WO2007055786A2

WO2007055786A2 - Ombi tool with guarded electrode current measurement

Info

Publication number: WO2007055786A2
Application number: PCT/US2006/034737
Authority: WO
Inventors: Myrick L. Monroe; Marian L. Morys; Carl Dodge
Original assignee: Halliburton Energy Services, Inc.
Priority date: 2005-11-04
Filing date: 2006-09-07
Publication date: 2007-05-18
Also published as: WO2007055786A3

Abstract

An apparatus and method for guarding the current injection sources of downhole logging tools utilized to determine the resistivity of an adjacent portion of a borehole wall. Two current electrodes are energized to create an oscillatory electric field in a borehole wall. The two current electrodes are each shielded with a conductive shield, maintained at the same electric potential as the current electrode, to prevent current leakage into the logging tool body. A current sensor is coupled between each current electrode and conductive shield to measure the actual current flow injected into the borehole wall from the current electrode and to compensate for current leakage. A voltage detector measures the differential voltage created by the electric field in the borehole wall, and the differential voltage is used in combination with the measured current flow to determine a resistivity value for the borehole wall.

Description

OMBI TOOL WITH GUARDED ELECTRODE CURRENT MEASUREMENT

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT Not applicable.

BACKGROUND

Modern oil field operations demand a great quantity of information relating to the paraineters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as "logging," can be performed by several methods including wireline logging and "logging while drilling" (LWD).

In wireline logging, a probe or "sonde" is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable or "wireline" that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well, hi accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.

In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.

In these and other logging environments, it is desirable to construct an image of the borehole wall. Among other things, such images reveal the fine-scale structure of the penetrated formations. The fine-scale structure includes stratifications such as shale/sand sequences, fractures, and non-homogeneities caused by irregular cementation and variations in pore size. Orientations of fractures and strata can also be identified, enabling more accurate reservoir flow modeling.

Borehole wall imaging can be accomplished in a number of ways, but micro-resistivity tools have proven to be effective for this purpose. Micro-resistivity tools measure borehole surface resistivity on a fine scale. The resistivity measurements can be converted into pixel intensity values to obtain a borehole wall image. However, oil-based muds can inhibit such measurements. U.S.

Patent No. 6,191,588 (Chen) discloses an imaging tool for use in oil-based muds. Chen's resistivity tool employs at least two pairs of voltage electrodes positioned on a non-conductive surface between a current source electrode and a current return electrode. At least in theory, the separation of voltage and current electrodes eliminates the oil-based mud's effect on voltage electrode measurements, enabling at least qualitative measurements of formation resistivity. In constructing an imaging tool for use in oil-based muds, certain engineering constraints on the structural strength of sensor pads will be recognized. These engineering constraints may be met by making the sensor pad base out of a metal such as steel. Though the steel can be insulated to present a non-conductive external surface, the electrical conductivity of the base creates potential current leakage paths to the metal body of the pad. These leakage paths affect the accuracy and stability of the tool's resistivity measurements, especially when the source current operating frequency increases. Accordingly, an improved method and system to compensate for and reduce the effect of the leakage current is needed.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed description, reference will be made to the accompanying drawings, in which:

Fig. 1 shows an illustrative logging while drilling (LWD) environment;

Fig. 2 shows an illustrative wireline logging environment;

Fig. 3 shows an illustrative first logging tool configuration; Fig. 4 shows an illustrative second logging tool configuration;

Fig. 5 shows a front view of an illustrative sensor pad;

Fig. 6 shows a cross section of the illustrative sensor pad;

Fig. 7 shows an illustrative current sensor configuration;

Fig. 8 shows an illustrative circuit model for the illustrative sensor pad; and Fig. 9 shows a flow diagram of an illustrative imaging method with guarded current electrode measurement.

The drawings show illustrative invention embodiments that will be described in detail.

However, the description and accompanying drawings are not intended to limit the invention to the illustrative embodiments, but to the contrary, the intention is to disclose and protect all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.

