Abstract:
An apparatus for logging a formation traversed by a borehole includes a plurality of logging tools adapted for conveyance inside the borehole. The plurality of logging tools includes a tool body, a sensing pad responsive to a density property of the formation coupled to the tool body, a current emitting measure electrode responsive to a lateral resistivity property of the formation incorporated on the sensing pad, a mechanism for urging the sensing pad in contact with a side of the borehole coupled to the tool body, and a pair of mass isolation bands disposed about the tool body to isolate a mass of the tool body adjacent the measure electrode.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to apparatus for obtaining subsurface measurements. 
   Subsurface formation logs contain data related to one or more properties of a formation as a function of depth. A formation log is typically recorded as a logging tool traverses a borehole penetrating a formation of interest. The logging tool may be conveyed in a number of ways, e.g., on cable, on drill pipe, or on 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, which measures formation density, porosity, deep and/or intermediate and/or shallow resistivity, natural gamma radiation, and borehole size in a single logging run. The standard triple-combo tool uses a separate tool to measure each type of formation property. While the individual tools are very modular, a tool string assembled from these modular tools is long, typically about 90 ft (27.4 m), and time consuming to setup and run into and out of the borehole. 
   Operating cost and equipment cost contribute to the cost of logging. Both may be reduced by making tools smaller and simpler. Smaller and lighter tools are easier to transport, setup, and operate. Simpler tools are cheaper to build. Integrating measurements and adopting novel approaches to implementing measurements can reduce tool size and complexity. Even a highly integrated tool can be broken down into several sections to optimize transport and handling. However, depending on the degree of integration, there may not be a one-to-one relationship between measurements and sections. Schlumberger offers an integrated wireline logging tool under the trade name Platform Express™ that is about half the length of the standard triple-combo tool. The integrated wireline logging tool includes an integrated gamma-ray and neutron sonde, a high-resolution mechanical sonde with associated electronics cartridge and pad-mounted measurements, and a high-resolution azimuthal laterolog sonde or array induction imager tool. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention relates an apparatus 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 sensing pad responsive to a density property of the formation coupled to the tool body, a current emitting measure electrode responsive to a lateral resistivity property of the formation incorporated on the sensing pad, and a mechanism for urging the sensing pad in contact with a side of the borehole coupled to the tool body. The apparatus further includes a pair of mass isolation bands disposed about the tool body to isolate a mass of the tool body adjacent the measure electrode. 
   In another 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 sensing pad responsive to a density property of the formation coupled to the tool body, a current emitting measure electrode responsive to a lateral resistivity property of the formation incorporated on the sensing pad, and a mechanism for urging the sensing pad in contact with a side of the borehole coupled to the tool body. 
   In yet another aspect, the invention relates to a method of logging a formation traversed by a borehole which comprises disposing in the borehole a tool body carrying a sensing pad responsive to a density property and lateral resistivity property of the formation, moving the tool body in the borehole while urging the sensing pad in contact with a side of the borehole, emitting current from a measure electrode incorporated on the sensing pad, measuring flow of current into the formation, wherein the flow of current is proportional to the lateral resistivity property, emitting gamma radiation from a gamma source incorporated in the sensing pad, and detecting gamma particles returning from the formation, wherein energies of the gamma particles are proportional to the density property. 
   Other features and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1A  shows a logging tool according to one embodiment of the invention. 
       FIGS. 1B and 1C  are cross-sectional views of sensing pads according to embodiments of the invention. 
       FIGS. 2A and 2B  show a tool string including the logging tool of  FIG. 1A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. 
     FIG. 1A  shows a logging tool  100  according to one embodiment of the invention. The logging tool  100  is shown in a borehole  102  drilled through a formation  104 . The logging tool  100  may be conveyed inside the borehole  102  in a number of ways, including, but not limited to, on the end of a wireline, slickline, coiled tubing, or drill pipe. The logging tool  100  has ends  100   a ,  100   b  for connection to other logging tools. In one embodiment, the logging tool  100  measures formation density and lateral formation resistivity. Examples of logging tools that may be combined with the logging tool  100  to form a tool string include, but are not limited to, sonic, propagation resistivity, neutron, gamma ray, nuclear magnetic resonance, formation pressure, imaging, dipmeter, and ultrasonic caliper tools. Logging tools combined with logging tool  100  can be suitably chosen to make desired logs, e.g., triple-combo logs. 
