Abstract:
A borehole caliper tool includes a tool body, a bow spring flexibly coupled to the tool body, a target coupled to the bow spring, and an ultrasonic transducer coupled to the tool body, wherein in operation the ultrasonic transducer transmits an acoustic pulse to the target and receives an echo of the acoustic pulse from the target.

Description:
FIELD OF INVENTION  
       [0001]     The invention relates to tools for obtaining subsurface measurements. More specifically, the invention relates to techniques for determining the dimensions of a borehole.  
       BACKGROUND OF INVENTION  
       [0002]     Various caliper tools for gauging the diameter of a borehole are known in the art. In one example, a caliper tool includes one or more bow springs coupled to a tool body. When the tool body is disposed in a borehole, the bow spring engages the borehole wall and expands and contracts as the tool body traverses the borehole and the borehole diameter changes. The motion of the bow spring can provide an indication of the borehole diameter. In this case, a sensing device can be attached to the bow spring and used to monitor the motion of the bow spring. This is taught, for example, in U.S. Pat. No. 2,639,512. Some caliper tools further include one or more rigid arms coupled between the tool body and the bow spring. The rigid arm deflects as the bow spring expands and contracts, and the motion of the rigid arm provides an indication of the borehole diameter.  
         [0003]     An electronic sensing device having a movable part is usually used to monitor the motion of the rigid arm. Typical examples of these electronic sensing devices include linear variable differential transformer (LVDT) and potentiometer sensors. An LVDT sensor includes a ferromagnetic core disposed within a series of inductors and produces electrical output proportional to the physical position of the ferromagnetic core within the series of inductors. A potentiometer sensor includes a slider attached to a resistor and produces electrical output proportional to the contact position of the slider on the resistor. The caliper tool uses a mechanical linkage to couple the movable part of the sensing device to the rigid arm so that the electrical output generated by the sensing device is representative of the motion, or deflection, of the rigid arm.  
         [0004]     The mechanical linkage is required to satisfy various requirements. For example, the mechanical linkage is required to fit in a small space on the tool body and work in the hydrostatic pressure of the borehole, which frequently exceeds 20,000 psi (138 MPa), and in the presence of drilling mud, which typically contains debris. The mechanical linkage must be mechanically tight to avoid introducing errors in translating the position of the rigid arm to the sensing device. To allow attachment to the mechanical linkage, the movable part of the sensing device would either have to be exposed to borehole pressure and drilling fluid or be located in a compensator filled with oil at borehole pressure.  
         [0005]     As evident from conventional configurations, physically linking the sensing device to a rigid arm complicates the design and operation of a caliper tool. A caliper tool that does not require a mechanical linkage to directly translate motion of an arm to a sensing device is desired.  
       SUMMARY OF INVENTION  
       [0006]     In one aspect, the invention relates to a borehole caliper tool which comprises a tool body, a bow spring flexibly coupled to the tool body, a target coupled to the bow spring, and an ultrasonic transducer coupled to the tool body, wherein in operation the ultrasonic transducer transmits an acoustic pulse to the target and receives an echo of the acoustic pulse from the target.  
         [0007]     In one aspect, the invention relates to a borehole caliper tool which comprises a tool body; a bow spring disposed on the tool body; an ultrasonic transducer coupled to the bow spring; and an ultrasonic transducer coupled to the tool body, wherein in operation an acoustic pulse is transmitted from one of said ultrasonic transducers for receipt by the other ultrasonic transducer.  
         [0008]     In another aspect, the invention relates to a method for gauging a diameter of a borehole, comprising deploying a borehole caliper tool in the borehole, the borehole caliper tool comprising a tool body, a bow spring flexibly coupled to the tool body, a target coupled to the bow spring, and an ultrasonic transducer coupled to the tool body, the borehole caliper tool being deployed such that the bow spring engages with a surface of the borehole; and generating an acoustic pulse using the ultrasonic transducer; receiving an echo of the acoustic pulse from the target; determining a time elapsed between generating the acoustic pulse and receiving the echo of the acoustic pulse; and relating the time elapsed to the diameter of the borehole.  
         [0009]     In another aspect, the invention relates to a method for gauging a diameter of a borehole, comprising deploying a borehole caliper tool in the borehole, the borehole caliper tool comprising a tool body, a bow spring flexibly coupled to the tool body, an ultrasonic transducer coupled to the bow spring, and an ultrasonic transducer coupled to the tool body, the borehole caliper tool being deployed such that the bow spring engages with a surface of the borehole; and generating an acoustic pulse using the ultrasonic transducer coupled to the tool body; receiving the acoustic pulse using the ultrasonic transducer coupled to the bow spring; determining a time elapsed between generating the acoustic pulse and receiving the acoustic pulse; and relating the time elapsed to the diameter of the borehole.  