DETAILED DESCRIPTION

Disclosed herein are various current electrode guarding methods and systems for imaging in nonconductive fluids such as an oil-based mud. In some embodiments, disclosed logging systems include a logging tool in communication with surface computing facilities such as a personal computer, server, or digital signal processing board, or some other form of computing circuit. The logging tool is provided with a sensor array having at least two voltage electrodes positioned between at least two current electrodes that create an electric field in a borehole wall, and is further provided with an electronic circuit to determine a differential voltage between the voltage electrodes and two current flows from separate ones of the current electrodes. Conductive shields for the current electrodes and the lines that feed the current electrodes are included. The use of the shields drastically reduces the effect of the current leakage in the current measurements, which allows for the use of higher frequencies in the operation of the logging tool.

Fig. 1 shows an illustrative logging while drilling (LWD) environment. A drilling platform 2 supports a derrick 4 having a traveling block 6 for raising and lowering a drill string 8. A kelly 10 supports the drill string 8 as it is lowered through a rotary table 12. A drill bit 14 is driven by a downhole motor and/or rotation of the drill string 8. As bit 14 rotates, it creates a borehole 16 that passes through various formations 18. A pump 20 circulates drilling fluid through a feed pipe 22 to kelly 10, downhole through the interior of drill string 8, through orifices in drill bit 14, back to the surface via the annulus around drill string 8, and into a retention pit 24. The drilling fluid transports cuttings from the borehole into the pit 24 and aids in maintaining the borehole integrity.

An LWD resistivity imaging tool 26 is integrated into the bottom-hole assembly near the bit 14. As the bit extends the borehole through the formations, logging tool 26 collects measurements relating to various formation properties as well as the bit position and various other drilling conditions. The logging tool 26 may take the form of a drill collar, i.e., a thick-walled tubular that provides weight and rigidity to aid the drilling process. A telemetry sub 28 may be included to transfer tool measurements to a surface receiver 30 and to receive commands from the surface receiver.

At various times during the drilling process, the drill string 8 may be removed from the borehole. Once the drill string has been removed, logging operations can be conducted using a wireline logging tool 34, i.e., a sensing instrument sonde suspended by a cable 42 having conductors for transporting power to the tool and telemetry from the tool to the surface. A resistivity imaging portion of the logging tool 34 may have sensing pads 36 that slide along the borehole wall as the tool is pulled uphole. A logging facility 44 collects measurements from the logging tool 34, and includes computing facilities for processing and storing the measurements gathered by the logging tool.

Fig. 3 shows a cross-sectional view of LWD resistivity imaging tool 26 in a borehole 16. A biasing mechanism 302 de-centralizes tool 26 to minimize the standoff between the tool's sensors and the borehole wall. The tool's sensors may be located in a pad on biasing mechanism 302, or alternatively the sensors may be located in the main body of the tool opposite the biasing mechanism. As the tool 26 rotates and progresses downhole at the drilling rate, the sensors will trace a helical path on the borehole wall. Orientation sensors within the tool may be used to associate the resistivity measurements with the sensors' positions on the borehole wall. Surface computing facilities may collect resistivity measurements, orientation (azimuth) measurements, and tool position measurements, and may process the collected measurements to create a resistivity image of the borehole wall. Fig. 4 shows a cross-sectional view of one embodiment of the wireline resistivity imaging tool 34 in a borehole 16, depicting a possible configuration of a LWD imaging tool. Sensing pads 36 are deployed against the borehole wall to minimize standoff. Multiple pads may be used to obtain measurements over a greater fraction of the borehole's circumference. In some embodiments, the pads are provided in axially-offset groups to increase circumferential coverage without undue crowding in the undeployed configuration.

In the logging scenarios described above with respect to Figs. 1 and 2, the drilling fluid present in the borehole is an electrically nonconductive fluid such as an oil-based mud. Some of the fluid may mix with drill cuttings or material from the borehole walls to form a viscous semi-solid layer on the borehole walls. This layer is commonly termed "mudcake," and it prevents intimate contact between logging sensors and uncontaminated formation material. In addition, motion of the logging instruments may create a fluid flow layer that further separates the logging sensors from the uncontaminated formation materials.

The mudcake and fluid flow layers have a very low conductivity, which creates some difficulty for high-resolution measurements of borehole wall resistivity. Measurements through the low-conductivity layers may be improved by (1) using an alternating current, (2) separating the electrodes that source a current from the electrodes that measure a voltage, and (3) using a source current with a higher operating frequency.

