Apparatus for detecting high temperature in wells
United States Patent Showalter et al.  
 [ 1 Apr. 29, 1975 Hebenstrcit 73/362 AR X Holmes [66/64 X Primary E.ruminerierry W. Myracle Armrncy. Agent. or Firm-Dean Sandford; Richard C. Hartman; Lannas S. Henderson [57] ABSTRACT A device for detecting the occurrence. at any point along a substantial length of a well. of a temperature that exceeds a predetermined value. An elongated temperature sensor having the capability of detecting the occurrence of a high temperature at any point along its entire length is placed in the well so as to extend the length of the zone to be observed. The sensor is connected to a detector at the surface. which on the occurrence of a high temperature can optionally activate an alarm. record the occurrence of the high temperature. and initiate corrective action.  
 15 Claims. 5 Drawing Figures Pmimlimvnz ms 3.880.234  
 SHEET 2 OF 4 iiwi| m vfllllgg1:5  
 a w L a APPARATUS FOR DETECTING HIGH TEMPERATURE IN WELLS This invention relates to wells, and particularly to apparatus for detecting and extinguishing fires in wells employed in the in situ combustion of carbonaceous materials in permeable subterranean formations to recover petroleum therefrom.  
  In situ combustion of a portion of the carbonaceous material in a permeable subterranean reservoir to increase or promote the production of petroleum therefrom is becoming a more widely used oil recovery technique. In this technique of production, air is injected through one or more wells penetrating the oil-bearing stratum to be stimulated. and combustion initiated in the stratum. The resulting combustion zone is caused to move through the stratum by either inverse or direct air drive, whereby the heat of combustion of a proportion of the carbonaceous materials in the stratum mobilizes and usually upgrades a substantial proportion of the unburned material.  
  In the direct drive mode of conducting an in situ combustion operation, air is injected into a petroleumbearing stratum through one or more injection wells in communication therewith and the carbonaceous material ignited in the stratum adjacent to the injection wells. Air injection through the ignition wells is continued to form a combustion zone that progressively moves through the reservoir. The hot combustion gases exiting the combustion zone mobilize petroleum in the stratum and displace the mobilized petroleum toward one or more production wells. In some applications inverse combustion is employed. In this mode of operation, a combustion zone is established around a well by injecting air through the well and into the oil-bearing stratum and, thereafter, air injection through the well is discontinued and air fed through the stratum to the combustion zone from one or more surrounding wells. Also, in situ combustion is employed in the so-called huff and puff&#34; combustion process. In this operation, air is injected through a well and into the oil-bearing stratum. and ignition established. Air injection is continued to form a combustion zone. After this step has been conducted for a period of time, air injection is discontinued and the injection well returned to production. Several cycles of combustion and production can be employed.  
  The in situ combustion processes all require that air be injected through injection wells in communication with the stratum to be stimulated. Experience has shown that combustion can occur in the well or in the formation sufficiently close to the well, to cause damage to the sub-surface well equipment, such as the easing, liner and tubing. Moreover, high temperatures can occur in the well during interruptions in the flow of air through the tubing. such as occur on shut down of the air compressors. High temperature problems can also be encountered in wells producing from a combustion operation, either because of burn through to the producing well, or because of the production of high temperature fluids.  
  Apparatus has been proposed to extinguish well fires and cool hot wells This apparatus typically employs a temperature sensor in the well that detects the occurrence of a high temperature, and a control mechanism at the surface that initiates the flow of cooling water or steam into the well to extinguish the fire or cool the well so as to prevent damage to the well equipment.  
 The use of both thermocouples and thermometers as the temperature sensing element have been proposed. However. both of these divices are subject to serious problems that markedly decrease their effectiveness in detecting high well temperatures. One major problem encountered is that the oil-bearing interval open to the well may be several hundred to several thousand feet thick. Since a thermocouple and some thermometers measure the temperature at one point location, a large number of such devices are nessary to detect the existence of high temperatures in a long interval. While certain thermometers, such as the resistance thermometers, can be utilized to measure temperatures along a substantial distance, such devices measure only the average temperature along the length of the resistance element. Thus, a localized high temperature of sufficient magnitude to damage the well can occur, without raising the average temperature significantly.  
