US 2836248 A
Description (OCR text may contain errors)
y 27, 1953 J. M. COVINGTON 2,
WELL HEATER Filed NOV. 13, 1951 United States Patent WELL HEATER James M. Covington, Long Beach, Calif., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application November 13, 1951, Serial No. 255,961
7 Claims. (Cl. 166-61) This invention relates generally to an apparatus for electrically heating and stimulating oil wells. More particularly, this invention relates to an improved apparatus for heating oil wells wherein a resistance heating coil is immersed in a substantially non-conducting high dielectric fluid.
Many methods have been employed in the prior art for heating oil wells, e. g., by electrical means, by injecting heat transfer agents into the well such as steam, hot oil, etc., and by burning natural gas in the well bore. Considerable difliculty has been encountered in the application of electric well heating owing to the highly corrosive nature of oil field brine, sulfur compounds, and other components of the fluids produced through the well bore.
These fluids penetrate the innermost parts of the well heater, and even traces of moisture cause short-circuiting of the apparatus necessitating shutdown, removal and repair. These difliculties materially reduce the operating efficiency of such heating processes. Well bore liquids also penetrate and permeate the customary insulating materials rendering them practically useless. These problems are peculiar to the Well heating art and are not generally encountered in other applications of electrical heating.
It has now been found that an eflicient well heater can be prepared by immersing and submerging a bare or uninsulated resistance heating coil or coils in a body of Dowtherm or other non-conducting high dielectric heat transfer fluid. Local overheating of the coils is prevented and heat transfer to the sun'ounding formation is greatly improved. When Dowtherm is employed as the fluid, any traces of water which penetrate the interior of the heater are floated on the surface of the Dowtherm away from the submerged resistance coils. Little or no problem is therefore presented by water penetration into the heater provided by this invention.
It is therefore an object of this invention to provide an improved apparatus for the electric heating of oil wells.
It is another object of this invention to provide a well heater comprising electric resistance coils immersed in a body of a high dielectric, substantially non-conducting fluid wherein the thermal energy generated in the coils is efliciently removed by the surrounding fluid and whereby local overheating, thermal breakdown and parting of the resistance coils are prevented.
It is another object of this invention to heat an electrically inert fluid by means of submerged resistance coils, and to transfer heat from such heated fluid to an oilbearing formation.
It is another object of this invention to prevent shortcircuiting of a well bore heater resulting from moisture penetration by maintaining a body of electrically inert fluid in and about the resistance coils of said heater, which fluid tends to float any contaminating water and prevent direct physical contact between the water and resistance coils.
It is another object of this invention to provide a simflow from oil-bearing sands.
Other objects and advantages of this invention will become apparent to those skilled in the art as the description thereof proceeds.
Briefly, this invention relates to an apparatus for heating an oil-bearing formation in the vicinity of a well bore wherein thermal energy is generated in resistance coils immersed in a body of electrically inert fluid and such thermal energy istransferred indirectly from the body of fluid to the oil-bearing formation. In the preferred modi-' fication of the invention, the fluid employed for immersion of the coils is a eutectic mixture of diphenyl and diphenyl other such as is commercially available under the trade name Dowtherm.
In generalized form, the apparatus comprises a fluidtight container having an outer tubular heat radiating shell, a body of an electrically inert fluid disposed within such container, and one or more resistance coils submerged in the body of fluid. Electrical conductive means are provided for supplying electrical energy to the resistance coils. In the preferred modification the one terminal of each resistance coil is grounded and the other terminal is connected to an insulated cable which emerges from the wall or closure of the fluid-tight container through an appropriate pressure sealing means. heat radiating shell is generally a cylindrical or fluted metal section located, mounted and supported for substantially coaxial movement within the bore hole.
