Abstract:
An apparatus for logging a formation traversed by a borehole includes a tool adapted for conveyance inside the borehole, a tool bias mechanism comprising a flexible member coupled to a first side of the tool body and adapted to urge a second side of the tool body into contact with a side of the borehole, a sensing pad responsive to a property of the formation, and a pad bias mechanism independent of the tool bias mechanism coupling the sensing pad to the second side of the tool body and adapted to urge the sensing pad into contact with the side of the borehole.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to apparatus for obtaining subsurface measurements. More specifically, the invention relates to logging tools that sense formation characteristics using a pad-type structure.  
         [0002]     Formation evaluation logs contain data related to one or more properties of a formation as a function of depth. A formation evaluation log is typically recorded as a logging tool containing appropriate instrumentation traverses a borehole penetrating the formation. The logging tool may be conveyed inside the borehole in a number of ways, e.g., on cable, drill pipe, or coiled tubing. For operational efficiency, it is common to include a combination of logging tools in a single logging run. One example of a combination of logging tools is a triple-combo tool string, which measures formation density, porosity, deep and/or intermediate and/or shallow resistivity, natural gamma radiation, and borehole size in a single logging run.  
         [0003]     Logging tools include sources and/or detectors which can emit or respond to various types of signals, including, for example, electrical, nuclear, or acoustic stimulus. The sources and/or detectors may be located in the tool body or on a sensing pad movably coupled to the tool body. In the latter case, it is desirable that the sensing pad maintain contact with the borehole wall even in the presence of irregularities. The pad assembly typically includes a mechanism for urging the sensing pad in contact with the borehole wall. Pad-based measurements are typically sensitive to standoff, i.e., distance between the pad face and the borehole wall. It is desirable that any mechanism that urges the sensing pad in contact with the borehole wall minimizes standoff under various operating conditions resulting from the varied geometrical shapes of borehole walls.  
         [0004]     Typically, sensing pads are urged in contact with borehole walls using spring-loaded backup or caliper arms. For example, U.S. Pat. No. 4,594,552 (Grimaldi et al.) discloses a pad assembly including an arm at the end of which is mounted a sensing pad. The arm is pivotally connected to the tool body and has an integral extension on the end farthest from the sensing pad. The extension is resiliently connected to the tool body. A backup arm is pivotally connected to the extension and resiliently biased away from the tool body. In the extended position, the backup arm engages one side of the borehole wall while urging the sensing pad in contact with the opposite side of the borehole wall.  
         [0005]     To avoid the backup arm becoming wedged in the borehole, the backup arm typically would have to be in the retracted position when the logging tool is run into the borehole or out of the borehole, depending on which direction the backup arm opens relative to the tool body. Measurements cannot be made effectively using the sensing pad when the backup arm is in the retracted position. However, it would be advantageous if measurements could be made effectively using the sensing pad, regardless of which direction the logging tool is running, i.e., into or out of the borehole. Therefore, a need remains for a more robust mechanism that would urge the pad in contact with the borehole wall regardless of which direction the logging tool is running.  
       SUMMARY OF THE INVENTION  
       [0006]     In one aspect, the invention relates to an apparatus for logging a formation traversed by a borehole which comprises a tool body adapted for conveyance inside the borehole, a tool bias mechanism comprising a flexible member coupled to a first side of the tool body and adapted to urge a second side of the tool body into contact with a side of the borehole, a sensing pad responsive to a property of the formation, and a pad bias mechanism independent of the tool bias mechanism coupling the sensing pad to the second side of the tool body and adapted to urge the sensing pad into contact with the side of the borehole.  
         [0007]     In another aspect, the invention relates to a tool string for logging a formation traversed by a borehole which comprises a plurality of logging tools adapted for conveyance inside the borehole. The plurality of logging tools comprises a tool body, a tool bias mechanism comprising a flexible member coupled to a first side of the tool body and adapted to urge a second side of the tool body into contact with a side of the borehole, a sensing pad responsive to a property of the formation, and a pad bias mechanism independent of the tool bias mechanism coupling the sensing pad to the second side of the tool body and adapted to urge the sensing pad into contact with the side of the borehole.  
         [0008]     In yet another aspect, the invention relates to a method of measuring a property of a formation traversed by a borehole which comprises disposing in the borehole a tool body carrying a sensing pad responsive to a property of the formation. The method further includes moving the tool body in the borehole, wherein as the tool body is moved in the borehole a tool bias mechanism comprising a flexible member urges a side of the tool body adjacent to the sensing pad in contact with a side of the borehole and a pad bias mechanism independent of the tool bias mechanism urges the sensing pad in contact with the side of the borehole. The method further includes measuring a formation property through the sensing pad.  