   The logging tool  100  includes a tool body  106 , which is typically made of metal and is rugged enough to withstand the borehole environment. A pad assembly  108  is coupled to the tool body  106 . The pad assembly  108  includes a sensing pad  110 , which carries one or more detectors that respond to acoustic, nuclear, or electrical stimuli. The sensing pad  110  may also carry one or more sources that emit acoustic, nuclear, or electrical stimuli. To measure borehole/formation properties, the sensing pad  110  contacts a surface of the borehole  102  and emits an appropriate stimuli into the formation  104  and/or responds to an appropriate stimuli from the formation  104 . An electronics cartridge  112 , which may be located inside or external to the tool body  106 , cooperates with the sensing pad  110  to make desired measurements. The electronics cartridge  112  includes appropriate circuitry to power the source(s)/detector(s) on the sensing pad  110  and to process and transmit signals. The measurement data may be sent to the surface in real-time or stored in tool memory and retrieved when the logging tool  100  is pulled to the surface as known in the art. 
   The pad assembly  108  includes a mechanism that urges the sensing pad  110  in contact with a side of the borehole  102 . This mechanism may be any mechanism that effectively urges the sensing pad  110  in contact with a surface of the borehole  102 . Preferably, the mechanism minimizes standoff, i.e., distance between the face  110   a  of the sensing pad  110  and the side of the borehole  102 , under various operating conditions resulting from the varied geometrical shapes of the borehole wall. One example of a mechanism that may be used in the invention is a back-up arm mechanism such as disclosed in U.S. Pat. No. 4,594,552 (Grimaldi et al.). In this patent, the arm mechanism includes an arm at the end of which is mounted a sensing pad. The arm is pivotally connected to the tool body. The end of the arm farthest from the sensing pad includes an integral extension that 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. 
     FIG. 1A  discloses another mechanism for urging a sensing pad in contact with a borehole wall. The mechanism includes a pad bias mechanism for urging the sensing pad  110  in contact with a side of a borehole  102  and a tool bias mechanism for urging the side of the tool body  106  adjacent the sensing pad  110  in contact with the side of the borehole  102 . The pad bias mechanism works independently of the tool bias mechanism. In one embodiment, the pad bias mechanism includes at least one resilient member or spring  114 , such as a leaf spring, and at least one linkage arm  118  coupled between the tool body  106  and a distal end of the sensing pad  110 . Preferably, the pad bias mechanism includes an additional resilient member or spring  116  and linkage arm  120  coupled between the tool body  106  and the other distal end of the sensing pad  110 . The linkage arm  120  may be coupled to the other distal end or middle of the sensing pad  110 . The spring  116  would also urge the sensing pad  110  away from the tool body  106  while the linkage arm  120  would limit how far the sensing pad  110  can move away from the tool body  106 . 
   The linkage arm  118  may be coupled to the sensing pad  110  and the tool body  106  by joints  118   a ,  118   b , respectively. The linkage arm  120  may be coupled to sensing pad  110  and tool body  106  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, but they are generally pivot or hinge joints so as to allow the sensing pad  110  to pivot relative to the tool body  106 . The pivot or hinge joints may be provided by mating pins and holes or other suitable structures. In one embodiment, at least one of the linkage arms  118 ,  120  is slidable relative to the tool body  106 , thereby providing flexibility in positioning the sensing pad  110  relative to the tool body  106 . For example, it may be desirable to move the sensing pad  110  between a retracted position, wherein the face  110   a  of the sensing pad  110  is flush or nearly flush with the tool body  106 , and a deployed position, wherein the sensing pad  110  can make contact with irregularities, such as depression  102   a , in a side of the borehole  102 . In one embodiment, the joint  120   b  includes a slot  120   c  that mates with a pin  120   d  coupled to the tool body  106 . Thus, the linkage arm  120  may slide relative to the tool body  106  by simply allowing the pin  120   d  to ride in the slot  120   c  as the tool body  106  traverses the borehole  102 . 
   It may be desirable to control sliding of the linkage arm  120  relative to the tool body  106 . In one embodiment, sliding of the linkage arm  120  is controlled through the use of an actuator  122  located within the tool body  106 . 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 arm  120  to slide relative to the tool body  106 . In another embodiment, sliding of the linkage arm  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 arm  120  is latched to the tool body  106  using the one-shot release system. The one-shot release system prevents sliding of the linkage arm  120  until a desired time when the one-shot release system is activated or released. 
