Borehole imaging tool

The disclosed embodiments include systems and methods to image a borehole. In one embodiment, a borehole imaging system having a borehole imaging tool and a processor is provided. The borehole imaging tool includes a magnetic field source and an array of electrode buttons. The borehole imaging tool also includes a galvanic source operable to inject an electrical current through one or more electrode buttons of the array of electrode buttons into the formation. The processor is operable to determine a differential voltage between at least two electrode buttons of the array of the electrode buttons, and determine the current through the one or more electrode buttons. The processor is also operable to determine a magnetic susceptibility and a resistivity of the formation based on the differential voltage and the current, respectively, and construct a visual representation of the formation based on the resistivity and the magnetic susceptibility of the formation.

BACKGROUND

The present disclosure relates generally to borehole imaging tools, borehole imaging systems, and methods to generate images of a formation proximate a borehole.

Borehole imaging tools are sometimes deployed in downhole environments to measure material properties of a formation along the borehole. These measurements are often interpreted to determine a lithology of the formation, such as, but not limited to the composition of the formation, physical characteristics of one or more types of rocks of the formation, mineralogy of the one or more types of rocks, as well as other properties of the rocks of the formation. Borehole imaging tools sometimes employ resistivity-based measurements to measure electrical properties of the formation along the borehole. However, it may be difficult for resistivity-based imaging tools to differentiate different types of rocks of the formation that have similar resistivity.

DETAILED DESCRIPTION

The present disclosure relates to borehole imaging tools, borehole imaging systems, and methods to generate visual representations of a formation. More particularly, the present disclosure relates to a borehole imaging tool having a resistive imaging component operable to measure the resistivity of a formation proximate the borehole imaging tool and also having a magnetic imaging component operable to measure the magnetic susceptibility of the formation. The borehole imaging tool is also operable to generate visual representations of the formation based on the resistivity and magnetic susceptibility measurements. The borehole imaging tool includes a magnetic imaging component, a resistivity imaging component, and an array of button electrodes operable to detect the magnetic susceptibility and the resistivity of the formation at the array of button electrodes.

The magnetic imaging component includes a magnetic field source that is constructed from coils, permanent magnets, solenoids, or other materials operable to induce an alternating magnetic field to flow from the magnetic field source into the formation. The induced alternating magnetic field in turn generates an induced electric field. The induced alternating magnetic field and the induced electric field are detected by different electrode buttons (such as a first electrode button and a second electrode button) of the array of electrode buttons. In some embodiments, where the magnetic field source is formed from multiple coils, the coils are axially symmetrically deployed on the borehole imaging tool. In such embodiments, the electrode buttons are placed along an axis of symmetry of the coils.

A differential voltage at the first electrode button and the second electrode button is calculated based on differences between the induced alternating magnetic field and the induced electric field detected at the first electrode button and the induced alternating magnetic field and the induced electric field detected at the second electrode button. The magnetic susceptibility of the formation is then calculated based on the differential voltage between the first electrode button and the second electrode button. In some embodiments, the magnetic imaging component operates at various frequencies within a range of frequencies (for example, between 100 Hz and 100,000 Hz). In some embodiments, the magnetic imaging component is further operable to eliminate a resistivity effect in the magnetic susceptibility based on measurements indicative of the magnetic susceptibility at multiple frequencies. In one of such embodiments, the magnetic imaging component utilizes a model containing a Maxwell equation that expresses the electrical permittivity and the magnetic permeability as being mutually coupled. Moreover, the magnetic imaging component applies multi-frequency data to the model to eliminate the resistivity effect in the magnetic susceptibility. In such embodiments, the magnetic imaging component is operable to measure the differential voltage at a plurality of frequencies. In one of such embodiments, the magnetic imaging component is operable to separate the differential voltage into a real component and an imaginary component. In such embodiments, the magnetic imaging component is also operable to determine the differential voltage of the real component as well as the differential voltage of the imaginary component of the differential voltage at different frequencies. For example, the magnetic imaging component, upon determining a real component of multiple voltage measurements made at different frequencies, determines a difference (first difference) between the real component of the plurality of the voltage measurements at two different frequencies. The magnetic imaging component also determines an imaginary component of the plurality of the voltage measurements. The magnetic imaging component then determines a difference (second difference) between the imaginary component of the plurality of voltage measurements at the two different frequencies. The magnetic imaging component then estimates the resistivity of the formation based on the first difference and the second difference. Additional examples of operations performed by the magnetic imaging component to determine the real and imaginary components of differential voltage at different frequencies and to estimate the resistivity of the formation are provided in the paragraphs below and are illustrated in at least Tables 1-3.

