Patent Publication Number: US-10329898-B2

Title: High temperature downhole gauge system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/884,478, filed Nov. 16, 2011, which is a National Stage Application under 35 U.S.C. 371 of International Application No. PCT/GB2011/52230, filed Nov. 16, 2011, which claims priority to Great Britain Patent Application No. 1019567.5, filed Nov. 19, 2010, the contents of all of which are incorporated herein by reference. 
    
    
     The present invention relates to permanent monitoring in oil and gas wellbores and in particular, though not exclusively, to a gauge system for monitoring well pressure at temperatures in excess of 300° C. 
     Downhole pressure and temperature gauges are routinely installed in wellbores to provide continuous reservoir monitoring. The gauge is typically mounted in a side pocket on the tubing. A cable is run up the outside of the tubing to connect the gauge to surface where signal processing and control is carried out. Current gauges have short life spans and give erroneous results when installed in very high temperature environments where the point of measurement is a long way from the measurement apparatus. Such environments are found in deep geothermal wells, deep oil wells, and in oil production schemes which use heating methods to reduce the viscosity of oil such as steam injection and in situ combustion. 
     Further disadvantages are found in using current technology to measure pressure in deep well bores and at extremely high temperatures. While transducers exist that can operate at very high temperatures, they are not able to withstand the hostile downhole environment nor can they be operated across a long cable. The transducers use wire connections which are difficult to form or operate reliably in very high temperature environments. Long cables are currently only available with either rubber of plastic insulation which does not survive at high temperatures. Some long cables using glass fibre exist but the glass becomes conductive and as such not suitable for this application. As cable length increases, greater errors and variable offsets in analogue measurements occur, producing poor quality measurements. 
     We define a long cable as having a length in excess of 1000 meters, and a high temperature being in excess of 300° C. 
     It is an object of the present invention to provide a downhole pressure gauge operable in deep wellbores at high temperatures. 
     It is a further object of the present invention to provide a method of obtaining a pressure measurement in a deep wellbore at high temperatures. 
     According to a first aspect of the present invention there is provided a gauge system for use in wellbores, the system comprising an analogue output transducer and a long cable, wherein the cable is an extruded mineral insulated multi-core with a seam welded corrosion resistant metal outer sheath; the transducer is enclosed in a pressure tight corrosion resistant housing; and the housing is pressure sealed to the metal outer sheath. 
     In this way, a transducer which is operational at high temperatures, coupled to a specially adapted mineral insulated (MI) cable can be used to continuously monitor in a high temperature wellbore. 
     Preferably, the transducer is an analogue output pressure sensor. More preferably, the pressure sensor is a silicon-on-insulator sensor. Preferably also, the sensor is configured in a Wheatstone bridge arrangement. Such a mono-crystalline silicon sensor has no pn junction and is dielectrically isolated. Accordingly, the temperature is not limited to 150° C. and can operate at temperatures over 600° C. 
     Preferably, the transducer is configured as a strain wire type. More preferably, the transducer has a ceramic mounting. 
     In this way, operation at extremely high temperature can be achieved. 
     Preferably the cable comprises at least one elongate electrical conductor within a metal sheath, the conductor being insulated from the sheath by means of a compacted mineral insulating powder. Such cables are known, at short lengths narrow diameters, for use in systems operating in fires. These cables are unsuitable for use in deep wells as the metal sheath is highly corrosive. Advantageously, the cable includes a further sheath of corrosion resistant metal, wrapped as a thin strip around the cable and seam welded. Preferably, the cable is drawn down to a diameter of approximately 0.25 inch. 
     Preferably, the cable includes at least one additional core and at least one analogue temperature sensor, wherein the temperature sensor is mounted on the cable. In this way, temperature measurements can be made along the cable to assist in signal correction at surface. 
     Preferably, connectors within the housing are plastic and rubber free. More preferably, the connectors are formed from metal contacts and ceramic insulators. In this way, the connections will not be affected by the high temperatures. 
     Preferably, the seals are plastic and rubber free. More preferably, the seals are metal seals which are able to withstand the high temperatures. 
     Preferably, the corrosion resistant materials and metals are stainless steel. Alternatively, the corrosion resistant materials and metals are Inconel alloys. The selection of material can be made based on the nature of the fluids likely to be in the well. 
     Preferably, the system includes an AC power supply. Alternatively, the system includes a slow DC reversing power supply. By alternating the power supply, offsets created by permanent charge build-ups over a long cable, can be measured and removed. 
     Preferably, the cable includes at least one passive sense wire. By measuring voltage drops in the cable and insulation loss across the cable, the actual voltage delivered to the sensor can be determined. 
     Preferably the system includes a signal conditioning and processing unit. In this way, the output from the transducer can be corrected to provide accurate measurements over periods of time to provide continuous monitoring. 
     According to a second aspect of the present invention there is provided a method of continuous monitoring in a wellbore, the method comprising the steps:
         (a) connecting an analogue output transducer to a long cable of extruded mineral insulated multi-core with a seam welded corrosion resistant metal outer sheath;   (b) mounting the transducer and connector in a pressure tight corrosion resistant housing;   (c) pressure sealing the housing against the metal outer sheath;   (d) mounting the housing on tubing and running the tubing into a wellbore, to a high temperature location;   (e) taking measurements from the transducer;   (f) applying signal conditioning and processing to the measurements to compensate for characteristics of the transducer, the cable and the environment and thereby provide continuous monitoring of the wellbore.       

