Apparatus and method for time measurement in downhole measurement processes

A time measurement device for a geologic downhole measurement tool is provided. The device includes a plurality of oscillators for measuring a time value. At least one of the plurality of oscillators has a first temperature range that is different from a second temperature range of at least another of the plurality of oscillators. A time measurement system and a method for providing a time measurement are also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The teachings herein relate to formation evaluation tools used in subterranean exploration, and in particular, to devices and techniques for measuring properties of geologic formations.

2. Description of the Related Art

Various tools are used in hydrocarbon exploration and production to measure properties of geologic formations during or shortly after the excavation of a borehole. The properties are measured by formation evaluation tools and other suitable devices, which are typically integrated into a bottomhole assembly.

Such tools provide for the measurement versus depth and/or time of one or more physical quantities in or around a borehole. The taking of these measurements may be referred to as “logging”, and a record of such measurements may be referred to as a “log”.

Examples of logging processes include measurement-while-drilling (MWD) and logging-while-drilling (LWD) processes, during which measurements of properties of the formations and/or the borehole are taken downhole during or shortly after drilling. The data retrieved during these processes may be transmitted to the surface, and may also be stored with the downhole tool for later retrieval.

The tools used in MWD and LWD processes are typically included as part of a bottomhole assembly located at the lower portion of a drillstring, and may include various sensors or transducers for continuously or intermittently measuring properties of the formations and/or borehole.

LWD formation evaluation tools use real-time clocks that, when post-processing the logged data, allow the data to be correlated with associated times and depths. Such clocks allow individual measurements performed during LWD to be assigned specific depths. One pre-condition for assuring accurate time (and thus depth) assignments is that both downhole and uphole clocks run synchronized.

One drawback associated with the use of LWD tools is that the downhole clock is typically subject to great temperature variations. Such temperature variations may occur in the range of, for example, 20 degrees Celcius to 175 degrees Celsius.

It is very difficult to provide a clock or other time measurement device that is accurate over such a large temperature range. Thus, in typical LWD processes, synchronization between the uphole and downhole clocks may be compromised due to inaccuracies in the downhole clock. This results in misalignment of log features recorded during the LWD process.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed herein is a time measurement device for a geologic downhole measurement tool. The device includes a plurality of oscillators for measuring a time value. At least one of the plurality of oscillators has a first temperature range that is different from a second temperature range of at least another of the plurality of oscillators.

Also disclosed herein is a time measurement system for a geologic downhole measurement tool. The system includes: a plurality of oscillators, at least one of the plurality of oscillators having a first temperature range that is different from a second temperature range of at least another of the plurality of oscillators; and a processor for individually selecting one of the at least one and the at least another of the plurality of oscillators to measure a time value, based on an oscillator temperature.

Further disclosed herein is a method for providing a time measurement associated with a geologic downhole measurement. The method includes: positioning a measuring device at a depth of a borehole in a geologic formation, the measuring tool comprising a plurality of oscillators, at least one of the plurality of oscillators having a first temperature range that is different from a second temperature range of at least another of the plurality of oscillators; and selecting one of the at least one and the at least another of the plurality of oscillators based on an oscillator temperature and measuring a time value with the selected oscillator.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description of one or more embodiments of the disclosed system and method are presented herein by way of exemplification and not limitation with reference to the Figures.

Referring toFIG. 1, an exemplary embodiment of a well logging apparatus10includes a drillstring11that is shown disposed in a borehole12that penetrates at least one earth formation14for making measurements of properties of the formation14and/or the borehole12downhole. As described herein, “formations” may refer to the various features and materials that may be encountered in a subsurface environment. Accordingly, it should be considered that while the term “formation” generally refers to geologic formations of interest, the term “formations,” as used herein, may, in some instances, include any geologic points or volumes of interest (such as a survey area).

A downhole tool15may be disposed in the well logging apparatus10at or near the downhole portion of the drillstring11, and may include various sensors or receivers16to measure various properties of the formation14as the tool15is lowered down the borehole12. Such sensors16include, for example, nuclear magnetic resonance (NMR) sensors, resistivity sensors, porosity sensors, gamma ray sensors, seismic receivers and others.

The tool15may also include a clock18or other time measurement device for indicating a time at which each measurement was taken by the sensor16. The tool15may further include an electronics unit20. The sensor16and the clock18may be included in a common housing22. The electronics unit20may also be included in the housing22, or may be remotely located and operably connected to the sensor16and/or the clock18. With respect to the teachings herein, the housing22may represent any structure used to support at least one of the sensor16, the clock18, and the electronics unit20.