   The pad bias mechanism has been described with respect to springs  114 ,  116  biasing the sensing pad  110  away from the tool body  106 . 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  106 . 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  106 . Initially, the coil spring can be latched to the tool body  106  using, for example, a one-shot electrical latch. This would also serve to prevent sliding of the linkage arm  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 arm  120 . Thus, extension of the actuator rod  122   b  would serve to bias the sensing pad  110  away from the tool body  106 . In this case, it is not necessary that the linkage arm  120  has the slot  120   c , and a simple pin and hole connection between the linkage arm  120  and the actuator rod  122   b  would suffice. 
   To minimize surface wear of the sensing pad  110 , particularly if the sensing pad  110  is run into the borehole  102  in a deployed position, easily replaceable wear buttons, plates, or housings may be used to protect the sensing pad  110 . 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. 
   In one embodiment, the tool bias mechanism that urges the side of the tool body  106  adjacent to the sensing pad  110  in contact with a side of the borehole  102  includes a flexible member  124 , such as a bow spring, located opposite the sensing pad  110 . The ends  126 ,  128  of the bow spring  124  are coupled to the tool body  106  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/or sliding of the bow spring ends  126 ,  128  relative to the tool body  106 . 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  106 . The mating pin and slot at joint  128   a  allow pivoting and sliding of the bow spring end  128  relative to the tool body  106 . Thus, the bow spring  124  can expand and contract as the tool body  106  traverses the borehole  102 . 
   When the bow spring  124  engages one side of the borehole  102 , it presses the tool body  106  against the opposite side of the borehole  102 . A wall-engaging pad (not shown) may be attached to the middle portion of the bow spring  124 . As the tool body  106  traverses the borehole  102  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  106  against a side of the borehole  102 . The force of the springs  114 ,  116  (or coil spring if used) is designed to maintain the sensing pad  110  in contact with the formation  104  even in the presence of local irregularities, such as depression  102   a  shown in a side of the borehole  102 . 
   In one embodiment, the logging tool  100  is configured to measure density of the formation  104  using, for example, a conventional dual-detector gamma-gamma measurement configuration. Referring to  FIG. 1B , this configuration includes a gamma ray source  134  mounted in the body  136  of the sensing pad  110 . The gamma ray source  134  is surrounded by a shield  138  made of a high density shielding material, such as tungsten. Gamma ray detectors  140 ,  142  are also mounted in the body  136  of the sensing pad  110 . The detectors  140 ,  142  are longitudinally aligned with the source  134 . The detector  140  closest to the source  134  is known as the short-spaced detector, and the detector  142  farthest from the source  134  is known as the long-spaced detector. Intermediate and backscattering detectors may also be provided in the pad body  136  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  146  made of a high density shielding material, such as tungsten, is mounted on the pad body  136 . The source  134  and detectors  140 ,  142  communicate with the formation ( 104  in  FIG. 1A ) through windows  148 , made of material transparent to gamma rays, such as epoxy resin, in the shield  146 . 
   Returning to  FIG. 1A , the logging tool  100  configured as described above measures formation density in a conventional manner. To measure formation density, the logging tool  100  is lowered to a desired depth in the borehole  102 . Also, the sensing pad  110  is pressed against a side of the borehole  102 . As the logging tool  100  ascends the borehole  102 , the source ( 134  in  FIG. 1B ) emits gamma radiation and the detectors ( 140 ,  142  in  FIG. 1B ) 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  134  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  140 ,  142  to a count rate. The count rate is then expressed for each detector  140 ,  142  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  140 ,  142 . 
   In addition to measuring density of the formation  104 , the logging tool  100  is also configured to measure lateral resistivity of the formation  104 . In one embodiment, the pad assembly  108  includes a current emitting measure electrode  150 , which is built on a non-conductive pad  152 , e.g., made from rubber, fiberglass, plastic, or ceramic, and is installed on the face  110   a  of the sensing pad  110 . The measure electrode  150  could be a single electrode or multiple electrodes. Multiple electrodes would provide degrees of freedom in establishing various focusing conditions and/or could be used to mitigate effects of contact impedance. Wires connected to the electrodes are fed into the sensing pad  110 . These wires in turn connect to the resistivity electronics. The electronics may be housed in the sensing pad  110  itself, in the electronics cartridge  112 , or in another tool in the tool string. A pair of mass isolation (or insulating) bands are placed at the ends of the logging tool  100 . This allows the isolated mass of the logging tool  100  to be used as a bucking electrode. In this figure, only one of the mass isolation bands, e.g., mass isolation band  154 , is integrated with the end  100   a  of the logging tool  100 . The other mass isolation band is integrated with a logging tool that would be attached to the end  100   b  of the logging tool  100 ; although, it is also possible to integrate the other mass isolation band at the end  100   b  of the logging tool  100 . An alternative to installing the measure electrode  150  on the sensing pad  110  is to isolate the sensing pad  110  from the tool body  106  and then use the isolated sensing pad  110  as a measure electrode. This could be done, for example, by integrating mass isolation bands on the pad assembly  108 . For example,  FIG. 1C  shows mass isolation bands  154   a ,  154   b  integrated on the pad assembly  108 , about the sensing pad  110 . 
