X-ray downhole tool with at least two targets and at least one measurement detector

The current disclosure is related to a downhole tool that comprises an electronic photon generator and at least one detector. The electronic photon generator comprises a cathode configured to emit electrons, a first target configured to generate photons when struck by the electrons, a second target configured to generate photons when struck by the electrons, and a beam steering device that directs the electrons to a first or second target. The at least one detector is configured to detect at least some of the photons emitted by the first target and at least some of the photons emitted by the second target.

BACKGROUND

This disclosure relates generally to a downhole tool that generates x-rays to measure formation properties and, more particularly, to a downhole tool that includes at least two targets to generate x-rays from different locations in the downhole tool.

To locate and extract oil, water, natural gas, or other liquids, a hole, referred to as a borehole, may be drilled into a surface of a geological formation. To form the borehole, a drill bit may excavate a portion of the geological formation. A drilling fluid, commonly referred to as “mud” or “drilling mud,” may be pumped into the borehole, for example, to cool and/or lubricate the drill bit. Generally, the drilling mud may include solid particles, such as dirt, suspended in liquid, such as water. When the geological formation is porous, the liquid component of the drilling mud may be pushed into the geological formation, leaving the solid particles on the borehole wall. Over time, a layer of the solid particles, commonly referred to as “mud cake,” may form along the wall of the borehole.

A formation density tool may be deployed within the sub-surface to measure physical properties of a surrounding geological formation. The formation density tool may be moved within a borehole drilled in the geological formation. For example, the formation density tool may be pushed farther into the borehole and/or pulled to remove it from the borehole. The formation density tool may include a source to emit high-energy photons into the geological formation. Some of the high-energy photons may interact with the geological formation and may then be detected by detectors on the formation density tool. The physical properties of the geological formation may be determined from the number and characteristics of the detected high-energy photons.

Because the photons interact at various depths of investigation (DOI) acquiring data associated with multiple DOIs may improve the accuracy of the determined physical properties of the geological formation. To acquire data at multiple DOIs, the formation density tool may include multiple detectors, such that each detector may provide data regarding an additional DOI. However, each of the detectors may include additional hardware that increases the complexity, cost, and may be difficult to accommodate given the space constraints of the formation density tool.

SUMMARY

In some embodiments, there is disclosed a downhole tool that comprises an electronic photon generator and at least one detector. The electronic photon generator comprises a cathode configured to emit electrons, a first target configured to generate photons when struck by the electrons, a second target configured to generate photons when struck by the electrons, and a beam steering device that directs the electrons to a first or second target. The at least one detector is configured to detect at least some of the photons emitted by the first target and at least some of the photons emitted by the second target.

The current disclosure also discloses an electronic photon generator comprising a cathode configured to emit electrons, a first target configured to generate photons when struck by the electrons, and a second target configured to generate photons when struck by the electrons.

A method is also disclosed, which comprises lowering a downhole tool into a wellbore penetrating a subterranean formation. The downhole tool comprises an electronic photon generator having a cathode configured to emit electrons, a first target configured to generate photons when struck by the electrons, a second target configured to generate photons when struck by the electrons, a beam steering device that is configured to direct the electrons to a first or second target, and a detector configured to detect at least some of the photons emitted by the first target and at least some of the photons emitted by the second target. The method further comprises directing photons emitted by the first target out of the downhole tool at a first location, directing photons emitted by the second target out of the downhole tool at a second location, and determining a property of the downhole tool, the wellbore or the subterranean formation based on signals detected from the photons emitted by the first and second targets.