  While the foregoing problems have been described in conjunction with an in situ combustion operation, similar problems are encountered any time that temperatures in a well can exceed an allowable temperature. Hence. need exists for a device to detect the occurrence of a high temperature at any point along a substantial length of a well, and to initiate corrective action to prevent damage to the well.  
  Accordingly, a principal object of this invention is to provide a temperature sensing device capable of detecting the occurrence of a high temperature at any point along a substantial length of the well.  
  Another object of the invention is to provide a temperature sensing device capable of detecting the occurrence of a high temperature at any point along a substantial length of a well employed in an in situ combustion operation.  
  Still another object of the invention is to provide a temperature sensing device capable of detecting the occurence of a high temperature at any point along a substantial length of an air injection well employed in an in situ combustion operation.  
  A further object of the invention is to provide a device for detecting and extinguishing fires occurring in a well.  
  A still further object of the invention is to provide a device for protecting wells from excessive temperatures.  
  The manner of accomplishing the foregoing objects as well as further objects and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawings, wherein-like numerals refer to corresponding parts, and in which:  
  FIG. 1 is a vertical cross-sectional view through a subterranean earth formation schematically illustrating the apparatus of this invention installed in an air injection well completed in a permeable pertroleum reservoir;  
  FIG. 2 is an elevation view in cross-section showing the connector employed to connect the temperature sensing element to the supporting cable;  
  FIG. 3 is a horizontal cross-sectional view of the temperature sensing element taken along the line 33 of FIG. 2;  
  FIG. 4 is a block diagram schematically illustrating the electrical circuitry employed in the monitoring and control unit; and  
  FIG. 5 is a schematic circuit diagram illustrating the electrical circuitry employed in the monotoring and control unit.  
  Referring now to FIG. 1, an injection well is completed in a permeable subterranean reservoir comprised of permeable strata 12, 14 and 16, which can have the same or different permeablilities, interposed between substantially impermeable strata 18. These strata are overlain by overburden lying beneath surface 22. Well 10 is drilled through these strata and provided with casing 24 cemented in place by outer cement sheath 26 around the periphery of the casing. Casing 24 is provided with a conventional well head assembly 28 that forms a closure at the surface end of the casing, and the casing and surrounding cement sheath are provided with perforations 30 that provide communication between the well and the adjacent permeable strata. Tubing 32 extends from the surface to a point adjacent the bottom of well 10. Air, from high pressure source 34, which can be a compressor or other available source of high pressure air, is normally introduced into well 10 through tubing 32 by means of conduit 36, valve 38 and check valve 40. The air is introduced into well 10 at a pressure sufficient to cause it to flow through perforation 30 and into the permeable strata in communication therewith. Also, if desired, air can be diverted directly into the well casing through valve 42 and conduit 44 connected to casing port 46. While FIG. 1 depicts a conventional air injection well completed in a permeable strata. it is to be reconginzed that the foregoing is used for illustrative purposes, and that various modifications of well apparatus and air injection techniques can be employed.  
  ln accordance with the present invention, well 10 is modified by providing a second relatively small diameter tubing 50 extending from the surface through the strata that will be exposed to air. Tubing 50 can be conventional lVz-inch tubing string closed at the lower end and provided with packing gland 52 at the surface end. It is preferred that tube 50 be sealed to prevent the discharge of well fluids to the atmosphere in the event that the tubing develops a leak. Pressure gauge 54 and bleeder valve 56 are provided at the surface end of tubing 50 to indicate the pressure in the tubing, which provides an indication that a leak has been developed. Temperature sensor 58 is suspended within tubing 50 by means of armored conductor cable 60 connected to the sensor by means of connector 62. which will be hereinafter more fully described. Cable 60 provides support for temperature sensor 58 while in the well, provides means for lowering the sensor into position and for withdrawing the sensor from tubing 50, and electrically connects sensor 58 to control unit 64 located at the surface.  
