Nuclear logging tool and its applications

Abstract

A nuclear logging tool has a housing, one or more neutron sources, one or more shields, and two or more detectors disposed about the housing. Each of the one or more neutron sources is configured to produce neutrons in a pulsed or continuous manner, and each of the two or more detectors is operable to detect neutrons and gamma rays. The two or more detectors include a first detector disposed at a first distance from the first neutron source and a second detector disposed at a second distance from the first neutron source. The first distance is shorter than the second distance. The first distance and the second distance are measured in a longitudinal direction of the housing. Each shield is operable to absorb neutrons and gamma rays and is disposed within the housing between one of the one or more neutron sources and one of the one or more detectors.

Description

Technical Field

[0001] This disclosure provides methods and systems for nuclear logging and formation evaluation, and in particular methods and systems for nuclear logging and data analysis to obtain formation parameters. Background Technology

[0002] In oil and gas exploration, porosity, mineralogical properties, density, and gas / oil saturation are important formation parameters for evaluating the total oil and gas reserves in an oil and gas field. Various wireline and LWD (logging while drilling) logging tools have been developed to measure downhole formation parameters.

[0003] Formation density is obtained by measuring backscattered gamma rays from a gamma radiation source (e.g., a Cs-137 source), which are received by two detectors (e.g., two sodium iodide scintillation detectors) positioned at different distances from the gamma ray source. These two detectors are typically referred to as the near detector and the far detector based on their relative distance from the gamma ray source.

[0004] Neutron porosity logging tools study formation porosity by measuring the ratio of the neutron count rate of the near-detector to that of the far-detector after fast neutrons from an isotopic neutron source (e.g., an Am-Be source) are slowed down and scattered back to the detector by the surrounding environment (e.g., wellbore fluids and formation). This ratio is then converted to porosity based on the specific formation mineralogy (e.g., sandstone, limestone, or dolomite). Using the ratio of the count rates from the two detectors reduces the influence of variations in the near-wellbore environment (wellbore fluids, wellbore size, etc.) on porosity measurements.

[0005] Pulsed neutron tools employ a pulsed neutron source (e.g., a DT or DD neutron generator) and one, two, or three detectors to detect neutrons or neutron-induced gamma rays. The energy spectrum of neutron-induced gamma rays from each element is unique. Therefore, by measuring the energy spectrum of gamma rays from inelastic scattering and / or neutron capture reactions, elements can be identified and the relative percentage of gamma rays from each element in the formation, i.e., elemental yield, can be obtained. Inelastic spectroscopy is the basis for carbon-oxygen (C / O) ratio logging, but it can also provide information on other elements such as hydrogen (H), silicon (Si), calcium (Ca), iron (Fe), sulfur (S), and chlorine (Cl).

[0006] Since elemental yield logging only provides the relative concentrations of elements, they are typically expressed as ratios, such as C / O, Cl / H, Si / (Si+Ca), H / (Si+Ca), and Fe / (Si+Ca). These ratios are indices for oil, salinity, mineralogical properties, porosity, and clay, respectively. Elemental yield logging, along with the reaction cross-sections of neutron inelastic scattering and neutron capture reactions of these elements, can also be used to determine the elemental concentrations in the formation.

[0007] In addition, by measuring the thermal neutron time decay curve or the captured gamma ray time decay curve after one or more neutron pulses, the macroscopic thermal neutron absorption cross section (sigma) of the formation can be obtained, which can be used to estimate the gas / oil saturation.

[0008] In most of these applications, neutrons and gamma rays are detected by their respective detectors / sensors. For example, He-3 gas detectors are used to detect thermal neutrons. The He-3 isotope has a high thermal neutron absorption cross section. After fast neutrons emitted from a neutron source are slowed down by the formation and scattered back to the detector, the neutrons are absorbed and produce other detectable ions, such as protons (p) and tritium (T), thus ionizing the gas. Ions and electrons multiply and drift in an electric field to form an electrical signal. Various scintillation detectors, such as NaI, CsI, BGO, GSO, LaBr3, YAP scintillators, and photomultiplier tubes (PMTs), can be used to detect gamma rays. These scintillators convert the accumulated energy of the gamma rays into scintillation light. PMTs convert the scintillation light into electrons and amplify them to form an electronic signal.

[0009] Existing nuclear logging tools typically employ single-function detectors to detect neutrons or gamma rays. For example, to simultaneously obtain formation density and neutron porosity, the conventional approach is to combine density and neutron porosity tools in a toolchain. A density tool might have one gamma-ray source and two gamma-ray detectors. A neutron porosity tool might have one neutron source and two neutron detectors. Therefore, measuring formation density and neutron porosity requires two different radiation sources and four radiation detectors. To obtain other parameters, such as gas saturation, a third detector might be needed. Furthermore, a neutron monitoring detector might be required to monitor the source intensity of the neutron generator, as the source intensity can decrease or fluctuate over time. Thus, nuclear logging tools need to carry multiple different types of radiation sources and detectors to measure various formation parameters. The use of such logging tools is limited by their high cost, low reliability, and large size.

[0010] Recently, scintillator materials sensitive to both neutrons and gamma rays have been developed, such as Cs₂LiYCl₆ (CLYC) and Cs₂LiLaBr₆ (CLLB). By coupling crystals of these materials with scintillator photosensitive elements (such as PMTs), bifunctional scintillators capable of detecting both neutrons and gamma rays can be generated. Based on the fact that the electron signal of gamma rays decays faster than that of neutrons, the neutrons and gamma rays received by the bifunctional detector can be distinguished from each other using pulse shape discrimination (PSD) technology.