The tool15may be operably connected to a surface processing unit24, which may act to control the sensor16and/or the clock18, and may also collect and process data generated by the sensor16during the LWD or MWD process. The surface processing unit24may include components as necessary to provide for processing of data from the tool. Exemplary components include, without limitation, at least one processor, storage, memory, input devices, output devices and the like. As these components are known to those skilled in the art, these are not depicted in any detail herein.

The tool15may be equipped with transmission equipment to communicate ultimately to the processing unit24. Connections between the tool15and the processing unit24may take any desired form, and different transmission media and methods may be used. Examples of connections may include wired, fiber optic, wireless connections or mud pulse telemetry. Further examples of connections may also include direct, indirect or networked connections between the tool15and the processing unit24.

Referring toFIG. 2, the clock18is shown schematically to provide a frame of reference for the description following herein.

The clock18includes a plurality of oscillators30and32, a temperature sensor34, and control circuitry36(or other processor) connected to the temperature sensor34. The temperature sensor may measure an oscillator temperature, which may include a temperature of one or more of the oscillators30,32, the clock18, the tool15and any components thereof The clock18may also include processing circuitry38to process data received from the oscillators30,32. In one embodiment, the oscillators30,32are crystal oscillators, such as oscillators including quartz crystals. Although in the embodiment ofFIG. 2, the clock18includes two oscillators30,32, any number “n” of oscillators (oscillators1through n as shown inFIG. 2) may be used.

Each oscillator30,32includes an associated temperature range, in which the oscillator30,32is at least substantially temperature independent or has a known temperature dependency. As described herein, a “temperature range” associated with a respective oscillator30,32represents a range of temperatures in which the oscillator30,32is at least substantially temperature independent or has a known temperature dependency. The respective oscillator30,32performs most accurately when the respective oscillator has a temperature within the associated temperature range.

In one embodiment, one or more of the oscillators30,32has a temperature range that is defined as a range of temperatures within which the frequency of the oscillator30,32is substantially temperature independent. Accordingly, changes in temperature within the temperature range do not cause any significant change in the output frequency of the respective oscillator30,32.

In another embodiment, one or more of the oscillators30,32includes an associated temperature range in which the frequency of the oscillator30,32has a known temperature dependency. Thus, the oscillation frequency of the oscillator30,32can be accurately obtained based on the temperature of the oscillator30,32within the temperature range, and/or changes in the oscillation frequency can also be accurately obtained based on temperature changes within the temperature range. For example, if the temperature dependency is well characterized in a certain temperature range and the temperature is known (e.g., by measurement), then an accurate time measurement in the certain temperature range can be performed.

In one embodiment, the clock18includes a number of the oscillators30,32, each of which have a respective temperature range. The number and type of the oscillators30,32is selected so that the temperature ranges of each oscillator30,32, when combined, represent a selected overall temperature range. In one example, the oscillators30,32are selected to represent an overall temperature range of approximately 20 degrees Celcius to approximately 175 degrees Celcius. As the temperature ranges of each type of oscillator30,32may be known, each individual oscillator30,32may be selected to cover, when combined, the overall temperature range. In one embodiment, at least one oscillator30,32has a temperature range that overlaps with one or more other oscillators30,32.

The control circuitry36is electrically connected to the temperature sensor34, and is also connected to the oscillators30,32via a switch40. In one embodiment, the control circuitry36, in response to a temperature measurement from the temperature sensor34, actuates the switch40as necessary to select the appropriate oscillator30,32. The “appropriate oscillator” is an oscillator30,32whose temperature range includes the value of the temperature measurement. The appropriate oscillator30,32is thus connected to the circuitry via the switch40

In one embodiment, each oscillator30,32may be individually connected to one or more power sources, such as a battery. The connection to the power source may be controlled by the control circuitry36to selectively power only the appropriate oscillator for a given temperature. This may be useful, for example, in preserving battery life.

The processing circuitry38may be operably connected to the oscillators30,32via the switch40and receive data, such as a time signal, from the oscillator30,32that was selected by the control circuitry36. The processing circuitry38may process the data, for example, by applying any required corrections or compensations to the data. For example, an oscillator30,32may have known compensations associated therewith. The data may also be converted to a real-time format. In addition, the processing circuitry38is in operable communication with a tool processor (not shown) for controlling the tool15, referred to as the “toolmaster” of the FE (Formation Evaluation) tool inFIG. 2. The tool processor may be incorporated with the tool15or may be located remotely, such as at a surface. The tool processor, in one embodiment, includes sufficient storage and processing components to receive data including time signals from the clock18and the sensor16and to process the data, for example, to associate data from the sensor16and data from the clock18at a given depth.