  Control unit 64 detects the occurrence ofa high temperature at any place along the length of temperature sensor 58 and activates various control and warning devices in the event of a high temperature. In the illustrated embodiment, control unit 64 activates alarm horn 66, recorder 68 and opens valve 70 to cause cooling water to flow from high pressure water source 72 through conduit 74 to the well. Cooling water can be introducted into the well through tubing 32 by means of valve 76 and directly into the casing through valve 78 and conduit 80 connected to casing port 82.  
  FIGS. 2 and 3 more specifically illustrate the construction of a preferred embodiment of temperature sensor 58 and the means for supporting the tempera ture sensor within tubing 50. Temperature sensor 58 is comprised of sensing element suspended within a thin outer protective tubing 102. Both sensing element 100 and protective tubing 102 are supported from connector 62, which in turn is attached to the lower end of armored cable 60. Connector 62 provides both support for temperature sensor 58 and electrically connects the sensor to the electrical conductors in the cable.  
  Sensing element 100 is of the type that can detect specific overheat conditions at any point along the length of the sensing element without regard for rate of temperature rise or average ambient temperature. One suitable sensing element is marketed by Fenwal, lncorporated of Ashland, Massachusetts and consists of a small diameter lnconel tube 108 and a coaxial nickel wire 104. A thermally sensitive, eutectic insulating salt composition is packed into the annular space 106 between tube 108 and wire 104, and hermitically sealed therein. These elements are available in various lengths up to 15 feet, and can be coupled together to provide a unit of any desired length.  
  Under normal operating conditions, the salt packed into annulus 106 of the sensor electrically insulates center wire 104 from outer tubing 108. A small voltage is impressed on the sensor element, but the flow of electrical current between the outer tubing and the central wire is restricted by the insulating properties of the eutectic salt compositon. However, when an overheat condition occurs at any point along the entire length of the element, the resistance of the eutectic salt composition drops sharply, allowing a higher current flow between center conductor 104 and outer tubing 108 of the sensor. This current flow is sensed by control unit 64 which produces an output signal to actuate a visible or audible alarm. When the overheat condition is corrected, the resistance of the eutectic salt compound returns to its original value, thereby decreasing the current flow between the inner and outer conductors and returning the unit to standby alert.  
  With this type of sensing element, the control point is determined by the composition of the eutectic salt employed in packing the annulus of the sensing element. Sensing elements are available that function at specified temperatures between about 255 and 900F. The selection of a specific temperature setting, or control point, will depend upon the well environment and the maximum temperature to be protected against. For wells, such as air injection and producing well, employed in in situ combustion operations, it is preferred that the control point of the detector be between about 250 and 400F. Sensors responsive to a selected temperature within this range minimize false alarm conditions, yet rapidly detect localized overheating.  
  Connector 62 is comprised of an outer sleeve 110 provided with internal threads 112 at its upper end and a smaller diameter bottom opening so as to form inwardly protruding peripheral shoulder 114. Externally threaded nut 116 threadably engages the upper end of sleeve 110. Nut 116 is provided with axial bore 118 for cable 60 to pass through and with a peripheral groove 120 to facilitate the grasping of the nut with a fishing tool should it ever be necessary to fish the temperature detector from the well. Radial bores 120 are provided to receive the lugs ofa spanner wrench employed in assembling and disassembling the device. External collar 122 is welded or otherwise fixedly attached to the upper end of tubing 102 to mate with shoulder 114, O ring 124 providing a fluid-tight seal therebetween.  