DETAILED DESCRIPTION

The present disclosure relates to a downhole tool that measures properties of a geological formation by steering an electron beam towards different targets to generate photonic radiation. In this disclosure, the photonic radiation is described as including a spectrum of x-rays, but any suitable form of photonic radiation may be generated. The different targets may have different locations within the downhole tool at different spacings from a detector. With different spacings of the targets from the detector, the downhole tool may acquire data at different depths of investigation (DOI). As described below, DOI refers to a depth or range of depths in a formation that is being probed by the photons. Thus, some embodiments of the downhole tool may avoid using more than one detector, yet still obtain measurements at different DOIs. However, more than one detector may be used if desired. The photonic radiation emitted by the downhole tool lends itself well to a formation density measurement. However, it should be understood that the downhole tool may obtain any suitable measurement and is not meant to be limited to measuring formation density. As such, this disclosure will refer to the downhole tool as a downhole formation density tool, but the disclosure should be understood to encompass downhole tools that generate photonic radiation using multiple possible targets.

The formation density tools of this disclosure may measure different DOIs using multiple spacings due to multiple targets, rather than or in addition to positioning different detectors at different distances from the source. For example, the system may have a short-spaced target located in closer proximity to the detector than a long-spacing target. The properties of the geological formation may be determined based on data related to the photons detected from each of the targets to the detector, which may include energy level of the detected photons and count rate of photons. The properties of the geological formation and the borehole in the formation may include for example a formation density, a formation photoelectric factor (PEF), a mud cake thickness, a mud cake density, and a mud cake PEF. The measurements of each of the properties may have varying sensitivities depending on the energy of the emitted or the detected photons, the count rate of the photons, the source-detector spacing, or the like. Using various data analysis techniques, such as spine-and-ribs techniques, forward model techniques, inversion techniques, neural networks, or other suitable approaches, a data processing system may determine the properties of the geological formation based on the energy of the detected photons (e.g., spectral information), the count rate of the detected photons, the DOI associated with the detected photons, or the like. As such, having data related to more than one DOI may improve the accuracy of the data produced using the various techniques.

Although additional detectors assist in data analysis, each detector adds additional complexity to the system and/or may take additional space within the tool. For example, each detector may include hardware, such as a scintillator, a photomultiplier tube, and a high voltage power supply connected to the photomultiplier tube. To gain information for each additional DOI, an additional detector may be added, thereby increasing the complexity and/or have additional radial or axial space in the downhole tool.

Thus, embodiments of the disclosure include a downhole tool having a detector used to acquire data from photons interacting with the geological formation at different DOIs. That is, the downhole tool may provide data at multiple DOIs without including additional detectors (e.g., additional scintillators, additional photomultipliers, etc.). For example, the downhole tool may include an electron accelerator within the downhole tool that accelerates electrons towards a selectable set of at least two targets. Each of the targets, which may be composed of different materials, have different collimation to direct the outgoing radiation or have different angles with respect to the electron beam, may emit photons (e.g., x-rays) of different energy when struck by the electrons. Thus, the different targets may cause photons to exit the downhole tool at different positions, different emission angles and, when the photons are detected by a photon detector (e.g., an x-ray detector), provide data relating to the formation at a different DOI. The downhole tool may also include a steering control system that steers the electron beam from the cathode towards a selected target. The electron beam impinges on the target and creates photons that interact with the geological formation at various depths of investigation.

With this in mind,FIG. 1illustrates a well-logging system10that may employ the systems and methods of this disclosure. The well-logging system10may be used to convey a downhole tool12through a geological formation14via a wellbore16. The downhole tool12is conveyed on a cable18via a logging winch system20. Although the logging winch system20is schematically shown inFIG. 1as a mobile logging winch system carried by a truck, the logging winch system20may be substantially fixed (e.g., a long-term installation that is substantially permanent or modular). Any suitable cable18for well logging may be used. The cable18may be spooled and unspooled on a drum22and an auxiliary power source24may provide energy to the logging winch system20and/or the downhole tool12.

Although the downhole tool12is described as a wireline downhole tool, it should be appreciated that any suitable conveyance may be used. For example, the downhole tool12may instead be conveyed as a logging-while-drilling (LWD) tool as part of a bottom hole assembly (BHA) of a drill string, conveyed on a slickline or via coiled tubing, and so forth. For the purposes of this disclosure, the downhole tool12may be any suitable measurement tool that uses a detector to obtain measurements of properties of the geological formation14.