         [0010]     In another aspect, the invention relates to a method for gauging a diameter of a borehole, comprising deploying a borehole caliper tool in the borehole, the borehole caliper tool comprising a tool body, a bow spring flexibly coupled to the tool body, an ultrasonic transducer coupled to the bow spring, and an ultrasonic transducer coupled to the tool body, the borehole caliper tool being deployed such that the bow spring engages with a surface of the borehole; and generating an acoustic pulse using the ultrasonic transducer coupled to the tool bow spring; receiving the acoustic pulse using the ultrasonic transducer coupled to the tool body; determining a time elapsed between generating the acoustic pulse and receiving the acoustic pulse; and relating the time elapsed to the diameter of the borehole.  
         [0011]     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]      FIG. 1  shows a caliper tool according to one embodiment of the invention in a borehole.  
         [0013]      FIG. 2  shows a cross-section of a caliper tool according to one embodiment of the invention.  
         [0014]      FIG. 3  shows a transducer configuration in accord with an embodiment of the invention.  
         [0015]      FIG. 4  is a schematic diagram illustrating certain operation principles of the caliper tool of  FIG. 2 .  
         [0016]      FIG. 5  shows a cross-section of a caliper tool according to another embodiment of the invention.  
         [0017]      FIG. 6  shows an enlarged view of the caliper tool of  FIG. 5 .  
         [0018]      FIG. 7  is a plot illustrating a measurement relationship between a target and borehole diameter in accord with an embodiment of the invention.  
         [0019]      FIG. 8  shows a caliper tool according to another embodiment of the invention.  
         [0020]      FIG. 9  shows another caliper tool according to one embodiment of the invention in a borehole. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0021]     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in great detail in order to avoid obscuring the invention.  
         [0022]      FIG. 1  shows a caliper tool  100  according to an embodiment of the invention. The caliper tool  100  is deployed in a borehole  102 . Typically, the borehole  102  will be filled with drilling fluid. The caliper tool  100  can measure and log the diameter of the borehole  102  as it traverses the borehole. The caliper tool  100  could be deployed alone in the borehole  102  on the end of a logging cable  104 . Alternatively, the caliper tool  100  could be deployed with a downhole tool (not shown) that performs downhole operations in the borehole  102 . The caliper tool  100  includes an elongated tool body  106  attached between an upper body  108  and a lower body  110 . The upper body  108  and/or the lower body  110  may include the circuitry needed to record caliper tool measurements and transmit the measurements to the surface. The tool body  106  carries an arm assembly  112 . In this embodiment, the arm assembly  112  and the tool body  106  engage or contact the borehole wall while the caliper tool  100  is used to measure and log the diameter of the borehole  102 . The arm assembly  112  expands and contracts in response to the changing diameter of the borehole  102 . The motion of the arm assembly  112  is tracked to determine the diameter of the borehole  102 .  
         [0023]      FIG. 2  shows a cross-sectional view of the tool body  106  and the arm assembly  112  according to one embodiment of the invention. In this embodiment, the arm assembly  112  includes a bow spring  114  having ends  116 ,  118  coupled to the tool body  106  by joints  120 ,  122 , respectively. The invention is not limited by how the joints  120 ,  122  are implemented, but the joints preferably allows pivoting of the bow spring ends  116 ,  118  relative to the tool body  106 . In the illustrated embodiment, the joint  120  is shown as a pin-in-hole joint, while the joint  116  is shown as a pin-in-slot joint. Both the (pin-in-hole) joint  120  and the (pin-in-slot) joint  116  allow pivoting of the bow spring ends  116 ,  118 . In addition, the (pin-in-slot) joint  122  allows sliding of the bow spring end  118  along the tool body  106 . Thus, the bow spring  114  can expand and contract as the tool body  106  and arm assembly  112  traverse a borehole. A pad  123  is attached to a middle section of the bow spring  114 . The pad  123  has a surface  125  for engaging a borehole wall and a surface  141  for reflecting the ultrasonic wave.  
         [0024]     An ultrasonic transducer  136  located in a cavity  138  in the tool body  106  is used to track the motion of the arm assembly  112 . The ultrasonic transducer  136  generates acoustic pulses, which are transmitted to a target, and then echoed back from the target. The ultrasonic transducer  136  converts the echoes received from the target into electrical signals that are representative of the time elapsed between generation of the acoustic pulses and receipt of the echoes. Electronic circuitry for controlling the ultrasonic transducer  136  and receiving signals from the ultrasonic transducer  136  may be located in the upper body ( 108  in  FIG. 1 ) or the lower body ( 110  in  FIG. 1 ) or in the tool body  106 . The transmitting and receiving functions of the ultrasonic transducer  136  may be performed by one sensor element or two sensor elements. An example of an ultrasonic transducer suitable for use in the invention is disclosed in U.S. Pat. No. 5,130,950. However, the invention is not limited to this particular ultrasonic transducer. Any ultrasonic transducer that can transmit and receive acoustic pulses in a borehole environment may be used.  