The heater is employed in one of two ways depending upon the particular temperatures produced within the immersing fluid. In the one modification, the pressure and temperature of the immersing fluid are controlled so as to maintain the fluid in a substantially liquid state, and the thermal energy is transferred from the body of heated liquid through a tubular wall whence it is transferred through an intervening liquid or gaseous phase to the oil-bearing sand. In an alternative modification of the invention, a relatively larger vapor chamber is provided above the body of immersing fluid and the temperature and pressure of such body of fluid are controlled to effect appreciable boiling thereof. The vaporized liquid carrying latent heat flows to appropriate heat transfer surfaces or other retaining walls surrounding the vapor chamber. The thermal energy is removed from the vaporized liquid and transferred to the formation through an intervening gaseous or liquid phase. Removal of latent heat from the vaporized liquid restores it to a liquid state and it flows by gravity back to the main body of boiling liquid for reboiling. The first modification of the invention is particularly advantageous where relatively shallow formations are involved, such as those of 5 to 20 feet in thickness. The second modification is more advantageously employed when it is desired to heat a relatively larger section of earth strata such as an oil-bearing formation of 20 feet or more in thickness.
The heating coil or coils may be wound on suitable electrically insulated coil forms such as grooved ferroenameled pipe, asbestos forms and the like. Alternatively, the coils may be simply suspended in the immersing fluid. In one modification the coils may be suspended in fluidfilled channels formed between the electrically insulated surfaces of a coaxially positioned fluted tube and pipe. The coils may be employed in any suitable form including helices and distorted helices such as were the central axis of the helix is made U-shaped, helical, circular, etc. In the preferred modification the windings are non-inductively wound as by doubling the windings back in the second channel of a double channelled coil form.
,The preferred immersing liquid for submerging the resistance heating coils is the eutectic mixture of diphenyl and diphenyl ether which is available commercially under The outer tubular the trade name Dowtherm A. Other electricallyinert liquids may be employed, such as those which boil generally between about 300 and 600 F. and preferably between about 400 and 550 F. These'liquids should be non-ionizable and substantially non-conduc tive under the conditions of operation. Liquids which can be employed include hydrocarbons, halogenated hydrocarbons, ethers and the like.
A particular feature of the invention resides in the fact that the metallic surfaces of the heater may be coated with a layer of ferro-enamel and the resistance wire coils may be merely suspended in the body of liquid bounded by the coated walls. Ferro-enamel coat ings of this type normally fuse and are destroyed at temperatures above about 600 F. to 800 F. The use of an immersing fluid maintains the temperatures satisfactorily below the particular upper temperature limit so that this type of coating can be successfully employed in conjunction therewith.
Another important feature of the invention resides in the fact that smaller diameter wire may be employed to produce a given thermal output without danger of overheating and blowout. Larger wire must be employed where only an air bath surrounds the coils.
Figure 1 shows an overall partly sectional view of an apparatus for electrically heating an oil-bearing sand to stimulate production therefrom.
Figure 2 shows a partial cross-sectional view of a well heater employing a fluted cylindrical heat radiating shell.
Figure 3 shows a cross sectional view of the heater of Figure 2 through plane 3-3.
Figure 4 shows an alternative heater employing an immersing fluid wherein the resistance coils are noninductively wound on a double channeled central tubular form or pipe.
Figure 5 shows a partial cross-sectional view of a heat r having an internal insulation member to minimize transmission of heat from the heater coil to the oil flowing upwardly through the central pipe.
Figure 6 shows an alternative modification of Figure 5 wherein insulation is located externally to the heater coil to eifect a maximum heat transfer to the oil flowing within the central pipe.
Referring now more particularly to Figure 1, well casing 11 extends downwardly from the earth surface 12 into bore hole 13 to the vicinity of upper boundary of oil-bearing sand 14. Well casing 11 is capped with a tubing head 15. Tubing 16 is suspended through packing ring 17 and tubing head downwardly within casing 11.
