Abstract:
An apparatus, method and computer-readable medium for evaluating an earth formation are disclosed. The apparatus includes at least one coil on a logging tool conveyed in a borehole in the earth formation. Passage of a current through the coil induces an electrical current in the earth formation. At least two electrodes associated with the logging tool and in proximity to a wall of the borehole have a potential difference in response to the induced electrical current that is indicative of a property of the earth formation. The at least one coil may be mounted on a mandrel of a downhole assembly. The electrodes may be positioned on a first pad extendable from a mandrel of the downhole assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to exploration 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. 
     2. Background of the Art 
     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 is a hybrid of the two. 
     There are several modes of operation of prior art devices: in one, the current at the measuring electrode is maintained constant and a voltage is measured while in the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. Ohm&#39;s law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current. 
     Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Patent 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. and U.S. Pat. No. 6,714,014 to Evans et al, each of which provide additional background information to this disclosure. 
     In the prior art devices, current is actively focused in the direction perpendicular to the borehole wall. There is a technical challenge to provide stable focusing conditions during the logging if the borehole walls are rough or the mud is very conductive. As soon as the focusing conditions are not met, the measurements are responsive to a considerable extent to the properties of the mud. The prior art devices do not specifically address the problems due to irregularities in the wall surface of the wellbore. If the wall of the wellbore is irregular, the measuring current path becomes distorted and the relationship between measured impedance and earth formation resistivity changed as result. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention is an apparatus for evaluating an earth formation. The apparatus includes at least one coil on a logging tool conveyed in a borehole in the earth formation. Passage of a current through the coil induces an electrical current in the earth formation. At least two electrodes associated with the logging tool and in proximity to a wall of the borehole have a potential difference that is indicative of a property of the earth formation. The at least one coil may be mounted on a mandrel of a downhole assembly. The at least one coil may include at least three planar coils with their normals distributed azimuthally about an axis of the logging tool. At least one of the three coils may operate at a different frequency from another one of the at least three coils. The electrodes may be positioned on a first pad extendable from a mandrel of the downhole assembly. A second pad may be provided on an opposite side of the mandrel from the first pad. The two electrodes may further include three pairs of electrodes, each pair being at a different azimuthal position on the pad. The apparatus may further include a processor which uses the difference to estimate a resistivity property of the earth formation. The logging tool may be conveyed into the borehole using a wireline, a drilling tubular and/or a slickline. The difference may be in quadrature with the current in the coil. 
     Another embodiment of the invention is a method of evaluating an earth formation. A current is passed through at least one coil on a logging tool conveyed in a borehole in the earth formation so as to induce an electrical current in the formation. A difference in electrical potential between at least two electrodes in proximity to a wall of the borehole is detected. 
     Another embodiment of the invention is a computer-readable medium for use with an apparatus for evaluating an earth formation. The apparatus includes at least one coil on a logging tool conveyed in a borehole in the earth formation. Passage of current through the coil induces an electrical current in the earth formation. The apparatus also includes two electrodes in proximity to a wall of the borehole. The medium includes instructions which enable a processor to determine from a difference in electrical potential between the two electrodes a property of the earth formation. The medium may include a RAM, a ROM, an EPROM, an EAROM, a flash memory, and/or an optical disk. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention is best understood with reference to the accompanying figures in which like numerals refer to like elements and in which: 
         FIG. 1  shows an exemplary logging tool suspended in a borehole; 
         FIG. 2  is a mechanical schematic view of an exemplary imaging tool; 
         FIG. 3   a  is a schematic illustration of three coils on a tool of the present invention; 
         FIG. 3   b  illustrates an embodiment of the present invention showing a single coil on a mandrel and pad mounted electrodes; 
         FIG. 3   c  is an equivalent circuit diagram of a resistivity imaging tool; 
         FIGS. 4   a  and  4   b  shows an arrangement of pad mounted electrodes and a pad mounted coil; 
         FIGS. 5   a  and  5   b  show exemplary models used for evaluation of the tool configuration of  FIG. 3   b;    
         FIG. 6  Shows the response at different azimuths for the model of  FIG. 4 , with no rugosity; 
         FIG. 7  Shows the K-factor for the curves of  FIG. 6  for azimuths of 10° and 20°; 
         FIG. 8  shows the results of applying the K-factor to the 10° and 20°; azimuth curves; 
         FIG. 9  shows the response at different standoffs for the model of  FIG. 4 ; and 
         FIG. 10  shows the effect of rugosity on measurements made by an exemplary tool of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       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 . 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. 
       FIG. 2   a  is a schematic external view of a borehole sidewall imager system. The tool  10  comprising the imager system includes resistivity arrays  26 . Optionally, the imager system may include other sensors, such as a mud cell  30  or a circumferential acoustic televiewer  32 . Electronics modules  28  and  38  may be located at suitable locations in the system and not necessarily in the locations indicated. The components may be mounted on a mandrel  34  in a conventional well-known manner. The outer diameter of the assembly may be about 5 inches and about fifteen feet long. An orientation module  36  including a magnetometer and an accelerometer or inertial guidance system may be mounted above the imaging assemblies  26  and  32 . The upper portion  38  of the tool  10  contains a telemetry module for sampling, digitizing and transmission of the data samples from the various components uphole to surface electronics  22  in a conventional manner. If acoustic data are acquired, they are preferably digitized, although in an alternate arrangement, the data may be retained in analog form for transmission to the surface where it is later digitized by surface electronics  22 . Also shown in  FIG. 2A  are three resistivity arrays  26  extendable from the tool  10  via bowsprings  42  (a fourth array is hidden in this view. 