         [0025]     Some embodiments of the invention may be implemented to perform pulse-echo type measurements, where the transducer on the tool body is energized to emit acoustic energy in the borehole fluid such that an acoustic wave travels to a target to be reflected. The transducer is adapted to receive the reflected wave. Other embodiments may be implemented in a “pitch-catch” arrangement. In the pitch-catch arrangement, a receiver  103  replaces the target (See  FIG. 9 ). The receiver  103  may be disposed on the arm with the wiring running through the arm  112  (not shown). Alternatively, the receiver  103  transducer may be adapted to operate as the transmitter and transducer  136  adapted to operate as the receiver. Benefits of pitch-catch configurations include: reducing the path the wave must travel from a round-trip to a one-way and having a dedicated sensor as a receiver; halving the distance traveled results in less attenuation of the signal resulting in a larger received signal which is easier to analyze; and having a dedicated sensor reduces difficulty in discriminating the reflected signal from oscillations (ringing) that may occur in a pulse-echo sensor from the pulse.  
         [0026]      FIG. 3  shows the cross section of a transducer  136  that may be used to implement the invention in either a pulse-echo or pitch-catch embodiment. The transducer  136  can be mounted in the tool body  106  as shown in  FIG. 2  or in any suitable manner as known on the art. A piezoelectric ceramic disk  142  acts alternatively as the signal source and receiver. An inner window  143  made of PEEK™ seals the disk from well fluids. An outer window  144 , also made of PEEK™, protects the transducer from physical damage. The stacked windows  143 ,  144  match the acoustic impedance of the disk  142  to that of the well fluid to optimize energy transfer and minimize internal reflections. A backing  145  damps the oscillations after the transducer excitation is removed. In one embodiment, the backing  145  may be made of rubber loaded with heavy particles such as tungsten carbide. Metal pins  146  connect the disk  142  to the transmitter and receiver electronics (See  108  in  FIG. 1 ). The transducer body  147 , preferably made of stainless steel, contains all the pieces and seals against the tool body  106 . As known in the art, elements within the transducer  136  may be set at wellbore pressure and vacuum filled with a suitable material. The catch transducer in a pitch-catch embodiment of the invention would not require a large backing  145  and could be sealed in rubber to eliminate the body  147 . The dimensions of the transducer  136  may be chosen to balance the overall size against the measurement parameters. The sensor may be operated at any suitable frequency depending on the subsurface conditions as known in the art.  
         [0027]     In one embodiment, the surface  141  of pad  123  attached to the bow spring  114  acts as a target for the acoustic pulses generated by the ultrasonic transducer  136 . The surface  141  provides a high contrast to fluids in the borehole, thereby reflecting a clear signal and extending the measurement range of the ultrasonic transducer  136 . The pad surface  141  may be formed of a metal such as stainless steel. The acoustic impedance of steel (47,000,000 Rayle) is much larger than that of typical borehole fluids (in the neighborhood of 1,500,000 Rayle), so nearly all the incoming acoustic energy is reflected back to the source. In conventional caliper applications of ultrasonics, the reflector is the borehole wall itself, which has a reduced impedance contrast (7,700,000 Rayle for sandstone and shale) and whose rugosity can significantly diminish the amount of energy reflected. Thus in a comparison between steel to perfect rock, steel reflects almost 40% more energy. Seldom is the borehole perfect rock. The sensing end of the ultrasonic transducer  136  preferably faces the surface  141  such that acoustic pulses travel in a generally perpendicular direction between the sensing end of the transducer  136  and the surface  141 . Because the pad  123 , and therefore the surface  141 , moves relative to the ultrasonic transducer  136  during measurements, a surface  141  large enough to receive acoustic pulses from the ultrasonic transducer  136  during movement is preferable.  
         [0028]     In this embodiment, the ultrasonic transducer  136  measures the travel time of an acoustic pulse transmitted from the ultrasonic transducer  136  to the surface  141  and echoed back to the transducer  136 . From the travel time measured by the transducer  136 , the distance from the transducer to the surface  141  can be determined using the sonic velocity of the fluid in the borehole. The borehole diameter, D, may be expressed as follows:  
             D   =         d   pu     +     d   ut     +     d   pd       =       vT   2     +     d   ut     +     d   pd                 (   1   )             
 
         [0029]     As illustrated in  FIG. 4 , d pu  is the distance from the surface  141  to the ultrasonic transducer  136 , d ut  is the distance from the ultrasonic transducer  136  to the edge of the tool body  106  (in contact with the borehole wall), d pd  is the distance from the surface  141  to the pad surface  125  (in contact with the borehole wall), v is the sonic velocity of the drilling fluid, and T is the travel time of an acoustic pulse from the ultrasonic transducer  136  to the surface  141  and back to the ultrasonic transducer  136 .  
