US2771140A - Subsurface igniter - Google Patents

Description

Nov. 20, 1956 H. A. BARCLAY ET AL 2,771,140

SUBSURFACE IGNITER Filed Aug. 28, 1953- 5 Sheets-Sheet 2 FIG. 8

/0- HARRY A. BARCLAY DEAN P. NICHOLS INVENTORS 'A TTORNEY 'bon material.

' ment of our invention.

United States Patent SUBSURFACE IGNITER Harry A. Barclay, Dallas, Tex., and Dean P. Nichols, Comanche, Okla., assignors, by mesne assignments, to Socony Mobil Oil Company, Inc., a corporation of New York Application August 28, 1953, Serial No. 377,155

9 Claims. (Cl. 166-60) This invention relates to an igniter and relates more particularly to an electrical igniter for initiating combustion of hydrocarbon materials in a subterranean formation.

It has been recently proposed to recover hydrocarbon materials from subterranean formations by a process involving combustion of a portion of the hydrocarbon material in place within the subterranean formation. this process, oxidizing gas is supplied to the subterranean formation undergoing combustion through an input well leading thereto, and the combustion gases, the hydrocarbons, and distillation and viscosity breaking products of the hydrocarbons migrate through the formation to an output well or output wells leading from the formation, from which these fluids are removed and thereafter treated for recovery of the desired valuable constituents. Initiation of combustion within the formation is effected by application of heat to the formation along the face thereof at the input well.

It is an object of this invention to provide an igniter for initiating combustion of hydrocarbon material within a subterranean formation. It is another object of this invention to provide an electrical igniter which will initiate combustion along the face of an input well penetrating a subterranean formation containing hydrocar- It is another object of this invention to provide an electrical igniter for use in a well leading to a subterranean formation which may be suspended in the well by means of a cable. These and other objects'of the invention will become apparent from the following detailed description thereof.

In the drawings Figure 1 is a plan view of an embodi- Figure 2 is a longitudinal sectional view of the upper portion of the igniter of Figure 1. Figure 3 is a cross-sectional view taken along line 3-3 of Figure 2. Figure 4 is a cross-sectional view taken along line 44 of Figure 2. Figure 5 is a crosssectional view taken along line 5-5 of Figure 2. Figure 6 is a cross-sectional view taken along line 66 of Figure 2. Figure 7 is a longitudinal, broken plan view of the intermediate portion of the igniter of Figure 1, :showing the casing of the heating element in section. Figure 8 is a simplified diagram of the electrical circuits of the heating elements and the temperature sensing means. Figure 9 is a cross-sectional view taken along the line 9-9 of Figure 7. Figure 10 is a cross-sectional view taken along line 10-10 of Figure 7. Figure 11 is a longitudinal plan and sectional view of the lower portion of the igniter of Figure 1. Figure 12 is a sectional view taken along line 12- 12 of Figure 11. Figure 13 is .a cross-sectional view taken along line 1313 of Figure 11.

In Figure 1, an igniter 1 embodying the features .of

our invention is shown supported at the lower end of a.

cable 2.

The upper portion of igniter 1 is shown-in detail in Figures 2-6. Cable 2, which serves both to support igniter 1 in a well bore and to carry power leads and leads to temperature sensing means within igniter 1, comprises outer armor 3, inner armor 4, power leads 5, 11 and 12, and leads 13, 14 and 15 to temperature sensing means. Lifting head 20 is provided with bore 21 which is of sufifb cient diameter that cable 2 will readily pass through it.

Adapter 22, which is threaded at 23 into the lower end of head 20, is provided with screw 24 threaded into adapter 22. Ring 34 may be made of a rubber or any other satisfactory sealing material which will withstand temperatures of about 200 F.

Sleeve 35 is screw threaded into the upper end of adapter 31 and extends upwardly into adapter 22, the external surface of sleeve 35 fitting in close spaced relation with the inner surface of adapter 22. Screw holes 41, three in number at 120 intervals, are provided through the wall of the upper end of sleeve 35. Plug assembly 42 is held in position in sleeve 35 and adapter 22 by means of set screws 43 which screw through holes 41 in sleeve 35 into screw holes 44 provided in plug assembly 42. Plug assembly 42 comprises sleeve 45 and body 51 which is pressed into sleeve 45 substantially as shown. Body 51 in the preferred embodiment of the invention is formed of a hard, electrical and heat insulating material such as a phenol condensation product sold under the trade names of Bakelite or Micarta. The lower end of body 51 is provided with hole 52 which contains screw threads at its lower end and with holes 53, which in this embodiment are six in number spaced at 60 intervals. Screw 54 is threadedly engaged in hole 52. Near its upper end sleeve 45 is provided with external annular channel 55 adapted to receive 0 ring 56 which functions to form a seal between sleeve 45 and adapter 22. O ring 56 is formed of a rubber or similar material which is capable of withstanding temperatures of about 200 F.