  Cylindrical insert 126 is fitted within sleeve 110 and is provided with a smaller diameter section 128 adapted to fit into the upper end of tubing 102. lnsert 126 is provided with a longitudinal bore 130 having a necked-down, or small-diameter section 132 intermediate its length. Both ends of insert 126 are provided with internal threads, and the insert is provided with peripheral groove 134 fitted with 0&#34; ring 136 to provide a fluid-tight seal between the insert and the inner wall of sleeve 110.  
  Cable 60 is a two conductor electrical cable having a central core of two electrical conductors 140 and 142 protected by spirally wound armor wires 144. Threaded bushing 146 having a cylindrical lower section 148 is screwed into insert 126. Bushing 146 is provided with a longitudinal bore to receive cable 60. Cable 60 passes through the longitudinal bores of nut 116 and bushing 146, and armor wires 144 are crimped up around the cylindrical lower section 148 of bushing 146. These wires are clamped between bushing 146 and the necked-down section 132 of insert 126 so as to maintain connector 62 securely, but removably, attached to cable 60. Conductor 142 serves as a ground wire and is also clamped between bushing 146 and necked-down section 132. Alternatively. a single wire cable can be employed and the outer armor wires used for grounding. Conductor 140 passes downwardly through bore 130 of insert 126 and is connected to cable connector 150 which is threadably attached to the lower end of insert 126. Sensing element 100 is threadably attached to cable connector 150 by means of sensing element connector 152. Cable connector 150 electrically connects conductor 140 to central conductor 104 of the sensing element, and connects outer tubing 108 to ground.  
  The electrical circuitry of control unit 64 is illustrated in FIGS. 4 and 5. The unit is battery powered and includes a battery charger to convert 1 volt a.c. to 12 volt d.c. The 12 volt d.c. is converted to approximately 8 volt a.c. and amplified for power input to the detector circuitry, which includes an isolating transformer having an output of about 24 volts a.c., a wheatstone bridge and a full wave rectifier to convert the a.c. output to a dc signal. The dc. output signal is amplified and employed to actuate alarm horn 66 and cooling water valve 70, and this output is recorded on recording microampmeter 68 and indicated on microampmeter 316.  
  Referring particularly to FIG. 5, a 115 volt a.c. supply 200 is connected by means of conductors 202 and 204 through fuse 206 and a.c. power switch 208 to the primary of transformer 210 which reduces the l 15 volt a.c. Neutral power conductor 204 is connected to ground by conductor 212. The secondary of transformer 210 is connected by means of conductor 212 and 214 across full wave bridge recitifier 216 which consistis of four bridge-connected diodes. Capacitor 222 and first stage series regulator 224 are parallel connected across the output of rectifier 216 by means of conductor 218 and negative d.c. supply conductor 220. The output of the first stage rectifier is connected in series through second stage rectifier 226 to battery 228. First stage series rectifier 224 includes Zener diodes 230 and 232 and resistor 234 series connected between positive and negative d.c. conductors 218 and 220 by means of conductors 236 and 238, and NPN transistors 240 and 242. The base of transistor 240 is connected to conductor 238, the collector to conductor 218, and the emitter is connected by conductor 244 to the base of transistor 242. The collector of transistor 242 is connected to conductor 218 and the emitter is connected by conductors 246 and 248 through parallel connected resistors 250 and 252 to the collectors of NPN transistors 254 and 256, and through resistor 258 and conductor 260 to the base of transistor 254 and through series connected diode 262 and Zener diode 264 to negative d.c. supply conductor 220. The emitter of transistor 254 is connected to the base of transistor 256 by conductor 266. The emitter of transistor 256 is connected to positive d.c. supply conductor 268, which is connected through fuse 270 and dc. power switch 272 to the positive terminal of battery 228, and dc. power supply conductor 220 is connected to the negative terminal of battery 228.  