[0011] There is a need to reduce the number of radiation sources and detectors while improving the accuracy and precision of formation parameters obtained through logging tools in wireline or logging-while-drilling environments. This disclosure provides a novel logging tool that combines a neutron source and a dual-function detector, opening up new avenues for nuclear logging, new logging tools, measurement methods, and new data processing approaches. Summary of the Invention

[0012] This invention provides a simplified introduction to a series of concepts, which are further described in the detailed description below. It is not intended to identify key or essential features of the subject matter of the claims, nor is it intended to serve as an aid to defining the scope of the claims.

[0013] According to one embodiment of this disclosure, a nuclear logging tool has a housing, one or more neutron sources, one or more shields, and two or more detectors disposed around the housing. Each of the one or more neutron sources is configured to generate neutrons in a pulsed or continuous manner, and each of the two or more detectors is operable to detect neutrons and gamma rays. The two or more detectors include a first detector disposed at a first distance from a first neutron source and a second detector disposed at a second distance from the first neutron source. The first distance is shorter than the second distance. The first and second distances are measured in the longitudinal direction of the housing. Each shield is capable of absorbing neutrons and gamma rays and is disposed within the housing between one of the one or more neutron sources and one of the one or more detectors.

[0014] According to other embodiments, one or more neutron sources in the nuclear logging tool are independently a deuterium-tritium (DT) neutron generator, a deuterium-deuterium (DD) neutron generator, or a radioactive isotope neutron source.

[0015] According to some embodiments, the nuclear logging tool has a third detector disposed at a third distance from the first neutron source in the longitudinal direction of the casing, and the third distance is greater than the second distance.

[0016] According to another embodiment, each of the two or more detectors has a scintillator made of Cs2LiYCl6 or Cs2LiLaBr6 and can detect neutrons and gamma rays.

[0017] According to another embodiment, the nuclear logging tool includes a second neutron source disposed around a housing. The second neutron source is spaced apart from a first neutron source in the longitudinal direction of the housing. In some embodiments, the first neutron source is an isotopic neutron source, and the second neutron source is a pulsed neutron generator, or vice versa.

[0018] According to another embodiment, two or more detectors are positioned between a first neutron source and a second neutron source in the nuclear logging tool.

[0019] In other embodiments, the nuclear logging tool includes at least two detectors positioned at a first distance approximately equal to that of the first detector or a second distance approximately equal to that of the second detector.

[0020] According to further embodiments, at least two of the two or more detectors are mounted in two different radial directions around the housing and have two different azimuth directions when the nuclear logging tool is deployed in the underground formation.

[0021] In addition, at least one detector is positioned off-axis or in the center relative to the axis of the logging tool.

[0022] In some embodiments, the shielding element is made of a material capable of absorbing neutrons and gamma rays. This material may be gadolinium (Gd), samarium (Sm), tungsten boride, alloys containing Gd or Sm, or oxides (such as Gd₂O₃, Sm₂O₃, and B₂O₃).

[0023] In some embodiments, the detector has a housing that houses the scintillator crystal and electronics. A portion of the housing may be made of a shielding material, or a portion of the housing may be covered by a shielding material, allowing neutrons and gamma rays to pass through the remainder of the housing to reach the scintillator crystal.

[0024] This disclosure also provides a method for measuring formation properties. The method includes the steps of deploying a nuclear logging tool into the formation; causing one or more neutron sources to emit neutrons into the formation; converting neutrons and gamma rays received from the formation by two or more detectors into electrical signals; and analyzing the electrical signals to obtain multiple formation properties.

[0025] According to certain embodiments of the method, the nuclear logging tool has a second neutron source disposed around the outer casing. The second neutron source is spaced apart from the first neutron source in the longitudinal direction of the outer casing, and one or more detectors are disposed between the first and second neutron sources.

[0026] This method can be used to obtain formation parameters, including formation density, formation porosity, gas and oil saturation, and / or formation element concentrations.

[0027] This disclosure also provides a downhole drilling system. The system includes a drill string having a downhole drill assembly (BHA) disposed at its lower portion, a crisscross driver configured to deliver the drill string into the wellbore, a top drive configured to rotate the drill string, and a controller. The BHA has a drill bit disposed at one end of the BHA, a downhole motor, and a measuring sub configured to measure formation properties and operating parameters. Nuclear logging tools are mounted in the measuring sub.

[0028] This disclosure also provides a cable logging tool that includes a currently disclosed nuclear logging tool and a cable connecting to a piece of equipment (such as a cable truck) on the surface. Attached Figure Description

[0029] The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.

[0030] Figure 1A , Figure 1B , Figure 1C and Figure 1D Four exemplary configurations of a nuclear logging tool are shown, which has a neutron source (S1) and three detectors (D1, D2 and D3) arranged along the longitudinal direction of the tool housing.

[0031] Figure 2A , Figure 2B and Figure 2C A cross-sectional view of an exemplary nuclear logging tool with S1, D1, D2, and D3 is shown.

[0032] Figure 3A and Figure 3B Cross-sectional views of exemplary nuclear logging tools with four detectors (D1, D2, D31, D32) and six detectors (D1, D21, D22, D31, D32, and D33) are shown respectively.

[0033] Figure 4A This is an example diagram of a nuclear logging tool with two neutron sources (S1, S2) and two detectors (D1, D2); Figure 4B and Figure 4C Cross-sectional views of exemplary configurations of this embodiment are provided respectively.

