Abstract:
An unfocused resistivity imaging device horizontal and vertical currents conveyed into the borehole. Pad mounted electrodes are used to make voltage measurements in orthogonal directions. The voltage measurements are then rotated to give principal resistivity measurements in an anisotropic formation with dipping beds.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention generally relates to explorations for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this invention relates to highly localized borehole investigations employing the introduction and measuring of individual survey currents injected into the wall of a borehole by capacitive coupling of electrodes on a tool moved along the borehole with the earth formation.  
         [0003]     2. Background of the Art  
         [0004]     Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a measure electrode (current source or sink) are used in conjunction with a diffuse return electrode (such as the tool body). A measure current flows in a circuit that connects a current source to the measure electrode, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present invention belongs to the first category.  
         [0005]     Techniques for investigating the earth formation with arrays of measuring electrodes exist. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Pat. No. 685727 to Mann et al. U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. U.S. Pat. No. 6,348,796 to Evans et al., having the same assignee as the present invention and the contents of which are incorporated herein by reference, discloses an apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a focusing electrode. The focusing electrode is maintained at a slightly higher potential than the measure electrodes. A modulated electrical current with a carrier frequency of 1 MHz is injected into the formation. When used with a non-conducting fluid, capacitive coupling between the electrode and the conductive formation is provided by the dielectric of fluid. When used with a conducting borehole fluid, an additional capacitor may be incorporated into the circuit. The current in the measure electrode is indicative of the conductivity of the formation.  
         [0006]     In oil-based muds, unfocused methods provides an alternative to the focused methods such as the device of Evans. In such devices, the electrodes are voltage measuring electrodes on a non-conducting pad. A current source and a current return provide a current flow in the formation parallel to the pad and voltage differences between electrodes are indicative of the formation resistivity. See, for example, U.S. Pat. No. 6,191,588 to Chen and WO2005/006023 of Cheung et al. With such a device, the problems caused by variations in standoff of the individual electrodes from the borehole wall are substantially eliminated. We refer to such devices as “four-terminal” devices and the corresponding methods as “four-terminal” methods.  
         [0007]     However, because the four-terminal methods measures the formation resistivity with electrodes arranged in a direction parallel to the borehole wall, the image will depend on the direction of measurement for a layered or other inhomogeneous formation. The conventional resistivity imaging tools like that of Evans which measures the radial formation resistivity does not have this limitation. Specifically, if the bedding plane is inclined to the plane defined by a row of electrodes in an unfocused device, the measured resistivity will be a combination of the horizontal and vertical resistivity (defined here as parallel to and perpendicular to the bedding plane).  
         [0008]      FIG. 2  (prior art, from Evans) illustrates an exemplary array of measure electrodes  115   a,    115   b,    115   c  . . . set within a substantially rectangular guard electrode  103  with gaps  107   a  (that contain insulating material therein). The guard electrode  103  is separated from the pad or body  101  by a substantially rectangular insulating gap  107   b.  In one embodiment of the invention, the spacing between the measure electrodes is selected as to provide overlap in azimuth and depth, i.e., the diameter D of the measure electrode is greater than the horizontal spacing d 1  of the electrodes  115   b,    115   c  in adjacent rows and the vertical spacing d 2  between the rows of electrodes. In another embodiment of the invention, the electrodes do not have this azimuthal and vertical overlap, but due to a broadening of the measure beam used in Evans, overlap in azimuth and borehole depth of the region of investigation is obtained.  
         [0009]      FIG. 3  depicts a borehole  121  penetrating a dipping-bed formation. The dipping beds are denoted by  123 . When a focused imaging tool such as that of Evans is conveyed in the borehole, the tool should read the horizontal resistivity of the formation (the resistivity parallel to the bedding plane) when the pad surface is normal to the y-axis. This is because the focused currents travel radially into the formation, i.e., along the y-axis and parallel to the bedding. If, on the other hand, the pad surface is normal to the x-axis, the radial currents from the borehole will cut across the bedding plane and will be effected by both horizontal and vertical formation resistivities. However, since the current flowing into the formation is governed mainly by the conductive beds, the resistivity parallel to bedding again controls the measurement made by the focused imaging tool except at very steep dips. At intermediate pad face angles (between the x- and y-axes, both horizontal and vertical resistivities will affect the measure current of the tool.  
         [0010]     The present invention is directed towards a method and apparatus that are relatively insensitive to the formation dip and may be used with an unfocused tool within a borehole, and may also be used with oil-based mud.  
       SUMMARY OF THE INVENTION  