Studs 61 are screw threaded into the upper end of sleeve 45 permitting the securing of body 62 against the upper end of sleeve 45 by means of set screws 63 which aflix body 62 to studs 61 as shown more clearly in Figure 4. The upper end of body 62 is held in contact with the upper end of the inside of lifting head 21. Outer armor 3 of cable 2 is stripped away from the cable for a short distance permitting armor 3 to be held firmly within the body 62 by cone 64 which fits around cable 2 and has external annular teeth 65 to contact armor 3. Cone 64 is heat treated to provide a very hard surface to grip armor 3, which in this embodiment is formed of steel, and must, along with inner armor 4, support the weight of the igniter. Cone 64 is held in position by nut 66 which has internal axial bore 71 and is screw threaded into body 62 at 72. Actual contact of nut 66 against cone 64 is made by washer 73 pressed into the end of nut 66. Washer 73 is formed of a metal such as brass. Inner armor 4 of cable 2 is stripped back from the core of cable 2 and positioned within bore 71. Collar 74,

which fits around cable 2 between inner armor 4 and the core of the cable, prevents the ends of armor 4 contacting and damaging the core of cable 2. Collar 74 in this embodiment of the invention is formed of a phenol condensation product such as is sold on the market under the trade name of Bakelite.

Heater support 75 having external annular shoulder 81 'is positioned within head adapter 31 with shoulder 81 resting on shoulder 32. Positioned within the upper end of support 75 is insulator 82 and positioned within the lower. end of support 75 is insulator 83. Insulators 82 and 83 are formed of a material having good dielectric qualities, high compressive strength, and relatively high shear strength. Insulators 82 and 83 are secured to support 75 by means of bolts 84. Support 75 and insulators 82 and 83 are provided with holes 85 adapted to receive rods 86, 87, and 88 and with holes 92 adapted to receive leads 131 and 132. Holes 85 through support 75 are enough larger than rods 86, 87, and 88 to prevent electrical shorts through support 75 between the rods. Rods 86, 87, and 88, which are of substantial length, are screw threaded at both ends and are secured at the upper ends thereof to support 75 and insulators 82 and 83 by means of nuts 93 and washers 94. Washers 94 are countersunk into insulators 82 and 83 to keep rods 86, 87, and 88 centered as they pass through holes 85 in support 75 to provide even clearance around the rods so that no electrical shorting will occur between therods through support 75. Rods 86, 87, and'88 serve to support heating means in the igniter and to conduct electrical power to said heating means. The rods are formed of stainless steel in this preferred embodiment of the invention but may be formed of any other suitable electrically conductive material which will maintain its strength and shape and. will have a high resistance to oxidation at temperatures within the range of 1.400, F.

Terminals 95, having screw threads on their lower ends and a shoulder on their upper ends, are positioned in holes 53 of body 51 and leads 5, 11, and 12 and 13, 14, and 15 are secured by soldering or other satisfactory means to the upper ends of terminals 95. For purposes of clarity, in Figure 2 only leads 11, 12, 13, and 15 are shown secured to four of terminals 95. Leads and 14 are similarly secured to the remaining two terminals 95. Seals 101 are positioned around terminals 95 in the lower end of body 51 as shown. formed of a rubber-like material such as is sold on the market under the trade name of Neoprene. Secured on those terminals 95 which are connected to leads 5, 11, and 12 are brackets 102 which are held in place by means of nuts 103. Conductors 104 are connected at one end thereof to brackets 102 and at the other end to brackets 105 by means of bolts 111. Brackets 105 are held on rods 86, 87, and 88 by nuts 112.. Thus, by the means just described, power leads 5, 11, and 12 are connected to rods 86, 87, and 88.

