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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable.  
       BACKGROUND  
       [0003]     The present invention relates generally to methods and apparatus for operating electronic components on a downhole tool within a wellbore. More particularly, the present invention relates to methods and apparatus for controlling the temperature of downhole electronic components.  
         [0004]     Many wellbore logging and evaluation tools utilize electronic components to gather data from the wellbore and surrounding formation and transmit that data back to the surface. Because the temperature within a wellbore increases with depth, these electronic components are routinely exposed to very high ambient temperatures. The temperature of the electronic components is also increased by the consumption and production of power by the electronic components themselves.  
         [0005]     Many of these electronic components may be temperature sensitive components that may face degrading performance with increasing temperatures. Further, some of the electronic components may only satisfactorily operate within a certain range of temperatures. Therefore, as the complexity and sophistication of the electronic components disposed within downhole tools increases, methods and apparatus for cooling these components take on greater importance.  
         [0006]     Several downhole electronic component cooling systems have been developed that use an array of temperature control technologies. Some of these systems are passive systems that seek to insulate the electronic components to delay the inevitable temperature increase. These passive systems extend the operating life of the tool but may or may not provide sufficient operating life to accomplish the desired analysis.  
         [0007]     Active systems are also available that cool the electronic components through refrigeration or some other temperature control technique. Active systems require a source of power, such as a supply of chilled fluid from the surface or electricity from a battery or turbine located downhole. The sources of power are often limited and the power consumed by the cooling system reduces the power available to the electronic components to perform the desired monitoring.  
         [0008]     There remains a need to develop more efficient methods and apparatus for controlling the temperature of downhole electronic components that overcome some of the foregoing difficulties while providing more advantageous overall results.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0009]     The problems noted above are solved in a large part by apparatus and methods for operating an electronics assembly of a downhole tool. A method comprises disposing a temperature-sensitive electronic component within an insulated chamber contained within a downhole tool. The temperature of the temperature-sensitive electronic component is monitored and a temperature control system is selectively activated to regulate the temperature of the temperature-sensitive electronic component. A downhole electronic assembly comprises a temperature-sensitive electronic component and a temperature-tolerant electronic component in electrical communication with the temperature-sensitive electronic component. An insulating chamber provides a thermal barrier between the temperature-sensitive electronic component and the temperature-tolerant electronic component. A temperature control apparatus in thermal communication with the temperature-sensitive component.  
         [0010]     Thus, the present invention comprises a combination of features and advantages that enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For a more detailed description of the preferred embodiment of the present invention, reference will now be made to the accompanying drawings, wherein:  
         [0012]      FIG. 1  is an illustration of a drilling rig including a measurement while drilling tool;  
         [0013]      FIG. 2  is a schematic illustration of an electronic assembly of a downhole tool; and  
         [0014]      FIG. 3  is a schematic illustration of a cooling system constructed in accordance with embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Referring now to  FIG. 1 , a drilling rig  10  supports and drives a drill string  12  to form a wellbore  14 . Located at the distal end of drill string  12  is a bottom hole assembly  16  comprising drill bit  18  and monitoring tool  20 . Monitoring tool  20  includes power supply  22  and electronics module  24 . Electronics module  24  includes data acquisition module  26  and data storage and transfer module  28 . Acquisition module  26  gathers data, such as seismic data or pressure data, from wellbore  14  and/or the surrounding formation. This data is stored in storage and transfer module  28  and communicated to surface module  30 , via cable connections, sonic signals, or wireless telemetry.  
         [0016]     Referring now to  FIG. 2 , electronics assembly  200  is shown including power supply  202 , temperature-tolerant electronic component  204 , temperature-sensitive electronic component  206 , and temperature control system  208 . Temperature-sensitive electronic component  206  is isolated within insulated chamber  210 . Temperature control system  208  includes temperature sensor  212 , temperature regulator  214 , and controller  216 . Sensor  212  and regulator  214  are partially disposed within, or in thermal communication with, insulated chamber  210 . Controller  216  may be located outside of chamber  210 .  
         [0017]     Temperature-tolerant component  204  includes those electronics that have operating envelopes including relatively high temperatures and those components that tend to generate large amounts of heat during operation. Temperature-sensitive component  206  includes one or more components that have operating characteristics significantly dependent on temperature. This temperature dependence may be manifested in a variety of ways, including a degradation of performance, an inability to fully function, and a limitation on stability.  
         [0018]     In operation, controller  216  monitors the temperature inside chamber  210  via sensor  212 . Controller  216  operates regulator  214  that adds or removes heat from chamber  210  in order to maintain a desired temperature for temperature-sensitive component  206 . Once the temperature within chamber  210  reaches a desired level, controller  216  shuts down regulator  214 . Regulator  214  may be periodically activated to keep the temperature within chamber  210  within a desired range. By isolating temperature-sensitive component  206  within chamber  210 , the mass of the temperature controlled components and the required heating load can be reduced.  
         [0019]     Assembly  200  may include one or more chambers  210  isolating separate temperature-sensitive components  206 . Each separate chamber  210  may have its own temperature control system  208  such that each temperature-sensitive component  206  can be maintained within a selected temperature range independent of the temperature ranges for the other components.  