[0034] Figure 5A This is another exemplary embodiment of a nuclear logging tool having two neutron sources (S1, S2) and four detectors (D11, D12, D21, D22); Figure 5BA cross-sectional view of this embodiment is provided.

[0035] Figure 6 This is a block diagram of an exemplary drilling system that implements embodiments of the present disclosure.

[0036] Figure 7 A schematic diagram is shown of the neutron pulse, neutron count rate, and the inelastic energy spectrum and capture energy spectrum of neutron-induced gamma rays.

[0037] Figure 8 A flowchart of an exemplary method for obtaining various formation parameters is shown.

[0038] Throughout the accompanying drawings and detailed description, unless otherwise stated, the same reference numerals will be understood to refer to the same elements, features, and structures. For clarity, illustration, and convenience, the relative sizes and descriptions of these elements may be exaggerated. Detailed Implementation

[0039] The following specific embodiments are provided to help the reader gain a comprehensive understanding of the methods, apparatus, and / or systems described herein. Examples of specific embodiments of this disclosure are shown in the accompanying drawings. Similar or related reference numerals may be used in the drawings and may indicate similar or related elements.

[0040] The features described herein may be embodied in different forms and should not be construed as limiting the embodiments described herein. Rather, the embodiments described herein and depicted in the accompanying drawings are provided to make this disclosure thorough and complete, and to convey the full scope of this disclosure to those skilled in the art, so that they may readily recognize from the following description that alternative embodiments exist without departing from the general principles of this disclosure.

[0041] Therefore, the scope of the invention is not defined by specific embodiments, but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be construed as included in this disclosure.

[0042] In this disclosure, unless otherwise stated, the detector refers to a dual-function detector capable of detecting both neutrons and gamma rays. Such detectors employ scintillation crystals (e.g., Cs₂LiYCl₆ (CLYC) or Cs₂LiLaBr₆ (CLLB)) and associated electronics (e.g., PMTs). When deployed downhole, the detector can be actively or passively cooled. For example, a detector using CLLB and a high-temperature PMT can be used at high temperatures without additional cooling.

[0043] Figure 1A-1DThis is a schematic diagram (not to scale) of four exemplary configurations of a cylindrical nuclear logging tool 200, which has a neutron source (S1) and three dual-function detectors (D1, D2, D3) arranged along the housing of the logging tool suitable for logging-while-drilling (LWD) operations. The mud channel (MC) is arranged along the axis of the logging tool, while the detectors are eccentrically positioned along the longitudinal direction of the logging tool. Figure 1A Also shown are: a high-voltage power supply (HV); electronic instruments (e.g., a controller) for sending commands, receiving and processing data from the neutron source and the detectors; and a telemetry device for transmitting data between the logging tool and the surface. The high-voltage power supply powers the detectors (D1, D2, D3) and the pulsed neutron source (S1). For simplicity, the required power supply, electronic instruments, and telemetry device are not shown. Figure 1B-1D As shown in the image.

[0044] As shown in the figure, D1 is the near detector with the shortest distance to the neutron source in the longitudinal direction, D3 is the far detector with the longest longitudinal distance to the neutron source, and D2 is the middle detector with the longitudinal distance in the middle.

[0045] like Figure 1A As shown, all three detectors are located on one side of the neutron source along the logging tool 200. This side can be either the proximal side or the distal side of the neutron source. The proximal side is the side closer to the surface when the nuclear logging tool 200 is deployed downhole, while the distal side is the side farther from the surface. A high-voltage power supply powers the detectors (D1, D2, D3) and the pulsed neutron source (S1). Signals from each detector are processed by electronic instruments and acquired and transmitted via telemetry. Figure 1B , 1C As shown in Figure 1D, the neutron source has at least one detector disposed on both the distal and proximal sides.

[0046] In wireline logging, this tool can be mounted in a probe that does not include a mud channel. The probe can be mounted along or off-axis of the tool body. Power and control signals can also be supplied to the logging tool from the surface, while data from the logging tool can be transmitted to the surface via cable.

[0047] exist Figure 1A-1DThe neutron source S1 in each logging tool depicted is a pulsed neutron generator. However, isotopic neutron sources can also be used. The pulsed neutron source can be a deuterium-tritium (DT) pulsed neutron generator, which can operate in pulsed mode with various pulse principles (e.g., frequency, pulse duration). For example, the frequency of the neutron pulse can be approximately 10 kHz (period of 100 μs), and the duration of the neutron pulse can be approximately 20 μs. In another embodiment, the frequency of the neutron pulse can be approximately 1 kHz (period of 1000 μs), and the duration of the neutron pulse can be 50 μs. Depending on the method and measurement, the DT neutron generator can also operate in continuous mode. In this case, the start-up frequency of the neutron generator is high enough that neutrons are emitted continuously. Neutrons from the DT neutron generator have an initial energy of approximately 14.1 MeV.

[0048] The pulsed neutron source can also be a deuterium-deuterium (DD) pulsed neutron generator, which can operate in pulsed mode with different pulse principles (e.g., frequency, pulse duration). For example, the frequency of the neutron pulse can be approximately 20 kHz (period of 50 μs), and the duration of the neutron pulse can be approximately 20 μs. Alternatively, the frequency of the neutron pulse can be approximately 1 kHz (period of 1000 μs), and the duration of the neutron pulse can be 40 μs. Depending on the method and measurement, the DD neutron generator can also operate in continuous mode. Neutrons from the DD neutron generator have an initial energy of approximately 2.5 MeV.