     Turning to  FIG. 3   a , a simplified exemplary diagram of three vertical coils  101 ,  103 ,  105  of the present invention on a mandrel (not shown) of the present invention is shown.  FIG. 3   b  show one of the coils  103  mounted inside or on the surface of mandrel  121 . Arms depicted schematically by  131 ,  137  extend a pad  133  radially outward from the mandrel to make contact with the borehole wall (not shown). Disposed on the pad  133  are electrodes depicted schematically by  135   a ,  135   b . Another pad (not shown) may be positioned on the opposite side of the coil  103  from the pad  133 . In an alternate embodiment of the invention, a single coil may be mounted on the mandrel with its axis along the tool axis.  FIGS. 4   a  and  4   b  shows an arrangement of pad mounted electrodes and a pad mounted coil.  FIG. 4   a  shows coil  141  disposed on pad  133 ′. 
     This tool may be referred to as a “mixed mode” tool in that an inductive source is used and galvanic currents are detected by the electrodes. Specifically, a plurality of long transversal rectangular coils with the magnetic moment perpendicular to the axis of the borehole are used. Each transmitter loop is centered in the borehole and electrode pairs are placed on the pad attached to the borehole wall. This is a generic design and further variants are identified below. In a practical design each transmitted coil serves two pads with a number of electrode pairs on each pad. Each transmitter coil may have its own operating frequency to avoid the interference of the neighboring induction coils. By using an induction transmitter, an electric current can be injected into the formation. 
     At a low frequency and relatively close to the induction loop, the electric field does not depend on the conductivity of the formation and can be increased simply by increasing the operating frequency ω. In the case of a galvanic injection and non-conductive mud the injection current must go through quite a large capacitive resistance. This can be better understood from the simplified schematics in  FIG. 3   c  where the capacitor C m  represents the capacitance between the injection electrodes and the formation, and R f  corresponds to the resistivity of the formation. The current I f  injected into the formation can then be expressed as 
                       I   f     =       U   ab         2   ⁢     X   c       +     R   f           ,       X   c     =     1     i   ⁢           ⁢   ω   ⁢           ⁢     C   m           ,       C   m     ≈     ɛ   ⁢     S   d         ,           (   1   )               
where S is the area of the electrode, U ab  is the applied potential difference between the injection electrodes a and b. Because C m  is inversely proportional to the distance d between the current electrode and the formation, the amount of the current injected into the formation will drop with increasing standoff. A long induction transmitter is free of such high sensitivity to the standoff value and well suited to the nonconductive environment.
 
     If only electric field is measured, the measurements will be very sensitive to a relative variation of resistivity in the adjacent formation. To derive the absolute resistivity of the formation, some additional induction measurements and their combination with the galvanic readings are helpful. 
     The response of the tool design of  FIG. 3   b  was tested on a number of different models. Those versed in the art and having benefit of the present disclosure would recognize that the antenna would produce a magnetic field oriented in a horizontal plane. This magnetic field would induce eddy currents in the vertical direction so that the voltage difference between electrodes  135   a  and  135   b  would be indicative of the vertical formation resistivity.  FIG. 4   a  shows a configuration of vertically spaced apart electrodes on a pad while  FIG. 4   b  shows azimuthally spaced apart electrodes on a pad for measuring horizontal resistivity. One of these models is illustrated in  FIG. 5   a . Shown therein is a borehole  151  with a diameter of 8.5 inches (21.6 cm). The mandrel is shown as  121 , a pad by  132  and an arm on which the pad is carried by  131 . The tool has a variable standoff  133 . The formation comprises beds of thickness 0.5 inches, 1 inch, 2 inches, 3 inches and 4 inches (1.27 cm, 2.54 cm, 5.08 cm, 7.62 cm and 10.16 cm respectively). The layers had resistivities ρ and relative dielectric constant ∈ of (10 Ω-m, 10) and (1 Ω-m, 20) respectively. 
     In a second model shown in  FIG. 5   b , the formation had a uniform p=H2−m, £=20, The standoff was fixed at ⅛ inches (3.18 mm). However, the borehole was rugose, with a depth of rugosity varied between ¼ inches and ¾ inches (6.35 mm and 19.1 mm). Response to other models which represented a combination of the features of the models of  FIGS. 5   a ,  5   b  were also simulated. 
     In the modeling, a 0.914-m long transmitter with a width of 0.1524 m was used. The operating frequency was 100 kHz. In the case of lower or higher frequencies (up to several MHz), the response can be approximately derived simply by linear resealing of the signal corresponding to 100 kHz frequency. A transmitter loop is placed in the nonconductive borehole environment with the radius of the borehole 10.795 cm. An electrode spacing of 0.25 inches or 0.5 inches (0.63 cm and 1.27 cm) was used to measure a potential drop U z  in the vertical direction parallel to the borehole axis. 