As previously described, leads 13, 14 and 15 are secured to the upper ends of three of terminals 95. Secured to the lower ends of these same terminals 95 in a conventional manner are leads 113, 114 and 115 which are connected respectively to terminals 121, 122 and 123 positioned within the chamber 124. Connected between terminals 121 and 122 is thermistor element 125. Connected to terminal 121 is thermocouple lead 131 and connected to terminal 123 is thermocouple lead 132. The temperature sensing leads and their connections have been shown in Figure 2 in diagrammatic form for purposes of clarity. Leads 131 and 132 pass into insulating tubing 133 and run through one of holes 92 downward into the lower portion of the igniter to thermocouple means hereinafter described. Tube 133 is formed of electrical insulating and heat resistant material and has small separate bores to accommodate thermocouple leads 131 and 132. In the preferred embodiment of the invention, lead 131 is formed of chromel and lead 132 is formed of alumel, but it will be apparent to those skilled in the art that leads 131 and 132 may be formed of any dissimilar metals satisfactory for thermocouple use and capable of withstanding temperatures in the range of 1400 F. Though in this embodiment of the invention only one of holes 92 is used to pass leads 131 and 132 to the lower portion of the igniter, it will be apparent that lead 131 may go downward through one of holes 92 and lead 132 may go downward through another of holes 92 or in this embodiment the presently unused holes 92 may be used for additional thermocouple leads if desired.

The previously described parts: which are positioned Seals 101 are 'block 152 has given satisfactory performance.

within chamber 124 and those within plug assembly 42 are coated during assembly of the igniter with an electrically insulating grease to prevent moisture condensation upon them. The interior of plug assembly 42 is filled with the grease by injecting it through hole 52. Screw 54 is screwed into hole 52 to retain the grease within the plug assembly.

Casing 134 is secured by welding or other suitable means to the lower end of adaptor 31. Casing 134 is of relatively thin-walled construction in order that it will have low heat capacity and conductance. The low heat capacity of casing 134 reduces the amount of heat required to bring the igniter up to the desired operating temperature and permits rapid dissipation of heat to the adjacent oil formation. The low heat conductance feature of casing 134 lessens the amount of heat conducted up the casing to the upper end of the igniter. In this embodiment of the invention, casing 134 is formed of stainless steel, but it may be made of any other suitable material which will permit thin-walled construction that will retain its strength and shape at the high operating temperatures used.

Referring to Figure 11, cap 135, provided with central bore 136, is secured by welding or other suitable means to the lower end of casing 134. Secured to cap 135 by means of bolts 137 is flange 141. Gasket 142 is positioned between cap 135 and flange 141. Flange 141 is provided with central screw threaded bore 143 having therein seat 144 and radial screw threaded bore 145 providing communication between bore 143 and the exterior of flange 141. Screw 151 in bore 143 will, when fully screwed into bore 143, contact seat 144 and close off communication through bore 145 into bore 143. Screw 151 may readily be backed ofl until communication can be had from the exterior of flange 141 through bores 145 and 143 into the inside of casing 134. Thus, the combination of bores 143 and 145 and screw 151 comprises a valve through which communication may be had into the interior of casing 134.

In Figures 7 and 11, the disposition of rods 86, 87, and 88 with respect to each other has been shown out of scale for purposes of clarity. Figures 2, 6, 9, 10, 12, and 13 show the true relation of the rods to each other.

Referring to Figure 7, positioned on rods 86, 87, and 88 downward from their upper ends is heat block 152 which in this embodiment of the invention is in the form of a lamination having alternate layers of a heat insulating and electrically non-conductive material such as asbestos and -a heat resistant, electrically non-conductive material such as is sold on the market under the trade name Transite. Heat block 152 may also be formed of fiber glass wrapped around a ceramic or refractory core or of any other suitable heat resistant,

electrically non-conductive material. Heat block 152 functions to insulate the upper end of the igniter from the heating element means and should, therefore, form a fairly close fit with the inside of casing 134, though in practice a clearance of as much as one-sixteenth inch between the'inside of casing 134 and the outside of heat The heat block is held in place on rods 86, 87, and 88 by means of cylindrical shaped spacers 153 and collars 154 which fit on the' rods as indicated. Spacers 153 are formed of a ceramic or refractory electrically non-conductive material and fit loosely on the rods. In this embodiment collars 154 are made of stainless steel and secured to the rods by means of set screws. As collars 154 are electrically conductive, it is imperative that they be secured on the rods in staggered positions, as shown in the drawings, to prevent any possible electrical shorting between the rods which might occur during operation.