[0049] Depending on the target formation parameters and measurement methods, isotopic neutron sources (e.g., Am-Be, Pu-Be, Cf-252) can be used instead of pulsed neutron sources. Neutrons from these isotopic neutron sources have different energy spectra. For example, neutrons emitted from an Am-Be source have energies ranging from 0 MeV to approximately 10 MeV, with an average energy of about 4.2 MeV.

[0050] exist Figure 1A-1D The neutron source S1 and detectors D1, D2 and D3 in the tool 200 are only shown in their relative positions along the longitudinal direction of the tool housing, but not in their radial positions in the cross-section of the tool housing.

[0051] In some embodiments, S1, D1, D2 and D3 can be set in the same radial direction or different radial directions, that is, when deployed in the formation, they have the same or different measurement azimuth angles. Figure 2A , 2B And 2C shows in Figure 1A-1D The exemplary cross-sectional views are shown in the AA, BB, CC, and DD directions. Figure 2AS1, D1, D2, and D3 are set at the same azimuth angle. However, in Figure 2B In the diagram, S1, D1, and D3 have the same azimuth angle, while D2 is at a different azimuth angle. Figure 2C In the diagram, S1 and D1 have the same azimuth angle, while each of D2 and D3 has a different azimuth angle.

[0052] Other embodiments of the logging tool may have more than three detectors. For example, Figure 3A A variation of the logging tool in Figure 1 is depicted, featuring four detectors: D1, D2, D31, and D32. D31 and D32 are approximately equidistant from S1 but positioned at two different azimuth angles. Similarly, Figure 3B Another variation of the tool shown in Figure 1 is depicted, featuring six detectors: D1, D21, D22, D31, D32, and D33. In this embodiment, D21 and D22 are positioned opposite each other across the cross-section of the logging tool, with azimuth angles of 0° and 180°, respectively. D31, D32, and D33 are spaced 120° apart across the cross-section of the logging tool, meaning the difference in azimuth angle between any two of D31, D32, and D33 is 120°. Having different azimuth angles allows the detectors to preferentially receive neutrons and gamma rays from specific incident angles of the formation. This embodiment also improves the detection efficiency of neutrons and gamma rays by increasing the overall count rate of all detectors.

[0053] In addition, Figure 3A In the diagram, the distances of D31 and D32 from S1 are approximately the same. Figure 3B In this configuration, the intermediate detectors D21 and D22 are at approximately the same distance from S1, as are the far detectors D31, D32, and D33. "Approximately the same distance" means that the distance from S1 to the center of the scintillator of each detector (e.g., D31 and D32) is roughly the same. For example, the difference is less than half an inch or a quarter an inch. With this arrangement, the intermediate detector as a whole and the far detector as a whole have a higher count rate than if only one intermediate detector or only one far detector were used. Therefore, the neutron generator S1 can be a lower-power source, and its confinement can be less than that of a higher-power neutron source. Furthermore, the count rates of individual detectors can be recorded and processed separately. The differences in distance and azimuth among the various detectors can be used to obtain stratigraphic information in a specific azimuth direction.

[0054] In some embodiments, the logging tool 200 has multiple shields (not shown) capable of absorbing neutrons and gamma rays. The shields can be placed between the neutron source and each detector within the logging tool, so that the detectors receive neutrons and gamma rays from the formation, rather than neutrons and gamma rays passing through the logging tool body. Alternatively, the detectors can also be partially shielded by shielding materials capable of absorbing neutrons and gamma rays from certain directions.

[0055] The shielding material is made of or contains one or more materials that can effectively attenuate thermal neutrons and gamma rays. The shielding material may contain materials selected from heavy elements with high thermal neutron absorption cross sections, including metals such as gadolinium (Gd) and samarium (Sm), metal oxides (such as Gd2O3, Sm2O3, B2O3), alloys containing Gd or Sm combined with other heavy metals (such as Fe, Pb, or W), or boron-containing materials (such as tungsten boride (WB, WB2, etc.)).

[0056] The shielding element can be a separate metal component inserted into the logging tool or an integral part of the detector housing. For example, the portion of the detector housing facing inward toward the logging tool can be made of shielding material, while the portion facing the formation can be made of a material transparent to neutrons and gamma rays, forming windows through which neutrons and gamma rays can pass. Therefore, neutrons and gamma rays from certain incident angles can be absorbed by the shielding material, while those passing through the windows are received by the detector. Thus, by adjusting the size and orientation of the windows in the detector housing, the detector can be made more sensitive to certain incident angles. During operation, data collected by various detectors may reveal formation properties in specific directions, which can be used to guide drilling direction.

[0057] Nuclear logging tools may have more than one neutron source. Figure 4AAnother embodiment of a logging tool with two neutron sources (S1, S2) is shown, one neutron source at the near end and the other at the far end, with two detectors (D1 and D2) arranged between S1 and S2. Alternatively, depending on engineering considerations, S1 and S2 can be arranged in series near one end of the logging tool, while D1 and D2 are arranged in series near the other end. In both embodiments, the distance between S1 and D1 is d1, the distance between S1 and D2 is d2, the distance between S2 and D2 is d3, and the distance between S2 and D1 is d4. When both S1 and S2 are pulsed neutron generators, they can be alternately turned on or off, thereby alternately inducing neutrons and gamma rays from the formation for reception by D1 and D2. Because there are four different source-to-detector distances (d1-d4), data generated in D1 and D2 can better compensate for near-wellbore effects, such as wellbore size, tool spacing, mud weight and / or salinity, casing size, cement thickness, etc., compared to tools with only two or three source-to-detector distances. Ultimately, the obtained formation parameters can be more accurate.

