This invention relates generally to the provision of a stabilized timing reference such as is provided by solid state timers, crystal oscillators and the like. More particularly, the present invention is directed to the provision of a stable timing reference in downhole well logging instruments for provision of a stabilized time reference signal for effective instrument operation over a large spectrum of operating temperatures while eliminating typical loss of accuracy due to temperature variations, line loss and skew caused by the resistance and capacitance of the lengthy wireline required for a well logging operation.

Timing devices (i.e., solid state timers, such as crystal oscillators and the like) are well known to be quite sensitive to changes in temperature. Though stable at a certain temperature, changing the temperature degrades their stability. It is desirable, therefore, to provide means for maintaining the timing device within a designated, quite narrow temperature range that permits the timing device to operate at maximum efficiency and accuracy. It is well known that the ambient temperature of the downhole well environment varies with depth and well location. Thus, during a downhole well logging operation, the logging instrument will typically encounter a wide variety of ambient temperatures depending upon its location in the well bore. Some of these temperatures may be above or below the stable operating temperature range of crystal oscillators and other timing devices. It is desirable, therefore, to provide novel means for providing a stable timing reference by maintaining the timing device at a fixed, elevated temperature above that of the maximum ambient temperature of use, while operating in a well logging environment where the ambient temperature varies with depth and well location. It is a principal feature of the present invention to provide a novel high temperature stabilized time base for well logging operations which generates a stable

reference signal at the logging instrument and maintains the signal generating device of the logging instrument within a predetermined, narrow temperature range within which the timing device maintains its operational stability.

It is another feature of this invention to provide a novel high temperature stabilized time base for well logging instruments which maintains the timing device or devices thereof within a predetermined, narrow temperature range regardless of the ambient temperature that is encountered at any particular depth within the well bore.

It is an even further feature of this invention to provide a novel high temperature stabilized time base for logging instruments wherein the timing device or devices are maintained within a narrow temperature range which is an elevated temperature range above that of the maximum ambient temperature of use while operating in a well logging environment where the ambient temperature varies with depth and well location.

It is also a feature of the present invention to provide a novel stabilized timing base system for operational control of well logging instruments which permits controlled maintenance of a timing device such as a crystal oscillator within a temperature range that is less than the highest temperature that will be encountered by a logging instrument at operational depth within a well bore.

Briefly, the various features of the present invention are achieved through the provision of a system for maintaining a high temperature stabilized time base which comprises a low thermal-conduction oven or isolation enclosure of sufficient dimension to accommodate the time base and control circuitry. The low thermal-conduction oven is provided with a heating and/or cooling element capable of maintaining a required mass within a predetermined temperature range. The heating/cooling oven is provided with a control circuit having a temperature sensor that is designed to monitor internal oven temperature and regulate the supply of electrical power to the heater or to the cooling element as the case may be. To provide for stability of the timing device, such as a crystal oscillator for example, a timing device is provided which is specifically designed to have frequency stability at a predetermined elevated temperature range. The circuit components of the time base is maintained at

the predetermined elevated temperature regardless of well temperature to thus ensure the stability of the time base in the downhole environment. Where excessively hot well conditions are encountered, or if the time base circuitry is stable only at a temperature lower than the ambient temperature of the well the protective enclosure for the circuitry may also be provided with controlled cooling to maintain the temperature of the time base within the range of its stability.

So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

Fig. 1 is a block diagram schematic illustration of a high temperature stabilized time base for well logging instruments which is constructed in accordance with the features of the present invention.

Fig. 2 is a sectional view of the heat maintaining oven of the stabilized time base of Fig. 1, showing the temperature generation and control devices together with the time base signal generating device being located within the temperature control oven.

Fig. 3 is a block diagram electronic schematic representing an alternative embodiment of the present invention, for selectively heating or cooling the electronic circuitry.

Referring now to Figs. 1 and 2 of the drawings, a stabilized time base system shown generally at 10 is provided for incorporation within a downhole well logging instrument, a portion of which is shown at 12. The stabilized time base includes a low thermal-conductive oven or isolation enclosure 14 is located within the logging instrument 12 and functions as a thermal isolator which is used to contain heat for the purpose of maintaining the efficiency of electronic circuitry disposed within an internal compartment 16 thereof. The low thermal- conductive oven also functions to

isolate the heat contained therein to lessen the effects of thermal transience to which the instrument 12 might be subjected. The oven structure is also employed as the means of mounting the hardware to a backbone. The cavity or compartment 16 of the oven structure is of a dimension and design for effective support of the electronic time base and heat control circuitry. The oven structure is composed of a material such as polyether ether ketone having a low thermal conductivity and having a characteristic of acceptable thermal expansion. The oven structure is composed of a material that is easily machinable or molded for the complex shapes that may be required within the oven cavity 16. Within the oven cavity 16 is located an electrically energized heating element 18 having the capability of producing sufficient heat to maintain an internal oven temperature at the lowest required ambient temperature that is expected to be encountered within the well bore. The electronic heating element has the capability of being energized from an electric power circuit 23 by electrical energy from a suitable source 25 such as a supply cable, onboard battery, etc.