Casing 11 above earth surface 12 is fitted with gas discharge line 18. Gas discharge line 18 is fitted with presassaass 1 is carried through tubing 16, skeeter bill 28a, and heater 28 which are thus maintained at ground potential. Line 33 passes through insulator 34 to the interior of casing 11 whence it passes to and connects with cable 35 which in turn connects with heater 28 providing a complete electrical circuit therefor.
sure tap 19 and motor valve 20 which is controlled by pressure controller 21. Motor valve 20 is regulated by the pressure controller so as to maintain a positive and controlled back pressure in line 13 at all times. Alternatively, gas discharge line 18 may be fitted with any suit able pop-off valve to maintain a suitable back pressure in line 18. Such valves are generally of the spring loaded variety which open when a given set pressure is exceeded. This type of valve may be substituted for pressure tap 19, motor valve 20, and pressure controller 21.
Tubing 16 at its lower end is attached to pump lock shoe 23 by means of joint 22. Above the earth surface 12, tubing 16 is fitted with stuffing box 24 through which sucker rod 25 is permitted to reciprocate. Sucker rod 25 in turn operates pump 26 which is secured within tubing 16 by the engagement of pump lock 27 within pump lock shoe 23. A skeeter bill 28a below the pump lock shoe carries heater 28.
The extension of the skeeter bill 28a below heater 28 contains port 29 for the intake of oil from on pool 30. The location of port 29 controls the level of oil pool 3% below heater 23 such that heat is transferred from The oil flow is removed from tubing 16 above the earth surface through line 36 by suitable manipulation of valve 37.
In the operation of the apparatus of the invention,
' heater 28 is run in on tubing 16 and the pump and rods are locked into position by means of pump lock shoe 23. Electric current is supplied to heater 28 with or without simultaneous operation of pump 26. In the referred modification, motor valve 20 is positioned by pressure controller 21 to maintain a suitable positive back pressure on the annular zone between casing 11 and tubing 16.
Following the initial heating period, the well production will be increased markedly and such production can be maintained at the new increased level by (1) contiming the application of heat during the pumping and/ or by (2) maintaining the back pressure within suitable limits. Where the back pressure is maintained, such heating may be conducted either intermittently or continuously, as desired.
.be maintained at a high production level for as long as three to six months after a single heating period of a few days or weeks to remove accumulated deposits. The accumulation of well deposits is controlled or even prevented by suitable regulation of the back pressure; Where a slow accumulation tends to build up over a' period of time, intermittent heating is preferably employed to remove periodically the accumulated deposits and to restore the well to a high level of production.
Where such heating is applied to a previously unheated oil well, the production rate is increased several fold in the case of marginal wells. Often marginal wells initially producing only 5 to 15 B./D. can be made to yield as much as 40 to 100 BJD. However, only slight permanent improvement in production is obtained unless the heating is continuous, or unless a back pressure is maintained on the well annulus to minimize gas expansion cooling. With most wells which are amenable to heating, the formation pressure is generally less than 500 p. s. i., and in these cases the back pressure is maintained between the range of about 5 to p. s. i. and preferably in the range of about 15 to 4-0 p. s. i. Such pressures are predicated upon conditions in the formation, and in all events must be less than the formation pressure in order induce oil flow into the bore hole. In the application of the invention to reiatively high pressure formations, such as those above 500 p. s. i., the back pressure is increased correspondingly and generally varies between about 50 and 200 p. s. i.
In the preferred mode of operation, heat transmitted to the oil-bearing formation through the medium of In cer-' an intervening gas phase, as shown in Figure l. taln cases, the heat may be transmitted through the medium of the intervening oil of the oil pool. Such heat transfer is sometimes desirable, such as in the case where sand conditions prohibit pumaing off the bottom and/or high gas-oil ratios cause gas-locking of the pump. Under these conditions, the use of the oil pool as a heat tranfer medium serves to direct the heat toward the very lowest portion of the bore hole providing the bottom of the heater is placed at or below the bottom 'of the formation. This method suifers the disadvantage of inherent coking resulting from even temporary local overheating.
Referring now more particularly to Figure 2, heater 28 is supported coaxially on skeeter bill 50. The external heat radiating tubular shell 51 of heater 28 is a fluted or grooved cylinder which is at least partially lined on the inside with a ferro-enamel coating 52. Skeeter bill 50 is similarly lined externally with ferroenamel coating 53. Closure plate 54 seals the lower annular zone between shell 51 and skeeter bill 50 while closure plate 55 seals the corresponding annular space at the top of the heater.
The interior of heater 28 is filled with a body of electrically inert fluid 49, such as Dowtherm. The level of the fluid is such that it covers the resistance coil 59 and also provides a vapor space above the liquid level to provide for liquid expansion and for condensation of vaporized liquid according to the ambient temperatures developed by the heater coil or coils.
Closure plate 55 is equipped with removable plug 56 for the introduction or discharge of the immersing fluid, and with a suitable sealing means 57 which admits insulated electric cable 60 to the interior of the heater.
Skeeter bill 50 is fitted with a ground connection 58 for grounding the one terminal of the resistance coil 59. Other ground connections are employed for other resistance coils not shown. The insulated electric cable 60 passing through sealing means 57 connects with thermostat 61 which makes or breaks electrical contact, depending on the temperature of its environment, as described hereinafter. The electrical output of thermostat 61. flows to conductor ring 62 which is prevented from physically abrading ferro-enamel coating 53 by means of insulation ring 63. The other terminal of resistance coil 59 is electrically connected to conductor ring 62. The second terminals of other resistance coils not shown in Figure 2 are also connected to conductor ring 62.
Referring now more particularly to Figure 3 in connection with Figure 2, resistance coil 59 is disposed in the pair of longitudinal fluid-filled channels in a zone bounded generally by fluted tubular shell 51 with its attached internal ferro-enamel coating 52. Other resistance coils 64 and 65 respectively, are correspondingly disposed within other pairs of fluid-filled channels.
Referring again to Figure 2, oil is drawn into the interior of skeeter bill 50 through port 66 and flows through the skeeter bill or central pipe to a pump not shown. The thermal energy generated in resistance coil 59, and in the corersponding coils 64 and 65 shown in Figure 3, is radiated to tubular shell 51 whence it is transmitted by radiation, conduction and/or convection to the oil-bearing sand, as shown in Figure 1.
The facility of construction of the fluted tube and pipe type heater is at once evident. The resistance coils need simply be placed within the insulated channels of the irregular annular zone and the heater filled with fluid. The rate of heat transfer from the fluted exterior is considerably greater than in the case of a circular cylindrical section.
It is apparent that the skeeter bill may in certain cases be fabricated from a fluted tubular section. Thus the channels may be formed between an outer pipe having a circular cross section an inner fluted pipe. This arrangement is especially desirable in those cases wherein a high heat transfer rate to the oil flowing upwardly through the skeeter bill is desired in order to permit pumping of extremely low gravity crude oils.
In another modification cf the invention two fluted pipes may be employed to form the channels. The two fluted pipes are placed in coaxial alignment and are rotated and positioned so that the crests of the one fluted pipe are radially aligned with the troughs of the other fluted pipe. The resistance coils can then be disposed in the channels formed thereby.
While Figures 2 and 3 depict a fluted shell having 6 crests, it is apparent that a greater or lesser number of crests may be employed. Preferably tubes and pipes having even numbers of crests are employed to permit the resistance coil to run down the one and back up the other channel of each pair of channels so as to give a non-inductive circuit. Thus 4, 6, 8 or 10 crested fluted tubes may be employed.
The preferred immersing fluid for submerging the resistance heating coils is the eutectic mixture of diphenyl and diphenyl oxide which contains about 73.5 wt. per cent of diphenyl oxide and is commercially available un der the trade name Dowtherm A. This composition has the following vapor pressures:
Temperature, F.: Pressure, p. s. i.