[0058] Figure 4B and Figure 4C Two exemplary embodiments of the design are shown, in which the two sources and two detectors are arranged at the same azimuth angle or different azimuth angles. For example... Figure 4B As shown, when the source and detector have the same azimuth angle, the measurement covers the same sector in the formation at any given time. Figure 4C As shown, when the source and detector have different azimuth angles, the data generated in D1 and D2 reflect different sectors of the formation, thus revealing the differences between different stratigraphic sectors at any given time by comparing the measurements of D1 and D2.

[0059] Figure 5A and 5B A logging tool with four detectors (D11, D12, D21, D22) and two neutron sources (S1 and S2) is shown. It is important to note that detectors D11 and D12 (and D21 and D22) are positioned at substantially the same distance as S1 or S2. As previously mentioned, having more than one detector at a given distance increases the count rate at that distance, thus allowing the use of lower-power neutron sources. The count rate of a single distant detector may be too low to provide reliable measurements. By using two or more distant detectors, the count rate can be significantly increased, allowing for reliable measurements to be obtained by processing data from multiple distant detectors as a whole.

[0060] In some embodiments, S1 and S2 can be turned on or off simultaneously. Doing so increases the count rates of D1 and D2, thereby reducing the uncertainty of the statistical measurement.

[0061] In another embodiment, both S1 and S2 are isotopic neutron sources. Compared to pulsed neutron sources, isotopic neutron sources do not require a power source, making logging tools more compact. Furthermore, isotopic neutron sources have longer lifetimes and are more reliable. For example, an isotopic Am-Be neutron source has a half-life of 432 years, significantly longer than the average tube lifetime of neutron generators with 500 to 4000 hours.

[0062] In another embodiment, S1 and S2 can be two different types of neutron sources. For example, S1 can be a DT neutron generator, while S2 can be an Am-Be neutron source. In the field, the DT neutron generator can be shut down, allowing the Am-Be neutron source to operate independently for neutron porosity logging. Alternatively, the Am-Be neutron source can be removed from the logging tool so that the DT neutron generator can independently emit neutron pulses into the surrounding formation. In this case, the DT neutron generator can be used to obtain neutron porosity logging and other measurements (density, hydrocarbon saturation, etc.).

[0063] Porosity logging obtained using Am-Be and DT sources differs slightly. Comparing these porosity logs for the same well obtained using two different neutron sources reveals a correlation between the two types of logging. Since historically porosity logging has primarily used isotopic neutron sources, this correlation helps update past porosity logs, making them comparable to newer logs obtained using pulsed neutron sources. Similarly, new pulsed neutron porosity logs can be adapted to past porosity logs to continue applying reservoir models built using past logs in production prediction.

[0064] The logging tool 200 may be part of a wireline logging tool or may be included in downhole equipment as a LWD logging tool in drilling operations. Figure 6 This is a schematic diagram of an oil drilling system 10 used in directional drilling of wellbore 16. The oil drilling system 10 can be used for drilling both onshore and underwater. A rotary drilling rig, including a derrick 12, drill platform 14, winch 18, traveling block 20, hook 22, rotary joint 24, kelly joint 26, and rotary table 28, is used to drill wellbore 16 into the formation. The drill string 100 includes multiple drill pipes connected in series and fixed to the bottom of the kelly joint 26 at the surface. The rotary table 28 is used to rotate the entire drill string 100, while the winch 18 is used to lower the drill string 100 into the wellbore 16 and apply a controlled axial compressive load. A downhole tool assembly 150 is located at the distal end of the drill string 100.

[0065] Drilling fluid (also known as mud) is typically stored in a mud pit or mud tank 46 and transported using a mud pump 38. The mud pump 38 forces the drilling fluid through a surge suppressor 40, then through a kerb hose 42, and through a rotary joint 24, thus entering the top of the drill string 100. The drilling fluid flows through the drill string 100 at a rate of approximately 150 gallons per minute to approximately 600 gallons per minute and into the downhole assembly 150. The drilling fluid then returns to the surface through the annular space between the outer surface of the drill string 100 and the wellbore 16. When the drilling fluid reaches the surface, it is transported back to the mud tank 46 via a mud return line 44.

[0066] The pressure required to maintain drilling fluid circulation is measured by a pressure sensor 48 on the angular drill pipe hose 42. The pressure sensor detects pressure changes caused by pressure pulses generated by a pulse generator. The amplitude of the pressure waves from the pulse generator can reach 500 psi or higher. The measured pressure is transmitted as an electrical signal to a ground computer 52 via a sensor cable 50, which decodes and displays the transmitted information. Alternatively, the measured pressure is transmitted as an electrical signal via the sensor cable 50 to a decoder, which decodes the electrical signal and transmits the decoded signal to the ground computer 52, which displays the data on a screen.

[0067] As described above, the lower portion (“far end portion”) of the drill string 100 includes a downhole assembly (BHA) 150, which includes a non-magnetic drill collar in which a MWD system (MWD equipment or MWD tool) 160 is installed, a logging-while-drilling (LWD) instrument sub 165 containing an LWD instrument, a downhole motor 170, a near-bit measurement sub 175, and a drill bit 180 with a borehole nozzle (not shown). Drilling fluid flows through the drill string 100 and exits through the borehole nozzle of the drill bit 180. During drilling operations, the drilling system 10 can operate in a rotary mode, in which the drill string 100 is rotated from the surface by an electric motor (i.e., a top drive) in the rotary table 28 or traveling block 20. The drilling system 10 can also operate in a sliding mode, in which the drill string 100 does not rotate from the surface, but the drill bit 180 is rotated by the downhole motor 170. Drilling fluid is pumped from the surface to the drill bit 180 via the drill string 100 and injected into the annular space between the drill string 100 and the wall of the wellbore 16. The drilling fluid carries rock cuttings from the wellbore 16 to the surface.