     In this way, continuous monitoring of a high temperature wellbore can be achieved. 
     Preferably, the transducer is a pressure transducer. More preferably, the method includes the steps of characterising the transducer behaviour over a range of temperatures and pressures. In this way, the measurements can be mathematically compensated to provide more accurate results. 
     Preferably, the method includes the step of measuring temperature in the wellbore. In this way, the mathematical compensation can be more correctly applied. 
     Preferably, the method includes the step of monitoring insulation leakage and compensating for this. In this way, any loss measured can be used to compensate the transducer output for the same loss. 
     Preferably, the method includes the step of alternating the power supply, measuring the permanent charge build-ups on the cable and removing this error from the measurements. This removes the offsets caused by the residual charge effect which occurs over long cables. 
     Preferably, the method includes the step of filtering the received signal at a frequency of the AC power supply. This assists in noise reduction on the measurements. 
    
    
     
       Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, of which: 
         FIG. 1  is a schematic illustration of a gauge system for use in a wellbore, according to an embodiment of the present invention; 
         FIGS. 2( a ) and ( b )  are schematic illustrations of a cable, for use in the gauge system of  FIG. 1 ; 
         FIGS. 3( a )-( c )  are schematic illustrations of a connector, for use in the gauge system of  FIG. 1 ; 
         FIG. 4  is a schematic illustration of a housing, for use in the gauge system of  FIG. 1 ; and 
         FIG. 5  is a schematic illustration of a gauge system, illustrating a method of monitoring, according an embodiment of the present invention. 
     
    
    