         [0009]     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]      FIG. 1A  shows a logging tool incorporating a pad assembly according to one embodiment of the invention.  
         [0011]      FIG. 1B  shows the sensing pad of  FIG. 1A  in a retracted position.  
         [0012]      FIGS. 2A-2C  show a tool string including the logging tool of  FIGS. 1A and 1B . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in accompanying drawings. 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 detail in order to avoid unnecessarily obscuring the invention.  
         [0014]      FIG. 1A  shows a pad assembly  100  according to one embodiment of the invention. The pad assembly  100  is coupled to a body  102  of a logging tool  104 . The pad assembly includes a sensing pad  106 , which carries one or more detectors that respond to acoustic, nuclear, or electrical stimuli. The pad  106  may also carry one or more sources that emit acoustic, nuclear, or electrical stimuli. The logging tool  104  may be lowered into a borehole  108  traversing formation  110  to allow measurements of the borehole/formation through the sensing pad  106 . The logging tool  104  may be conveyed inside the borehole  108  in a number of ways, including, but not limited to, on the end of a wireline, slickline, coiled tubing, or drill pipe. To make borehole/formation measurements, the sensing pad  106  contacts the borehole  108  surface. An electronics cartridge  112 , which may be located inside or external to the tool body  102 , cooperates with the sensing pad  106  to make measurements. The electronics cartridge  112  includes appropriate circuitry to power the detector(s) on the sensing pad  106  and to process and transmit signals. The measurement data may be sent to the surface in real-time or stored in a memory tool and retrieved when the logging tool  104  is pulled to the surface.  
         [0015]     The pad assembly  100  includes a pad bias mechanism for urging the sensing pad  106  in contact with a side of the borehole  108 . In one embodiment, the pad bias mechanism includes springs  114 ,  116 , such as leaf springs, coupled between the tool body  102  and the sensing pad  106 . The pad bias mechanism further includes linkages  118 ,  120  coupled between the tool body  102  and the pad  106 . The linkages  118 ,  120  allow the position of the sensing pad  106  relative to the tool body  102  to be adjustable. While the springs  114 ,  116  urge the sensing pad  106  away from the tool body  102 , the linkages  118 ,  120  limit how far the sensing pad  106  can move away from the tool body  102 . Linkage  118  is coupled to the sensing pad  106  and tool body  102  by joints  118   a ,  118   b , respectively. Linkage  120  is coupled to the sensing pad  106  and tool body  102  by joints  120   a ,  120   b , respectively. The joints  118   a ,  118   b ,  120   a ,  120   b  could be implemented in any number of ways. In one embodiment, the joints  118   a ,  118   b  are pivot or hinge joints, which may be provided by mating pins and holes or other suitable structures. In one embodiment, the joints  120   a ,  120   b  are pivot or hinge joints, which may be provided by mating pins and holes or other suitable structures.  
         [0016]     In one embodiment, at least one of the joints  118   b  and  120   b  is also capable of sliding relative to the tool body  102 . This provides flexibility in positioning the sensing pad  106  relative to the tool body  102 . For example, it may be desirable to move the sensing pad  106  between a retracted position, wherein the sensing pad  106  is flush or nearly flush with the tool body  102  ( FIG. 1B ), and a deployed position, wherein the sensing pad  106  can make contact with irregularities in a side of the borehole  108 . In one embodiment, the joint  120   b  includes a slot  120   c  that mates with a pin  120   d  coupled to the tool body  102 . Thus, the linkage  120  may slide relative to the tool body  102  by simply allowing the pin  120   d  to ride in the slot  120   c  as the tool body  102  traverses the borehole  108 .  
         [0017]     It may be desirable to control sliding of the linkage  120  relative to the tool body  102 . In one embodiment, sliding of the linkage  120  is controlled through the use of an actuator  122  located within the tool body  102 . The actuator  122  could include a motor  122   a  which drives an actuator rod  122   b , such as a lead screw. In this example, the pin  120   d  is coupled to the actuator rod  122   b . The motor  122   a  may then be operated as needed to extend or retract the actuator rod  122   b , thereby moving the pin  120   d  inside the slot  120   c , thereby causing the linkage  120  to slide relative to the tool body  102 . In another embodiment, sliding of the linkage  120  is controlled through the use of a one-shot release system (not shown), such as a one-shot electrical latch, e.g., a solenoid and hook linkage. In this case, the linkage  120  is latched to the tool body  102  using the one-shot release system. The one-shot release system prevents sliding of the linkage  120  until a desired time when the one-shot release system is activated or released.  
