resulting in the generation of substantial amounts of heat. Virtually all of the heat is produced within a relatively small radius of each well since a short distance from the wells the conductor effectively becomes one of indefinite area and substantially zero resistance. However, the heat generated is conducted away from the wells and can result in a substantial reduction in viscosity of fluids to be produced. Also, for a reason not completely understood, the combination of high current flow and heat has been found: to be a highly effective emulsion breaker. In many fields this is very important.

It will be noted that the pair of wells 10a and 10b of FIG. 3 are each equipped substantially in the same manner as that described with reference to FIG. 1 of t-hedra'wings in order that the necessary supply of water can be maintained in contact with the tubing members- 16a and 16b. Moreover, it will not be necessary to insulate the tubing of either well from ground or casing if the ground is dry and no water bearing strata such as the formation 64 are present. However, in most instances, such water bearing formations are present, rending it necessary to insulate the tubing of one well, such as well a from the formation 64. This can be accomplished by coating the casing 14a with an insulating material, such as an epoxy resin, but generally this is not an acceptable solution due to damage which is frequently done to such coatings in the course of setting the casing. One preferred manner for insulating the tubing of one well from the formation 64 or other water bearing formation is shown in FIG. 3. Thus, in accordance with another embodiment of the invention, packer 64 is set in the annulus 42a between the formation 12 containing the oil to be produced and water bearing formation 64. The casing 14a is cut at 66 and oil 68 is pumped into annulus 42a. A portion of the oil 68 is held within the annulus'42a by the insulating packer 19a, with the remainder permeating the formation adjacent the casing 14a. Insulators 18a are provided and the well head is insulated as described with reference to FIG. 1.

Since well 10b is not insulated, water can be supplied as shown in FIG. 1 through either the tubing 16b or annulus 42b. However, it is preferred that the annulus be used for supplying water to permit a pump to be used ifv desired. On the other hand, the presence of packer 65 precludes the use of annulus 42a for supplying water to well 10a, and tubing 16a must be used. Moreover, the water supply must be insulated from ground since the tubing 16a is necessarily insulated from ground.

It will also be noted that the installation of FIG. 3 is not dangerous in that only one wire of the electrical circuit is present at either of the two wells and that this wire or any portion of thewell connected thereto can be touched with complete safety.

The embodiment of the invention shown in FIGS. 1 and 3 of the drawings utilize wells completed in conventional fashion except for provision of the insulating members 18a and cutting of casing 1411.

Turning now to FIGS. 4 and 5 of the drawings, there is shown an installation especially equipped for use as an electrode well in practicing the present invention. Thus, as shown in FIG. 4, a well bore -10 extends from the surface of the ground into oil containing strata 12. As mentioned previously with reference to FIGS. 1 and 3 of the drawings, the

formations

60 and 62 above and below the oil bearing strata are normally of a material substantially impervious to the liquids such that oil and gas are trapped in the formation 12. Also, water will normally be present in the formation 12 and underlying the oil bearing sand.

There is also provided a. string of tubing which passes through a string of casing 101. Insulating members 102 are provided for positioning the tubing 100 within the casing 101 and insulating the two from one another. The tubing'100 includes a portion 104 formed of insulating material. The insulating portion 104 of tubing 100 extends upward into the casing 14 to a point 105 established by the water level it is desired to maintain. In some applications in which it is not necessary that the tubing extend to a point adjacent the .formation 12, it is practical to terminate the tubing 100 at the point 105 at which it becomes insulated. The casing 14 also includes an insulating lower portion, designated by reference character 103. The insulating portion 103terminates above point 105. It is practical for the portion 103 to be positioned partially in the formation 12 and partially in the overlying strata. A high density cement 107 is provided in the region of the, upper end of portion 103 to obviate the possibility that a conductive path may be provided to strata 12 from an upper water bearing strata. Positioned within the tubing 100 and insulated therefrom by insulating members 106 is an electrode member 108, also suitably formed of tubing. For instance, in a practical installation, the tubing100 may be 2 /8" diameter and the tubing 108 may be of 1" diameter. The tubing 108 extends below the portion 103, as shown.