         [0030]     As discussed above, the sonic velocity (or an estimate of the sonic velocity) of the fluid in the borehole is used to determine the borehole diameter. The sonic velocity varies with fluid density and temperature and is preferably measured while the borehole diameter measurements are made. One simple method for measuring the sonic velocity includes installing a second ultrasonic transducer in the tool body  106  (not shown). The second ultrasonic transducer would have a fixed length acoustic travel path, i.e., a known distance from the transducer to the target. With the distance to the target and travel time known, the sonic velocity can be determined. It will be appreciated that any suitable means for determining the borehole fluid velocity may be use to implement the invention as known in the art. For example, one conventional technique for deriving the fluid sonic velocity uses mud parameters and temperature measurements.  
         [0031]      FIG. 5  shows the tool body  106  and the arm assembly  112  according to another embodiment of the invention. In this embodiment, the arm assembly  112  further includes a rigid follower arm  124  having an end  126  coupled to the bow spring  114  and an end  128  coupled to the tool body  106 . A pad  130  attached to the middle portion of the bow spring  114 , opposite the pad  123 , couples the end  126  of the follower arm  124  to the middle portion of the bow spring  114 . The pad  130  includes a slot which cooperates with a pin on the end  126  of the follower arm  124  to form a pin-in-slot joint  132 . The joint  132  allows the end  126  to both slide and pivot relative to the bow spring  114 . The end  128  of the follower arm  124  is coupled to the tool body  106  via a joint  134 , which preferably allows pivoting of the end  128 .  FIG. 6  shows a more detailed view of this embodiment.  
         [0032]      FIG. 5  also shows a pad  140  attached to the follower arm  124 . The pad  140  may act as a target for the ultrasonic transducer  136 . As in the previous embodiment, when the pad  140  acts as a target for the ultrasonic transducer  136 . This embodiment reduces the distance the acoustic wave travels, and subsequently the signal attenuation, compared to the embodiment shown in  FIG. 2 . A typical transducer&#39;s response function half-power point is +/−15 degrees, so it is preferable to maintain the target perpendicular to the axis of the transducer  136 . This may be accomplished with a concave surface  148  on the pad  140 . Embodiments may be implemented with other configurations to maintain the target perpendicular to the axis of the transducer  136  as known in the art (e.g. by adding a second arm to make a parallelogram mechanism (not shown)). The pad  140  can be positioned on the follower arm  124  such that the distance between the surface  148  and the transducer  136  is within the measurement range of the transducer  136 .  
         [0033]     In the embodiment described above where the pad  140  surface  148  on the follower arm  124  is used as a target, the travel time measured by the ultrasonic transducer  136  is indicative of the distance between the ultrasonic transducer  136  and the concave surface  148  of pad  140  on the follower arm.  FIG. 7  shows the relationship between this measurement and the borehole diameter graphically. The input to the algorithm is the one-way travel time. This is determined by taking half of the two-way time and using the borehole fluid sonic velocity to convert this time to distance. The algorithm may be performed analytically by a suitable processor located in the tool  106  or on the surface as known in the art.  
         [0034]     Various modifications are possible to the embodiments described herein. For example,  FIG. 1  shows a single caliper arm assembly  112  coupled to the tool body  106  such that the caliper arm assembly  112  and the tool body  106  both engage or make contact with the borehole wall while making borehole diameter measurements. In an alternate embodiment, as illustrated in  FIG. 8 , multiple caliper arm assemblies  112  may be coupled to the tool body  106  such that the tool body  106  is centered in the borehole  102  and does not make contact with the borehole wall. In this case, multiple ultrasonic sensors  136  are also mounted in cavities  138  in the tool body  106  to track the motion of the arm assemblies  112 . The measurements made by the ultrasonic sensors  136  can be integrated to determine the overall diameter of the borehole. In the embodiment of  FIG. 5  where a follower arm  124  is coupled between the bow spring  114  and the tool body  106 , the pad  130  or pad  123  may also be adapted to serve as a target for acoustic pulses.  
         [0035]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. For example, embodiments of the invention may be implemented to run on a “slick-line” with the measured data stored in memory for retrieval when the tool is brought back to the surface. In such embodiments, the data may be sent to a memory interface that stores the data in non-volatile memory for later retrieval. In real-time applications, some basic processing may be done in the caliper tool as known in the art. The resulting data being sent to a telemetry interface (that could be in a separate downhole instrument) and sent up to a surface acquisition system (e.g. via a wireline).