Thermostat 61 can be set to control any desired temperature within the bath of immersing fluid. In the preferred modification of the invention the thermostat is set to control to some temperature between about 150 and 400 F., such as 250, 300 or 350 F. Under these conditions no boiling of the Dowtherm occurs and the heat flows from liquid Dowtherm through the shell to the formation. In another modification of the invention, the thermostat may be set to control to some temperature above 400 F., such as 450 F., 500 F. or 550 F. in this case the Dowtherm is vaporized and the vapor condenses on the wall of the tubular shell above the main body of the boiling liquid. The heat flows from the condensed vapor through the tubular shell to the formation. Higher temperatures promote coking of the oil and vapors in contact with the shell and such temperatures are therefore generally undesirable. Furthermore, the ferroenamel coatings are generally thermally unstable at temperatures above about 600 F.
Referring now more particularly to Figure 4, skeeter bill 70 is coated with ferro-enamel coating 71 and is fitted with a system of double grooved channels 72 within which is wound non-inductive resistance coil 73. The one terminal of resistance coil 73 is grounded to skeeter bill 70 through a suitable ground connection 74. The other terminal of resistance coil 73 is connected to thermostat 75 which is in turn connected to electric cable 76 which passes out of the heater through sealing means 77. The operation of thermostat 75 is analogous to the operation of the thermostat described in connection with Figure 2.
The heater is fitted with external heat radiating shell 78 which is coated interiorly with ferro-enamel coating 79. Plate 80 closes the lower annular zone between skeeter bill 70 :and shell 78, and plate 81 closes the corresponding upper annular zone. Plug 82 provides for introduction or discharge for the immersing fluid.
In the operation of the heater of Figure 4, the oil preferably flows upwardly through skeeter bill 70 through a pump not shown. Fluid body 83 submerges resistance coil 73 and removes heat therefrom while maintaining the coil at a relatively low ambient temperature. Heat is transmitted from liquid body 83 through tubular shell 78 to the oil-bearing sand, as shown generally in Figure 1.
Referring now more particularly to Figure 5, in certain cases it is desirable to direct a greater proportion of thermal energy to the oil-bearing sand than to the oil flowing through the skeeter bill. Skeeter bill 90 is surrounded by thermal insulation shell 91 which is in turn surrounded by resistance coil form 92 supporting ferroenamel coating 93. A resistance coil, not shown, is wound non-inductively in the double grooved channels of resistance form 92. The tubular shell 94 is preferably interiorly coated with a ferro-enamel coating 95 and the annular zone between skeeter bill and tubular shell 94 is suitably closed at the top and bottom. A sealing means is provided for admission of an inxrlated electric cable while maintaining a pressure seal. The annular space between resistance coil form 92 and tubular shell 94 is filled with an immersing fluid to cover the resistance coil, not shown.
In the operation of the apparatus shown in Figure 5, thermal insulation shell 91 provides an impediment for the transmission of thermal energy to the fluids passing through skeeter bill 96. Accordingly, a greater proportion of the thermal energy finds its way through tubular shell 94 to the surrounding oil-bearing sand.
Referring now more particularly to Figure 6, Skeeter bill 100 is fitted with double grooved channels 101 for the non-inductive winding of a resistance coil, not shown. Skeeter bill 100 is coated with ferro-enamel coating 102. Tubular shell 103 is coaxially disposed about skeeter bill 100 and thermal insulation shell 104 is coaxially disposed in the annular zone between skeeter bill 100 supporting the resistance coil and tubular shell 103.
The annular space between skceter bill 100 and thermal insulation shell 104 is filled with an immersing fluid, not shown, to submerge the resistance coil, not shown.
Thermal insulation shell 194 offers an impediment to the transmission of thermal energy to and through tubular shell 103 and accordingly a greater proportion of the thermal energy is transmitted to the fluids flowing through skeeter bill 100.
The advantages and operation of the apparatus of this invention will be further understood by reference to the following specific example:
Example A heater was fabricated by coaxially assemblying a 2 in. outer diameter pipe and a 4 /2 in. outer diameter pipe having a in. wall thickness and a resistance coil consisting of 73 ft. of gage 20 Nichrc-me wire. The coil was wound to form a helical coil of about A in. outer diameter and about 12 ft. long. The coiled Nichrome wire was in turn wound on A in. grooved insulated Transite strips and placed inside the annular zone between the two pieces of pipe. An iron-constantan thermocouple was placed inside the coiled Nichrome wire to determine the ambient temperatures thereof during the subsequent testing. The annular space at the lower end was suitably plugged to prevent ingress and egress of fluids.