As discussed further below, the downhole tool12may emit photons, such as x-rays, into the geological formation14, which are detected by the downhole tool12as data26relating to the wellbore16and/or the geological formation14. The data26may be sent to a data processing system28. The data processing system28may be any electronic data processing system that can be used to carry out the systems and methods of this disclosure. For example, the data processing system28may include a processor30, which may execute instructions stored in memory32and/or storage34. As such, the memory32and/or the storage34of the data processing system28may be any suitable article of manufacture that can store the instructions. The memory32and/or the storage34may be read-only memory (ROM), random-access memory (RAM), flash memory, an optical storage medium, or a hard disk drive, to name a few examples. A display36, which may be any suitable electronic display, may display the images generated by the processor30. Part or all of the data processing system28may be located in the downhole tool, or may be a local component of the logging winch system20, may be a remote device that analyzes data from other logging winch systems20, may be a device located proximate to the drilling operation, or any combination thereof. In some embodiments, the data processing system28may be a mobile computing device (e.g., tablet, smartphone, or laptop) or a server remote from the logging winch system20. In some embodiments, the processing system28and storage34and/or memory32may be located in the downhole tool. In some cases, the downhole tool12may not communicate with the surface unit or have limited communication and the recorded data may be retrieved from the tool memory32or storage34once the tool is returned to the surface.

FIG. 2is a schematic diagram of the downhole tool12that detects physical characteristics of the geological formation14. The downhole tool12may include a voltage generator40that may generate voltages of 150 kV, 200 kV, or any other voltage suitable to form an electric field and electrical potential difference between a cathode48and different targets (e.g., anodes). In the illustrated embodiment, the downhole tool12includes six targets50,52,54,56,58, and60. WhileFIG. 2includes six targets50,52,54,56,58, and60, this is meant to be illustrative and the downhole tool12may include any suitable number of targets.

To determine the properties of the geological formation14, the downhole tool12may emit photons into the geological formation14to interact with the geological formation14which then are detected by a detector62. For example, the cathode48may emit electrons in an electron beam49(i.e., stream of electrons) between the cathode48and the first target50. Due to a voltage difference between the cathode48and the first target50, the electrons of the electron beam49may travel from the cathode48, through the electric field, to the first target50.

As the electrons in the electron beam49decelerate due to contacting (e.g., colliding with) the first target50, which may be gold (Au), Tungsten, or the like, the deceleration may cause photons51, such as high-energy photons (e.g., X-rays) to be emitted as Bremsstrahlung radiation. At least some of the photons51may be high-energy photons at an energy sufficient to cause at least a portion of the photons51to elastically (e. g. Compton scatter) scatter off electrons of the geological formation14and to interact with a detector62(e.g., through Compton scattering or photoelectric absorption), such as an x-ray detector. The detector62may include a scintillator66that detects the photons51and emits light based on the energy deposited by the interaction of the photons51. For example, each emission of light may count as a detected photon (e.g., thereby adding one to a number of counts of the detector62). Further, the detector62may include a photomultiplier68(or other suitable photon detector) operatively coupled to the scintillator66to detect the light emitted by the scintillator66. The photomultiplier68may output an electrical signal from the detected light of the scintillator66to the data processing system28. As mentioned above, the data processing system28may process the electrical signals from the photomultiplier68at the surface (e.g., as the data26), at the downhole tool12, or a combination thereof. As such, the downhole tool12may include hardware similar to the data processing system28(e.g., processor30, memory32, storage34, etc.) In some embodiments, the detector62may be communicatively coupled to the data processing system28to communicate the data26related to the electrical signals indicative of the detected photons51.