A flange member 110 is threadedly connected to the string of casing 14 at its upper end and a second flange member 112 is threadedly connected to the string 100 of tubing at its upper end. An insulating gasket 114 is positioned between the flange 110 and the flange 112 for electrically insulating the two one from the other and, also, for providing the necessary scaling function. Still a third flange member 116 is threadedly connected to the member 108, and a gasket 118, also of insulating material, is provided between

flanges

112 and 116 for insulating the two and sealing the juncture therebetween. Suitable means,

. 7 not shown, are also provided for connecting the

flanges

110, 112 and 116 into a unitary structure.

There is also provided a control circuit 120 having input leads 112 and 124 electrically connected to the tubing 100 and the member 108, respectively. A T 130 is mounted on the surface of casing 14 for connecting one end of a length of pipe 132 to the casing 14, the other end of the length of pipe 132 being connected to the output of pump 134. The inlet of pump 134 is connected to tank 136 adapted to contain a. supply of makeup water by pipe 138. When the pump 134 is actuated by the control circuit 120, it will pump water from the tank 136 into the annulus 140 between the casing 14 and the tubing 100. The water will then flow through the annulus 140 to the cavity 54 at the bottom of the well bore 10. It will 'be noted that all of the insulators used in all embodiments of the invention suitably are of a spider type configuration to permit the passage of fluids.

A source of alternating current supply voltage is shown connected between the member 108 and a similar member 108 positioned in a different well bore similarly equipped, producing a flow of electrical current in the formation 12 which is effective to heat the water present in the formation. Alternatively, the supply voltage can be connected between the tubing 108 and the upper conductive casing 101. In such an instance, it is preferred that the insulating portion 103 extend into the formation.

When the water level in the annulus 140 falls below point 105 of the tubing, pump control 120 will be effective to apply power to pump 134, causing makeup water to flow into annulus 140. At such time as the water in the annulus 140 again rises to a point above point 105, pump 134 will be shut off. If desired, a time delay may be incorporated into the starting and stopping of the pump to increase the time intervals between starting and stopping of the pump.

A control circuit 120 suitable for use in practicing the invention is shown in FIG. of the drawings wherein the resistor 150 represents the resistance between tubing mem her 100 and member 108. It will be appreciated that when the water in the annulus 140 extends above point 105, the resistance of resistor 150 will be relatively low and when the water level in the annulus 140 is below point 105, the resistance of resistor 150 will be very high.

There are also provided three transistors, Q Q and Q Transistors Q and Q are connected in a conventional Schmitt trigger circuit. The base of transistors Q is connected to the juncture between resistor 150 and resistor 152, with the resistors 150 and 152 being connected between a source of volts and ground. The emitter of transistor Q is also connected to ground, the collector of transistor Q being connected to +15 volts through resistor 154.

There is also provided a relay 156 having a coil 158.

Coil 158 is connected in series with the power electrodes of a silicon controlled rectifier 160 between +15 volts and ground. Relay 156 also includes a normally open contact 162 which is connected in series with the motor 164 which drives pump 134 and a source of alternating current supply voltage. Thus, when relay 156 is energized, power will be applied to drive the pump 134, causing water to be pumped from the tank 136 into the annulus 140. The collector of transistor Q is connected through resistor 166 to the gate electrode of silicon controlled rectifier 160 and through capacitor 168 to the anode of silicon controlled rectifier 160.