     FIGS. 2A and 2B  together form a complete assembly of a tool string  200  including the logging tool  100  of  FIG. 1A  The tool string  200  is disposed in a borehole  202  traversing formation  204 . The tool string  200  may be conveyed inside the borehole  202  in a number of ways, including, but not limited to, on the end of a wireline, slickline, coiled tubing, or drill pipe. In one embodiment, the tool string  200  includes logging tools  300  ( FIG. 2A) and 400  ( FIG. 2B ) attached to either ends of the logging tool  100 . In one embodiment, the tool string  200  provides triple-combo logs. In one embodiment, the tool string  200  configured for triple-combo logging has a length on the order of 26 ft (7.9 m), which is considerably shorter than the length of the standard triple-combo tool. In this configuration, the logging tool  100  measures formation density and lateral formation resistivity, the logging tool  300  measures formation porosity and natural gamma radiation, and the logging tool  400  measures deep formation resistivity. In addition, any of these tools can be configured to measure borehole size without increasing the length of the tool string  200 . 
   In one embodiment, the logging tool  300  ( FIG. 2A ) includes a neutron source  302  and neutron detectors  304 ,  306  for measuring formation porosity. The logging tool  300  may also include a gamma ray detector  308  for measuring natural gamma radiation. The logging tool  300  may further include a telemetry cartridge  310  for sending measurements to the surface and receiving commands from the surface. A bow spring  312  may be attached to the logging tool  300  to bias the logging tool  300  towards a side of the borehole  202 , thereby improving response of the porosity and gamma radiation measurements. 
   In one embodiment, the logging tool  400  ( FIG. 2B ) is an induction tool. This tool  400  is preferably centralized within the borehole  202 . Centralizers  402  may be provided on the logging tool  400  to centralize the logging tool  400  within the borehole  202 . The logging tool  100  is a pad-based tool that makes contact with a surface of the borehole  202  to make measurements. To allow the logging tool  100  to contact a side of the borehole  202  while the logging tool  400  remains centralized within the borehole  202 , a hinge joint  500  is provided between the logging tools  100 ,  400 . 
   A mass isolation (or insulating) band  502  is placed at an end of the hinge joint  500 . The mass isolation band  502  forms a pair with the mass isolation band  154  on the logging tool  100 . The pair of mass isolation bands  154 ,  502  are placed generally symmetrically about the measure electrode  150 . This allows the metal body of the logging tool  100  between the mass isolation bands  154 ,  502  to act as a bucking electrode. For lateral resistivity measurements, the bucking electrode is held at the same potential as the measure electrode  150  and thereby forces the current from the measure electrode  150  to run approximately perpendicular to the logging tool  100 . This focuses the current emitted from the measure electrode  150  into the formation  204 . The bucking and measure currents return on the metal bodies of tools above and below the mass isolation bands  154 ,  502 . An isolated electrode ( 314  in  FIG. 2A ) at the top of the logging tool ( 300  in  FIG. 2A ) provides a distant reference voltage for the lateral resistivity measurement. 
   In operation, voltage on the metal surfaces of the tool string  200  between the mass isolation bands  154 ,  502  is maintained at a certain value, e.g., 1 V, and voltage above and below the mass isolation bands  154 ,  502  is maintained at a different voltage, e.g., at 0 V. Current flows from the measure electrode  150  to the formation  204  in proportion to the resistivity of the formation  204 . The bucking electrode, i.e., the portion of the logging tool  100  between the mass isolation bands  154 ,  502 , focuses the current flow from the measure electrode  150  in a direction generally perpendicular to the logging tool  100 . Any downward-going current returns on the metal bodies of the hinge  500  and logging tool  400 . Any upward-going current returns on the metal body of the logging tool ( 300  in  FIG. 2A ). The mass isolation bands  154 ,  502  separate the current return electrodes from the bucking electrode. The current emitted by the measure electrode  150  is measured by appropriate electronics in the logging tool  100  and translated to the resistivity of the formation  204 . Resistivity measurements may be conducted simultaneously or alternately with density measurements. 
   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.