The photon or x-ray detection system is not limited to a scintillation detector system, wherein the scintillation detector is coupled to a photomultiplier. The photomultiplier could be replaced by any other photon detector suitable to detect the scintillation light and converting it into an electrical signal. Also, the entire detector could be a solid state detector such as a silicon detector, a silicon carbide (SiC) detector, Cadmium Zinc Telluride (CZT) detector to name a few. In yet another approach the detector could be a gas detector such as a high pressure Xe-filled proportional counter or an ionization chamber.

As mentioned above, during the drilling process, drilling fluid, commonly referred to as “mud” or “drilling mud,” may be pumped into the borehole, for example, to cool and/or lubricate the drill bit. Over time, a layer of the solid particles, commonly referred to as “mud cake,” may form along parts of the wall of the borehole due to the drilling mud. In the illustrated embodiment, the borehole16includes mud cake74between the downhole tool12and the geological formation14. The data processing system28may determine the properties of the geological formation14and/or the mud cake74, such as such as formation density, formation photoelectric factor (PEF), mud cake thickness, mud cake density, and mud cake PEF, based at least in part on characteristics of photons51detected by the detector62. The PEF may indicate the lithology (e.g., composition) of the formation, such as the type of rocks. The data processing system28may receive data from the detector62indicating count rates and/or energy levels of the photons51at the detector62. Each of the properties of the geological formation14and/or the mud cake74may have varying sensitivities to the photons51. For example, the count rate of photons51at lower energies may depend more on the type of rock of the geological formation14, characterized by the PEF, of the geological formation12. Conversely, the count rate of photons51at higher energies may depend more on the electron density of the geological formation14than the count rate of the photons51at lower energies. The data processing system28may determine the properties of the geological formation14based on count rates of photons at various energies indicative of the interaction of photons with the elements in the formation. For example, the data processing system28may use spine-and-ribs techniques, forward model techniques, inversion techniques, neural networks, or other suitable approaches, to determine the properties of the geological formation14and the mud cake74.

To improve the accuracy of the values of the properties of the geological formation14and/or the mud cake74determined by the data processing system28, the processing system28may acquire data from the detector62at the different DOIs. For example, information at different DOIs may make inversion more robust and create new perspectives for more complex techniques, such as 3-layer inversion or radial profiling of density and/or PEF. For instance, information at various DOIs may enable better techniques in cased holes in the presence of a casing, a cement layer and the formation behind. As mentioned above, conventional systems may include multiple detectors at different distances from the source to provide multiple DOIs. However, each of the additional detectors may include additional hardware (e.g., scintillator, photomultiplier tube, etc.) thereby increasing the complexity of the downhole tool and taking additional space within the tool where axial and radial space is limited.

In the illustrated embodiment, the downhole tool12includes targets50,52,54,56, and58that direct photons51into the geological formation14as well as a target60that creates photons51within the downhole tool12to serve as an internal reference. To acquire data regarding at least two DOIs within the geological formation14, the downhole tool12may include an electron beam steering system76that steers the electrons of the electron beam49towards each of the targets50,52,54,56, and58. For example, the electron beam steering system76may steer the electron beam49by inducing an electric field in a direction perpendicular to the x-ray tube axis using electrodes at a variable potential to direct the electrons towards the first target50. That is, the electron beam steering system70may apply a voltage to control a direction of the electric field between the cathode48and the targets50,52,54,56, and58to steer the electron beam49towards the first target50. Further, the data processing system28may send signals to adjust the electric field generated by the electron beam steering system76to steer the beam49towards the second target52that corresponds to a second DOI, which is different from the first DOI of the first target50. This process may be performed for each of the five different targets50,52,54,56, and58corresponding to five different DOIs. In certain embodiments, an isolation transformer having a primary coil at a ground potential on the outside of insulation of the downhole tool12and a secondary coil inside the insulation may be used. The AC voltage at the output of the secondary coil may be rectified and the rectified and smoothed voltage may be used to control the direction of electrons of the electron beam49by applying the voltage between the electrostatic deflector electrodes118. In some embodiments, the electron beam steering system70may steer the electron beam49emitted by the cathode48by modulating the input voltage on the primary to obtain a variable deflection voltage on the deflector electrodes118without rectifying. In this case, one may be sweeping over all the deflection angles in a continuous way or in predetermined steps. In certain embodiments, the electron beam steering system70may steer the electron beam49by applying a magnetic field via one or more coils proximate to the cathode48. These coils will advantageously be outside of the electrical insulation surrounding the photon generating tube, but may also be placed inside such insulation. While these are provided as examples, the electron beam steering system70may steer the electron beam49using any suitable method to control the direction of electrons of the electron beam49. It should be understood that while the present embodiment indicates steering in a single plane across the axis of the generator tube deflection could be at any azimuthal angle. This may be achieved through the use of a second set of deflection electrodes, create an electric deflection field perpendicular to the one generated by the first set of deflector electrodes118.