     Referring initially to  FIG. 1 , there is illustrated a gauge system, generally indicated by reference numeral  10 , for use in a wellbore at high temperatures. Gauge system  10  comprises an analogue output transducer  12  enclosed in a pressure tight corrosion resistant housing  14 , and the housing  14  is pressure sealed against a long cable  16 , which connects the gauge system  10  to the surface of the wellbore. 
     Transducer  12  is a pressure transducer. It is a silicon-on-insulator semiconductor piezo-electric pressure sensor. It is formed from two wafers, one forming the p-type diffused mono-crystalline silicon piezoresistors in a Wheatstone bridge arrangement while the second acts as a silicon diaphragm. The wafers are fusion bonded together. The entire chip is mounted on Pyrex® and encapsulated to be hermetically sealed. The sensor can operate in temperatures in excess of 600° C. and measure pressures over 5000 psi. These sensors are manufactured by Kulite Semiconductor Products Inc., New Jersey, USA. 
     The sensor  12  output from the Wheatstone bridge is designed with a high impedance, typically several kilo ohms. This is in contrast to short range sensors of several hundred ohms. This improves the transmission of the signals to surface. 
     The long cable  16  is attached to the transducer  12 , as will be described hereinafter with reference to  FIG. 4 . In this specification we define long as in excess of 1000 meters. Reference is now made to  FIG. 2  of the drawings which illustrates an (a) cross-sectional view and (b) plan view of a cable  16 , according to an embodiment of the present invention. 
     Cable  16  comprises seven conductors  18 , encased in a mineral insulator  20 , enclosed in a metal sheath  22  which is surrounded by a non-corrosive metal outer sheath  24 . The metal sheath  22  may be copper. 
     Conductors  18  run the entire length of the cable  16  and are made of high temperature materials such as stainless steel or nickel based alloys. Examples of nickel based alloys include alloys of nickel with copper, e.g. having between 25% and 75% nickel, such as cupronickel, and Monel, and other alloys comprising nickel with chromium and/or with cobalt, e.g. Ni,Cr,Fe,Co alloys. Examples of such alloys include those sold under the trademarks ‘Inconel’ and ‘Incoloy’. Alternatively pure nickel may be employed. 
     The mineral insulator  20 , is silica combined with a binder material such as a metal oxide or boron nitride. The silica is selected to withstand the high temperatures while the binder is used to prevent deformation of the silica during the cable formation process which can cause splitting of the sheath  22 . 
     MI cables having a single metal sheath are currently produced by an extruding technique. For example, a strip of metal and one or more elongate conductors are transported along their length and the strip is continuously formed into a tube that encloses the or each conductor, opposed longitudinally extending edges of the strip are welded together, mineral insulant is inserted into the tube to form a cable preform. The cable preform is then continuously drawn through sets of shaped rollers which reduce the cross-sectional area of the preform by about 80 to 99%. Annealing stages are located between the sets of rollers and after the last set of rollers. Typically, the drawing stage will comprise three sets of rollers, each with 14 pairs of rollers arranged at 90° with respect to each other, and three annealing stages. The annealing temperature usually lies in the range of 350 to 6500 C, depending on the speed of the cable preform through the annealing stage. This process provides a typical seven core cable of diameter approx. 11 mm and length approx. 350 m. 
     In an embodiment of the present invention, the cable  16  is manufactured by a similar technique, however, an outer sheath  24  comprising a non-corrosive material such as stainless steel or Iconel, is formed from a strip, seam welded around the metal sheath  22 . In order to achieve the desired diameter of a quarter inch (6.35 mm) and continuous lengths in excess of 1000 m, a significantly larger preform is created and drawn through a greater number of sets of rollers and annealing stages. A resulting coil of cable  16  will have a diameter of a few meters. 
     Returning to  FIG. 1 , the cable  16  is attached to the transducer  12 , using a specially constructed connector  26 . Connector  26  is shown in greater detail in  FIGS. 3( a )-( c ) . Connector  26  is formed in two parts and based on crimping connector technology. Transducer  12  has a series of connector pins  28  ( FIG. 3( b ) ). A connector body  30 , formed of ceramic, has machined ducts  32 . High temperature cable strands  34  are arranged for connection to cable  16  (not shown). Strands  34  are fed through the ducts  32  and terminate at metal tubes  36 . This is shown in  FIG. 3( a ) . Each tube  36  locates over a corresponding connector pin  28 . The tube  36  is then crimped onto the connector pin  28 . Body  30  is pushed towards the transducer  12 , with the crimped tubes  36  sliding into the ducts  32 . When assembled, see  FIG. 3( c ) , all connections are encased in the ceramic body  30 . The connector  26  provides a field serviceable connection scheme which is then used in conjunction with the sensor  12  and cable  16  to allow large cable drums to be shipped separately from the wellbore sensor  12 . This arrangement also allows replacement of sensors  12 . 