         [0018]     The pad bias mechanism has been described with respect to springs  114 ,  116  biasing the sensing pad  110  away from the tool body  102 . In an alternate embodiment, the springs  114 ,  116  may be omitted and a coil spring may be used to bias the sensing pad  110  away from the tool body  102 . In the current embodiment shown in  FIG. 1A , the coil spring (not shown) could replace the motor  122   a . The coil spring would be coupled between the actuator rod  122   b  and the tool body  102 . Initially, the coil spring can be latched to the tool body  102  using, for example, a one-shot electrical latch. This would also serve to prevent sliding of the linkage  120 . At a desired time, the one-shot electrical latch can be activated or released. This would then allow the coil spring to extend the actuator rod  122   b . The actuator rod  122   b  is coupled to the linkage  120 . Thus, extension of the actuator rod  122   b  would serve to bias the sensing pad  110  away from the tool body  102 . In this case, it is not necessary that the linkage  120  have the slot  120   c , and a simple pin and hole connection between the linkage  120  and the actuator rod  122   b  would suffice.  
         [0019]     To minimize surface wear of the sensing pad  106 , particularly if the sensing pad  106  is run into the borehole  108  in a deployed position, easily replaceable wear buttons, plates, or housings may be used to protect the sensing pad  106 . These surface wear protectors would be long-wearing parts and provide a minimal standoff so that the measurement quality is not affected and may incorporate a time-to-replace-me indicator.  
         [0020]     The pad assembly  100  includes a tool bias mechanism for urging the side of the tool body  102  adjacent to the sensing pad  106  in contact with a side of the borehole  108 . The tool bias mechanism includes a flexible member  124 , such as a bow spring, located opposite the sensing pad  106 . The ends  126 ,  128  of the bow spring  124  are coupled to the tool body  102  by joints  126   a ,  128   a , respectively. The joints  126   a ,  128   a  can be implemented in any number of ways. In one embodiment, the joints  126   a ,  128   a  allow pivoting and sliding of the bow spring ends  126 ,  128  relative to the tool body  102 . In one embodiment, the joint  126   a  includes mating pin and hole, and the joint  128   a  includes mating pin and slot. The mating pin and hole at joint  126   a  allow pivoting of the bow spring end  126  relative to the tool body  102 . The mating pin and slot at joint  128   a  allow pivoting and sliding of the bow spring end  128  relative to the tool body  102 . Thus, the bow spring  124  can expand and contract as the tool body  102  traverses the borehole  108 .  
         [0021]     When the bow spring  124  engages one side of the borehole  108 , it presses the tool body  102  against the opposite side of the borehole  108 . A wall-engaging pad (not shown) may be attached to the middle portion of the bow spring  124  as the tool body  102  traverses the borehole  108 . The motion of the bow spring  124  may be monitored and translated into borehole caliper measurement. The force of the bow spring  124  is designed to hold the entire tool body  102  against a side of the borehole  108 . The force of the springs  114 ,  116  is designed to maintain the sensing pad  106  in contact with the formation  110  even in the presence of local irregularities, such as depression  129  shown in a side of the borehole  108 .  
         [0022]     The logging tool  104  may be configured to make any number of measurements. For example, the logging tool  104  may measure formation density, for example, using a conventional dual-detector gamma-gamma measurement configuration. This includes a gamma ray source  130  mounted in the body  132  of the sensing pad  106 . The gamma ray source  130  is surrounded by a shield  134  made of a high density shielding material, such as tungsten. Gamma ray detectors  136 ,  138  are also mounted in the body  132  of the sensing pad  106 . The detectors  136 ,  138  are longitudinally aligned with the source  130 . The detector  136  closest to the source  130  is known as the short-spaced detector, and the detector  138  farthest from the source  130  is known as the long-spaced detector. Intermediate and backscattering detectors may also be provided in the pad body  132  as taught in, for example, U.S. Pat. No. 5,390,115 (Case et al.) and U.S. Pat. No. 5,528,029 (Chapellat et al.), respectively. A shield  142  made of a high density shielding material, such as tungsten, is mounted on the pad body  132 . The source  130  and detectors  136 ,  138  communicate with the formation  110  through windows  144 , made of material transparent to gamma rays, such as epoxy resin, in the shield  142 .  
         [0023]     The logging tool  104  configured as described above measures formation density in a conventional manner. To measure formation density, the logging tool  104  is lowered to a desired depth in the borehole  108 . Also, the sensing pad  106  is pressed against a side of the borehole  108  using the mechanism previously described. As the logging tool  104  ascends the borehole  108 , the source  130  emits gamma radiation and the detectors  136 ,  138  detect gamma returning particles and generate output pulses in response. The energies of the detected gamma particles are representative of specific interaction phenomena between the gamma particles emitted by the source  130  and the atoms in the formation. The output pulses are received by the electronics cartridge  112 , which counts the output pulses for a predetermined time period at appropriate time intervals and converts the total count for each detector  136 ,  138  to a count rate. The count rate is then expressed for each detector  136 ,  138  as a function of the energy of each gamma particle. A calibration process is used to determine formation density from the count rates of each detector  136 ,  138 .  