The downhole tool12may be configured such that each of the targets50,52,54,56, and58may be used to obtain radiation with the desired DOIs. For example, each of the targets50,52,54,56, and58may have respective collimation channels80,82,84,86, and88to obtain the desired sensitivities to density, PEF or borehole and borehole fluid properties, as a function of DOI. Further, some or all of the targets50,52,54,56,58, and60may be at the same voltage potential or at different voltage potentials. The collimation channels80,82,84,86, and88may form a tube with a trapezoidal cross section along the tube axis or any other suitable shape to include an angular opening to have collimation openings such that the opening is surrounded mostly or completely by tungsten (or other dense high Z materials). The shape of the hole may have a smaller or larger angular aperture depending on the desired configuration, e.g. the desired DOI and/or a desired upper limit of the count rate in the detector.

The downhole tool12may include a collimation channel90within the downhole tool12to enable the electron beam49to contact an internal target60and direct photons51internally (e.g., within the downhole tool12) to the detector62to serve as a reference to regulate the generator40and/or the detector62. That is, the electron beam steering system76may direct the beam toward the target60using any suitable method described above. The generator40may be regulated by measuring a current on the target60and the spectrum itself or a total count of the spectrum via the detector62. The material (e.g., tungsten) of the target60may filter the low energies of photons51from the spectrum. For example, the data processing system28may regulate the generator40, as set forth according to the techniques disclosed by U.S. Pat. No. 7,960,687, entitled “Sourceless Downhole X-ray Tool” and filed on Sep. 30, 2010, which is incorporated by reference herein in its entirety.

FIG. 3is a graph94illustrating data received by the data processing system28related to the photons51detected by the detector62from the target60within the downhole tool12. While the graph94may be displayed on the display36, in certain embodiments, the data processing system28may process the data without displaying the graph94. The graph94shows an ordinate96representing counts of photons per second (cps) and an abscissa98representing energy levels of the photons51. The spectrum100detected by the detector62has attenuated low energies of photons51due to filtering from the target60material (e.g., tungsten). To regulate the gain of the detector62, the data processing system28may send signals to the detector62to adjust a voltage on the photomultiplier tube68to control a centroid102of the detected photon spectrum100, such that the centroid102is within a channel104.

FIG. 4is directed to the downhole tool12having a mud property detection target106, in accordance with an embodiment. The downhole tool12may include similar targets as those described with respect toFIG. 2on a first side108of the downhole tool12. Additionally, the downhole tool12includes the mud property detection target106on a second side110of the downhole tool12, which may be opposite the first side108, to enable shallow detection of mud properties (e.g., density and PEF), on the downhole tool12. This can be useful in inversion techniques to detect the presence of Barite in the mud and therefore in the mud cake112and may be used to account for the impact of these properties on the formation property measurement.