In one embodiment, the clock18does not include separate circuitry or processors, and processing of the data as described above is performed by the control circuitry36.

In one embodiment, in the instance that the temperature ranges of one or more oscillators30,32overlap, the processing circuitry or processor38is configured to select the outputs of the multiple overlapping oscillators30,32and apply at least one statistical operation to the outputs of the oscillators30,32, such as an average and/or a weighted average. A weighted average, in one embodiment, includes one or more weighing factors that depend on parameters such as accuracies of the oscillators30,32at the actual measured temperature. This configuration may allow for increased clock accuracy to compensate for potential temperature dependency of an oscillator30,32even within its temperature range.

Although the present embodiment provides the circuitry36and the processor38to both select the oscillator30,32and process the data received from the oscillator30,32, any number or types of processors, circuits or devices for controlling operation of the clock18and/or processing of data may be provided. Such devices may include any suitable components, such as storage, memory, input devices, output devices and others.

As used herein, generation of data in “real-time” is taken to mean generation of data at a rate that is useful or adequate for making decisions during or concurrent with processes such as production, experimentation, verification, and other types of surveys or uses as may be opted for by a user or operator. As a non-limiting example, real-time measurements and calculations may provide users with information necessary to make desired adjustments during the drilling process. In one embodiment, adjustments are enabled on a continuous basis (at the rate of drilling), while in another embodiment, adjustments may require periodic cessation of drilling for assessment of data. Accordingly, it should be recognized that “real-time” is to be taken in context, and does not necessarily indicate the instantaneous determination of data, or make any other suggestions about the temporal frequency of data collection and determination.

FIG. 3illustrates a method50for providing a time measurement associated with a geologic downhole measurement, such as a downhole measurement performed during a LWD process. The method50includes one or more stages52,54,56and58. The method50is described herein in conjunction with the oscillators30,32, although the method50may be performed in conjunction with any number and configuration of oscillators. The method50may be performed by one or more processors or other devices capable of controlling operation of the oscillators30,32and processing data. In one embodiment, the method includes the execution of all of stages52,54,56and58in the order described. However, certain stages may be omitted, stages may be added, or the order of the stages changed.

In the first stage52, the tool15is positioned at a depth of a geologic formation14. Positioning may include lowering the tool15during drilling of the borehole12or shortly thereafter.

In the second stage54, one or more properties of the formation14and/or the borehole12are measured at the depth of the tool15. This measurement may be accomplished using one or more of the sensors16.

In the third stage56, the temperature at and/or around the clock18is measured using, for example, the temperature sensor34. The control circuitry then selects the oscillator30,32having an associated temperature range that includes the measured temperature. In one embodiment, selection is accomplished by actuating the switch40to connect the selected oscillator30,32to the processing circuitry38. In another embodiment, power from a power source is connected only to the selected oscillator30,32, and power is removed from the remaining oscillators30,32until a new oscillator is selected.

In the fourth stage58, a time value is measured by receiving data from the selected oscillator30,32. In one embodiment, the time value is processed to apply any necessary compensations and/or convert the time value into a real-time format. Also in the fourth stage58, the time value is associated with property measurement data received from the sensor16. Such association may be performed by the tool processor, the processing unit24or any other suitable device.

The above method50may be performed continuously or intermittently as desired. As temperature values received from the temperature sensor34change, the circuitry36(or other suitable processor) may compare each temperature value and select the appropriate oscillator30,32to ensure that an accurate time value is being received for each temperature range.

The systems and methods described herein provide various advantages over existing LWD tools that utilize existing clocks. The systems and methods described provide a highly accurate measurement of time that is not susceptible to variations in temperature experienced as the tool is lowered through the borehole. Accordingly, these systems and methods reduce or eliminate the need to synchronize uphole or surface clocks with the downhole clock described herein. This may be further advantageous in that synchronization downhole is generally not feasible or not precise if mud pulse telemetry is used as a means of communication between uphole and downhole components. Other advantages include both ease of operation and production, especially over large variations in temperature, as the production of multiple oscillators having smaller temperature ranges is more feasible than the production of a single oscillator having a large temperature range.

Further, various other components may be included and called upon for providing aspects of the teachings herein. For example, a sample line, sample storage, sample chamber, sample exhaust, pump, piston, power supply (e.g., at least one of a generator, a remote supply and a battery), vacuum supply, pressure supply, refrigeration (i.e., cooling) unit or supply, heating component, motive force (such as a translational force, propulsional force or a rotational force), magnet, electromagnet, sensor, electrode, transmitter, receiver, transceiver, controller, optical unit, electrical unit or electromechanical unit may be included in support of the various aspects discussed herein or in support of other functions beyond this disclosure.