Fig. 5 shows the face of an illustrative sensor pad 502 having six pairs of voltage electrodes 504 positioned between current electrodes 506 and 508. In practice, the sensor pads may be provided with additional voltage and current electrodes, and in fact may operate on multiple axes. With uni-axial sensor pads such as pad 502, the length of the sensor pad is kept parallel to the long axis of tool 34. The distance between the current electrodes 506, 508 controls the depth of investigation, with greater distances providing greater depths of investigation. The distances between the voltage electrodes 504 control the spatial resolution of the tool, with smaller distances providing higher resolutions.

A cross-section of the illustrative sensor pad 502 is shown in Fig. 6. Sensor pad 502 comprises a metal substrate 602 to provide the pad with the needed rigidity and strength. The metal substrate 602 may include cavities 604 to hold sensor circuitry. For illustrative purposes, the electrode feeds are shown passing through the sensor pad 502, but the electrode feeds may alternatively connect to the sensor circuitry in cavities 604 or in a central cavity (not shown).

In some embodiments, metal substrate 602 comprises steel. The face of metal substrate 602 is covered with an insulating layer 606, which in some embodiments comprises a polyetheretherketone (PEEK) material. Current electrodes 506 and 508 are embedded on the face of the insulating layer 606. Shields 510 and 512 separate the current electrodes 506 and 508 from the body of pad 502, and the current electrode feeds are preferably also shielded, possibly with the shield feeds in a coaxial cable or triaxial cable configuration. Separating the current electrode 506, 508 from the shield 510, 512 are insulating inserts 608, which in some embodiments comprise a PEEK material. When tool 34 is operated at a very low source current frequency of excitation (i.e., approximately less than 2-5 IcHz), the capacitive coupling to the metal body of sensor pad 502 is negligible, meaning that the current leakage between current electrodes 506, 508 and the metal body of sensor pad 502 is veiy small. However, the operation of tool 34 at low current frequencies results in poor accuracy when measuring borehole wall resistivity due to the low voltage difference generated between voltage electrodes 504. The use of higher frequencies (e.g., in excess of 5 IcHz) can provide more accurate measurements of the adjacent borehole wall resistivity. Unfortunately, an increase in the source current frequency of operation produces a corresponding undesirable increase in current leakage from current electrodes 506, 508 to the metal body of sensor pad 502. As a result, the measured voltage difference between voltage electrodes 504 may not be helpful in creating an accurate indication of the true borehole wall resistivity adjacent to tool 34.

In order to reduce the amount of leakage current that results when the frequency of operation exceeds 5 kHz, in certain embodiments of the present invention a conductive shield 510, 512 is placed behind each of corresponding current electrodes 506, 508. Alternatively, it is contemplated that in certain embodiments only one conductive shield may be used with a single corresponding current electrode. For example, tool 34 may be configured such that current electrode 506 is shielded by the inclusion of conductive shield 510, while current electrode 508 remains unshielded.

Shield 510, 512 may alternatively be termed a "guard electrode." The shields 510, 512 may be maintained at the same electric potential as the corresponding current electrodes 506, 508, thereby preventing current flow between the current electrodes and guard electrodes. Further, any leakage currents from the current electrodes to the metal body of sensor pad 502 are minimized, and any current leakage into the metal body of tool 34 primarily originates from shields 510, 512. The ability to minimize the leakage of current from the current electrodes to the metal body of sensor pad 502 in the present invention becomes more important as higher source current operating frequencies are utilized. As the frequency of operation increases, the amount of capacitive coupling to the metal body of sensor pad 502 increases current leakage and (absent any guard electrodes) negatively affects the accuracy of tool 34 in determining borehole wall resistivity.