       [0011]     One embodiment of the invention is an apparatus for evaluating an earth formation. The apparatus includes a logging tool conveyed into a borehole in the earth formation. A first pair of current electrodes conveys a first current into the earth formation. A first pair of voltage electrodes on the tool provides a first voltage measurement in a first direction. A second pair of voltage electrodes provides a second voltage measurement in a second direction substantially orthogonal to the first direction. A processor uses the first and second voltage measurements to estimate a horizontal resistivity and a vertical resistivity of the earth formation, the estimation using a coordinate rotation.  
         [0012]     Another embodiment of the invention is a method of evaluating an earth formation. A current is conveyed into the earth formation. A voltage difference is measured in first and second directions substantially orthogonal to each other. Using the two voltage differences and a coordinate rotation, a horizontal resistivity and a vertical resistivity of the earth are estimated.  
         [0013]     Another embodiment of the invention is a computer readable medium for use with an apparatus for evaluating an earth formation. The apparatus includes a logging tool conveyed into a borehole in the earth formation. A first pair of current electrodes conveys a first current into the formation. Two pairs of voltage electrodes measure a voltage difference in two substantially orthogonal directions. The medium includes instructions which enable a processor to use the voltage measurements to estimate a horizontal resistivity and a vertical resistivity of the earth formation using a coordinate transformation. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0014]     The present invention is best understood with reference to the accompanying figures in which like numerals refer to like elements and in which:  
         [0015]      FIG. 1  shows the logging tool of this invention suspended in a borehole;  
         [0016]      FIG. 2  (prior art) is a mechanical schematic view of a prior art focused imaging tool;  
         [0017]      FIG. 3  illustrates a borehole in a dipping formation;  
         [0018]      FIG. 4  shows an electrode arrangement for measuring both vertical and horizontal potential differences with current excitation in the vertical direction;  
         [0019]      FIG. 5  shows an electrode arrangement for measuring both vertical and horizontal potential differences with current excitation in the horizontal or azimuthal direction;  
         [0020]      FIG. 6   a  shows front view of the pad facing the y-axis along with bedding planes;  
         [0021]      FIG. 6   b  shows front view of the pad facing the x-axis along with bedding planes;  
         [0022]      FIGS. 7   a  and  7   b  shows variations of  FIGS. 4 and 5  in which the electrodes are rotated; and  
         [0023]      FIG. 8  shows an electrode arrangement in which tensor potential differences are measured at multiple azimuthal positions using three rows of electrodes. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     In order to gain a proper understanding of the present invention, reference is made to  FIGS. 1-5 .  FIG. 1  shows an imaging tool  10  suspended in a borehole  12 , that penetrates earth formations such as  13 , from a suitable cable  14  that passes over a sheave  16  mounted on drilling rig  18 . By industry standard, the cable  14  includes a stress member and seven conductors for transmitting commands to the tool and for receiving data back from the tool as well as power for the tool. The tool  10  is raised and lowered by draw works  20 . Electronic module  22 , on the surface  23 , transmits the required operating commands downhole and in return, receives data back which may be recorded on an archival storage medium of any desired type for concurrent or later processing. The data may be transmitted in analog or digital form. Data processors such as a suitable computer  24 , may be provided for performing data analysis in the field in real time or the recorded data may be sent to a processing center or both for post processing of the data.  
         [0025]      FIG. 2  (prior art) illustrates an array of measure electrodes  315   a,    315   b,    315   c  . . . set within a substantially rectangular guard electrode  303  with gaps  307   a  (that contain insulating material therein). The guard electrode  303  is separated from the pad or body  301  by a substantially rectangular insulating gap  307   b.  In one embodiment of the invention, the spacing between the measure electrodes is selected to provide overlap in azimuth and depth, i.e., the diameter D of the measure electrode is greater than the horizontal spacing d 1  of the electrodes  315   b,    315   c  in adjacent rows and the vertical spacing d 2  between the rows of electrodes. The pad may be extended from the body of the logging tool by a suitable extension device.  
         [0026]     Turning now to  FIG. 4 , a tool mandrel  151  that is part of the logging tool is shown. To simplify the illustration, the borehole is not shown. Current is passed vertically through the formation (parallel to the wellbore) using a first pair of current electrodes  153 ,  155 . The electrode pair  161   a,    161   b  measures a voltage V zz  in the vertical direction and may be referred to as a first pair of voltage electrodes. If the pad surface is normal to the y-axis, the second voltage electrode pair  163   a,    163   b  measures a voltage V xx  in the horizontal (circumferential to the wellbore) direction. Here, the first subscript stands for the current injection direction and the second for the potential drop measurement direction. Similarly, for the configuration of  FIG. 5 , current is injected in a horizontal (circumferential to the wellbore) using a pair of horizontally separated current electrodes  173 ,  175 . Electrodes  181   a,    181   b  measured V xz  while azimuthally-separated electrodes  183   a,    183   b  measured V xx . The same voltage electrodes may be used for measuring the voltage resulting from the vertical current flow and the horizontal current flow. The measurements made by the electrodes in  FIGS. 4 and 5  can be assembled into a tensor  
               V   =     [           V   xx           V   zx               V   xz           V   zz           ]       ,           (   1   )             
 