Core 155 supported on'rods 86, 87, and 88 provides an electrically insulating structure which in turn supports electrical heating means. Power supplied to such heating means is transmitted through the igniter by way of the electric circuits, the structural features of which are shown in detail in Figures 2, 7, and 11. A simplified circuit diagram has been shown in Figure 8 and includes three electrical heaters 160, 161, and 162. The power supplied to the heaters 160-162 is provided by a threephase alternating current source 163 to the heating circuit which is connected in the form of a delta.

The structural features of the heating elements 160 162 and their mountings are shown in detail in Figures 7 and 912.

Also shown in Figure 8 is a simplified circuit diagram of the temperature sensing means, the structural features of which are shown in detail in Figures 212 and which include thermocouple element 164, thermistor element 125, voltmeter 170, and ohmmeter 171. Dissimilar conductors 131 and 132 of thermocouple element 164 extend upward into the upper portion of the igniter, as shown in Figure 2, where they connect to leads 13 and 15 which extend to the surface through cable 2. The point of termination of leads 131 and 132 within the upper portion of the igniter forms a cold junction for thermocouple element 164. Connected to and immediately adjacent to the upper end of lead 131 is thermistor element 125 which is also connected to lead 13 which extends to the surface through cable 2. Voltmeter 170 at the surface is connected to leads 13 and 15. Ohmmeter 171 at the surface is connected to leads 13 and 14. Lead 15 is provided with switch 172 which, when closed, permits a reading to be made on voltmeter 170 indicative of the temperature of element 164. Lead 14 is provided with switch 173 which, when closed, permits a reading to be made on ohmmeter 171 indicative of the temperature of element 125. Since the cold junction for thermocouple element 164 is located adjacent to element 125, the temperature as indicated by element 164 is relative to the temperature of its cold junction, or, in other words, relative to the temperature of element 125. The cold junction temperature is determined by thermistor element 125. Thus, the true temperature in the immediate vicinity of element 164 is determined by adding the temperature of element 125 to the temperature of element 164.

Referring to Figure 7, core 155 as previously stated is positioned on rods 86, 87, and 88. Core 155 is formed of a refractory type, electrically non-conductive material, is provided with a continuous spiral groove 174 to receive electrical resistance heating coil 160, and contains holes 175 for rods 86, 87, and 88, and holes 176 for the temperature sensing leads. Coil 160 is secured at its upper end to rod 86 by means of clamp 181 and is secured at its lower end to rod 88 by clamp 182. Core 155 is spaced downward from heat block 152 by spacers 153 'and clamps 154.

Core 183 having continuous spiral groove 184 is positioned on rods 86, 87, and 88 spaced downwardly from core 155, and contains holes 185 for rods 86, 87, and 88, and holes 186 for the temperature sensing leads. Spacers 153 and collars 154 are positioned on rods 86 and 87 between cores 155 and 183. Electrical resistance heating coil 161 is positioned in groove 184 of core 183 and is connected at its upper end to rod 88 by clamp 187 and at its lower end to rod 87 by clamp 191, shown in Figure 11.

As shown in Figure 11, core 192 is positioned on rods 86, 87, and 88 spaced downwardly from core 183 by means of spacers 153 and clamps 154 positioned on rods 86 and 88. Core 192 has continuous spiral groove 193 adapted to receive electrical resistance heating coil 162 which is connected at its upper end to rod 87 by clamp 194 and is connected at its lower end to rod 86 by clamp 195. Passing through core 192 axially are holes 196 for rods 86, 87, and 88, and holes 197 for the temperature sensing leads.

Cores 183 and 192 are identical in material, shape, and dimensions to core 155.

- Spacers 153 are positionedon rods 87 and 88 below.

core 192. Support 201 is secured on rods 86, 87, and 88 by means of nuts 202, which are screw threaded on the lower ends of the rods. Cap 203 is secured to support 201. Support 201 and cap 203 are formed of a heat resistant, electrically non-conductive material.

Shown in Figure 11 positioned within core 192 is thermocouple element 164 which is formed by fusing together the ends of conductors 131 and 132 just below the point of termination of tubing 133. Though thermocouple element 164 is shown here within core 192 which places it within the field of influence of heating coil 162, it will be recognized that element 164 may be placed anywhere within the heating section of the igniter.