In addition to minimizing the current leakage from current electrodes 506, 508 to the metal body of sensor pad 502, the manner of measuring the current flowing into current electrodes 506, 508 is adapted in view of the inclusion of shields 510, 512. Referring to Fig. 7, current sensors 702, 704 in illustrative embodiments of the present invention include transformers 706, 708 mat are used to assist in measuring the current associated with current electrodes 506, 508. Further, power amplifiers 710, .712 provide the source current in the present embodiment. In the illustrative embodiment, transformer 706 is coupled between power amplifier 710 and the left current electrode 506 so that the current measurement includes only the current that flows into current electrode 506 and not the current that flows into shield 510. Similarly, transformer 708 is. coupled between the power amplifier 712 and the right current electrode 508 so that the current measurement includes only the current that flows into current electrode 508 and not the current that flows into shield 512. In the embodiments discussed above where only one of current electrode 506, 508 is shielded, the inclusion of a current sensor may be limited to the one of current sensor 702, 704 that corresponds to the shielded current electrode. In this configuration, only the current associated with the current electrode that is shielded is measured by the single current sensor present.

In this configuration, the amount of leakage current into the metal body of sensor pad 502 can be compensated for during the current measurement process. The total current present in sensor pad 502 comprises the current flowing into current electrodes 506, 508 and the current leaking from shields 510, 512. Since the total current from the source is known, and the current flowing into current electrodes 506, 508 is measured as a result of the shield and current sensor configuration described above, the current leakage flowing from shields 510, 512 can be compensated for since it is isolated from the corresponding current sensor and not measured. As a result, current flow from the shields 510, 512 is not included as part of the measured current flow from the current electrodes 506, 508 and does not distort the measurement of the current injected into the formation.

Fig. 8 shows an illustrative circuit model for pad 502 as it operates to measure formation resistivity. Pad 502 comprises measurement circuitry 802 coupled to the voltage electrodes, current electrodes, and the electrode shields. The various electrodes and shields in turn couple to the measurement environment that is modeled as an equivalent circuit 804. The equivalent circuit 804 is a simplified approximation of the borehole wall's electrical characteristics, and is provided here as an aid to understanding the configuration of the measurement circuitry 802.

Measurement circuitry 802 comprises a current or voltage source 806 that drives an oscillating current between the current electrodes ("Right Electrode" and "Left Electrode"). Source 806 is also coupled between the shields/guard electrodes ("Right Shield" and "Left Shield") to maintain the shields at approximately the same potential as their corresponding current electrodes. Current sensors are coupled to the current electrodes to measure simultaneous current flows from the two current electrodes.

The measured currents may be corrected to compensate for baseline current flow (i.e., the current flow that would be measured if the tool were isolated in a vacuum or in air). In some embodiments, the voltage of each current electrode (relative to the tool body) is measured and multiplied by a vacuum calibration constant to determine the baseline current from that current electrode. Note that the current electrodes may be at different voltages, causing a different baseline current to be determined for each current electrode. The corrected current values are determined by subtracting each baseline current from the measured current for the respective current electrode, thereby excluding from the measurement the current leaking from the shields to the tool body.

In addition to current sense amplifiers for the current measurements, measurement circuitry 802 includes a detector 816 for each voltage electrode pair to measure the potential difference generated by the formation currents. Detector 816 may take the form of a differential voltage amplifier, and in alternative embodiments, may take the form of separate sense amplifiers for each voltage electrode. In both cases, circuitry 802 may include analog-to-digital converters to enable digital processing of the measured potential differences. These potential differences are associated with a position on the borehole wall and processed to estimate formation resistivity at that position.

Equivalent circuit 804 includes components 824-832 that approximate a theoretical current path between the current electrodes. Capacitor 824 represents a capacitive coupling between the left current electrode and the borehole wall. Resistors 826, 828, and 830 represent resistive portions of the borehole wall, and capacitor 832 represents a capacitive coupling between the borehole wall and the right current electrode. Capacitors 834 and 836 represent capacitive couplings between the voltage electrodes and the measured portion of the borehole wall. The shields minimize direct capacitive coupling between the current electrodes and the pad body, assuming the capacitive couplings 820 and 822 exist. Nevertheless, indirect coupling is present as represented by capacitors 838 and 840. The current labeled IC_F flows through resistor 828, and it is the current of interest for determining resistivity. Given the measured electrode currents and assuming that the voltage electrode currents are minimized and compensated for in light of the shield and current sensor configuration, it is possible to estimate the current of interest, I_CF, and hence the resistivity of the adjacent borehole wall formation.