As explained above, the downhole tool12may include the steering control system76to steer the electron beam49.FIG. 5is a schematic diagram of the downhole tool12that steers the electron beam49using electrostatic beam steering electrodes118, in accordance with an embodiment. The downhole tool12may include at least one limiting aperture electrode120disposed between the cathode48and the targets50,52,54,56,58,60, and106to ensure that the portion of the electron beam49that does not contact the limiting aperture electrode120instead contacts the selected target. In the illustrated embodiment, the limiting aperture electrode120includes apertures122corresponding to each of the targets50,52,54,56,58,60, and106. That is, the apertures122are disposed on the aperture electrode120such that the electron beam49passes through the aperture122to contact the selected target without contacting the limiting aperture electrode120. The apertures122may be aligned or distributed throughout a plane, as illustrated inFIG. 5. While multiple apertures122are disposed on the single limiting aperture electrode120in the illustrated embodiment, in other embodiments, individual limiting aperture electrodes may be used on each of the targets50,52,54,56,58,60, and106. The apertures in the one or more limiting aperture electrodes may be circular or may have any other shape suitable to insure that, when properly deflected, the beam passes through the respective aperture, while few if any electrons get intercepted by the electrode. The aperture electrode may be shaped and/or biased in such a way as to insure an accurate current measurement by limiting secondary electron emission or collection of scattered electrons from a target or other electrodes. This allows an accurate measurement of the electron current on the aperture electrode.

The downhole tool12may include a current sensor124disposed on the limiting aperture electrode120to measure the current IDIAGNOSTICSfrom the electron beam49that contacts the limiting aperture electrode120. The limiting aperture electrode120may be composed of a low-Z material, such as Beryllium, to limit an amount of undesired X-rays while the electron beam49hits the electrode. Moreover, the downhole tool12may include another current sensor126coupled to the targets50,52,54,56,58,60, and106to measure the current IXRAYfrom the electron beam49that contacts the targets50,52,54,56,58,60, and106.

FIG. 6is a schematic diagram of another embodiment of the downhole tool12that includes current sensors130,132,134,136,138,140, and142disposed on each of the targets50,52,54,56,58,60, and106, respectively. Moreover, the current sensors130,132,134,136,138,140, and142may measure the current on the respective targets50,52,54,56,58,60, and106. Further, the data processing system28may receive signals from each of the current sensors130,132,134,136,138,140, and142. This may be used to determine which target50,52,54,56,58,60, and106is being contacted by the electron beam49.

FIG. 7is a schematic diagram of an embodiment of a control system148that controls the electron beam and beam steering system76to acquire the properties of the geological formation14and/or mud cake74. The control system148may include the data processing system148or another processing system. Moreover, the control system148may send signal(s) indicating instructions to control the electron beam steering system76, such as the voltage across the electrostatic beam steering electrodes118. In addition, the knowledge of which target is contacted by the beam is used in the processor to associate count rates and/or spectra with the correct target-source spacing or DOI.

Depending on the direction of the electron beam49, some of the electrons may contact the limiting aperture electrode120, and thereby cause the current IDIAGNOSTICSto increase. Some or all of the electrons may pass through one of the apertures122and contact the corresponding target50,52,54,56,58,60, or106, thereby causing the current IX-RAYto increase. The sensors124,130,132,134,136,138,140, and142may send signals indicating the currents IDIAGNOSTICSand IX-RAYto the processing system28. The processing system28may send signals to control the beam position to contact the selected target based on the currents IDIAGNOSTICSand IX-RAY. For example, the control system may control the electron beam49based at least in part on the following equation:
F=IX-RAY/IBEAM(1)
where IX-RAYis the current on the selected target, IBEAMis the total current emitted by the cathode (e.g., IX-RAY+IDIAGNOSTICS), and F is the feedback value. That is, the data processing system28may send signals to the electron beam steering system76in a direction that increases the feedback value (e.g., maximizes at one). For example, the data processing system28may send signals to control the electron beam steering system76to cause the electron beam to be directed to the selected target (e.g., feedback value F of one or close to one). Further, the control system148may control the voltage of the voltage generator40as well as send signals to power the cathode48(e.g., based on photons51detected when using the internal target60). As mentioned above, the X-ray intensity may be maintained by regulating IX-RAYwith the power on the cathode48. Further, the X-ray energy is maintained by determining the x-ray spectrum on detector62while the electron beam is directed on the monitoring target60. It is during this same period of time that the gain of the detector62may be regulated based on the detected spectrum94ofFIG. 3. In another embodiment gain regulation may be done using internal radioactivity of the scintillation material as shown in U.S. Pat. No. 8,173,953, the entire contents of which are incorporated by reference into the current disclosure.