The heating coils may be formed of any suitable electrical resistance material capable of operating over long periods of time at temperatures which will maintain the heating portion of the igniter around 1400 F. It has been found with the currently available electrical resistance materials that the life of the heating coils may be materially extended by use of coils of a size requiring a relatively low watt density to produce the operating temperatures of the igniter.

While only three electrical resistance heating coils have been shown here, additional heating coils may be added in multiples of three to lengthen the igniter as desired. The use of three-phase power requires that the number of heaters be in multiples of three to keep the system balanced electrically. With the addition of heaters, casing 134 and rods 86, 87, and 88 will, of course, have to be lengthened proportionately. The length of the rods must be such that when fully expanded at operating temperature cap 203 will not contact cap and cause the rods to buckle.

In the operation of our igniter, formations will frequently be encountered wherein the pressures are of such a magnitude that the thin walls of casing 134 will tend to collapse inwardly. In those instances, the igniter may be pressured with an inert gas such as nitrogen to a value which will prevent any dangerous stresses being exerted upon casing 134. This is done by injecting enough gas into the igniter at the surface that the pressure will increase with the operating temperatures of the igniter to a value sufiicient to counteract the pressure of the formation in which the igniter is operating. The gas used to pressure up the igniter is injected in the lower end. of the igniter through bore 145, shown in Figure ll. During the initial stages of pressuring the ignite-r, the air present Within it may be bled out through hole 25, shown in Figure 2, which is closed by a screw 24 when all air has been removed. The pressuring of the igniter with such a gas also functions to place the heating coils and their related parts in an atmosphere which aids in reducing their tendency to oxidize.

In operation, the igniter, supported by cable 2, is lowered into a bore hole until it is adjacent to a hydrocarbonbearing formation penetrated by the bore hole. Heating coils 168162 are then energized from current source 163 to heat the igniter to the temperature necessary to initiate combustion in the hydrocarbon-bearing formation. The combustion supporting gas being supplied to the formation through the input well and the hydrocarbon material within the formation being at its ignition temperature, combustion of the hydrocarbon material within the formation will be effected. When combustion within the formation becomes self-sustaining merely with supply of combustion gas thereto, the supply of electrical current to the heater coils may be discontinued.

In supplying energizing current to the heater coils, it is usually desirable to supply current thereto at a slow rate so that the temperature of the various elements of the heating section of the igniter do not have a high, differential in temperature which would tend to cause damage by buckling or other conditions arising from thermal strain. For example, satisfactory results have been obtained without excessive thermal strain to the apparatus where the rise in temperature was not per-- mitted to exceed 60 F. per hour.

The supply of current to the heating coils to attain the desired temperature is controlled by Varying the energy output from source 163 and the rise in the temperature is indicated by the reading of meters 170 and 171. Similarly, after the igniter has reached operating temperature, abrupt changes in temperature either by discontinuance of the supply of electrical current to the heating coils or removal of the heater from its position adjacent to the heated walls of the bore hole should be avoided to prevent damage from thermal strain.

While the apparatus of the invention has been described as being an igniter for initiating combustion of hydrocarbon materials in a subterranean formation, it will be apparent that the apparatus has general utility with respect to applying heat to such a formation along the walls of a bore hole. Thus, for example, the apparatus may be employed for supplying heat to a subterranean hydrocarbon-containing formation along the walls of a bore hole penetrating the formation to remove parafiin therefrom. In procedures of this sort, of course, it may not be necessary to supply power to the apparatus to the same extent as it would be supplied where initiation of combustion of a hydrocarbon material is desired. These and other uses of the apparatus of the invention will be apparent to those skilled in the art.

Having thus described our 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.

We claim:

1. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises a plurality of spiral heating elements oriented in an end-to-end array to form an elongated heating unit, supporting structure for said plurality of elements including three metallic rods electrically insulated one from the other and spaced uniformly around the periphcry of a circle within and smaller than the dimension of said spiral heating elements, and electrical connections between the two extremities of each of said heating elements and selected pairs of said rods.

2. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises a head portion, a casing portion secured at its upper end to the lower end of said head portion, closure means secured to the lower end of said casing portion, a cable secured to the upper end of said head portion for supporting said apparatus within said bore hole, a plurality of spiral heating elements positioned within said casing portion and oriented in an end-to-end array to form an elongated heating unit, supporting structure for said plurality of elements including three metallic rods electrically insulated one from the other and spaced uniformly around the periphery of a circle within and smaller than the dimension of said spiral heating elements, electrical connections between the two extremities of each of said heating elements and selected pairs of said rods, means for securing said supporting structure pendent within said casing portion, a source of three-phase power, means for connecting said source of three-phase power through said cable and said head portion to said three rods, at least one temperature sensitive element positioned within said casing portion for determining the temperature of said heating elements relative to the temperature of said head portion, a temperature sensitive element within said head portion for determining the temperature of said head portion, and conductor means through said cable to said temperature sensitive elements.

3. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises a head portion; a thin walled casing portion secured at its upper end to the lower end of said head portion; closure means secured to the lower end of said casing portion; valve means in said closure means for injecting gas into said apparatus; port means in the wall of said head portion to permit air to bleed from said apparatus while said gas is being injected into said valve means; cable means secured to the upper end of said head portion for supporting said apparatus in said bore hole; a plurality of cores positioned within said casing portion and oriented in an end-to-end array spaced apart one from the other, the uppermost of said cores being spaced downwardly from the upper end of said casing portion, each of said cores having a continuous spiral groove cut into its outer surface to receive an electrical resistance heating element and being formed of heat resistant and electrically insulating material; supporting structure for said plurality of cores including three metallic rods electrically insulated one from the other and spaced uniformly around the periphery of a circle within and smaller than the diameter of said cores; a heat block supported on said rods intermediate said uppermost core and the upper end of saidcasing portion, said heat block being formed of a heat and electrically insulating material; means for supporting said rods pendent within said casing; a plurality of electrical resistance heating elements positioned in said spiral grooves of said cores; electrical connections between the two extremities of each of said heating elements and selected pairs of said rods; a source of three-phase power; conductor means through said cable and said head portion connecting said source of three-phase power to said rods; at least one temperature sensitive element positioned in said casing within the field of influence of said heating elements to determine the temperature of said heating elements with respect to said head portion; a temperature sensitive element positioned within said head portion to determine the temperature of said head portion; and conductor means through said cable and said head portion connected to said temperature sensitive elements.

4. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises three metallic rods positioned parallel to each other and electrically insulated one from the other, a plurality of spirally-grooved cores positioned on said rods in an end to end array spaced apart one from the other, said cores being formed of electrically non-conductive and heat resistant material and having a root diameter larger than the diameter of a circle encompassing said rods perpendicular to the axis of said rods, spacer members positioned on each of said rods between said cores to maintain said cores in position on said rods, electrical resistance heating elements positioned within said spiral grooves on each of said cores, said heating elements comprising coiled resistance wire, and electrical connections between the two extremities of each of said heating elements and selected pairs of said rods.

5. In an apparatus for supplying heat to a subterranean formation, the combination which comprises a tubular shaped housing closed at both ends, cable means secured to the upper end of said housing for supporting said housing within a bore hole, three metallic rods positioned within said housing parallel to the vertical axis of said housing and secured pendant from the upper end of said housing, a plurality of spirally grooved cores positioned on said rods in an end-to-end array spaced apart one from the other, said cores being formed of electrically non-conductive and heat resistant material and having a root diameter larger than the diameter of a circle encompassing said rods perpendicular to the axis of said rods, spacer members positioned on each of said rods between said cores to maintain said cores in position on said rods, electrical resistance heating elements positioned within said spiral grooves on each of said cores, said heating elements comprising coiled resistance wire, electrical connections between the two extremities of each of said heating elements and selected pairs of said rods, conductor means within said cable means connected to the upper ends of said rods, temperature sensing means positioned within said housing within the field of influence of said heating elements, and conductor means within said cable means connected to said temperature sensing means.

6. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises a head portion, a casing portion secured to the lower end of said head portion, closure means secured to the lower end of said casing portion, a cable secured to the upper end of said head portion for supporting said apparatus within a bore hole, three metallic rods positioned within said casing portion parallel to the vertical axis of said casing portion and secured pendant from said head portion, said rods being electrically insulated one from the other, a plurality of spirally grooved cores positioned on said rods in an end-to-end array spaced apart one from the other, said cores being formed of electrically non-conductive and heat resistant material and having a root diameter larger than the diameter of a circle encompassing said rods perpendicular to the axis of said rods, spacer members positioned on each of said rods between said cores to maintain said cores in position on said rods, electrical resistance heating elements positioned within said spiral grooves on each of said cores, said heating elements comprising coiled resistance wire, electrical connections between the two extremities of each of said heating elements and selected pairs of said rods, electrical conductor means through said cable connected to the upper ends of said rods, at least one temperature sensitive element within said casing portion for determining the temperature of said heating elements relative to the temperature of said head portion, a temperature sensitive element within said head portion for determining the temperature of said head portion, and conductors through said cable connected to said temperature sensitive elements.

7. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises a head portion, a thin-walled casing portion secured at its upper end to the lower end of said head portion, closure means secured to the lower end of said casing portion, valve means in said closure means for injecting gas into said apparatus, port means in the wall of said head portion to permit air to bleed from said apparatus while said gas is being injected into said valve means, cable means secured to the upper end of said head portion for supporting said apparatus in said bore hole, three metallic rods secured within said casing por tion pendant from said head portion, said rods being positioned parallel to the vertical axis of said casing portion and electrically insulated one from the other, a plurality of spirally grooved cores positioned on said rods in an end-to-end array spaced apart one from the other, the uppermost of said cores being spaced downwardly from the upper end of said casing portion, said cores being formed of electrically non-conductive and heat resistant material and having a root diameter larger than the diameter of a circle encompassing said rods perpendicular to the axis of said rods, spacer members positioned on each of said rods between said cores to maintain said cores in position on said rods, an electrical resistance heating element positioned within said spiral grooves on each of said cores, said heating element comprising coiled resistance wire, electrical connections between the two extremities of said heating element and selected pairs of said rods, a heat block supported on said rods intermediate the uppermost of said cores and the upper end of said casing portion, said heat block being formed of a heat and electrically insulating material,

conductor means through said cable and said head portion for connecting the upper ends of said rods to a source of three-phase power, at least one thermocouple positioned in said casing within the field of influence of said heating element to determine the temperature of said heating element with respect to said head portion, the cold junction of said thermocouple being positioned within said head portion, a thermistor positioned within said head portion adjacent the cold junction of said thermocouple to determine the temperature of said head portion and the cold junction of said thermocouple, and conductor means through said cable in said head portion connected to said thermistor and the cold junction of said thermocouple.

8. In an apparatus for supplying heat to a subterranean formation adjacent a bore hole, the combination which comprises three metallic rods positioned parallel to each other and electrically insulated one from the other, a plurality of spirally grooved cores positioned on said rods in an end-to-end array, said cores being formed of electrically non-conductive and heat resistant material and having a root diameter larger than the diameter of a circle encompassing said ends perpendicular to the axis of said rods, electrical resistance heating elements positioned within said spiral grooves on said cores, said heating elements comprising coiled resistance wire, electrical connections between the two extremities of each of said heating elements and selected pairs of said rods, and spacer members positioned on each of said rods between cores to maintain apart and in position on said rod cores containing within spiral grooves thereof each of said heating elements.

9. In an apparatus for supplying heat to a subterranean formation, the combination which comprises a tubular shaped housing closed at both ends, cable means secured to the upper end of said housing for supporting said housing within a bore hole, three metallic rods positioned within said housing parallel to the vertical axis of said housing and secured pendant from the upper end of said housing, a plurality of spirally grooved cores positioned on said rods in an end-to-end array, said cores being formed of electrically non-conductive and heat resistant material and having a root diameter larger than the diameter of a circle encompassing said rods perpendicular to the axis of said rods, electrical resistance heating elements positioned within said spiral grooves on said cores, said heating elements comprising coiled resistance wire, electrical connections between the two extremities of each of said heating elements and selected pairs of said rods, spacer members positioned on each of said rods to maintain apart and in position on said rod cores containing Within said spiral grooves thereof each of said heating elements, conductor means within said cable means connected to the upper ends of said rods, temperature sensing means positioned within said housing within the field of influence of said heating elements, and conductor means within said cable means connected to said temperature sensing means.

References Cited in the file of this patent UNITED STATES PATENTS 957,785 Linquest May 10, 1910 1,726,041 Powell Aug. 27, 1929 2,506,853 Berg et a1 May 9, 1950 2,632,836 Ackley Mar. 24, 1953 2,647,196 Carpenter et a1 July 28, 1953

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