A processor may be provided as part of measurement circuitry 802 to calculate resistivity values. Alternatively, current and voltage measurements may be communicated to surface computing facilities to calculate the resistivity values. The resistivity estimation can be expressed as a function:

R = ^V₅ ILE₅ IRE) (1)

The function can take a number of forms depending on experimentally measured sensor pad characteristics. In some embodiments, the resistivity estimation is the measured voltage difference divided by a weighted sum of the measured corrected electrode currents, which have been corrected through the compensation of shield leakage currents: R = k δV / (C₀ IMAX + c, IMIN), (2) where k is a calibration constant based on the sensor pad geometry, I_MAX is the greater of the corrected electrode currents, I_M]N is the lesser of the corrected electrode currents, and c₀ and C₁ are weight factors that sum to unity. In one embodiment, the weight factors equal 1/2, while in another embodiment, Ci=2/3. The weight factors may be determined in a manner that minimizes the mean square error in various calibration curves. In still other embodiments, the resistivity estimation is a weighted sum of resistivities determined for the separately measured currents:

R = C₀ RMIN + Ci RMAX = C₀ (k 5V / I_MAX ) + C₁ (Ic δV / I_MIN), (3) where, again, k is a calibration constant based on sensor pad geometry, IMAX is the greater of the corrected electrode currents, I_MIN is the lesser of the corrected electrode currents, and C₀ and C₁ are weight factors that sum to unity.

Fig. 9 shows a flow diagram of a resistivity imaging method. In block 902, the resistivity imaging tool is placed in a borehole. For LWD, the tool is part of the bottom hole assembly to perform logging as drilling operations are performed. For wireline logging, the tool is part of a sonde that is lowered to the bottom of the region of interest to perform logging as the logging tool is pulled uphole at a steady rate. hi block 904, the tool is placed in logging mode. For LWD, this operation may (or may not) involve deploying a de-centralizer that forces sensors in the tool body against the borehole wall.

Alternatively, the LWD resistivity imaging tool may have one or more sensor pads that are deployed against the borehole wall. For wireline logging, multiple sensor pads are deployed against the borehole wall.

Blocks 906-914 represent operations that occur during the logging process. Though shown and described in a sequential fashion, the various operations may occur concurrently, and moreover, they may simultaneously occur for multiple voltage electrode pairs and multiple sensor pads. In block 906, the tool measures the currents through the two current electrodes, which have been corrected through the compensation for shield current leakage as a result of the shield and current sensor placement. The tool further measures the voltage difference between the various voltage electrode pairs in this step. In block 908, the tool determines a compensated resistivity measurement for each voltage electrode pair in accordance with one of equations (1), (2), or (3). In block 910, the tool, or more likely, the surface logging facility coupled to the tool, associates the compensated resistivity measurements with a tool position and orientation measurement, thereby enabling a determination of borehole wall image pixel values. hi block 912, the tool moves along the borehole, and in block 914,^' a check is performed to determine whether logging operations should continue {e.g., whether the logging tool has reached the end of the region of interest). For continued logging operations, blocks 906-914 are repeated. Once logging operations are complete, the surface logging facility maps the resistivity measurements into borehole wall image pixels and displays a resistivity image of the borehole wall in block 916.

A variety of voltage electrode geometries are possible and may be used. A greater number of voltage electrodes may provide higher resolution at the expense of increased processing costs. The operating voltages and currents may vary widely while remaining suitable for the logging operations described herein. It has been found that source current frequencies above about 5 IcHz, and perhaps as high as lOOkHz or more, are desirable as they reduce the mud layer impedances and increase the voltage differences measurable between the voltage electrodes. Higher frequencies generally provide larger measurement signals, but they also increase leakage currents, making the guarding and compensation methods disclosed herein even more desirable. In some tool embodiments, the source current frequency may be switchable between low frequency (e.g., 10 kHz) and high frequency (e.g., 80 kHz) for measurements in formations of differing resistivity. Higher frequencies may be preferred for formations having a generally lower resistivity, and vice versa.