A low resistance of resistor 150, resulting from the water level in the annulus 140 being above point 105, will bias the transistor Q to be conducting, causing transistor Q to be biased off and transistor Q, to be biased on. Silicon controlled rectifier 160 will be in its normally high impedance state and relay 156 will not be energized. If the water level in the annulus 140 falls below the level 105, the resistance of resistor 150 will increase, causing transistor Q, to turn off. When transistor Q is turned off, transistor Q will be biased on and transistor Q will be biased off. When transistor Q turns off, a biasing signal will be applied to cause the silicon controlled rectifier 160 to switch to its low impedance state, permitting current to flow through the coil of relay 158, resulting in closure of contact 162. Upon closure of the contact 162, power will be applied to cause the motor 164 to drive pump 134, resulting in water being pumped from tank 136 in the annulus 146. When sufircient water is pumped into the annulus to cause the water level to rise again above point 105, the resistance of resistor will again become quite small, biasing transistor Q, on. As transistor Q turns on, transistor Q will turn off and transistor Q, will turn on, causing the collector of transistor Q, to become less positive. The change in potential at the collector of transistor Q is differentiated by capacitor 168 and applied to the anode of silicon controlled rectifier as a negative going pulse which is effective to turn the silicon controlled rectifier 160 off. When silicon controlled rectifier 160 turns off, it will prevent further current flow through the coil of relay 156 and power will no longer be applied to the motor 164. Suffcient water will therefore be maintained at all times for purposes of producing steam without interruption in the flow of current, thereby providing maximum efficiency.

As indicated previously, indvidual wells equipped in the manner as shown in the drawings can be utilized to produce steam or hot water. However, on a field-wide basis, best results are often obtained by connecting two wells in series in the manner described with reference to FIG. 3 of the drawings. Turning now to FIG. 6 of the drawings, many oil fields are arranged such that wells are drilled in a symmetrical, predetermined pattern with the wells generally grouped in squares, each well being equidistant from its adjacent well along either a vertical or horizontal line.

In practicing the present invention, it is practical to utilize a single well, such as well 149, for producing steam or hot water to increase the recovery from, for example, wells 141-148. At such time as substantially all of the oil has been recovered from that portion of the reservoir between well 149 and wells 141-148, separate current paths can be established between well 149 and each of wells 141-148. The entire area of the formation bounded by wells 141-148 then become effective as a large heat producing reservoir, with the heat produced in this reservoir serving to increase production of wells 125-140.

Over a period of time, the heat stored in the reservoir will slowly increase. Since the producing formation is normally a substantial distance below the surface of ground and since the heat is produced at the location at which it is to be utilized, the loss of heat is quite small. A large reservoir of steam or hot water with a large amount of stored thermal energy is thereby provided for purposes of increasing recovery from the wells surrounding the reservoir. As the surrounding wells are depleted, additional wells can be connected to electrical circuits to further increase the effective size of the heat reservoir.

In those fields in which a substantial quantity of the oil is in place in a water-oil emulsion, best results may be obtained utilizing an arrangement as shown in FIG. 7 wherein current is caused to flow between two

electrode wells

170 and 172 on opposite sides of a well 174 to be produced. A combination of heat and the effect of the flow of alternating current is an effective emulsion breaker which will facilitate the pumping of oil from well 174.

From the above, it can be seen that the present invention provides an improved method of thermal recovery utilizing electrical current to produce heating of water in the formation at the bottom of a bore hole immediately prior to the time that the water or steam is injected into the surrounding oil bearing formation. Since heating is accomplished in the formation rather than at the surface, thermodynamic losses are maintained at a very low level, making it feasible to use electricity to produce the heating of water even though it is a much more expensive source of energy than hydrocarbon fuels. Further, the cost of the installation is only a fraction of that required in conventional systems as large boilers are not required at the surface.

The combination of high thermodynamic efiiciency and low installation cost makes it practical to utilize the present invention for the secondary recovery of oil from fields in which conventional methods of steam injection or hot water flooding would not be economically practical. For example, the present invention can be applied to the oil bearing formations located at much greater depths than at which it would be practical to utilize a conventional steam injection or hot water flood. The low installation cost means that the system carii be applied on virtually an acre to acre basis, making it feasible to apply secondary recovery techniques either to small fields or to provide a pilot operation for determining the practicality of installing a much more expensive conventional steam injection or water flood system.