     Returning again to  FIG. 1 , the gauge system  10  having the transducer  12  connected  26  to the cable  16 , is now enclosed in a housing  14 . At a first end the cable  16  connects the gauge system  10  to surface and at a second end a pressure port  50  for measuring the pressure downhole. The housing  14  is constructed of a non-corrosive metal or metal alloy. Typically, stainless steel is used. The housing  14  has a body and a base  38  for assembly. The housing  14  must be both pressure tight and protective to the transducer  12 . 
     To make the housing  14  pressure tight, metal seals are used. Referring to  FIG. 4 , the housing is illustrated with the main seal  40  between the body  36  and the base  38 . The pressure sealing comprises a chamber  42  in which the pressure transducer  12  is housed, a base  38  to which the internal components are mounted, an outer pressure housing  36 , and a metal seal  40 , in the form of a ring, which has a lower yield strength either by material selection or mechanical size than the housing  14 . 
     The energy used to tighten the outer housing  36  to the base  38  energises the seal  40  and creates a pressure sealing contact between the seal  40 , base  38  and body  36 . This seal  40  has to be energised so that the seal remains sealed even when the metal parts are heated to very high temperatures. Accordingly, the body  36  is designed such that increasing hydrostatic pressure on the outside of the system  10  will apply greater compression force to the seal  40 . In this way, the sealing energy increases with increasing pressure. 
     An identical sealing arrangement is used to seal the housing  14  against the cable  16 . Returning to  FIG. 1 , a metal sealing ring  44  is located against the outer sheath  24  of the cable  16 . A seal nut  46  is tightened against the housing body to energise the seal  44 . As with the opposing seal  40 , between the body  36  and the base  38 , the seal is arranged so that hydrostatic pressure outside the system  10  applies greater compression force to the seal  44 . While the metal seals  40 , 44  are described as metal rings, hermetic welding could be used instead. As with the housing parts, the seals are plastic and rubber free, formed from a corrosion resistant material which can withstand high temperatures. 
     Reference is now made to  FIG. 5  of the drawings which illustrates a method of operating the gauge system  10 . It is recognised in downhole monitoring that operating a sensor at such distances from the power and signal processing, has inherent difficulties. The present invention provides a number of embodiments to overcome these disadvantages. The signal from the transducer is conditioned and processed to obtain accurate results from this system. 
     Initially, the sensor i.e. transducer  12  must first be calibrated for use in the high temperature and high pressure environment. This provides characterisation of the sensor behaviour over pressure and temperature and facilitates use of mathematical compensation to achieve more accurate results. Typically the transducer is tested at five different temperatures and ten pressures at each temperature. The output is then be used to generate a 3 rd  or 5 th  order polynomial curve fit over pressure and temperature. This both uses a temperature sensor  52  in the downhole module  54  to compensate the pressure output changes with temperature, and also compensates for any non-linearity in the transducer  12  behaviour over a range of temperatures. 
     As the sensor  12  is designed with a high impedance output, in the kilo-ohms range, this reduces the effect of the resistance of the cable wires which may also be hundreds of ohms and may change with temperature. 
     The sensor output is further enhanced by use of sense wires in the cable assembly  16  to sense the actual voltage delivered to the sensor  12 . These sense wires are used to sense insulation loss or leakage in the cable  16  by use of an open circuit wire for further compensation. This uses an unused wire  56  in the cable  16  and senses the loss of current to earth (or to the cable sheath  24 ). Any loss measured is used to compensate the live transducer output for the same loss. 
     At surface, a control and signal processing module  60  controls power to and processes the signals received from the downhole module  54 . 
     The power supply is an AC power supply  62 . It is known that continuous DC current on a long cable can cause permanent charge build ups, and creates offsets in the readings. By alternating the power supply these offsets are measured and removed. The offsets are measured using a mV sensor  68 . Those skilled in the art will appreciate that a slow DC reversing system could also be used to remove any residual charge effect from the long cable. 
     To remove noise on the signal created by other electrical machinery or electronics equipment near the sensor system, a selective tuned filter  64  is located at the input to the signal processing unit  66 . The filter  64  is tuned to the frequency of the AC supply  62 . In this way the sensor output is extracted from the received signal. 
     The principle advantage of the present invention is that it provides a downhole pressure gauge operable in deep wellbores (over 1000 m) at high temperatures (over 300° C.). 
     A further advantage of the present invention is that it provides a method of obtaining a pressure measurement in a deep wellbore at high temperatures. 
     A yet further advantage of the present invention is that it provides a downhole continuous monitoring system in deep wellbores at high temperatures. 
     Various modifications may be made to the invention herein described without departing from the scope thereof. For example, any HP/HT sealing arrangement may be used. Additional sensors may also be mounted in the housing.