In some embodiments, the targets50,52,54,56,58,60, and106may be positioned at an angle to limit the impact of the beam spot position on the measurement. The angle may be different for the different targets50,52,54,58,60and106.

FIG. 8is a graph160of a profile162of the F-ratio due to the currents IDIAGNOSTICSand IX-RAYwhile steering the beam between each of the targets50,52,54,56,58,60, and106. In the graph160, an ordinate164represents the magnitude of the F value and an abscissa166represents the deflection voltage between the deflection plates. As the electron beam49contacts the first target50, the profile162of the F ratio may include a local maximum168in which more of the current contacts the first target50than the limiting aperture electrode120. After a certain duration, the data processing system28may send signals to the electron beam steering system76to steer the electron beam49to the second target52. While the electron beam49is changing directions, the profile162includes a local minimum170in which some or all of the current contacts the limiting aperture electrode120before contacting the second target52. Additional local minima and local maxima are shown in the profile162as the electron beam49is steered towards the subsequent targets54,56,58,60, and106. The dashed line172in the graph indicates the maximum possible ratio of 1, where all of the beam is hitting one of the targets and no current is intercepted by an aperture electrode.

FIG. 9is a graph174of signals received by the data processing system28from the detector62and the total current measured on targets50,52,54,56,58,60, and106, in accordance with an embodiment. While the graph174may be shown on the display36, this is meant to be illustrative and the data processing system28may receive and process the signals without displaying the graph174. In the graph174, an ordinate176represents magnitudes of various subsections178,180,182, and184, and an abscissa186represents time.

To steer the electron beam49, the data processing system28may send signals to cause voltages differences between the electrodes118as shown in the first subsection178. For example, the data processing system28may send a signal to cause a first voltage186across the electrodes118for a first duration188to direct the electron beam49to contact the first target50. The data processing system28may send a second signal to cause a second voltage190across the electrodes118for a second duration192to direct the electron beam49to contact the second target52. This process may continue with the data processing system28sending signal(s) to control the voltage across the electrodes118for different durations to direct the electron beam49to contact the internal target60, targets56,58, and the mud property detection target106. In the illustrated example, the data processing system28may send signals to the electron beam steering system76to steer the electron beam49from the targets in close proximity to the first side108to the targets in close proximity to the second side110.

In some embodiments, the data processing system28may send signal(s) to steer the electron beam steering system76to steer the electron beam49on each target50,52,54,56,58,60, and106for a duration associated with the statistics from the measurements received at the detector62. For example, the electron beam steering system76may be steering the electron beam toward each target depending on whether the measurement statistics for a given target position with a shorter or longer accumulation time. In many cases, the counting statistics will be poorer for a deeper depth of investigation (larger spacing between target and detector) and acquisition at a deeper DOI may have a longer duration of the respective target exposure.

As shown in subsections182and184of the graph174, the feedback value as well as the IX-RAYvalue decreases to a local minimum between durations188and192because the electron beam steering system is steering the electron beam49between the first target50and the second target52. As such, some electrons generate current on the limiting aperture electrode120. At the second duration192, the feedback value as well as IX-RAYincrease to a local maximum due to the electrons contacting the second target52. A typical electron current for this embodiment could range from about 5 μA to more than 100 μA.

FIG. 10is a schematic diagram of another embodiment of the downhole tool12that operates using transmission of X-rays in place of reflection. That is, the x-rays generated in the downhole tool12need to traverse the target after the emission and this alters the X-ray spectrum entering the collimation channels. For example, the downhole tool includes a target196that is outside of the collimation channels and the beam position on the target196is selected in such a way as to make sure that the resulting x-ray emission is guided into the collimation opening.