[0068] In one or more embodiments, the MWD system 160 may include a pulse generator subsection, a pulse generator drive subsection, a battery subsection, a central storage unit, a motherboard, a power supply subsection, a orientation module subsection, and other sensor boards. In some embodiments, some of these devices may be located in other areas of the BHA 150. One or more of the pulse generator subsection and the pulse generator drive subsection may communicate with a pulse generator 300, which may be located below the MWD system 160. The MWD system 160 may transmit data to the pulse generator 300, causing the pulse generator 300 to generate pressure pulses.

[0069] The non-magnetic drill collar houses the MWD system 160, which includes a suite of instruments for measuring dip, azimuth, well trajectory (wellbore trajectory), etc. The nuclear logging tool 200 and associated electronics can be housed in the LWD instrument section 165. The nuclear logging tool 200 and other logging instruments can be electrically or wirelessly coupled together, powered by a battery pack or a drilling fluid-driven generator. All collected information is transmitted to the surface via a mud column in the drill string in the form of pressure pulses generated by the pulse generator 300.

[0070] A near-bit measurement sub 175 can be positioned between the downhole motor 170 and the drill bit 180. A nuclear logging tool 200 can be alternatively mounted in the near-bit measurement sub 175 to provide more accurate real-time formation parameters to guide directional drilling. Data can be transmitted via a cable embedded in the downhole motor 170 to the MWD system 160 in the downhole drill string assembly 150.

[0071] In one embodiment of this disclosure, a logging tool with a DT neutron generator and three dual-function detectors is used to obtain a variety of formation parameters. Figure 7 A schematic diagram is shown, illustrating the neutron pulse, neutron count rate, and the inelastic and trapping energy spectra of neutron-induced gamma rays. The neutron pulse frequency is 10 kHz (period 100 μs), and the neutron working time is 20 μs. Figure 7 The subgraph (b) is shown in the figure.

[0072] The neutron count rate measured by each of the three detectors is as follows: Figure 7Subfigure (a) shows the data used to obtain formation porosity. Neutrons from the three detectors can be further separated based on whether the neutron pulse is on or off, serving as coincidence or anti-coincidence signals for the neutrons from the three detectors, so that during the neutron pulse (when the neutron pulse is on), neutrons are primarily recorded as fast neutrons. Between neutron pulses (when the neutron pulse is off), neutrons are recorded as thermal neutrons. The fast and thermal neutrons recorded by the three detectors can be used to obtain the spatial distribution of fast and thermal neutrons. Neutrons from each detector can also be recorded together. In this case, all neutrons (from thermal to fast neutrons) are used to obtain the neutron spatial distribution.

[0073] Gamma rays from the three detectors can be further separated based on whether the neutron pulse is on or off, thus serving as coincidence or anti-coincidence signals for the gamma rays from the three detectors. This allows the gamma rays to be primarily recorded as an inelastic energy spectrum during the neutron pulse (when the neutron pulse is on), such as... Figure 7 Subgraph (c) is shown in the figure. Between neutron pulses (neutron pulses are off), gamma rays are recorded as a trapping energy spectrum, as shown in the figure. Figure 7 Subplot (d) is shown in the figure. An appropriate time window is selected so that the gamma rays measured in the capture time window all come from the thermal neutron capture reaction, and most of the gamma rays measured in the inelastic time window come from fast neutron inelastic scattering.

[0074] The detector background can be measured some time after the neutron generator is turned off and can be subtracted from the total signal of neutrons or gamma rays. The neutron background measured during a neutron pulse can be further subtracted using a small fraction of the neutrons measured between neutron pulses to obtain a “pure” fast neutron spectrum. Similarly, the trapped gamma rays measured during a neutron pulse can be further subtracted using a small fraction of the trapped energy spectrum measured between neutron pulses to obtain a “pure” inelastic energy spectrum.

[0075] The gamma rays detected by each detector can also be recorded in an energy spectrum (e.g., the total energy spectrum), regardless of whether they are caused by inelastic neutron scattering or neutron capture reactions. Thus, several formation measurements, such as formation porosity, elemental concentration, and formation hydrocarbon saturation, are feasible, but formation density may not be obtainable. This is because for measurement systems based on DT pulsed neutron generators, the energy spectrum of inelastic gamma rays is required to obtain formation density.

[0076] Figure 8This is an exemplary workflow demonstrating the steps of processing data from a logging tool 200, which has a DT neutron generator and three detectors: a near-field detector, a mid-field detector, and a far-field detector. First, pulse shape discrimination (PSD) technology is used to distinguish between neutron and neutron-induced gamma-ray signals from the three detectors. Then, the total count rate (CRN) is obtained using the neutron signals from the three detectors. n CRN m CRN f Fast neutron count rate (CRFN) n CRFN m CRFN f ), thermal neutron count rate (CRTN) n CRTN m CRTN f ), and based on them and using three ratios (Rn) m / f 、 Rn n / f 、Rn n / m ) and fast neutron distribution (e.g., Rfn) m / f 、Rfn n / f 、Rfn n / m ) and thermal neutron distribution (e.g., Rtn m / f 、Rtn n / f 、Rtn n / m To obtain neutron porosity.