 which can be written in terms of an apparent resistivity tensor R as  
               R   =         [           V   xx           V   zx               V   xz           V   zz           ]     ⁡     [           K   x                                       K   z           ]       =     [           R   xx           R   zx               R   xz           R   zz           ]         ,           (   2   )             
 
 where K x  and K z  are geometrical factors for horizontal (azimuthal) and vertical excitations respectively. These are obtained by modeling results or by making suitable calibration measurements in a test tank. 
 
         [0027]     The resistivity tensor R of eqn. (2) in the tool coordinate system can be obtained by successive rotations of the resistivity tensor in the earth coordinate system {circumflex over (R)} 
                   Θ   T     ⁢   R   ⁢           ⁢   Θ     =       [           R   h         0           0         R   v           ]     =     R   ^         ,     
     ⁢     Θ   =     [           cos   ⁢           ⁢   θ           sin   ⁢           ⁢   θ                 -   sin     ⁢           ⁢   θ           cos   ⁢           ⁢   θ           ]       ,           (   3   )             
 
 where R h  and R v  are the formation resistivities parallel to and perpendicular to the bedding planes respsectively, and θ is the angle between the bedding plane and the longitudinal axis of the tool. It is to be reiterated that the analysis above is for the situation illustrated in  FIG. 3  where the bedding plane is orthogonal to the y-axis. Those versed in the art would also recognize that what has been done by eqn. (3) is an diagonalization of the measured resistivity tensor R. Thus, if the measurements are made with the pad along the y-axis, it is possible to recover the horizontal and vertical resistivities as well as the dip angle θ. Usually, the larger eigenvalue corresponds to the vertical resistivity and the smaller eigenvalue corresponds to the horizontal resistivity. 
 
         [0028]     Referring to  FIG. 6   a,  the pad  203  is shown along with bedding plane  201  when the pad is oriented along the y-axis. R xx  and R zz  in this orientation will satisfy the relation: 
 
R v &gt;R xx &gt;R h ; 
 
R v &gt;R zz &gt;R h    (4). 
 