     The typical behavior of the electrical signal to the model is presented in  FIG. 6 . The three curves  201 ,  203  and  205  in this figure correspond to azimuths of 0°, 10° and 20° of the receiver&#39;s electrode pairs. The abscissa in the figure is the logging depth in inches and the ordinate is the signal (the voltage difference between the button electrodes). The 10° and 20° deg. azimuth curves can be shifted to the 0° deg. curve by applying a K-factor that is about 1.07 for the 10° curve and 1.27 for the 20-degree curve. The division result of 10° and 20° curves by the 0° curve is presented in  FIG. 7   as  the curves  221  and  223 , while the result of K-factor application to the original curves from  FIG. 6  is presented in  FIG. 8. 241  is the original 0° azimuth response to the model from  FIG. 6. 243  is the corrected 10° azimuth response to the model while 245 is the corrected 20° azimuth response. From  FIG. 8  it can be seen that it is possible to cover an azimuth range of 40° (from −20° to)+20° by having additional columns of electrodes on the pad of  FIG. 3   b.    
     The dynamic range, which is the ratio between the maximum and minimum reading along the logging depth, is changing between 5 and 6 considering layers  1  in. and larger. We define a Normalized Dynamic Range (NDR) as a ratio of a signal dynamic range to a resistivity contrast of the corresponding media. In the model of  FIG. 5  the resistivity contrast of the neighboring layers is 10, so that the NDR of the mixed mode arrangement is approximately 0.55. 
     Next, examples showing the influence of the distance between the receiver electrodes and the borehole wall are presented. The results of mathematical modeling for the same benchmark model of  FIG. 5   b  are presented in  FIG. 9 . The electrode spacing is 0.25 inches (6.35 mm). For the ⅛ inch (3.18 mm) standoff  263  the NDR drops to 0.3 in the 1 inch (2.54 cm) thick layer and decreases to 0.2 and 0.13 for the ¼ in (6.35 mm) 265 and ½ in (1.27 cm)  267  standoff, correspondingly. For the 2 in (5.08 cm) layer thickness the NDR parameter is as much as twice larger than for 1 in (2.54 cm) layers. The imperfections due to standoff are more noticeable in the conductive layers, and there is no signal imperfection in the resistive layers thicker than 2 in (5.08 cm). 
     Turning next to  FIG. 10 , the sensitivity of the measured electric field for the model of  FIG. 5   b  as a function of borehole rugosity is shown. The curves  281 ,  283  and  285  correspond to rugosity of ¼ inch (6.3 mm), ½ inch (1.27 cm) and ¾ inches (1.91 cm) respectively. This is a model with no resistivity contrasts, but the signal from the rugose wall has all the features of a structure-boundaries and resistivity contrast. Of course, these artifacts are more pronounced for the ½ inch (1.27 cm) and ¾ inches (1.91 cm) rugosity than for the ¼ inch (6.3 mm) rugosity. Based on extensive modeling results (not shown), we have concluded that in the case of a 0.25-in. rugosity depth, all I-in. beds are well resolved (NDR&gt;0.2) and the presence of the rugosity in some beds does not destroy the readings in front of neighboring beds. The situation deteriorates as the rugosity is increased to 0.5 in. and 0.75 in. 
     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. 
     The invention has been described with reference to a wireline conveyed logging tool. The principles discussed above may also be used in a measurement-while-drilling (MWD) implementation in which the logging tool is part of a bottomhole assembly (BHA) conveyed on a drilling tubular. The method may also be used with the logging tool conveyed on a slickline. For the purposes of the present invention, the term “downhole assembly” may be used to describe a BHA as well as configurations in which the logging tool is part of an assembly conveyed on a wireline or slickline. 
     The following definitions are helpful in understanding the present invention. 
     coil: one or more turns, possibly circular or cylindrical, of a current-carrying conductor capable of producing a magnetic field; 
     
         
         EAROM: electrically alterable ROM; 
         EPROM: erasable programmable ROM; 
         flash memory: a nonvolatile memory that is rewritable; 
         induction: based on a relationship between a changing magnetic field and the electric field created by the change; 
         machine readable medium: something on which information may be stored in a form that can be understood by a computer or a processor; 
         mandrel: A bar, shaft or spindle around which other components are arranged or assembled. The term has been extended in oil and gas well terminology to include specialized tubular components that are key parts of an assembly or system; 
         misalignment: the condition of being out of line or improperly adjusted; 
         Optical disk: a disc shaped medium in which optical methods are used for storing and retrieving information; 
         Position: an act of placing or arranging; the point or area occupied by a physical object 
         ROM: Read-only memory; 
         Resistivity: electrical resistance of a conductor of unit cross-sectional area and unit length. Determination of resistivity is equivalent to determination of its inverse (conductivity); 
         Rugosity: A qualitative description of the roughness of a borehole wall. Alternatively, the term pertains to a borehole whose diameter changes rapidly with depth. 
       
    
     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.