         [0024]      FIGS. 2A-2C  together form a complete assembly of a triple-combo tool string  200  according to an embodiment of the invention. The tool string  200  is disposed in a borehole  201  traversing a formation  203 . The tool string  200  includes logging tools  202  ( FIG. 2A ),  104  ( FIG. 2B ),  204  ( FIG. 2B ), and  206  ( FIG. 2C ). A hinge joint  208  ( FIG. 2B ) is provided between the logging tools  104 ,  204 . The hinge joint  208  allows the logging tools  204 ,  206  attached below the logging tool  104  to be centered in the borehole  201  even when the logging tool  104  is not centered within the borehole  201 . Centralizers  210  ( FIG. 2C ) are provided on the logging tool  206  to position the logging tool  206  in the center of the borehole  201 . Centralizers may also be provided on the logging tool  204  ( FIG. 2B ) as needed. In one embodiment, the logging tool  202  ( FIG. 2A ) measures porosity, the logging tool  104  ( FIG. 2B ) measures density, as described above, the logging tool  204  ( FIG. 2B ) makes a lateral measurement of resistivity, and the logging tool  206  ( FIG. 2D ) makes an induction-based measurement of resistivity. The logging tool  204  ( FIG. 2B ) employs parts of the tool string  200  above and below it in making its measurements. The tool string  200  may be run into the borehole  201  with the sensing pad  106  ( FIG. 2B ) of the logging tool  104  deployed or retracted.  
         [0025]     In one example, the logging tool  202  ( FIG. 2A ) includes a neutron source  212  and dual detectors  214 ,  216  to provide a compensated porosity measurement. The neutron tool  202  also includes a telemetry cartridge  218  for sending measurement data to the surface. An upper bow spring  220  is attached to the neutron tool  202 . The upper bow spring  220  engages a side of the borehole  201  while urging the neutron tool  202  towards the opposite side of the borehole  201 . The upper bow spring  220  may be configured for borehole caliper measurement. The upper bow spring  220  and lower bow spring  124  ( FIG. 2B ) co-operate to hold the connected bodies of the tool string  200  against a side of the borehole  201 . A single bow spring may be used instead of two bow springs  220 ,  124  if the single bow spring is positioned properly to avoid creating a moment that would tend to misalign the logging tool  200  in the borehole  201 .  
         [0026]     The lateral formation resistivity logging tool  204  ( FIG. 2B ) includes current emitting bucking electrodes  222  and  226  and current emitting measure electrode  224 . The measure electrode  224  is separated from the bucking electrodes  222  and  226  by insulating material  228 . The downward-going current returns on the metal body of the logging tool  206  ( FIG. 2C ), and the upward-going current returns on the metal bode of the hinge joint  208  ( FIG. 2B ) and metal body of the logging tool  104  ( FIG. 2B ). The returns are separated from the bucking electrodes  222 ,  226  by mass isolation (or insulating) bands  230 . An isolated electrode  232  at the top of the neutron tool  202  ( FIG. 2A ) provides a distant reference voltage for the lateral resistivity measurement. In this example, the logging tool  206  ( FIG. 2C ) may be an induction formation resistivity tool. The logging tool  206  may incorporate a natural gamma ray detector  234 .  
         [0027]     Advantages of the present invention include a pad assembly entailing a mechanism for urging a pad against a side of the borehole so that measurements can be made through the pad. The mechanism has two parts. The first part of the mechanism includes a flexible member such as a bow spring which urges the body of the logging tool, to which the pad is coupled, against a side of the borehole. The flexible member engages a side of the borehole and collapses as necessary as the logging tool traverses the borehole. The flexible member does not have the tendency to become wedged in the borehole regardless of the direction in which the logging tool is running inside the borehole. Therefore, measurements can be made with the pad regardless of the direction in which the logging tool is moving in the borehole. The second part of the mechanism urges the pad against irregularities such as depressions in a side of the borehole, thereby minimizing standoff between the pad and the formation. The pad assembly allows the logging tool to move up or down inside the borehole without retracting the sensing pad. The pad assembly makes the logging tool less complex and less costly than conventional pad-based logging tools. Reduced complexity allows the logging tool to be smaller, and smaller tools can be operated by fewer people at the well site.  
         [0028]     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. It will also be appreciated that conventional components, connectors, electronics, and materials can be used to implement embodiments of the invention. The components (e.g. linkages, hinges, springs) used to implement embodiments of the invention may be formed of non-metallic materials or insulated materials.