In the first experiment, no immersing liquid was employed and the annular zone accordingly comprised simply an air bath. The heater was immersed in a SO-gal. drum of water at 80 F. and the electrical leads were connected to 440 volt alternating current source. During the initial 18 minutes of heating, the thermocouple within the Nichrome wire reached a temperature of 511 F. The heating was continued for a total heating period of 3 hours, and the Nichrorne wire temperature remained constant. at about 511 F. during the latter 2 hours and 42 minutes of heating. At the end of the experiment, the water in die SO-gal. drum had reached a temperature of 117 F.
The experiment was repeated after the SO-gal. drum had been cooled to 80 F. and filled with water at a temperature of 80 F. In this experiment the heater was filled with Dc-wtherm A so as to submerge the Nichrome wire coil completely. The heater was placed in the- 50- gal. drum of water and during 3 hours of heating the assume temperature rose to 117 F. The temperature of the Nichrome wire during the same period reached .a maximum temperature of only 200 F.
At the end of the second 3-hour heating experiment, heating was continued until the water reached a temperature of 142 F. at which time the wire temperature was still only 200 F.
In all experiments the current flow remained constant at 5 amps. The heater capacity was therefore 2200 watts.
The foregoing example illustrates the relatively low temperatures which prevail in a Nichrome wire resistance heating coil which is immersed in an electrically inert fluid, such as Dowtherm, when compared to a corresponding air bath coil under the same heat load. Other experiments wherein smaller gauge resistance wire is employed for fabricating the heating element show that heaters employing Dowtherm are operable under very high current flow without danger of burn-out, and that corresponding air-filled heaters overheat and burn out the resistance coil when operated under these high current loads.
It is generally preferable to employ only limited amounts of heat during the heating of an oil-bearing formation. It has been found that heaters operating at the power level of between about 0.02 to 1.0 kilowatt per foot of formation undergoing heating are most efficient for wax bearing wells.
It has also been found that continuous heating of the lower portions of the oil-bearing sand gives a sustained high production rate from a given well. It has further been found that when the heating period is followed by production in a pressured controlled system so as to minimize gas expansion in thewell bore and the neighboring portion of the formation, a high production rate can be maintained even in the absence of continuous heating.
in operating the apparatus of this invention, alternating current is generally preferable to direct current for convenience of handling and availability. Electrolytic corrosion is generally greater when direct current is employed. The voltages employed in the case of alternating current are generally between about 10 to 1000 volts and preferably between about 30 to 600 volts..
Voltages between 5 to 600 volts and preferably between about 20 to 300 volts are employed in the case of direct current.
The foregoing disclosure of this invention is not to be considered as limiting since many variations may be made by those skilled in the art without departing from the spirit and scope of the following claims.
1. A unitary well heating apparatus comprising an imperforate central oil-conducting pipe, an outer heatradiating tubular shell, an upper closure, a lower closure, said upper and lower closures providing a fluid-tight seal between said pipe and said tubular shell, an electrical resistance heating coil disposed between said pipe and said tubular shell and in heat exchange relationship with said pipe and said tubular shell, a body of liquid consisting essentially of the eutectic mixture of diphenyl and diphenyl ether disposed between said pipe and said tubular shell, means for grounding one terminal of said heating coil, an insulated conductor attached to the other terminal of said heating coil, and sealing means for passing said insulated conductor through one of said closures.