         [0020]     Referring now to  FIG. 3 , temperature control assembly  300  is shown including temperature-sensitive component  302 , thermoelectric cooler  304 , heat sink  306 , insulated chamber  308 , temperature sensor  310 , thermostat  312 , and amplifier  313 . Temperature-sensitive component  302  is disposed within insulated chamber  308  and in thermal contact with thermoelectric cooler  304 . Heat sink  306  is located outside of insulated chamber  308  and in thermal contact with thermoelectric cooler  304 . Thermostat  312  monitors the temperature within chamber  308  via sensor  310  and provides power to thermoelectric cooler  304  through amplifier  313  and electrical connection  314 . Amplifier  313  may be a class-D amplifier, liner amplifier, variable switch-mode power supply, or a switching amplifier.  
         [0021]     Thermoelectric cooler  304  is a Peltier-type device comprising p-type semiconductor  316  and n-type semiconductor  318  sandwiched between two conductive plates  320 ,  322 . Semiconductors  316 ,  318  are connected electrically in series and thermally in parallel. Conductive plates  320 ,  322  have a high thermal conductivity and are often a ceramic material, such as a metallized beryllium oxide and/or an aluminum oxide. In certain embodiments, conductive plate  320  may be integrated into component  302 . A DC voltage is applied through electrical connection  314 .  
         [0022]     A positive DC voltage applied to n-type semiconductor  318  causes electrons to pass from p-type semiconductor  316  to n-type semiconductor  318 . As these electrons pass to n-type semiconductor  318  they absorb heat, essentially causing heat to flow from conductive plate  320  to conductive plate  322 . This, in effect, acts as a heat pump, transferring heat from temperature-sensitive component  302  to heat sink  306 .  
         [0023]     A negative DC voltage applied to n-type semiconductor  318  has the reverse effect and causes electrons to pass from n-type semiconductor  318  to p-type semiconductor  316 . As these electrons pass to p-type semiconductor  316  they absorb heat, essentially causing heat to flow from conductive plate  322  to conductive plate  320 . This, in effect, acts as a heat pump, transferring heat to temperature-sensitive component  302  from heat sink  306 .  
         [0024]     Semiconductors  316 ,  318  may be fabricated from an alloy of bismuth, telluride, selenium, and antimony and may be doped and processed to yield polycrystalline semiconductors with anisotropic thermoelectric properties. A plurality of thermoelectric coolers  304  may be stacked in a multistage or cascading arrangement to increase the potential thermal transfer through the cooler.  
         [0025]     Temperature control assembly  300  can be operated in a first mode where thermostat  312  is utilized to maintain the temperature within chamber  308  within a selected temperature range. For example, temperature-sensitive component  302  may be a temperature compensated zener diode being used as a voltage reference in a downhole application. Further, the zener diode may be specifically constructed to have a zero temperature coefficient (ZTC) at or near 150° C., normally the ZTC point is engineered to occurs at approximately 25° C. or ambient room temperature.  
         [0026]     Thermostat  312  is used to control the environment of the zener diode and other temperature sensitive components within chamber  308 . Thermostat  312  senses the temperature within chamber  308  via sensor  310  and operates thermoelectric cooler  304  to maintain the temperature at 150° C.+/−2° C. As operation of the downhole tool is initiated, the temperature within chamber  308  can be increased to within the desired range by operating thermoelectric cooler  304  as a heater. Once the tool is downhole and subjected to higher ambient temperatures, the thermoelectric cooler  304  can be operated as a cooler to maintain the temperature within the desired range.  
         [0027]     Operation in this mode allows the temperature sensitive components to operate at a relatively constant temperature and effectively shifts much of the burden of stabilization to the accuracy of the thermostat and thus away from having to perform higher order curvature corrections. Regulating the temperature of the selected components provides an efficient and cost effective way of stabilizing the output voltages of the zener diode voltage reference without any high order curvature correction schemes.  
         [0028]     Temperature control assembly  300  can also be operated in a second mode where thermostat  312  is utilized to intermittently maintain the temperature within chamber  308  within a selected temperature range. For example, temperature-sensitive component  302  may be a memory storage component in a downhole application. Many memory components can effectively store data at higher temperatures than are allowable for reading and writing to the memory.  
         [0029]     Thermostat  312  can be used to control the environment of the memory components within chamber  308 . Thermostat  312  senses the temperature within chamber  308  via sensor  310 . When data is ready to be written to, or read from, the memory thermoelectric cooler  304  is operated to reduce the temperature to within the allowable range. Once the read/write process is complete, thermoelectric cooler  304  is deactivated and the temperature within chamber  308  is allowed to increase.  
         [0030]     Batch cooling the memory modules in this manner allows for more efficient use of power from a limited supply of power often associated with a downhole application. This batch cooling method could also be used with a voltage reference to cool the reference only when being used or with a calibration reference that benefits from being calibrated to a controlled temperature. Batch cooling methods could also be used with other temperature control and refrigeration systems and are not limited to use with thermoelectric coolers.  
         [0031]     While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied, so long as the apparatus retain the advantages discussed herein. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.

Summary:
Apparatus and methods for operating an electronics assembly of a downhole tool. A method comprises disposing a temperature-sensitive electronic component within an insulated chamber contained within a downhole tool. The temperature of the temperature-sensitive electronic component is monitored and a temperature control system is selectively activated to regulate the temperature of the temperature-sensitive electronic component. A downhole electronic assembly comprises a temperature-sensitive electronic component and a temperature-tolerant electronic component in electrical communication with the temperature-sensitive electronic component. An insulating chamber provides a thermal barrier between the temperature-sensitive electronic component and the temperature-tolerant electronic component. A temperature control apparatus in thermal communication with the temperature-sensitive component.