         [0029]     When the pad is not orthogonal to the y-axis, it is still possible to do the 10 eigenvalue decomposition of the measured resistivity tensor R, but the recovered eigenvalues will no longer be the horizontal and vertical resistivities of the formation. Referring to  FIG. 6   b,  R yy  and R zz  in this orientation will satisfy the relation: 
 
R v &gt;R zz ; 
 
R yy =R h    (5). 
 
         [0030]     It can be shown that the ratio of the larger eigenvalue to the smaller eigenvalue is the greatest when the pad face is orthgonal to the y-axis. Based on a recognition of this fact, it is thus possible to use the measurements from a plurality of pads on the logging tool, each of the pads facing a different direction, to determine the horizontal and vertical resistivity of the formation. One simply goes through the eigenvalue decomposition of eqn. (3) for each of the pads, and identifies the particular set of electrodes on a particular pad for which the ratio of the eignevalues is a maximum. This pad is then identified as being perpendicular to the bedding plane, and the corresponding eigenvalues give the horizontal and vertical resistivities. Alternatively, for a Measurement-while-drilling (MWD) implementation, measurements made with a single pad as the tool rotates can be used to get the formation dip and the horizontal and vertical resistivities.  
         [0031]     When the pad surface is normal to the x-axis, the focused resistivity tool of Evans will measure a mixture of the resistivities parallel and normal to the bedding planes. The amount of current flowing into the formation will be dictated by the conductive beds. Hence, except for steeply dipping beds, it is the resistivity parallel to the bedding plane that controls the current measurement. Consequently, a measurement of the resistivity parallel to the bedding plane is again desirable to mimic the focused resistivity image. For other pad positions the measurement will be somewhere between the above two cases.  
         [0032]     Other electrode orientations may be used as long as the two measured components are orthogonal to each other. For example,  FIG. 7   a  shows an arrangement in which the current path is vertical and the voltage is measured at two paris of electrodes to give measurements V z1  and V z2 , while  FIG. 7   b  shows an arrangement in which the current is horizontal (orthogonal to the current in  FIG. 7   a ) and the same electrodes are used to give voltages V x1  and V x2 .  
         [0033]      FIG. 8  shows an arrangement in which multiple pairs of voltage measurement electrodes can be used to measure voltage differences at multiple horizontal and vertical positions.  
         [0034]     It should be noted that the invention has been described generally in terms of determination of horizontal and vertical resistivity of the earth formation. Those versed in the art would recognize that the method is also applicable to determination of horizontal conductivity and vertical conductivity; accordingly, determination of horizontal resistivity and vertical resistivity is to be construed as including a determination of horizontal conductivity and vertical conductivity. It should further be noted that the apparatus can also provide a resistivity image of the borehole using measurements from neighboring sets of electrodes.  
         [0035]     For all of the electrode configurations described above, the vertical current flow may be obtained by using current electrodes that are on the pad or off the pad. In the latter case, they may be on the mandrel or at a remote location on the logging string (or BHA). The horizontal current flow may be obtained using current electrodes on the pad.  
         [0036]     The processing of the data may be done with the use of a computer program implemented on a suitable machine readable medium that enables the processor to perform the control and processing. The term processor as used in this application is used in its traditionally-broad sense and is intended to include such devices as single-core computers, multiple-core computers, distributed computing systems, field programmable gate arrays (FPGAs) and the like. The machine readable medium referenced in this disclosure is any medium that may be read by a machine and may include magnetic media, RAM, ROM, EPROM, EAROM, flash memory and optical disks. The processing may be done downhole or at the surface. In an alternative embodiment, part of the processing may be done downhole with the remainder conducted at the surface.  
         [0037]     The invention has further been described by reference to logging tools that are intended to be conveyed on a wireline. However, the method of the present invention may also be used with measurement-while-drilling (MWD) tools, or logging while drilling (LWD) tools, either of which may be conveyed on a drillstring or on coiled tubing.  
         [0038]     While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.