[0077] Fast neutron spatial distribution and thermal neutron spatial distribution can be used to calculate formation elemental concentrations using inelastic and trapped energy spectra. They can also be used to obtain more accurate hydrocarbon saturation. Examples for obtaining formation porosity, density, elemental concentration, and gas and oil saturation are provided below.

[0078] like Figure 8 As shown, the neutron (CRN) count rate (CRN) measured from near-detector, mid-detector, and far-detector (during and between neutron pulses) is used. n CRN m CRN f Formation porosity (Φ) is obtained by using the ratio of count rates.

[0079] The far-to-middle ratio (Rn) is obtained using formulas 1, 2, and 3 respectively. m / f ), Near-far ratio (Rn) n / f ) and the ratio of near-near distance (Rn) n / m Because the three detectors are positioned at different distances from the neutron source, they have different detection depths. Therefore, the near-wellbore environment (such as wellbore fluids, cement, etc.) has different effects on these three ratios. m / fMore sensitive to formations, Rn n / m More sensitive to near-wellbore changes, Rn n / f It is sensitive to both formation and near-wellbore changes.

[0080]

[0081]

[0082]

[0083] Formation porosity Φ n It can be obtained as follows: First use Rn n / m and / or Rn n / f Correction Rn m / f Then use the corrected far-middle ratio Rnc m / f The formation porosity of a specific stratum (e.g., sandstone, limestone, or dolomite) is obtained. Formulas (4)-(6) illustrate the algorithm, where ΔR is the correction value.

[0084] Rnc m / f =Rn m / f +ΔR (4)

[0085] ΔR=f1(Rn m / f , Rn n / f , Rn n / m (5)

[0086] Φ n =f2(Rnc) m / f (6)

[0087] Based on an algorithm similar to that described in Equations 1-6, the formation porosity Φ can also be obtained by using the ratio of the three count rates of the captured gamma rays obtained from the three detectors. n .

[0088] Formation porosity Φ n It can also be obtained by combining the two porosities obtained separately from neutrons and captured gamma rays. Alternatively, it can be obtained directly from the three ratios of neutrons and the three ratios of captured gamma rays using other methods.

[0089] Mineralogical measurements can be obtained by using the same tools to measure the energy spectrum of gamma rays from neutron inelastic scattering and neutron capture reactions.

[0090] The gamma rays detected by each detector can be recorded in two separate energy spectra (inelastic and captured energy spectra) or in a single energy spectrum (total energy spectrum). In either case, elements can be identified, and the relative yield of characteristic gamma rays and elemental concentrations can be obtained from these elements.

[0091] Because the three detectors in the logging tool 200 simultaneously detect neutrons and gamma rays at three different locations, the neutron count rates from the three detectors can be used to obtain a more accurate neutron spatial distribution (fast neutron spatial distribution and thermal neutron spatial distribution). The measured neutron spatial distribution is then used to obtain a more accurate calculation of elemental (e.g., C, O, H, Cl, Si, etc.) concentrations.

[0092] The total initial inelastic gamma-ray count rate (CRIN) obtained from the near-detector n To compensate CRBS m and CRBS f Subsequently, the count rate of backscattered inelastic gamma rays (CRBS) from the intermediate and far detectors m CRBS f It can be used to obtain the apparent density (ρ) am ) and visual density (ρ) af For example, according to formulas 7 and 8. Then, the two apparent densities can be used to obtain the true formation density (ρ). t (Using formulas 9 and 10 as examples).

[0093]

[0094]

[0095] ρ t =ρ af +Δρ (9)

[0096] Δρ=f(ρ am , ρ af (10)

[0097] α m ,β m α f ,β f These are the calibration parameters for the mid-range and long-range detectors, respectively.

[0098] Formation gas saturation measurements can also be obtained using logging tool 200. Fast neutrons emitted from the neutron source in the tool are rapidly slowed into thermal neutrons as they pass through the formation. Hydrogen is the most effective at slowing down fast neutrons. Some thermal neutrons are captured by formation elements and emit captured gamma rays. Both thermal neutrons and captured gamma rays can be scattered back to three detectors and detected.

[0099] The detector's count rate is highly dependent on the downhole formation pores, which are filled with gas, oil, water, or mixtures thereof. All pores are rich in hydrogen. While water and oil have roughly the same hydrogen atom density, gas has a much lower hydrogen atom density. Therefore, the farther the detector is from the neutron source, the more sensitive it is to changes in gas saturation. As gas saturation increases, more thermal neutrons are generated, resulting in more trapped gamma rays at the detector. Furthermore, the increase in thermal neutrons or trapped gamma rays is greater in distant detectors than in near detectors. Therefore, the thermal neutron count rate (Rtn) from both near and distant detectors is used to determine the optimal range for the detector. n / f or captured gamma ray count rate (Rg) n / f The ratio of gas content to formation porosity (Φ) can be used to obtain the formation gas saturation.

[0100] Since both the near-detector and far-detector in tool 200 can detect and separate thermal neutrons and captured gamma rays, the ratio of the thermal neutron count rate (Rtn) at the two detectors and the ratio of the captured gamma ray count rate (Rg) at the two detectors can be obtained. From these, two apparent gas saturations can be derived, namely, Sg from thermal neutrons. n and Sg from captured gamma rays g Thus, the real gas saturation Sg is obtained independently. Real gas saturation is related to Sg. n and Sg g The function is . Exemplary algorithms for obtaining formation gas saturation are summarized in Equations 11 to 13.