  The oscillator includes resistor 280, variable resistor 282 and capacitor 284 connected in series by conductors 286 and 288 between positive d.c. supply conductor 268 and negative d.c. supply conductor 220. Conductor 288 is also connected to the base of NPN transistor 290 and to oscillator 292. One leg of oscillator 292 is connected to positive d.c. supply conductor 268 through resistor 294 and the other leg is connected through resistor 296 to negative d.c. supply conductor 220 and to test jack 302. Negative d.c. supply conductor 220 is connected to test jack 300 and positive d.c. supply conductor 268 is connected to test jack 304. The collector of transistor 290 is connected to positive d.c. supply conductor 268 and the emitter is connected through resistor 306 and parallel connected capacitor 308 and resistor 310 to the base of transistor 312. The emitter of transistor 312 is connected by conductor 314 to one side of microampmeter 316 and the collector is connected by conductor 318 through resistor 320 to positive d.c. supply conductor 268 and to the base of NPN transistor 322. The collector of transistor 322 is connected to positive d.c. supply conductor 268 and the emitter is connected through the primary of transformer 324 and series connected resistor 326 to negative d.c. supply conductor 220. The secondary of transformer 324 is connected by conductors 326 and 328 across Wheatstone bridge 330, which includes bridge connected resistors 332, 334 and 336, and parallel connected capacitor 337 and variable resistor 338. Sensing element 100, not shown, is connected in parallel with resistor 332 Variable resistor 338 is connected in the bridge circuit in parallel with capacitor 337 to provide means for balancing the bridge circuit. Full wave bridge rectifier 340 is connected by conductors 342 and 344 across Wheatstone bridge 330 to convert the a.c. output of bridge 340 to a dc signal. One junction of bridge rectifier 340 is connected to negative d.c. supply conductor 220 and the opposite junction is connected by conductor 346 to the base of NPN transistor 348, through resistor 352 to negative d.c. supply conductor 220, and through capacitor 354 to terminal 4 of recorder 68. The collector of transistor 348 is connected to positive d.c. supply conductor 368 and the emitter is connected through resistor 356 to the base of NPN transistor 358 and through resistor 360 to the other side of microampmeter 316. The collector of transistor 358 is connected by conductor 362 to the number 4 terminal of integrated circuit 364 and through resistor 366 to positive d.c. supply conductor 268.  
  Sensitivity adjustment is provided by variable resistor 368 connected between positive d.c. supply conductor 268 and negative d.c. supply conductor 220, the variable leg of resistor 370 being connected to the number terminal of integrated circuit 364. Positive d.c. supply conductor 268 is connected to the number ll terminal of integrated circuit 364 and negative d.c. supply conductor 220 is connected to the number 7 terminal. The number 10 terminal of integrated circuit 364 is connected through resistor 372 to the base of NPN transistor 374. The collector of transistor 374 is connected through the coil of relay 376 to positive d.c. supply conductor 268 and the emitter is connected to negative d.c. supply conductor 220. Relay contact 378 actuates alarm born 66 through conductors 380, and 384; and relay contact 386 operates cooling water valve 70 through conductors 388 and 390. Switch 392 is provided in positive d.c. supply conductor 268 to valve 70. The output signal from relay contact 386 is also supplied to meter 68 through reistor 394 by means of conductor 396.  
  In operation, the 12 volt power supply is converted to 8 volt a.c. and passed through transformer 324 to isolate any d.c. power from the bridge circuit. Wheatstone bridge circuit 330 is balanced to provide an output signal that is amplified and converted to a d.c. signal. The circuit is adjusted to provide a normal d.c. output of about milliamps. An&#39;alarm condition causes an output of about 70 milliamps. A normal output of less than 20 milliamps is indicative of a mechanical or electrical defect in the sensor or the sensing circuit, or imbalance of the bridge circuit.  