In some embodiments, the resistivity imaging tool includes multiple magnetometers, where each magnetometer of the multiple magnetometer is collocated with a different electrode button of the array of electrode buttons. In such embodiments, each of the magnetometers is operable to estimate the magnetic permeability of the formation proximate the electrode button of the array of electrode buttons that is collocated with the respective magnetometer. More particularly, the magnetic permeability is defined as (1+χ)μ0, where χ is the magnetic susceptibility and μ0is the free space magnetic permeability. Each magnetometer is operable to apply the foregoing equation to calculate the magnetic permeability. In some embodiments, each magnetometer is further operable to determine multiple voltage measurements of the differential voltage at multiple frequencies, and estimate the resistivity and the magnetic susceptibility of the formation based on the multiple voltage measurements of the differential voltage at different frequencies.

In some embodiments, the borehole imaging tool also includes a magnetic field receiver. In some embodiments, the magnetic field source is formed from permanent magnets. In other embodiments, the magnetic field source is formed from solenoids. In one of such embodiments, the magnetic field source forms a C-shape. In some embodiments, the magnetic imaging component is also operable to determine a differential voltage between the magnetic field source and the magnetic field receiver. In such embodiments, the magnetic imaging component is further operable to determine the magnetic susceptibility of the formation based on the induced voltage at the magnetic field receiver.

The resistivity imaging component includes a galvanic source that is operable to inject an electrical current through the array of electrode buttons into the formation. The resistivity imaging component also includes return electrodes operable to receive current returning from the formation. In some embodiments, the resistivity imaging component also includes a guard that is operable to facilitate the flow of the electrical current through the array of electrode buttons. The borehole imaging tool measures the amount of current flowing through the array of electrode buttons and determines the resistivity of the formation based on the amount of current flowing through the array of electrode buttons. In one of such embodiments, each electrode button includes or is coupled to a current measurement component operable to measure current flowing through the respective electrode button. In such embodiments, the resistivity imaging component calculates the impedance of each electrode button by dividing the applied voltage by the current flowing through the respective electrode button. In one of such embodiments, the resistivity imaging component accesses a look-up table to determine the formation resistivity-based on the impedance. In another one of such embodiments, the resistivity imaging component performs an inversion to determine the formation resistivity. In some embodiments, the resistivity imaging component operates at various frequencies within a range of frequencies (for example, between 100 Hz and 100,000 Hz). More particularly, the resistivity imaging component is operable to transmit the current into the formation at different frequencies to perform multi-frequency measurements. In some embodiments, the resistivity imaging component is operable to make multiple measurements of the current flowing through an electrode button at different frequencies, and is operable to estimate the resistivity of the formation based on the measurements of the current flowing through the electrode button at different frequencies.

In some embodiments, both the magnetic imaging component and the resistivity imaging component are operable to simultaneously perform the foregoing operations to determine the magnetic susceptibility and the resistivity of the formation. The borehole imaging tool then constructs a visual representation of the formation based on the resistivity and the magnetic susceptibility of the formation. In some embodiments, the borehole imaging tool determines a lithology of the formation based on the visual representation, where the visual representation is indicative of the magnetic susceptibility and the resistivity of the formation. In some embodiments, the borehole logging tool also generates a logging, completion and/or production-related decision based on the visual representation of the formation. In some embodiments, the borehole imaging tool includes a pad that extends towards a wall of the borehole, where the pad abuts or almost abuts against the borehole. In such embodiments, the array of electrode buttons is deployed on the pad to facilitate transmission of the electrical current to the formation, and also to facilitate detection of the resistivity and magnetic susceptibility of the formation. Similarly, in such embodiments, the magnetic field source (and in some embodiments) the magnetic field receiver also abut or almost abut the wall of the borehole to facilitate measurements of the differential voltage. Additional descriptions of borehole imaging tools, borehole imaging systems, and methods to generate visual representations of formation proximate a borehole are described in the paragraphs below and are illustrated inFIGS. 1-4.