[0101] Sg n =f1(Rtn n / f ,Φ) (11)

[0102] Sg g =f2(Rg n / f ,Φ) (12)

[0103] Sg=f3(Sg n Sg g (13)

[0104] Oil saturation can also be obtained using the ratio of the count rates of fast neutron inelastic gamma rays from carbon and oxygen (C / O ratio) in the formation. Fast neutron inelastic scattering of gamma rays on carbon and oxygen can be obtained by fitting and stripping the gamma ray energy spectrum from the inelastic energy spectrum or the total energy spectrum. The neutron source can be a DT neutron generator operating in pulsed or continuous mode, or an isotopic neutron source (e.g., Am-Be, Pu-Be, Cf-252, etc.).

[0105] From three detectors (So) n Som So f Exemplary algorithms for calculating apparent oil saturation are shown using Equations 14 through 16. Apparent oil saturation can be used to obtain corrected oil saturation (So) using Equation 17.

[0106] So n =f1(Rg n，c / o ,Φ) (14)

[0107] So m =f2(Rg m，c / o ,Φ) (15)

[0108] So f =f3(Rg f，c / o ,Φ) (16)

[0109] So = f4(So) n So m So f (17)

[0110] The corrected oil saturation can also be achieved by using the C / O ratio (Rg). n，c / o 、Rg m，c / o 、Rg f，c / o ), formation porosity (Φ) and the spatial distribution of fast neutrons from the three guides (Rfn) m / f 、Rfn n / f 、Rfn n / m The formula is as follows:

[0111] So=f5(Rg n，c / o Rg m，c / o Rg f，c / o ,Φ,Rfn m / f Rfn n / f Rfn n / m (18)

[0112] Although this disclosure has been described in conjunction with certain preferred embodiments thereof in the foregoing specification, and many details have been set forth for illustrative purposes, it will be apparent to those skilled in the art that this disclosure is readily adaptable and that certain other details described herein may vary considerably without departing from the fundamental principles of this disclosure. Furthermore, it should be understood that the structural features or methods shown or described in any embodiment herein may also be used in other embodiments.

Claims

1. A nuclear logging tool comprising: a housing, two or more neutron sources, one or more shields, and two or more detectors disposed about the housing, wherein each of the two or more neutron sources comprises a first neutron source and a second neutron source and each neutron source is configured to produce neutrons in a pulsed or continuous manner, and each of the two or more detectors is operable to detect neutrons and gamma rays, the two or more detectors comprise a first detector disposed at a first distance from the first neutron source and a second detector disposed at a second distance from the first neutron source, wherein the first distance is shorter than the second distance, each of the first distance and the second distance is in a longitudinal direction of the housing, and each shield is operable to absorb neutrons and gamma rays, and is disposed between one of the two or more neutron sources and one of the two or more detectors within the housing, wherein the second neutron source is spaced apart from the first neutron source in the longitudinal direction of the housing, and the two or more detectors are disposed between the first neutron source and the second neutron source, the first neutron source and the second neutron source are turned on or off simultaneously.

2. The nuclear logging tool of claim 1, wherein, each of the two or more neutron sources is independently a deuterium-tritium (D-T) neutron generator, a deuterium-deuterium (D-D) neutron generator, or a radioisotope neutron source.

3. The nuclear logging tool of claim 1 wherein, the nuclear logging tool further comprises a third detector, the third detector is located at a third distance from the first neutron source in the longitudinal direction of the housing, and the third distance is longer than the second distance.

4. The nuclear logging tool of claim 1 wherein, each of the two or more detectors comprises a scintillator made of Cs2LiYCl6 or Cs2LiLaBr6.

5. The nuclear logging tool of claim 1 wherein, the two or more neutron sources comprise an isotope neutron source and a pulsed neutron generator.

6. The nuclear logging tool of claim 1, wherein, at least two of the two or more detectors are mounted in different radial directions about the housing and have different azimuth angles when the nuclear logging tool is deployed in a subterranean formation.

7. The nuclear logging tool of claim 6, wherein, at least one of the two or more detectors is located on-axis or off-axis of the logging tool.

8. The nuclear logging tool of claim 1 wherein, the shield is made of a material selected from gadolinium (Gd), samarium (Sm), tungsten boride, an alloy containing Gd, an alloy containing Sm, Gd2O3, Sm2O3, B2O3, and mixtures thereof.

9. The nuclear logging tool of claim 8, wherein, a portion of a housing of at least one of the two or more detectors is made of the shield.

10. A method for measuring properties of a formation, comprising: deploying the nuclear logging tool of claim 1 into a formation; causing the two or more neutron sources to emit neutrons into the formation; converting neutrons and gamma rays from the formation received by the two or more detectors into electrical signals; and analyzing the electrical signals to obtain a plurality of properties of the formation. the plurality of properties of the formation comprise formation density, formation porosity, gas and oil saturation, and / or formation element concentration.

11. The method of claim 10, wherein, 12. A downhole drilling system, comprising: a drill string having a lower portion provided with a bottom hole assembly (BHA); ​ a kelly driver configured to deliver the drill string into the wellbore; a top drive configured to rotate the drill string; and a controller, wherein the BHA includes a drill bit disposed at an end of the BHA, a downhole motor, and a measurement assembly configured to measure formation properties and operating parameters, wherein the measurement assembly includes the nuclear logging tool of claim 1. each of the two or more neutron sources in the nuclear logging tool is independently a deuterium-tritium (D-T) neutron generator, a deuterium-deuterium (D-D) neutron generator, or a radioisotope neutron source.

13. The well system of claim 12, wherein, 14. A wireline logging tool comprising the nuclear logging tool of claim 1 connected to a wireline. ​

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