  A testing circuit consisting of resistor 400, push button 402, and timer 404 is connected across resistor 332 and sensing element 100, not shown, by means of conductors 406 and 408. On manually actuating test button 402, or at periodic intervals initiated by timer 404, an alarm condition is simulated. This alarm condition is detected by the sensor circuitry, and alarm horn 66 and cooling water valve 70 actuated and the alarm condition recorded by recorder 68.  
  Various embodiments and modifications of this invention have been described in the foregoing description and examples. and further modications will be apparent to those skilled in the art. Such modiciations are included within the scope of this invention as defined by the following claims.  
 Having now described the invention, I claim:  
  1. A device for detecting the occurrence of a high temperature at any point within a well penetrating a subterranean formation having at least one subsurface interval potentially susceptible to high temperatures. which comprises in combination:  
 a well penetrating said subterranean formation and passing into or through said subsurface interval potentially susceptible to high temperature;  
 a small diameter tubing suspended in said well and extending from the surface through that portion of said subsurface interval potentially susceptible to high temperature that is transversed by the well;  
 an elongated temperature sensor capable of sensing the occurrence of a high temperature at any point along its length, said temperature sensor being comprised of an elongated metallic outer tube and a substantially coaxial center conductor, the annulus between said outer tube and said center conductor being packed with a eutectic meterial having a first electrical resistance at temperatures below the eutectic temperature and a second different electrical resistance at temperatures above the eutectic temperature;  
 cable means for suspending said temperature sensor in said small diameter tubing so as to extend a substantial distance through said interval potentially susceptible to high temperature; and  
 detector means located at the surface and electrically connected to said temperature sensor for detecting the occurrence of a high temperature at any point along the length of said temperature sensor, said detector means including a means for impressing a controlled ac voltage between said center conductor and said outer tube of said temperature sensor, means for measuring the flow of electrical current between said center conductor and said outer tube; and means to generate a d.c. output signal having a value indicative of the current flow between said center conductor and said outer tube.  
  2. The device defined in claim 1 wherein the eutectic temperature of said eutectic material is between about 250 and 900F.  
  3. The device defined in claim I wherein the eutectic temperature of said eutectic material is between about 250and 400F.  
  4. The device defined in claim I wherein said temperature sensor is between about l0 feet and L000 feet in length.  
  5. The device defined in claim 1 including means responsive to said detector means to introduce cooling water into said well upon the occurence of a high temperature in said well.  
  6. The device defined in claim 1 including means responsive to said detector means to actuate an alarm horn upon the occurrence ofa high temperature in said well.  
  7. The device defined in claim 1 including recording means to record the amperage of said d.c. output signal.  
  8. A device for detecting the occurrence of a high temperature at any point within a well penetrating a subterranean formation having at least one subsurface interval potentially susceptible to high temperatures, which comprises in combination:  
 a well penetrating said subterranean formation and passing into or through said subsurface interval potentially susceptible to high temperature;  
 a small diameter tubing suspended in said well and extending from the surface through that portion of the subsurface interval potentially susceptible to high temperature that is tranversed by the well;  
 an elongated temperature sensor having a length between about 10 feet and L000 feet sufficient to extend at least through said subsurface interval potentially susceptible to high temperature and having the capability of sensing the occurrence of a high temperature at any point along its length, said temperature sensor being comprised of an elongated metallic outer tube and a coaxial center conductor, the annulus between said outer tube and said center conductor being packed with a eutectic material having a high electrical reisitanee at temperatures below the eutectic temperature and a low electrical resistance at temperatures above the eutectic temperature;  
 detector means located at the surface and electrically connected to said temperature sensor for detecting the occurrence of a high temperature at any point along the length of said temperature sensor. said detector means including a [2 volt battery, battery charging means to convert 1 volt ac power to regulated 12 volt d.c. power; means to convert said l2 volt d.c. power to a controlled ac. voltage; means for impressing said controlled a.c. voltage between said center conductor and said outer tube of said temperature sensor, means for measuring the flow of electrical current between said center conductor and said outer tube; and means to gener ate a d.c. output signal having a value indicative of the current flow between said center conductor and said outer tube;  
 an armored cable for suspending said temperature sensor in said small diameter tubing adjacent to said subterranean interval potentially susceptible to high temperature and having at least one center conductor electrically connecting said temperature sensor to said detector means; and  
 connector means for connecting said temperature sensor to said cable.  