Although the invention has been described with reference to particular preferred embodiments thereof, many changes and modifications will be obvious to those skilled in the art.

What is claimed is:

1. A method of secondary recovery of oil from an oil bearing formation comprising establishing an electrical current through water positioned in a well bore extending into an oil bearing strata, maintaining sufficient pressure on said Water as said water is heated by the flow of electrical current to prevent boiling of water at temperatures substantially in excess of the boiling point of water at the surface of the bore hole, and injecting water into said strata through said well bore to establish a water flood in the strata, the water injected being heated by said flow of electric current.

2. A method of secondary recovery as defined in claim 1 further including the step of controlling at least one of the amount of water injected and the amount of electrical current which flows to maintain the water flowing from the well bore into the strata ata desired temperature.

3. A method of secondary recovery as defined in claim 1 including the additional step. .of adding water soluble conductive salts to the water injected into the well bore to maintain the resistivity of the water through which current flows at a desired level.

4. A method of secondary recovery as defined in claim 1 wherein said electrical circuit includes a string of tubing extending from the surface through said well bore into the oil bearing strata and a string of casing extending from the surface through said well bore into said oil bearing formation and insulated from said string of tubing.

5. A method of secondary recovery as defined in claim 1 wherein the electrical circuit}; is established through a path comprising a string of tubing in one well bore and a string of tubing in a second well bore spaced apart from said first well bore.

6. A method of secondary recovery as defined in claim 5 further including the step ofirinsulating said first and second string of tubing from formations capable of conducting electrical energy in positions above the oil hearing strata whereby the primary path of current flow near the first and second string of tubing is in the oil bearing formation.

7. A method of promoting the flow of oil from an oil bearing formation having at least one well bore drilled into the formation, each said well bore having a string of casing positioned therein, and a string of tubing positioned within said casing comprising:

(a) connecting one side of a source of alternating current supply voltage to a string of tubing within one of said well bores;

(b) connecting the other side of said source of alternating current supply voltage to cause current to flow through said tubing and a supply of water in intimate contact with said tubing and said formation insulating said tubing from said casing.

9. A method as defined in claim 7 wherein the other side of said source of alternating current supply voltage is electrically connected to a second string of tubing in a wall spaced apartfrom said first mentioned well but electrically connected thereto by water of said formation and further including the step of electrically insulating the portion of the casing in said first mentioned well in contact with said supply water from any other formation capable of providing an electrically conductive path between said first mentioned well and said second mentioned well.

10. A method of promoting the flow of oil from an oil bearing formation that comprises:

(a) producing flow of electricity through water in intimate contact with an oil bearing formation to convert said water to steam; and

(b) adding water to maintain a path for said electrical current to flow. l

11. A method of promoting the flow of oil from an oil bearing formation into which at least two wells have been drilled, each of said wells having a string of casing extending from the surface to a point at least near the oil bearing formation and having a string of tubing extending from the surface through said casing into said oil bearing formation that comprises:

(a) cutting the casing of one of said wells at a point intermediate the bottom of the casing and any formation capable of providing an electrically conductive path between said one well and a second well;

(b) setting a packertin the annulus between the casing and tubing of said first well at a point below that at which said casing was cut;

(0) placing oil in the annulus between said casing and said tubing above'said packer to permit said oil to flow through the cut in said casing and permeate the formation adjacent to said cut;

((1) connecting a source of alternating current supply voltage between the tubing of said first mentioned well and one of said tubing and easing of a second well spaced apart from said first well; and

(e) supplying water through said first and second wells to said oil bearing formation to provide an electrically conductive path through said formation as said water is converted to steam responsive to the flow of electrical current therethrough.