While illustrative embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are illustrative and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. For example, though the disclosure and claims use the term "resistivity", it is widely recognized that conductivity (the inverse of resistivity) has a one-to-one correspondence with resistivity and, consequently, often serves as a functional equivalent to resistivity. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims which follow, the scope of which shall include all equivalents of the subject matter of the claims.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A logging method that comprises : creating an oscillatory electric field in a borehole wall using a first current electrode and a second current electrode; shielding the first current electrode from a pad substrate with a first conductive shield; measuring a first current to the first current electrode through a first current sensor, wherein the first current sensor is coupled between the first conductive shield and the first current electrode, and wherein the first current measured does not include a first leakage current from the first conductive shield to the pad substrate,

2. The method of claim 1, further comprising: shielding the second current electrode from the pad substrate with a second conductive shield; and measuring a second current to the second current electrode through a second current sensor, wherein the second current sensor is coupled between the second conductive shield and the second current electrode, and wherein the second current measured does not include a second leakage current from the second conductive shield to the pad substrate.

3. The method of claim 2, further comprising measuring a differential voltage with at least two voltage electrodes positioned between the first current electrode and the second current electrode.

4. The method of claim 3, further comprising determining a resistivity value from the first current measured, the second current measured, and the differential voltage.

5. The method of claim 4, further comprising correlating the resistivity value with a position on the borehole wall and displaying a borehole wall image that represents at least the position on the borehole wall associated with the resistivity value.

6. The method of claim 2 wherein the oscillatory electric field has a frequency between about 5 IcHz and about 100 IdHz.

7. The method of claim 2 wherein the first current sensor and the second current sensor are transformer-based.

8. The method of claim 2 wherein the first conductive shield and the second conductive shield are respectively maintained at the same electric potential as the first current electrode and the second current electrode.

9. A method of improving a current measurement in a downhole oil-based mud sensor array comprising: providing a sensor pad with a first current electrode and a second current electrode; shielding the first current electrode with a first conductive shield; and providing a first transfoπner-based current sensor, wherein the first current sensor is coupled between the first current electrode and the first conductive shield.

10. The method of claim 9, further comprising: shielding the second current electrode with a second conductive shield; and providing a second transformer-based current sensor, wherein the second current sensor is coupled between the second current electrode and the second conductive shield.

11. The method of claim 10 wherein a first insulating insert is provided between the first current electrode and first conductive shield, and a second insulating insert is provided between the second current electrode and the second conductive shield.

12. The method of claim 10 wherein the first conductive shield and the second conductive shield are respectively maintained at the same electric potential as the first current electrode and the second current electrode.

13. An oil-based mud imaging tool that comprises: a sensor array having at least two voltage electrodes positioned between a first current electrode and a second current electrode, wherein the first current electrode and second current electrode are energized to create an oscillatory electric field in a borehole wall; a first conductive shield, wherein the first conductive shield corresponds to the first current electrode; and a first current sensor coupled between the first current electrode and the first conductive shield to measure a first current flow that does not include a first leakage current originating from the first conductive shield.

14. The oil-based mud imaging tool of claim 13 further comprising: a second conductive shield, wherein the second conductive shield corresponds to the second current electrode; and a second current sensor coupled between the second current electrode and the second conductive shield to measure a second current flow that does not include a second leakage current originating from the second conductive shield.

15. The oil-based mud imaging tool of claim 14 further comprising at least one voltage detector coupled to the at least two voltage electrodes to measure a differential voltage created by the oscillatory electric field.

16. The oil-based mud imaging tool of claim 15 further comprising a circuit in communication with the first current sensor, the second current sensor, and the at least one voltage detector to deteπnine a resistivity value from the first current flow, the second current flow, and the differential voltage.

17. The oil-based mud imaging tool of claim 14 wherein the first current sensor and second current sensor are transformer-based.

18. The oil-based mud imaging tool of claim 14 wherein the first conductive shield and the second conductive shield are respectively maintained at the same electric potential as the first current electrode and the second current electrode.

19. The oil-based mud imaging tool of claim 14 wherein the oscillatory electric field in the borehole wall has a frequency between about 5 kHz and about 100 kHz.

20. The oil-based mud imaging tool of claim 14 wherein a first insulating insert is positioned between the first current electrode and the first conductive shield, and a second insulating insert is positioned between the second current electrode and the second conductive shield.