2. A unitary well heating apparatus comprising an imperforate central oil conducting pipe, an outer heatradiating tubular shell, an upper closure, a lower closure, said upper and said lower closures providing a fluid-tight seal between said pipe and said shell, a resistance heating coil disposed between said pipe and said tubular shell, said coil being spaced away from said pipe and in heat exchange relationship with said pipe and said tubular shell; a body of heat-conducting electrically insulating liquid consisting essentially of the eutectic mixture of diphenyl and diphenyl ether disposed between said pipe and said tubular shell and surrounding said heating coil, means for grounding one terminal of said coil, an insulated conductor attached to the other terminal of said heating coil, and sealing means for passing said conductor through one of said closures.
3. A unitary well heating apparatus comprising an imperforate central oil-conducting pipe, an outer heatradiating shell in the form of a fluted tube, an upper closure, a lower closure, said upper and lower closures providing a fluid-tight seal between said pipe and said shell, an electrical resistance heating coil disposed between said pipe and said shell and in heat exchange relationship with said pipe and said shell, a body of a heat-conducting electrically insulating liquid disposed between said pipe and said shell, means for grounding one terminal of said heating coil, an insulated conductor attached to the other terminal of said heating coil, and sealing means for passing said insulated conductor through one of said closures.
4. A unitary well heating apparatus comprising an imperforate central oil-conducting pipe, an outer heatradiating tubular shell, an upper closure, a lower closure; said upper and lower closures providing a fluid-tight seal between said pipe and said tubular shell, an electrical resistance heating coil disposed between said pipe and said tubular shell and in heat exchange relationship with said pipe and said tubular shell, a body of a heatconducting electrically insulating liquid disposed between said pipe and said tubular shell, means for grounding one terminal of said heating coil, an insulated conductor attached to the other terminal of said heating coil, sealing means for passing said insulated conductor through one of said closures, a ferro-enamel coating attached to the interior wall to said outer heat-radiating tubular shell, and a ferro-enamel coating attached to the exterior wall of said central oil-conducting pipe; said ferro-enamel coatings preventing electrical contact between said resist ance coil, said tubular shell, and said central oil-conducting pipe.
5. A unitary well heating apparatus comprising a central oil conducting pipe; an outer heat-radiating shell; an upper closure; a lower closure, said upper and lower closures providing a fluid-tight seal between said pipe and said tubular shell; a heat-conducting electrically resistant ferro-enarnel coating applied to the exterior surface of said pipe and the interior surfaces of said closures and said tubular shell; a resistance heating coil non-inductively wound on said ferro-enarnel coating on the exterior surface of said pipe; a body of a heatconducting electrically resistant fluid disposed between said pipe and said tubular shell; means for grounding one terminal of said heating coil; an insulated cable attached to the other terminal of said heating coil; and sealing means for passing said insulated conductor through one of said closures.
6. An apparatus according to claim 5 wherein said electrically inert fluid consists essentially of the eutectic mixture of diphenyl and diphenyl ether.
7. A unitary well heating apparatus comprising an imperforate central oil conducting pipe, an outer heatradiating tubular shell, an upper closure, a lower closure, said upper and said lower closures providing a fluid-tight seal between said pipe and said shell, a resistance heating coil disposed between said pipe and said tubular shell, said coil being spaced away from said pipe and in heat exchange relationship with said pipe and said tubular shell, 21 body of heat-conducting electrically insulating liquid disposed between said pipe and said tubular shell and surrounding said heating coil, means for grounding one terminal of said coil, an insulated conductor attached to the. other terminal of said heating coil, sealing means for passing said conductor through one of said closures; the exterior surface of said pipe and the interior surface of said closures and said tubular shell being coated with a layer of heat-conducting electrically resistant ferroenamel.
Reierences Cited in the file of this patent UNITED STATES PATENTS 1,291,302 Waring Jan. 19, 1919 1,426,407 Pennington Aug. 22, 1922 1,516,836 Williamson Nov. 25, 1924 1,525,656 Redficld -1 Feb. 10, 1925 1,535,776 Hollister Apr. 28, 1925 1,681,523 Downey et al. Aug. 21, 1928 1,835,400 Ingison et a1. Dec. 8, 1931 2,615,114 Colby Oct. 21, 1952 2,647,196 Carpenter et al July 28, 1953