12. A method of promoting the flow of oil from an oil bearing formation having a plurality of well bores drilled from the surface to said formation, each of said well bores having a string of casing set therein and a string of tubing passing through said casing from the surface into said formation that comprises:

(a) setting a stringof casing having its lowermost portion formed of an insulating material;

(b) cementing said well bore in the vicinity of said insulating portion;

(c) setting a string of tubing in said casing, said tubing being characterized by having a lower insulating portion, said lower insulating portion of said first string of tubing extending up said well bore a distance less than that which the insulated portion of said casing extends;

(d) positioning a conductive member within said first string of tubing with said conductive member extending into said formation;

(e) electrically insulating said first string of tubing from said casing and electrically insulating said conductive member from said first string of tubing;

(f) detecting when the level of water extends into said well bore above the insulated portion of said tubing;

(g) adding water to said formation in the vicinity of said conductive member responsive to the changes in the water level in said well bore relative to the insulated portion of said tubing;

(h) connecting a source of alternating current supply voltage between said conductive member and one of the casing and tubing of at least one other of said plurality of wells; and

(i) supplying water through said at least one other well into the vicinity of said formation to continuously provide an electrical circuit between said first and said at least one other well to convert water found in said formation to steam.

13. A method of promoting the fiow of oil from a formation that comprises causing the flow of alternating electrical current through the water of an oil bearing formation between two wells positioned at opposite sides of a third well to heat the formation in the vicinity of said third well and break water-oil emulsions which may be present in the vicinity of said third well.

14. Apparatus for promoting the flow of oil from an oil bearing formation comprising:

(a) a string of casing set in a well bore extending from the ground surface into said oil bearing formation;

(b) a string of tubing positioned within said casing extending from said surface into said formation;

() means for insulating said tubing from said casing;

(d) means for producing a flow of alternating current through said tubing and water in intimate contact with said formation to heat said water for converting said water to steam; and

(e) means for adding water to maintain a supply of water in intimate contact with said tubing and said formation.

15. Apparatus as defined in claim 14 wherein said last named means comprises:

(a) a sealed tank adapted to contain a supply of water and capable of withstanding the pressure present at the head of the well; and

(b) means including a valve and flow control means connecting said tank for communication with one of said tubing and the annulus between said tubing and said casing to permit water to flow from said tank into said formation at a controlled rate.

16. Apparatus as defined in claim 14 further includ- 12 ing means for insulating any portion of said casing in electrical contact with said water from any portion of said casing in contact with an electrically conductive formation located above said oil bearing formation.

17. Apparatus as defined in claim 14 wherein said last named means comprises:

(a) a tank for containing water; (b) means for pumping water from said tank to said formation; and

(c) control circuit means responsive to the height of the water in one of said tubing and casing for controlling operation of said last mentioned means.

18. Apparatus as defined in claim 14 wherein said casing is perforated along at least a portion of its length extending into said formation and further including packer means set in the annulus between said tubing and said casing for sealing the annulus, said tubing being perforated only above said packer means.

19. A method of promoting the fiow of oil from an oil bearing formation into which a well equipped for producing oil from said formation has been drilled that comprises producing a flow of alternating current through said formation between two points disposed on opposite sides of said well and spaced apart therefrom and preventing the flow of substantial quantities of said alternating current through formations other than said oil bearing formation.

20. A method as defined in claim 19 further including the step of injecting water into said formation to provide a conductive path for the flow of current through said formation.

21. A method as defined in claim 19 further including the step of controlling the resistivity of water injected into said formation to control the amount of current flowing through said formation.

References Cited UNITED STATES PATENTS 849,524 4/1907 Baker 166-248 1,372,743 3/1921 Gardner 16660X 1,784,214 12/1930 Workman 166-60X 2,597,261 5/1952 Rhoads 166-60X 2,801,090 7/1957 Hoyer et al. 16660X 3,106,244 10/ 1963 Parker 166-248 3,137,347 6/1964 Parker 16660X STEPHEN J. NOVOSAD, Primary Examiner US. Cl. X.R. 166250, 272,