Turning now to the figures,FIG. 1Ais a schematic, side view of a logging environment100with a borehole imaging tool120deployed in a borehole106to measure resistive and magnetic properties of a formation112surrounding the borehole106.FIG. 1Amay also represent another completion or preparation environment where a logging operation is performed. In the embodiment ofFIG. 1A, a well102having the borehole106extends from a surface108of the well102to or through a formation112. A conveyance116, optionally carried by a vehicle180, is positioned proximate to the well102. The conveyance116along with the borehole imaging tool120are lowered down the borehole106, i.e. downhole. In one or more embodiments, the conveyance116and the borehole imaging tool120are lowered downhole through a blowout preventer103. In one or more embodiments, the conveyance116may be wireline, slickline, coiled tubing, drill pipe, production tubing, downhole tractor or another type of conveyance operable to deploy the borehole imaging tool120. The conveyance116provides mechanical suspension of the borehole imaging tool120as the borehole imaging tool120is deployed downhole. In one or more embodiments, the conveyance116also provides power to the borehole imaging tool120as well as other downhole components. In one or more embodiments, the conveyance116also provides downhole telemetry. Additional descriptions of telemetry are provided in the paragraphs below. In one or more embodiments, the conveyance116also provides a combination of power and downhole telemetry to the borehole imaging tool120. For example, where the conveyance116is a wireline, coiled tubing (including electro-coiled-tubing), or drill pipe, power and data are transmitted along the conveyance116to the borehole imaging tool120.

The borehole imagining tool120includes an array of electrode buttons that are positioned to face the borehole106, a magnetic imaging component, and a resistivity imaging component that are operable to obtain measurements indicative of the magnetic susceptibility and the resistivity of the formation112proximate the borehole imaging tool120. Further, the borehole imaging tool120is operable to construct a visual representation of the formation112based on the resistivity and the magnetic susceptibility of the formation112. In that regard, the borehole imaging tool120includes a processor (not shown) operable to determine the magnetic susceptibility and resistivity of the formation112based on the measurements made by the magnetic imaging component and the resistivity imaging component, respectively. The processor is further operable to construct the visual representation. In some embodiments, the processor is a component of a surface based electronic device, such as controller184. In such embodiments, data obtained by the borehole imaging tool120are transmitted to the controller184and are processed by the processor of the controller184. In such embodiments, the foregoing operations of the processor are performed on the surface108. Additional descriptions of the processor and operations performed by the processor are described in the paragraphs below. In some embodiments, the borehole imaging tool120(more specifically, the processor of the borehole imaging tool120) is operable to obtain an estimate of a lithology of the formation112based on the visual representation of the formation112. Moreover, the borehole imaging tool120is also operable to generate at least one of a logging, completion, and production-related decision based on the visual representation of the formation112. In other embodiments, the estimate of the lithology of the formation112, as well as logging, completion, and production-related decisions are generated by the controller184. In one or more embodiments, where the processor is a component of the controller184, the controller184and the borehole imaging tool120form a borehole imaging system. In other embodiments, where the processor is a component of another surface based or downhole electronic device, such electronic device and the borehole imaging tool120form a borehole imaging system.

In some embodiments, the borehole imaging tool120is communicatively connected to the controller184via a telemetry system described herein and is operable to provide the visual representation of the formation112, as well as data indicative of other measurements (such as, but not limited to, an estimate of a lithology of the formation) and analysis (such as proposed logging, completion, and production decisions) performed by the borehole imaging tool120to the controller184. An operator may then access the controller184to analyze the visual representation of the formation112. As defined herein, the controller184represents any electronic device operable to receive the visual representation of the formation112from the borehole imaging tool120and provide the visual representation for display. In some embodiments, the borehole imaging tool120is also operable to transmit data indicative of the resistivity and the magnetic susceptibility of the formation112to the controller184. In such embodiments the controller184is also operable to construct a visual representation of the formation112based on the received data.

FIG. 1Bis a schematic, side view of a LWD/MWD environment150with another borehole imaging tool121deployed to measure the properties of the formation112during a drilling operation.FIG. 1Bmay also represent another completion or preparation environment where a drilling operation is performed. A hook138, cable142, traveling block (not shown), and hoist (not shown) are provided to lower a drill sting119down the borehole106or to lift the drill string119up from the borehole106.

At the wellhead136, an inlet conduit152is coupled to a fluid source (not shown) to provide fluids, such as drilling fluids, downhole. The drill string119has an internal cavity that provides a fluid flow path from the surface108down to the borehole imaging tool121. In some embodiments, the fluids travel down the drill string119, through the borehole imaging tool121, and exit the drill string119at the drill bit124. The fluids flow back towards the surface108through a wellbore annulus148and exit the wellbore annulus148via an outlet conduit164where the fluids are captured in container140. In LWD systems, sensors or transducers (not shown) are typically located at the lower end of the drill string119. In one or more embodiments, sensors employed in LWD applications are built into a cylindrical drill collar that is positioned close to the drill bit124. While drilling is in progress, these sensors continuously or intermittently monitor predetermined drilling parameters and formation data, and transmit the information to a surface detector by one or more telemetry techniques, including, but not limited to mud pulse telemetry, acoustic telemetry, and electromagnetic wave telemetry. In one or more embodiments, where a mud pulse telemetry system is deployed in the borehole106to provide telemetry, telemetry information is transmitted by adjusting the timing or frequency of viable pressure pulses in the drilling fluid that is circulated through the drill string119during drilling operations. In one or more embodiments, an acoustic telemetry system that transmits data via vibrations in the tubing wall of the drill string119is deployed in the borehole106to provide telemetry. More particularly, the vibrations are generated by an acoustic transmitter (not shown) mounted on the drill string119and propagate along the drill string119to an acoustic receiver (not shown) also mounted on the drill string119. In one or more embodiments, an electromagnetic wave telemetry system that transmits data using current flows induced in the drill string119is deployed in the borehole106to provide telemetry. Additional types of telemetry systems may also be deployed in the borehole106to transmit data from the borehole imaging tool121and other downhole components to the controller184.

The borehole imaging tool121, similar to the borehole imaging tool120shown inFIG. 1A, is also operable to obtain measurements of the resistivity and magnetic susceptibility of the formation112, generate a visual representation of the formation112based on the measurements of the resistivity and the magnetic susceptibility of the formation112, and provide the visual representation to the controller184. Additional descriptions of the operations performed by the borehole imaging tools120and121are provided in the paragraphs below. Further, additional illustrations of the borehole imaging tools120and121are provided in at leastFIGS. 2A, 2B, 3A, and3B. AlthoughFIGS. 1A and 1Beach illustrates a single borehole imaging tool120or121deployed in the borehole106, multiple borehole imaging tools, such as the borehole imaging tools120and121may be simultaneously deployed in the borehole106to perform operations described herein.

FIG. 2Ais a schematic, block diagram of a front view of the borehole imaging tool120ofFIG. 1A. The borehole imaging tool120includes an array of electrode buttons, including first electrode button222, second electrode button224, and third electrode button226that are deployed on a pad (not shown). The borehole imaging tool120includes a magnetic imaging component formed from magnetic field sources, including first magnetic field source210, second magnetic field source212, third magnetic field source214, and fourth magnetic field source216. In some embodiments, the first through fourth magnetic field sources210,212,214, and216are formed from one or more coils deployed on the borehole imaging tool120, such that the coils are axially symmetrically wrapped around the borehole imaging tool120. In other embodiments, the first through fourth magnetic field sources210,212,214, and216are formed from permanent magnets. In further embodiments, the magnetic field sources are formed from solenoids. The first through fourth magnetic field sources210,212,214, and216are operable to induce an alternating magnet field into the formation. In one of such embodiments, the alternating magnetic field traverses the formation112along a direction parallel to Z-axis202. The alternating magnetic field that flows into the formation112in turn induces an electric field. In some embodiments, the induced electric field travels in along a direction parallel to X-axis206. The alternating magnetic field and the induced electric field are measured by each of the first, second, and third electrode buttons222,224, and226.

The borehole imaging tool includes a processor (not shown) that calculates a differential voltage between two different electrode buttons (such as between the first electrode button222and the second electrode button224) based on differences between the induced alternating magnetic field and the induced electric field detected at the first and second electrode buttons. Further, the processor calculates the magnetic susceptibility of the formation based on the differential voltage between the first and second electrode buttons222and224. In some embodiments, the magnetic imaging component also includes one or more magnetometers (not shown) that are collocated with the first, second, and third electrode buttons222,224, and226. In such embodiments, each of the magnetometers is operable to estimate a magnetic permeability of the formation112proximate an electrode button222,224, or226that is collocated with the respective magnetometer.

In some embodiments, the magnetic imaging component operates at different frequencies within a range of frequencies to produce different alternating magnetic field and the induced electric field measurements, and evaluates the differences in the alternating magnetic field and the induced electric field to determine the magnetic susceptibility of the formation112. Table 1 is an example of measurements made by the first electrode button222of the induced electric field due to excitation of the first and the fourth magnetic field sources210and216, where the frequency column represents different frequencies at which the magnetic imaging component operates, resistivity represents the resistivity of the formation112, and permeability represents the magnetic permeability of the formation112. Further, Re[Eϕ] represents the value of the real component of the induced electric field in a phi ϕ direction that is parallel to the Y-axis204, and Im[Eϕ] represents the value of the imaginary component of the induced electric field in the phi ϕ direction.

As shown in Table 1, both the real and imaginary component of the electric field in the phi ϕ direction are sensitive to the frequency of the magnetic imaging component, the resistivity of the formation112, and the permeability of the formation112. As shown in Table 1, the Im[Eϕ] has little sensitivity to the formation resistivity. However, the Im[Eϕ] is sensitive to the formation permeability and varies with the same order of magnitude. In one of such embodiments, the formation permeability and susceptibility may be estimated by applying the Im[Eϕ] in a look-up table. In another one of such embodiments, an inversion process may be performed to determine the formation permeability and susceptibility.

Table 2 is an example of measurements made by the first electrode button222of a component of the magnetic field orientated along the Z-axis202due to excitation of the first and the fourth magnetic field sources210and216, where the frequency column represents different frequencies at which the magnetic imaging component operates, resistivity represents the resistivity of the formation112, permeability represents the magnetic permeability of the formation112, Re[Hz] represents the value of the real component of the alternating magnetic field, and Im[Hz] represents the value of the imaginary component of the alternating magnetic field.

As shown in Table 2, both the real (Re[Hz]) and imaginary (Im[Hz]) components of the alternating magnetic field orientated along the Z-axis202are sensitive to the frequency of the magnetic imaging component, the resistivity of the formation112, and the permeability of the formation112. In such embodiments, Re[Hz] has limited or almost no sensitivity to both formation resistivity and permeability. However, Im[Hz] is influenced by both formation resistivity and permeability. In such embodiments, the resistivity of the formation112is utilized to determine the formation permeability and susceptibility from the Im[Hz]. As shown in Tables 1 and 2, the imaginary part of the induced electric field in Table 1 is only sensitive to the changes of magnetic permeability and varies with the same order of magnitude. On the other hand, the induced magnetic field that is tabulated in Table 2 is sensitive to both resistivity and magnetic permeability. As such, a multi-frequency measurement of the resistivity of the formation112is obtained to determine the magnetic susceptibility.

The borehole imaging tool120also includes a resistivity imaging component. The resistivity component includes a galvanic source (not shown) that is electrically coupled to each of the electrode buttons222,224, and226. Moreover, the galvanic source is operable to inject an electrical current into the formation112in a direction parallel to the X-axis206. The resistivity imaging component also includes a guard220that facilitates the flow of the electrical current through the first, second, and third electrode buttons222,224, and226. The resistivity imaging component further includes first and second return electrodes232and234. The resistivity imaging component measures the current as it flows through the first, second, or third electrode buttons222,224, and226, and determines the resistivity of the formation112based on the current flowing through the first, second, or third electrode buttons222,224, or226. In the depicted embodiment, the resistivity imaging component determines the resistivity of the formation112based on the current that flows through the first, second, or third electrode buttons222,224, or226, into the formation112, and returns to first or second return electrodes232or234.

In some embodiments, the magnetic imaging component and the resistivity imaging component of the borehole imaging tool120are operable to simultaneously determine the magnetic susceptibility and the resistivity of the formation112. The borehole imaging tool120(more particularly, the processor of the borehole imaging tool120), upon determining both the magnetic susceptibility and the resistivity of the formation112, constructs a visual representation of the formation112based on the resistivity and the magnetic susceptibility. AlthoughFIG. 2Aillustrates four magnetic field sources210,212,214, and216, a different number of magnetic field sources may be deployed on the borehole imaging tool120. Similarly, althoughFIG. 2Aillustrates three electrode buttons222,224, and226, a different number of electrode buttons may be deployed on the borehole imaging tool120. Further, in some embodiments, the magnetic imaging component also includes one or more magnetic field receivers (not shown) that are deployed on the borehole imaging tool120. In some embodiments, where both the magnetic field sources and the magnetic field receivers are deployed on the borehole imaging tool120, measurements indicative of the magnetic susceptibility of the formation112are measured at the magnetic field receivers.

As described herein, and in some embodiments, the magnetic field sources are formed from coils (such as coils210,212,214, and216). In certain embodiments, some of the coils210,212,214, and216are utilized as transmitter coils and other coils are utilized as receiver coils.FIG. 2Bis a schematic, front view of another borehole imaging tool190that is similar to the borehole imaging tool120. The borehole imaging tool190and the borehole imaging tool120contain almost identical components, which are described in the foregoing paragraphs. However, in the embodiment ofFIG. 2B, coils250and256are utilized as a transmitter coils, whereas coils252and254are utilized as receiver coils. In such embodiments, the magnetic imaging component excites a transmitter coil (such as, for example, coil250) to induce an alternating magnetic field into the formation112. The alternating magnetic field in turn induces an electric field. The induced electric field in turn excites receiver coil256, thereby inducing a voltage at the receiver coil256(which is the differential voltage between the transmitter coil250and the receiver coil256). The magnetic imaging component (the processor) determines the differential voltage at the receiver coil256and determines the magnetic susceptibility of the formation112based on the differential voltage.

Table 3 is an example of measurements of real and imaginary components of the differential voltage at coil254, where the frequency column represents different frequencies at which the magnetic imaging component operates, resistivity represents the resistivity of the formation112, permeability represents the magnetic permeability of the formation112, Re[V] represents the value of the real component of the differential voltage, and Im[V] represents the value of the imaginary component of the differential voltage.

As shown in Table 3, the imaginary part of the differential voltage Im[V] at coil254is sensitive only to magnetic permeability at 100 Hz but may be influenced by the changes of resistivity at higher frequencies. The results shown in Table 3 illustrate the sensitivity of the borehole imaging tool190by using coil252as transmitting antenna and coil254as receiving antenna.

FIG. 3Ais a schematic, front view of the borehole imaging tool121ofFIG. 1B.FIG. 3Bis a schematic, side view of the borehole imaging tool121ofFIG. 1B. In the depicted embodiment ofFIGS. 3A and 3B, the borehole imaging tool121includes four outer magnetic field sources310,312,314, and316and four inner four magnetic field sources340,342,344, and346. In the depicted embodiment, the outer magnetic field sources310,312,314, and316and the inner magnetic field sources340,342,344, and346are formed from permanent magnets and from solenoids, respectively. As depicted inFIG. 3B, the outer and inner magnetic sources form C-shapes along a plane formed by the X-axis206and the Z axis202. In such embodiments, any of the four outer magnetic field source310,312,314, and316or the four inner magnetic field sources340,342,344, and346operates as a transmitting antenna. Although the embodiment depicted inFIGS. 3A and 3Billustrate four outer magnetic field sources and four inner magnetic field sources, the borehole imaging tool121may be fitted with a different number of outer and inner magnetic field sources to perform operations described herein.

The borehole imaging tool121also includes a resistivity imaging component similar to the resistivity imaging component of the borehole imaging tool120and operable to determine the resistivity of the formation112. Further, the borehole imaging tool121(the processor), upon determining the magnetic susceptibility and the resistivity of the formation112, performs operations similar to the operations of the borehole imaging tool120to construct the visual representation of the formation112, determine a lithology of the formation based on the visual representation, and generate logging, completion, and/or production-related decisions based on the visual representation. Although the foregoing paragraphs describe operations performed by the processor of the borehole imaging tool121, in one or more embodiments, the processor is a component of a surface based electronic device, such as the controller184. In such embodiments, data indicative of downhole measurements obtained by the borehole imaging tool121are transmitted via telemetry to the controller184. The processor of the controller184then performs the foregoing operations based on the data obtained from the borehole imaging tool121.

FIG. 4is a flow chart of a process400to generate a visual representation of the formation112surrounding the borehole106. Although the operations in the process400are shown in a particular sequence, certain operations may be performed in different sequences or at the same time where feasible.

As described herein, the borehole imaging tool120,121, or190contains an array of electrode buttons, a magnetic imaging component, and a resistivity imaging component. At block S402, the borehole imaging tool120,121, or190induces an alternating magnetic field into the formation112. As described herein, the induced alternating magnetic field in turn induces an electric field, which is picked up by the electrode buttons of the borehole imaging tool120,121, or190. At block S404, the borehole imaging tool120,121, or190determines, at two electrode buttons of an array of electrode buttons, a differential voltage between the two electrode buttons. In some embodiments, the borehole imaging tool120,121, or191determines a strength of the alternating magnetic field and a strength of the induced electric field at or proximate the two electrode buttons. In one of such embodiments, the borehole imaging tool120,121, or191determines the differential voltage between the two electrode buttons based on the induced electric field and the alternating magnetic field detected at each of the two electrode buttons. Further, in some embodiments, the borehole imaging tool120,121, or190operates at different frequencies to obtain the differential voltage at each of the different frequencies. At block S406, the borehole imaging tool120,121, or190excites the array of electrode buttons with a galvanic source. At block S408, the borehole imaging tool120,121, or190determines a current through the array of electrode buttons. In some embodiments, the borehole imaging tool120,121, or190also operates at different frequencies to determine the current at the different frequencies. In some embodiments, the borehole imaging tool120,121, or190simultaneously performs the operations described in blocks S404and S408.

At block S410, the borehole imaging tool120,121, or190estimates, based on the differential voltage between the electrode buttons and the current flowing through the array of electrode buttons, the magnetic susceptibility and the resistivity of the formation112. Further, in some embodiments, where the borehole imaging tool120,121, or190obtains the differential voltage at different frequencies, the imaging tool120,121, or190is further operable to determine, based on the differential voltage, the magnetic susceptibility at each of the different frequencies. In some embodiments, the borehole imaging tool120,121, or190also utilizes the estimated resistivity and the magnetic susceptibility to calculate or correct the magnetic property. At block S412, the borehole imaging tool120,121, or190constructs a visual representation of the formation112based on the resistivity and the magnetic susceptibility of the formation112. In some embodiments, the borehole imaging tool120,121, or190also generates an estimate of a lithology of the formation112based on the visual representation. In further embodiments, the borehole imaging tool120,121, or190also generates suggestions, such as logging, completion, and production-related suggestions based on the visual representation. In one or more embodiments, the operations described in blocks S404, S408, S410, and S412are performed by a processor of the controller184or a processor of another surface based electronic device. In one or more embodiments, the operations are performed by a processor of the downhole imaging tool120,121, or190. In one or more embodiments, the operations are performed by a combination of processors of the downhole imaging tool120,121, or190and the controller184.

The above-disclosed embodiments have been presented for purposes of illustration and to enable one of ordinary skill in the art to practice the disclosure, but the disclosure is not intended to be exhaustive or limited to the forms disclosed. Many insubstantial modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. For instance, although the flowcharts depict a serial process, some of the steps/processes may be performed in parallel or out of sequence, or combined into a single step/process. The scope of the claims is intended to broadly cover the disclosed embodiments and any such modification. Further, the following clauses represent additional embodiments of the disclosure and should be considered within the scope of the disclosure:

Clause 1, a borehole imaging system, comprising: a borehole imaging tool comprising: a magnetic field source operable to induce an alternating magnetic field to flow into a formation, the alternating magnetic field in turn inducing an induced electric field; an array of electrode buttons, each electrode button of the array of electrode buttons being operable to detect the induced electric field; a galvanic source operable to inject an electrical current through one or more electrode buttons of the array of electrode buttons into the formation; and a processor operable to: determine a differential voltage between at least two electrode buttons of the array of the electrode buttons; determine the current through the one or more electrode buttons; determine a magnetic susceptibility and a resistivity of the formation based on the differential voltage and the current, respectively, and construct a visual representation of the formation based on the resistivity and the magnetic susceptibility of the formation.

Clause 2, the borehole imaging system of clause 1, wherein the borehole imaging tool further comprises: one or more magnetometers, each of the one or more magnetometers being collocated with a different button of the array of buttons, wherein each of the one or more magnetometers is operable to estimate a magnetic permeability of the formation proximate a button of the array of buttons that is collocated with the respective magnetometer.

Clause 3, the borehole imaging system of clause 1 or 2, wherein the magnetic field source is formed from one or more coils.

Clause 4, the borehole imaging system of any of clauses 1-3, wherein the one or more coils comprise at least two coils, and wherein the at least two coils are axially symmetrically deployed on the borehole imaging system, and wherein the one or more electrode buttons are placed along an axis of symmetry of the one or more coils.

Clause 5, the borehole imaging system of any of clauses 1-4, wherein the magnetic field source comprises one or more permanent magnets.

Clause 6, the borehole imaging system of any of clauses 1-5, wherein the magnetic field source comprises one or more solenoids.

Clause 7, the borehole imaging system of clauses 1-6, wherein the one or more solenoids are C-shaped.

Clause 8, the borehole imaging system of any of clauses 1-7, wherein the borehole imaging tool further comprises a pad that extends towards a wall of the borehole, wherein the array of electrode buttons is positioned on the pad.