  9. The device defined in claim 8 wherein said d.c. output signal has a first current value of about milliamps at well temperatures below the eutectic temperature of said eutectic material. and wherein said output signal has a second value of about 70 milliamps at well temperatures above the eutectic temperature.  
  l0. The device defined in claim 8 wherein the eutectic temperature of said eutectic material is between about 250 and 400F.  
  ll. The device defined in claim 8 including means responsive to said detector means to introduce cooling water into said well upon the occurrence ofa high temperature in said well.  
  12. The device defined in claim 8 including means responsive to said detector means to actuate an alarm horn upon the occurrence ofa high temperature in said well.  
  13. The device defined in claim 8 including recording means to record the amperage of said d.c. output signal.  
  14. A device for detecting the occurrence of a high temperature at any point within a well penetrating a subterranean formation having at least one subsurface interval potentially susceptible to high temperatures, which comprises in combination:  
 a well penetrating said subterranean formation and passing into or through said subsurface interval potentially susceptible to high temperature;  
 a small diameter tubing suspended in said well and extending from the surface through that portion of the subsurface interval potentially susceptible to high temperature that is traversed by the well;  
 an elongated temperature sensor having a length between about l0 feet and 1,000 feet sufficient to extend at least through said subsurface interval potentially susceptible to high temperature and having the capability of sensing the occurrence of a high temperature at any point along its length, said temperature sensor being comprised of an elongated metallic outer tubular member and a coaxial center conductor the annulus between said outer tubular member and said center conductor being packed with a eutectic material having a high electrical resistance at temperatures below the eutectic temperature and a low electrical resistance at temperatures above the eutectic temperature; detector means located at the surface and electrically connected to said temperature sensor for detecting the occurrence of a high temperature at any point along the length of said temperature sensor, said detector means including a 12 volt battery, battery charging means to convet ll5 volt a.c. power to regulated l2 volt d.c. power, means to convert said l2 volt d.c. power to a controlled a.c. voltage, means for impressing said controlled a.c. voltage between said center conductor and said outer tubular member of said temperature sensor, means for measuring the flow of electrical current between said center conductor and said outer tubular memher, and means to generate a d.c. output signal hav ing a value indicative of the current flow between said center conductor and said outer tubular memher;  
 an armored cable for suspending said temperature sensor in said small diameter tubing adjacent to said subterranean interval potentially susceptible to high temperature and having at least one center conductor electrically connecting said temperature sensor to said detector means;  
 a fluid tight seal surrounding said armored cable at the upper end of said small diameter tubing; and connector means for connecting said temperature sensor to said armored cable.  
  15. The device defined in claim [4 wherein said connector means includes (1) an outer sleeve internally threaded at its upper end and having an internal bore of reduced diameter at its lower end; (2) an externally threaded nut having a central bore to receive said armored cable threadably attached at the upper end of said sleeve; (3) a cylindrical insert adapted to slidably fit within said sleeve, said insert having a longitudinal bore, the upper and lower ends of said bore being internally threaded and said bore having a restricted crosssection at a point intermediate its length; (4) a bushing threadably attached to the upper end of said insert, said bushing having a longitudinal bore to receive said armored cable and a smaller diameter shank terminating in a seat to bear against the restricted section of said insert and clamp the outer armor wires of said armored cable therebetween to attach said connector means to said cable; (5) a tubular member extending downwardly from said sleeve to house said temperature sensor; and (6) a threaded cable connector to threadably attach said temperature sensor to said insert and to electrically connect said center conductor of said armored cable to said temperature sensor.