While a grounded target generator may be shown in the illustrated embodiments, this is meant to be illustrative, and the present disclosure may be applied to a bipolar generator or a grounded cathode generator as well. In an embodiment with a bipolar generator, the X-ray tube may include a limiting aperture having a low-Z material suitable for scattering X-ray beams (e.g., Beryllium) positioned in a middle of the X-ray tube and, in some embodiments, at or close to ground potential. For example, the limiting aperture may be positioned as set forth according to the techniques disclosed by U.S. Pat. No. 7,564,948, U.S. Patent Pub. 2015/0055747, which are incorporated by reference herein in their entirety. Further, in some density tools, there may be a desire for a window in the pressure housing (or drill collar) made of a low-Z/density material, such as Beryllium or Titanium) for each measurement detector. If the energy accelerator voltage in case of an x-ray generator is sufficient (e.g., above about 250 kV), there may not be a need to provide a window in the pressure housing for the exiting x-ray radiation.

Keeping the above in mind,FIG. 11is a flowchart210representing a method for obtaining downhole properties using a downhole tool12having at least two targets (e.g.,50and52) that emit radiation with exit points from the downhole tool12at different spacings from a radiation detector66. The downhole tool12may be placed in the wellbore (block212) and an electron beam steered toward the first target (e.g.,50), causing radiation to be emitted out of the downhole tool12and toward the geological formation14(block214). The detector66may detect the radiation that returns (block216).

Because the detector66has a first spacing from the exit point where the radiation was emitted from the downhole tool12by the first target (e.g.,50), the radiation detected at block216may be understood to have penetrated the materials outside of the downhole tool12to a first depth of investigation (DOI). To measure a different DOI, the electron beam may be steered toward the second target (e.g.,52), causing radiation to be emitted out of the downhole tool12and toward the geological formation14from a different exit point and possibly at a different angle (block218). The detector66may detect the radiation that returns (block220), which may be understood to have probed to a second DOI. The measurements by the detector66based on radiation emitted from the two different targets (e.g.,50and52) at the first and second DOIs may be used to determine any suitable properties, such as density, of the geological formation14, the mud cake, or wellbore fluid, which may be output to a well log for viewing by an operator (block222).

In one embodiment, the downhole tool12may have two targets, one that emits radiation directed outside of the downhole tool (e.g.,50) and one that emits radiation directed internally to the downhole tool (e.g.,60). Some embodiments of the downhole tool12may have just those two targets, while other embodiments of the downhole tool have other targets as well (e.g.,52).FIG. 12is a flowchart230representing a method for obtaining downhole properties using such a downhole tool12. The downhole tool12may be placed in the wellbore (block232) and an electron beam steered toward at least the first target (e.g.,50), causing radiation to be emitted out of the downhole tool12and toward the geological formation14(block234). The detector66may detect the radiation that returns (block236). Additional targets may be struck at different times to emit radiation to probe different DOIs if the downhole tool12includes the additional targets.

To enable gain regulation of the detector66, the electron beam may be steered to the second target (e.g.,60), causing radiation to be emitted internally within the downhole tool12and toward the detector66(block238). The detector66may detect this radiation, which will not have interacted with the materials outside the downhole tool12, and thus may be well suited to be used for gain regulation (block240) and for the stabilization and regulation of the accelerator high voltage. The measurements by the detector66based on radiation emitted from at least the first target (e.g.,50) may be used to determine any suitable properties, such as density, of the geological formation14, the mud cake, or wellbore fluid, which may be output to a well log for viewing by an operator (block242).

In a different embodiment the radiation emitted internally may come from more than one target (60and another target (not shown)), where the absorber between the target and the detector is different for the two targets. The two different spectra generated by the s-ray radiation traversing the two targets may be used for improved gain stabilization and/or improved accelerator high voltage regulation.