Clause 9, the borehole imaging system of clauses 1-8, wherein the borehole imaging tool further comprises a magnetic field receiver, and the processor is further operable to determine an induced voltage at the magnetic field receiver, the induced voltage being a differential voltage between the magnetic field source and the magnetic field receiver; and determine the magnetic susceptibility of the formation based on the induced voltage at the magnetic field receiver.

Clause 10, the borehole imaging system of any of clauses 1-9, wherein the processor is further operable to determine the differential voltage based on the induced electric field and the alternating magnetic field detected at each of the two electrode buttons.

Clause 11, a method to generate a visual representation of a formation, comprising: inducing alternating magnetic field into a formation, the alternating magnetic field in turn inducing an induced electric field; determining, at two electrode buttons of an array of electrode buttons, a differential voltage between the two electrode buttons; exciting the array of electrode buttons with a galvanic source; determining a current through the array of electrode buttons; estimating, based on the differential voltage between the electrode buttons and the current flowing through the array of electrode buttons, a resistivity and magnetic susceptibility of the formation proximate the array of buttons; and constructing a visual representation of the formation based on the resistivity and magnetic susceptibility of the formation.

Clause 12, the method of claim11, further comprising obtaining an estimate of a lithology of the formation based on the visual representation of the formation.

Clause 13, the method of clause 11 or 12, further comprising generating at least one of a logging, completion, and production-related decision based on the visual representation of the formation.

Clause 14, the method of any one of clauses 11-13, further comprising determining, at the two electrode buttons, a strength of the alternating magnetic field and a strength of the induced electric field proximate the two electrode buttons, wherein determining the differential voltage between the different electrode buttons comprises determining the differential voltage based on the strength of the alternating magnetic field and the strength of the induced electric field proximate the two electrode buttons.

Clause 15, the method of any of clauses 11-14, further comprising eliminating a resistivity effect in the magnetic susceptibility based on at least one of measurements indicative of the magnetic susceptibility at multiple frequencies and measurements indicative of the resistivity at multiple frequencies.

Clause 16, the method of any of clauses 11-15, wherein determining the differential voltage comprises determining a plurality of voltage measurements of the differential voltage at a plurality of frequencies, and estimating the resistivity and the magnetic susceptibility of the formation comprises estimating the magnetic susceptibility of the formation based on the plurality of voltage measurements of the differential voltage at the plurality of frequencies.

Clause 17, the method of clauses 11-16, further comprising: determining a real component of the plurality of the voltage measurements; determining a first difference between the real component of the plurality of the voltage measurements at two different frequencies of the plurality of frequencies; determining an imaginary component of the plurality of the voltage measurements; and determining a second difference between the imaginary component of the plurality of voltage measurements at the two different frequencies, wherein estimating the resistivity of the formation is based on the first difference and the second difference.

Clause 18, the method of any of clauses 11-17, wherein determining the current through the array of the electrode buttons comprises determining a plurality of current measurements of the current at a plurality of frequencies, and wherein estimating the resistivity and the magnetic susceptibility of the formation comprises estimating the resistivity of the formation based on the plurality of current measurements of the current measurements of the current at the plurality of frequencies.

Clause 19, the method of any of clauses 11-18, wherein inducing the alternating magnetic field and exciting the array of the electrode buttons comprises simultaneously inducing the alternating magnetic field and exciting the array of the electrode buttons.

Clause 20, a borehole imaging tool, comprising: a pad; one or more coils axially symmetrically deployed on the pad; an array of electrode buttons deployed on the pad, wherein one or more electrode buttons of the array of electrode buttons are placed along an axis of symmetry of the one or more coils, and wherein each electrode button of the array of electrode buttons being operable to detect the induced electric field; one or more magnetometers, each of the one or more magnetometers being collocated with a different button of the array of buttons; a galvanic source operable to inject an electrical current through one or more electrode buttons of the array of electrode buttons into the formation; and at least one return electrode operable to receive the current.

Although certain embodiments disclosed herein describes transmitting electrical currents from electrodes deployed on an inner string to electrodes deployed on an outer string, one of ordinary skill would understand that the subject technology disclosed herein may also be implemented to transmit electrical currents from electrodes deployed on the outer string to electrodes deployed on the inner string.

As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” and/or “comprising,” when used in this specification and/or the claims, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. In addition, the steps and components described in the above embodiments and figures are merely illustrative and do not imply that any particular step or component is a requirement of a claimed embodiment.