As shown in the alternative embodiment of Fig. 3, in addition to the heating element 18, there is provided in the oven cavity 16 a cooling element 20 which is electrically energized and controlled through a control circuit 22. The cooling element is utilized where the ambient borehole temperature is above that of the oscillator's stable operating temperature, thus requiring cooling in order to maintain the oven cavity 16 within a predetermined temperature range for stability and efficiency of the timing device. The cooling element may employ Peltier- Junction cooling to lower the oscillator's temperature when required to maintain its operational stability.

Within the oven cavity is provided a temperature sensor 24 which provides temperature signals via conductor 26 to the control circuit 22. In the embodiment of Fig. 1 the control circuit induces energization of the heating element 18 periodically in order to maintain the temperature of the cavity 16 and its circuit components within a narrow predetermined temperature range, being the temperature range within which the timing signal generator is stable. As shown in Fig. 3, the control circuit 22 is selectively coupled via control conductors 28 and 30 to the cooling

element 20 and to the heating element 18. Thus the heating and cooling elements are selectively energized by the control circuit 22 to introduce heating or cooling into the cavity 16 to compensate for heat gain or loss and thereby maintain a predetermined typically quite narrow temperature range within the cavity. Within the oven cavity is also located a signal generator circuit 32 which may conveniently take the form of a crystal oscillator circuit having a predetermined timing frequency output signal via output conductor 34 which extends through the oven wall structure. As shown in Fig. 2 the control circuit 22 may conveniently be defined by a circuit board having appropriate electrical components for receiving the temperature signal of the sensor 24 and providing operation and control signals to the heating element 18 or cooling element 20. The temperature sensor of the control circuit will typically sense temperature of the time base substrate within a range of about 1° C. The temperature is monitored and feedback signals of the temperature sensor are used to control the heater/cooler circuits, thus maintaining the desired temperature within the oven compartment. To maintain the accuracy of the control circuit, the power source for the temperature sensor must also be maintained at the elevated temperature of the oven compartment to eliminate circuit temperature induced circuit drift. Temperature fluctuations outside the oven affect the heat loss of the oven. The control circuitry must sense this change and compensate accordingly. The crystal oscillator is a specially designed part that has integrated temperature stability at the desired elevated temperature. The specific crystal oscillator design is not a part of this invention except to the extent that it and its temperature for effective stability of operation are maintained within an appropriate typically narrow predetermined temperature range and it thus provides a stable timing reference over a large spectrum of operating temperatures ranging from 20° C to 300° C and above, which temperature ranges are common in deep petroleum wells. The crystal oscillator or other frequency output generator is thus capable of providing a consistent and stable output signal which maintains its accuracy even though the apparatus is subjected to temperature variations. Since the electronic timing signal is generated at the downhole well logging instrument, it is not subjected to temperature variations, line loss and

skew that might otherwise be caused by the resistance and capacitance of the lengthy wireline that is ordinarily required for well logging operations. Even though significant changes in ambient temperature are encountered by the well logging instrument during its traverse of the borehole, or its use at different borehole depths, the crystal oscillator or other frequency output device is capable of maintaining a consistent and accurate timing frequency output signal because it is not adversely influenced by ambient temperature changes. Typically, the temperature within the internal cavity of the low thermal conductive oven is maintained at a narrow temperature range that is above the maximum temperature that is expected to be encountered by the logging instrument at any depth in the well bore. In circumstances where the well bore temperature exceeds the predetermined temperature range for which the crystal oscillator is designed to have its frequency stability or time base circuitry is employed which is stable only at a temperature range that is lower than the ambient wellbore temperature at formation depths, the temperature control system will be also provided with a cooling element capable of cooling the temperature of the oven compartment and its circuit components to the desired narrow temperature range.

In view of the foregoing, it is evident that the present invention is one well adapted to attain all of the objects and features hereinabove set forth, together with other objects and features which are inherent in the apparatus disclosed herein. As will be readily apparent to those skilled in the art, the present invention may be produced in other specific forms without departing from its spirit or essential characteristics. The present embodiment, is therefore, to be considered as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within the meaning and range of the equivalence of the claims are therefore intended to be embraced therein.

What is claimed is: