Method and apparatus for obtaining real-time downhole oil saturation

Abstract

A method of measuring oil saturation values in a subterranean formation is performed using a tool having a plurality of dual-function detectors capable of detecting neutrons and gamma rays. The method includes emitting neutrons into the formation, detecting neutron and gamma ray signals in the formation using the detectors, determining formation parameters including formation type and formation porosity, further determining parameters such as a C / O ratio for each detector, a total neutron count rate for each detector, a fast neutron count rate for each detector, and a thermal neutron count rate for three or more detectors, and using the determined parameters to calculate oil saturation values.

Description

TECHNICAL FIELD

[0001] The present disclosure provides methods and systems for nuclear logging and formation evaluation, in particular, methods and systems for nuclear logging and data analysis using a pulsed neutron logging tool with multiple dual-function detectors to obtain real-time formation oil saturation. BACKGROUND

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

[0003] A neutron porosity logging tool investigates formation porosity by measuring the ratio of near-to-far detector neutron count rates after fast neutrons from an isotope neutron source (e.g., an Am-Be source) are slowed down and scattered back to the detectors by the environment around the tool (e.g., wellbore fluid and formation). This ratio is then converted to porosity according to a specific formation mineralogy (e.g., sandstone, limestone, or dolomite). Using the ratio of count rates from the two detectors can reduce the effect of near-wellbore environment (wellbore fluid, borehole size, etc.) variations on porosity measurements.

[0004] Formation mineralogy can be obtained by a pulsed neutron tool that employs a pulsed neutron source (e.g., a D-T neutron generator) and one, two, or three detectors that detect neutrons or neutron-induced gamma rays. The energy spectrum of neutron-induced gamma rays from each element is unique. Thus, by measuring the energy spectrum of gamma rays from fast neutron inelastic scattering and / or thermal neutron capture reactions, elements can be identified and the relative percentage of gamma rays from each element in the formation, i.e., elemental yields, can be obtained. The inelastic spectrum is the basis for measuring Mg, Fe, S, C, Al, Si, Ca, and O elements. The capture spectrum can provide information on other elements, such as Mg, S, Ti, Al, K, Ca, Si, Gd, Fe, Cl, and H elements.

[0005] Since elemental yield logs only provide the relative concentration of elements, they are expressed in ratios, such as C / O, Cl / H, Si / (Si+Ca), H / (Si+Ca), and Fe / (Si+Ca). These ratios are indicators of oil, salinity, mineralogy, porosity, and clay, respectively. The elemental yield logs, as well as the cross sections for neutron inelastic scattering and neutron capture reactions for these elements, can also be used to obtain the elemental content in the formation.

[0006] Additionally, by measuring the thermal neutron time decay curve or capture 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 gas / oil saturation when the formation salinity is high.

[0007] When the formation salinity is low, the C / O ratio is the primary method to obtain the formation oil saturation. The C / O ratio can be the ratio of the elemental yield of C to O, the ratio of the total count rate of inelastic gamma rays for C to O, or the ratio of inelastic gamma rays in two energy windows selected for C and O.

[0008] To accurately estimate the oil saturation, the formation type (e.g., limestone, sandstone, or dolomite), the formation porosity, and the wellbore environment (e.g., wellbore size, wellbore fluid) are needed. For a particular formation type and wellbore environment, the oil saturation can be a function of the C / O ratio and the porosity. Figure 1A and 1B is an example plot showing how the oil saturation depends on the C / O ratio and the formation porosity in limestone for the near and far detectors. Figure 1C and 1D is an example plot showing how the oil saturation depends on the C / O ratio and the formation porosity in sandstone for the near and far detectors. The C / O ratio for limestone is higher than that for sandstone when the porosity and oil saturation are the same because limestone (CaC03) contains carbon elements, while sandstone (Si02) does not contain carbon elements.

[0009] One method to obtain the oil saturation is to use a calibration model to perform a series of simulations and obtain the C / O ratio for the near and far detectors at different formation porosities for a particular formation type (e.g., with a known oil saturation) under a particular wellbore condition. An algorithm is then developed to relate the C / O ratio and the formation porosity to the oil saturation. In field applications, when the C / O ratio is obtained and the formation porosity is determined, the algorithm can be used to obtain the oil saturation.

[0010] 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 a fast neutron emitted from the neutron source is slowed down and scattered back to the detector by the formation, the neutron is absorbed and produces other detectable ions, such as protons (p) and tritium (T), which ionize the gas. The ions and electrons are multiplied and drifted 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 deposited energy of the gamma rays into scintillation light. The PMTs convert the scintillation light into electrons and amplify them to form an electrical current signal.

[0011] Existing nuclear logging tools typically use single function detectors to detect either neutrons or gamma rays. For example, to obtain accurate oil saturation information, knowledge of mineralogy, such as sandstone (Si02), limestone (CaC03), dolomite (CaMg(C03)2), formation porosity, and C / O ratio is needed. Traditionally, because porosity, mineralogy, and C / O logging data are obtained by at least one neutron porosity tool and one pulsed neutron tool, respectively, oil saturation cannot be estimated in real time because all logging data must be obtained before oil saturation can be estimated.

[0012] There is a need to reduce the number of logging tools and logging runs while still obtaining the various formation parameters needed for drilling operations. For example, there is a need to simultaneously measure formation type, porosity, and C / O ratio to accurately estimate formation oil saturation while drilling. The present disclosure provides a new logging tool that combines a neutron source and dual function detectors that can more quickly estimate formation oil saturation and other formation parameters in real time by simultaneously measuring formation mineralogy, porosity, and C / O ratio and determining formation type and oil saturation. SUMMARY

[0013] This section is intended to provide a brief overview of some concepts that are further explained in the detailed description section below. This section does not seek to determine the key or essential features of the claimed subject matter nor is it meant to limit the scope of the claimed subject matter.

[0014] According to one embodiment of the present disclosure, a method of measuring one or more parameters of a subterranean formation includes the steps of: deploying a nuclear logging tool into a subterranean formation, the nuclear logging tool having one or more neutron sources and three or more detectors configured to simultaneously detect neutrons and gamma rays; causing the one or more neutron sources to emit neutrons toward the subterranean formation; receiving neutrons and gamma rays from the subterranean formation at the three or more detectors and converting them into electrical signals; separating the electrical signals of the neutrons and gamma rays for each of the three or more detectors; obtaining an energy spectrum of inelastic gamma rays and an energy spectrum of capture gamma rays; obtaining a single element gamma ray spectrum for each of a plurality of elements; calculating an element yield for each of the elements to obtain a concentration of each of the elements in the formation; and determining a formation type based on the concentrations of the elements in the formation.

[0015] In certain embodiments, the plurality of elements is selected from the group consisting of Mg, Fe, S, C, Al, Si, Ca, O, Ti, K, Gd, Cl, and H. In addition, the fast and thermal neutron count rates of the three detectors and the resulting inelastic and capture spectra are used in calculating the element concentrations.

[0016] The present disclosure further provides a method of obtaining a formation porosity of a subterranean formation, comprising the steps of: after obtaining a formation type, obtaining a count rate of total neutrons, fast neutrons, and thermal neutrons for each of three or more detectors; calculating a ratio of neutron count rates for each of two of the three or more detectors to obtain a plurality of ratios of neutron count rates; and obtaining the formation porosity from the plurality of ratios of neutron count rates and the formation type.

[0017] In some other embodiments, after determining the formation type and the formation porosity, a method comprising the step of: using the formation type, the formation porosity, and a parameter selected from the group consisting of a C / O ratio for each of the three or more detectors, a total neutron count rate for each of the three or more detectors, and a thermal neutron count rate for each of the three or more detectors to calculate an oil saturation of the subterranean formation can be used.

[0018] In certain embodiments, the calculating step further comprises using the formation type, the formation porosity, and the C / O ratio for each of the three or more detectors to calculate a plurality of apparent oil saturation values; and using the plurality of apparent oil saturation values to calculate a corrected oil saturation. BRIEF DESCRIPTION OF DRAWINGS

[0019] The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.

[0020] Figure 1A 、 Figure 1B 、 Figure 1C and Figure 1D show that for near, far-near detectors, formation oil saturation in a particular wellbore environment depends on formation type, porosity, and C / O ratio.

[0021] Figure 2A 、 Figure 2B 、 Figure 2C and Figure 2D show four exemplary configurations of a nuclear logging tool having one neutron source S1 and three detectors D1, D2, and D3 arranged along a longitudinal direction of a tool housing.

[0022] Figure 3A 、 Figure 3B and Figure 3C show cross-sectional views of exemplary nuclear logging tools having S1, D1, D2, and D3.

[0023] Figure 4A and Figure 4B show cross-sectional views of exemplary nuclear logging tools having four detectors D1, D2, D31, D32 and six detectors D1, D21, D22, D31, D32, D33, respectively.

[0024] Figure 5A An embodiment of a nuclear logging tool with two neutron sources S1, S2 and two detectors D1, D21 is shown, Figure 5B and 5C A cross-sectional view showing an exemplary configuration of this embodiment is shown.

[0025] Figure 6A Another exemplary embodiment of a nuclear logging tool with two neutron sources S1, S2 and four detectors D11, D12, D21, D22 is shown, Figure 6B A cross-sectional view of this embodiment is shown.

[0026] Figure 7 is a block diagram of an exemplary drilling system in which embodiments of the present disclosure can be implemented.

[0027] Figure 8 A schematic diagram showing a neutron pulse, a neutron count rate, and inelastic and capture spectra of neutron-induced gamma rays is shown.

[0028] Figure 9 is a flowchart showing an exemplary method of obtaining oil saturation in a formation in accordance with the present disclosure.

[0029] Figure 10 is another flowchart showing an exemplary method of obtaining oil saturation in a formation in accordance with the present disclosure.

[0030] Throughout the drawings and the detailed description, unless otherwise noted, like reference labels will be understood to refer to like components, features, and structures. The relative dimensions of these elements can be exaggerated for clarity, illustration, and convenience. DETAILED DESCRIPTION

[0031] The following detailed description is provided so that the reader can obtain an exhaustive understanding of the methods, apparatuses, and / or systems described herein. The detailed description includes specific details for the purpose of providing a thorough understanding of the described embodiments. The described embodiments are not limited to the specific details set forth herein and can be practiced with alternative embodiments.

[0032] The features described herein can be embodied in different forms, and should not be construed as limited to the embodiments described herein. Rather, the embodiments described herein are provided as examples so that the disclosure will be thorough and complete, and will fully convey the full scope of the disclosure to those skilled in the art, and will fully convey the full scope of the disclosure so as to enable others skilled in the art to appreciate the embodiments and to realize its usefulness and to recognize possibilities of alternatives and / or modifications in the embodiments described herein. Accordingly, the description is not intended to limit the disclosure. Except as otherwise indicated, same reference numbers indicate same components in the attached drawings and the detailed description.

[0033] Therefore, the scope of the disclosure is not defined by the specific embodiments part, but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be construed as being included in the disclosure.

[0034] In the present disclosure, a detector refers to a dual-function detector that can detect both neutrons and gamma rays, unless otherwise specified. Such a detector employs a scintillation crystal, such as Cs2LiYCl6(CLYC) or Cs2LiLaBr6(CLLB), and related electronic components, such as a PMT. When the detector is deployed downhole, it can be actively cooled or non-actively cooled. For example, a detector using CLLB and a high-temperature PMT can be used at high temperatures without additional cooling.

[0035] Figure 1A 、 Figure 1B 、 Figure 1C and Figure 1D show the correlation between the formation oil saturation and the C / O ratio of the near and far detectors with respect to the formation type, porosity, and the near and far detector in a specific wellbore environment. Figure 1A and 1B are exemplary plots showing the correlation between the C / O ratio and the formation porosity for the near detector and the far detector, respectively, in limestone. Figure 1C and 1D are exemplary plots showing the correlation between the C / O ratio and the formation porosity for the near detector and the far detector, respectively, in sandstone. The C / O ratio rate is higher in limestone than in sandstone with the same porosity and oil saturation, which can be due to the fact that limestone (CaCO3) contains carbon elements while sandstone (SiO2) does not. In addition, the C / O ratio of the near detector and the far detector is also different. The reason is that the near detector is closer to the neutron source and is more affected by the wellbore fluid than the far detector. Therefore, by using multiple detectors to measure the C / O ratio and developing a correction algorithm for the wellbore fluid, the effect of the wellbore environment (such as the wellbore fluid) on the measured formation oil saturation can be reduced.

[0036] To obtain oil saturation, a series of simulations can also be performed using a calibrated model to obtain the C / O ratio for the near and far detectors for different formation porosities for a particular formation type under specific wellbore conditions. The calibrated model is a model that has been calibrated (i.e., adjusted) from experimental data. The calibrated model largely eliminates systematic errors, but statistical errors remain. An algorithm can then be developed to relate the C / O ratio and formation porosity to oil saturation for typical formation types and specific wellbore environments (e.g., wellbore size, borehole fluid, etc.). The algorithm can be implemented in the tool's firmware or software. In field applications, once the formation type, formation porosity, and C / O ratio are determined, the algorithm can be used to obtain the oil saturation.

[0037] Figures 2A-2D is a schematic illustration (not to scale) of four exemplary configurations of a cylindrical nuclear logging tool 200 having a neutron source S1 and three dual-function detectors D1, D2, D3 disposed along the housing of the logging tool suitable for a logging-while-drilling (LWD) operation. A mud channel (MC) is disposed along the axis of the logging tool, while the detectors are disposed eccentrically along the longitudinal direction of the logging tool. Figure 2A Also shown are a high-voltage power supply (HV); electronics (e.g., a controller) for sending commands, receiving, and processing data from the neutron source and the detectors; and telemetry 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, electronics, and telemetry are not shown in Figures 2B-2D .

[0038] As shown, D1 is the near detector with the shortest distance from the neutron source in the longitudinal direction, D3 is the far detector with the longest longitudinal distance from the neutron source, and D2 is the middle detector with a longitudinal distance in between.

[0039] As Figure 2A shown, all three detectors are disposed on one side of the neutron source along the logging tool 200. This side can be the near side or the far side of the neutron source. The near side is the side closer to the surface when the nuclear logging tool 200 is deployed downhole, while the far side is the side farther from the surface. The high-voltage power supply powers the detectors D1, D2, D3 and the pulsed neutron source S1. Signals from the detectors are processed by the electronics, and measurements / data are acquired and transmitted through the telemetry. As Figure 2B , 2C and 2D show, both the far side and the near side of the neutron source have at least one detector disposed thereon.

[0040] In wireline logging, the tool can be installed in a probe that does not contain a mud channel. The probe can be mounted along or offset from the axis of the tool body. Power and control signals can also be provided to the logging tool from the surface, and data from the logging tool can be transmitted to the surface via the wireline.

[0041] exist Figures 2A-2D The neutron source S1 in each logging tool depicted in FIG is a pulsed neutron generator. However, isotope neutron sources may also be used. The pulsed neutron source may be a deuterium-tritium (DT) pulsed neutron generator, which can be operated in a pulsed mode with a variety of pulse output modes (e.g., frequency, pulse duration). For example, the frequency of the neutron pulses may be approximately 10 kHz (period of 100 μs), and the duration of the neutron pulses may be approximately 20 μs. Depending on the method and measurement conditions, the DT neutron generator may also be operated in a continuous mode. In this case, the starting frequency of the neutron generator is high enough so that neutrons are emitted continuously. The neutrons from the DT neutron generator have an initial energy of approximately 14.1 MeV.

[0042] Depending on the target formation parameters and measurement method, isotope neutron sources (such as Am-Be, Pu-Be, and Cf-252) can also be used instead of pulsed neutron sources. Neutrons from these isotope neutron sources have different energy spectra. For example, Am-Be sources emit neutrons with energies ranging from 0 MeV to approximately 10 MeV, with an average energy of approximately 4.2 MeV. However, due to the lower neutron energy, the gamma-ray signal generated by rapid inelastic scattering from carbon and oxygen using isotope neutron sources is much lower than the gamma-ray signal triggered by a DT neutron generator.

[0043] like Figures 2A-2D The neutron source S1 and detectors D1 , D2 and D3 shown only indicate their relative positions along the longitudinal direction of the housing of the tool 200 , but do not indicate their positions in the radial direction in the cross section of the tool housing.

[0044] In certain embodiments, S1 , D1 , D2 , and D3 may be arranged in the same radial direction or in different radial directions, ie, have the same or different measurement azimuths when deployed in the formation. Figure 3A 、 3B and 3C shows the Figures 2A to 2D Exemplary cross-sectional views in directions AA, BB, CC and DD are shown in FIG. Figure 3A S1, D1, D2 and D3 are set at the same azimuth. Figure 3B In , S1, D1 and D3 have the same azimuth, while D2 is at a different azimuth. Figure 3C In the figure, S1 and D1 have the same azimuth angle, while D2 and D3 have different azimuth angles.

[0045] Other embodiments of the logging tool can have more than three detectors. For example, Figure 4A One variation of the logging tool in Figure 2A is depicted, which has four detectors, D1, D2, D31, and D32. D31 and D32 are substantially the same distance from S1, but are positioned at two different azimuth angles. Likewise, Figure 4B Another variation of the tool in Figure 2A is depicted, which has six detectors, D1, D21, D22, D31, D32, and D33. In this embodiment, D21 and D22 are positioned opposite each other in the cross-section of the logging tool, i.e., the azimuth angles of D21 and D22 are 0° and 180°, respectively. D31, D32, and D33 are positioned 120° apart in the cross-section of the logging tool, i.e., the difference between the azimuth angles of any two of D31, D32, and D33 is 120°. Having different azimuth angles allows the detectors to preferentially receive neutrons and gamma rays from a particular incident angle from the formation. This embodiment also improves the detection efficiency of neutrons and gamma rays by increasing the total count rate of all the detectors.

[0046] Additionally, in Figure 4A , D31 and D32 are substantially the same distance from S1. In Figure 4B , the intermediate detectors D21 and D22 are substantially the same distance from S1, while the far detectors D31, D32, and D33 are substantially the same distance from S1. By "substantially the same distance" is meant that the distance from S1 to the center of the scintillator of the detector (e.g., D31 and D32) is approximately the same. For example, the difference is less than one-half or one-quarter of an inch. With this arrangement, the intermediate detectors as a whole or the far detectors as a whole have a higher count rate than if only one intermediate detector or only one far detector were used. Thus, the neutron generator S1 can be a less powerful source, which can not be subject to the stringent regulations of a more powerful neutron source. Moreover, the count rates of the individual detectors can be recorded and processed separately. The differences in distance and azimuth angle of the various detectors can be used to obtain formation information in particular azimuthal directions.

[0047] In some embodiments, the logging tool 200 has a plurality of shields (not shown) that can absorb neutrons and gamma rays. The shields can be placed between the neutron source and the detectors in the logging tool so that the detectors receive neutrons and gamma rays from the formation, rather than neutrons and gamma rays that pass through the logging tool itself. Alternatively, the detectors can also be partially shielded by a shielding material that can absorb neutrons and gamma rays from certain directions.

[0048] The shield is made of or includes one or more materials that are effective at attenuating thermal neutrons and gamma rays. The shielding material can include a material selected from heavy elements having a high thermal neutron absorption cross section, including metals such as gadolinium (Gd), samarium (Sm), metal oxides (such as Gd2O3, Sm2O3, B2O3), alloys containing Gd or Sm in combination with other heavy metals (such as Fe, Pb, or W), or boron-containing materials (such as tungsten boride (WB, WB2, etc.).

[0049] The shield can be a separate piece of metal inserted into the logging tool, or it can be an integral part of the detector housing. For example, the portion of the detector housing that faces inward toward the logging tool can be made of shielding material, while the portion that faces the formation is made of a material that is transparent to neutrons and gamma rays, forming a window through which neutrons and gamma rays can pass. Thus, neutrons and gamma rays from certain angles of incidence can be absorbed by the shielding material, while those that pass through the window are received by the detector. By adjusting the size and orientation of the window in the detector housing, the detector can be made more sensitive to certain angles of incidence. The data collected by the various detectors can be used to infer formation properties in certain directions, which can be used to guide the direction of drilling.

[0050] The nuclear logging tool can have more than one neutron source. Figure 5A Another embodiment of a logging tool having two neutron sources S1 and S2 is shown, with one source near the proximal end and the other near the distal end, and two detectors D1 and D2 disposed between S1 and S2. Alternatively, S1 and S2 can be disposed in series and near one end of the logging tool, and D1 and D2 can also be disposed in series and near the other end, as desired for engineering considerations. In both embodiments, the distance between S1 and D1 is dl, 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 S1 and S2 are both pulsed neutron generators, they can be alternately turned on and off, thereby alternately inducing neutrons and gamma rays from the formation, which are received by D1 and D2. Because there are four different source-to-detector distances (dl through d4), the data generated by D1 and D2 can better compensate for near- borehole effects, such as borehole size, tool spacing, mud weight and / or salinity, casing size, cement thickness, etc., than a tool having only two or three source-to-detector distances. Thus, the resulting formation parameters can be more accurate.

[0051] Figure 5B and Figure 5C Two exemplary embodiments of a logging tool are shown, with two sources and two detectors disposed at the same tool face angle or at different tool face angles. As Figure 5BAs shown in , when the source and detector have the same tool face angle, the measurement will cover the same sector in the formation at any given time. Figure 5C As shown in FIG, when the source and the detector have different tool face angles, the data generated in D1 and D2 reflect different sectors of the formation. By comparing the measurement results of D1 and D2, the differences between different formation sectors at any given time can be found.

[0052] Figure 6A and 6B A well logging tool is shown with four detectors D11, D12, D21, and D22 and two neutron sources S1 and S2. Note that a pair of detectors D11 and D12 (as well as D21 and D22) are positioned at substantially the same distance from either S1 or S2. As previously mentioned, placing more than one detector at a given distance can increase the count rate at that distance, allowing the use of less powerful neutron sources. The count rate of a single remote detector may be too low to provide reliable measurement data. By using two or more remote detectors, the count rate can be significantly increased, enabling reliable measurements to be obtained by processing the combined data from multiple remote detectors.

[0053] In some embodiments, S1 and S2 can be turned on or off simultaneously, which can increase the count rates of D1 and D2, thereby reducing the uncertainty of statistical measurements.

[0054] In another embodiment, both S1 and S2 are isotope neutron sources. Compared to pulsed neutron sources, isotope neutron sources do not require a power source, allowing for more compact logging tools. Furthermore, isotope neutron sources have longer lifespans and are more reliable. For example, the half-life of an isotope Am-Be neutron source is 432 years, far longer than the average tube life of a neutron generator, which ranges from 500 to 4000 hours.

[0055] In another embodiment, S1 and S2 can be two different types of neutron sources. For example, S1 can be a DT neutron generator, and S2 can be an Am-Be neutron source. In the field, the DT neutron generator can be turned off and the Am-Be neutron source can be left to work on its own to perform neutron porosity logging. Alternatively, the Am-Be neutron source can be removed from the logging tool so that the DT neutron generator can emit neutron pulses into the surrounding formation alone. In this case, the DT neutron generator can be used to obtain neutron porosity logs and other measurement results (density, oil and gas saturation, etc.)

[0056] Porosity logs obtained using Am-Be neutron sources and D-T neutron sources are slightly different. By comparing porosity logs obtained using two different neutron sources in the same well, a correlation between the two logs can be obtained. Since historical porosity logs were primarily obtained using isotopic neutron sources, this correlation can be helpful in updating historical porosity logs to make them comparable to new logs obtained using pulsed neutron sources. Likewise, new pulsed neutron porosity logs can also be converted to match historical porosity logs for continued use of reservoir models established from historical logs in production forecasting.

[0057] Logging tool 200 can be part of a wireline logging tool, or included in downhole equipment as an LWD logging tool in a drilling operation. Figure 7 is a schematic illustration of a petroleum drilling system 10 applied in directional drilling of a wellbore 16. The petroleum drilling system 10 can be used to drill wells on land as well as underwater. A rotary drilling rig is used to drill the wellbore 16 in the earth formation, including a derrick 12, a drill floor 14, a drawworks 18, a traveling block 20, a hook 22, a rotary

[0058] Drilling fluid, also known as mud, is typically stored in a mud pit or tank 46 and delivered using a mud pump 38 that forces the drilling fluid to flow through a surge suppressor 40, then through a kelly hose 42, and through the rotary

[0059] The pressure required to maintain circulation of the drilling fluid is measured by a pressure-sensing transducer 48 on the kelly hose 42. The pressure-sensing transducer detects pressure changes caused by pressure pulses generated by a pulse generator. The pressure wave from the pulse generator can have an amplitude of 500 psi or more. The measured pressure is transmitted as an electrical signal through a sensor cable 50 to a surface computer 52, which decodes and displays the transmitted information. Alternatively, the measured pressure is transmitted as an electrical signal through a sensor cable 50 to a decoder, which decodes the electrical signal and transmits the decoded signal to a surface computer 52, which displays the data on a display screen.

[0060] As described above, the lower portion ("distal portion") of the drill string 100 includes a bottom hole assembly (BHA) 150 that 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 that contains LWD instruments, a downhole motor 170, a near-bit measurement sub 175, and a drill bit 180 having a borehole nozzle (not shown). Drilling fluid flows through the drill string 100 and out of the borehole nozzle of the drill bit 180. During drilling operations, the drilling system 10 can be operated in a rotary mode in which the drill string 100 is rotated relative to the surface by a motor (i.e., top drive) in the rotary table 28 or the traveling block 20. The drilling system 10 can also be operated in a sliding mode in which the drill string 100 is not rotated relative to the surface, but the drill bit 180 is rotated by the downhole motor 170. Drilling fluid is pumped from the surface through the drill string 100 to the drill bit 180, which is injected into the annular space between the drill string 100 and the wall of the borehole 16. The drilling fluid carries cuttings from the borehole 16 to the surface.

[0061] In one or more embodiments, the MWD system 160 can include a pulse generator sub, a pulse generator drive sub, a battery sub, a central memory unit, a main board, a power supply sub, a directional module sub, and other sensor boards. In some embodiments, some of these devices can be located in other areas of the BHA 150. One or more of the pulse generator sub and the pulse generator drive sub can be in communication with a pulse generator 300, which can be located below the MWD system 160. The MWD system 160 can transmit data to the pulse generator 300 so that the pulse generator 300 generates pressure pulses.

[0062] The non-magnetic drill collar houses the MWD system 160, which includes a suite of instruments for measuring inclination, azimuth, well trajectory (hole trajectory), etc. The nuclear logging tool 200 and associated electronics can be located in the LWD instrument sub 165. The nuclear logging tool 200 and other logging instruments can be electrically or wirelessly coupled together, powered by a battery pack or by a drill fluid driven generator. All information collected is transmitted to the surface through the mud column in the drill string in the form of pressure pulses generated by the pulser 300.

[0063] A near-bit measurement sub 175 can be disposed between the downhole motor 170 and the drill bit 180. The nuclear logging tool 200 can alternatively be installed in the near-bit measurement sub 175 to provide more accurate real-time formation parameters to guide directional drilling. Data can be transmitted through a cable embedded in the downhole motor 170 to the MWD system 160 in the downhole drilling assembly 150.

[0064] In one embodiment of the present disclosure, a logging tool with a D-T neutron generator and three dual-function detectors is used to obtain a variety of formation parameters. Figure 8 A schematic diagram showing the neutron pulse, the neutron count rate, and the non-elastic spectrum and capture spectrum of the neutron-induced gamma rays is shown. The frequency of the neutron pulse is 10 kHz (period of 100 μβ), and the neutron-on time is 20 μβ, as shown in subfigure (b) of Figure 8 .

[0065] The neutron count rate measured by each of the three detectors, as shown in subfigure (a) of Figure 8 , is used to obtain formation porosity. The neutrons of the three detectors can be further separated according to whether the neutron pulse is on or off, and the neutron pulse can be used as a coincidence or anticoincidence signal of the neutrons of the three detectors, so that during the neutron pulse (when the neutron pulse is on), the neutrons are mainly recorded as fast neutrons. Between the neutron pulses (when the neutron pulse is off), the neutrons are recorded as thermal neutrons. The fast and thermal neutrons recorded by the three detectors can be used to obtain fast and thermal neutron spatial distributions. The neutrons from each detector can also be recorded together. In this case, all the neutrons (from thermal to fast) are used to obtain a neutron spatial distribution.

[0066] The gamma rays from the three detectors can be further separated according to whether the neutron pulse is on or off, and the neutron pulse can be used as a coincidence or anticoincidence signal of the gamma rays from the three detectors, so that during the neutron pulse (the neutron pulse is on), the gamma rays are mainly recorded as the non-elastic spectrum induced by the non-elastic scattering fast neutrons, as shown in subfigure (c) of Figure 8 . Between the neutron pulses (the neutron pulse is off), the gamma rays are recorded as the capture spectrum induced by the thermal neutrons, as shown in subfigure (d) of Figure 8The time window is chosen so that all gamma rays measured in the capture time window come from thermal neutron capture reactions, and most gamma rays measured in the inelastic time window come from fast neutron inelastic scattering.

[0067] The background noise in each detector can be measured after the neutron generator has been turned off for a period of time and subtracted from the total neutron or gamma-ray signal. A "pure" fast neutron spectrum can be obtained by using a small fraction of the neutrons measured between neutron pulses as background, further subtracting the neutron background measured during the neutron pulses. Similarly, a "pure" inelastic spectrum can be obtained by using a small fraction of the capture spectrum measured between neutron pulses as background, further subtracting the captured gamma-rays measured during the neutron pulses.

[0068] The gamma rays detected by each detector can also be recorded in a single energy spectrum (i.e., a total energy spectrum), regardless of whether they originate from inelastic neutron scattering or neutron capture reactions. This allows for a variety of formation measurements, such as formation porosity, element concentrations, and formation oil / gas saturation, but may not be able to obtain formation density. This is because for measurement systems based on DT pulsed neutron generators, the inelastic gamma ray spectrum is required to obtain formation density.

[0069] Figure 9 The following is an exemplary workflow illustrating the steps involved in processing data from a well logging tool 200 equipped with a DT neutron generator and three detectors (near, intermediate, and distal) to obtain real-time oil saturation in a formation. In step 1001, the DT neutron generator emits a neutron pulse into the formation surrounding the logging tool. In step 1002, the fast neutrons are decelerated to thermal neutrons, generating inelastic gamma rays and capture gamma rays.

[0070] In step 1003, the three detectors detect neutrons and neutron-induced gamma rays. In step 1004, the signals of neutrons and neutron-induced gamma rays are distinguished from each other using, for example, pulse shape discrimination (PSD) technology. In step 1009, the neutron signals from the three detectors are used to obtain the total count rate (CRN) 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), after which they are utilized in step 1010 and the formation porosity is obtained by the ratio of the total neutrons (Rn f / m , Rn f / n , Rn m / n ) or the ratio of the thermal neutrons (Rtn f / m , Rtn f / n , Rtn m / n ).

[0071] On the other hand, in step 1005, the total energy spectrum of the inelastic gamma rays and the total energy spectrum of the capture gamma rays are determined after the separation in step 1004. In step 1006, the total energy spectrum in step 1005 is stripped using standard energy spectra of individual elements (such as C, O, Fe, etc.). Thus, the C / O ratio can be obtained in step 1007.

[0072] Further, in step 1011, the formation element concentrations are calculated using the fast and thermal neutron count rates of the three detectors in step 1010 and the inelastic and capture energy spectra obtained in step 1006. Once the formation element concentrations are known, the formation type can be determined in step 1008. With the formation type in step 1008 and the count rates in step 1010, the formation porosity can be obtained in step 1012.

[0073] Finally, in step 1013, the formation oil saturation can be determined using information about the formation type, the C / O ratio, the formation porosity, or the thermal neutron count rate ratio, more details of which are found in Figure 10 and the following description. It should be noted that most of the neutrons detected by the detectors are thermal neutrons, but some epithermal neutrons are also detected.

[0074] Figure 10 Another embodiment of the present disclosure is introduced using mathematical symbols and equations, which is performed using an exemplary logging tool 200 (e.g., as shown in Figures 2A to 2D ). Using the neutron count rates CRNmeasured during and between the neutron pulses from the near, middle, and far detectors, i.e., CRN n , CRN m , CRN f , the formation porosity (Φ) can be obtained using the count rate ratios.

[0075] The middle-far ratio (Rn m / f ), the near-far ratio (Rn n / f ), and the near-middle ratio (Rn n / m ) can be derived from Equations 1, 2, and 3, respectively. Since the three detectors are placed at different distances from the neutron source, their detection depths are also different. Thus, the near wellbore environment (such as the borehole fluid, cement, etc.) affects the three ratios differently. Rnm / f more sensitive to the formation n / m more sensitive to near wellbore changes than Rn n / f sensitive to both.

[0076]

[0077] Formation porosity Φ n can be obtained by first using Rn n / m and / or Rn n / f to correct Rn m / f and then using the corrected far-to- near ratio Rnc m / f to obtain the formation porosity for a particular formation (e.g., sandstone, limestone, or dolomite). Equations 4 through 6 illustrate this algorithm, where ΔR is the correction.

[0078] Rnc m / f = Rn m / f + ΔR (4)

[0079] ΔR = f1(Rn m / f , Rn n / f , Rn n / m ) (5)

[0080] Φ n = f2(Rnc m / f ) (6)

[0081] Alternatively, formation porosity Φ n can also be obtained using three ratios of capture gamma ray count rates obtained from the three detectors, following an algorithm similar to that described in Equations 1 through 6.

[0082] Formation porosity Φ n can also be obtained by combining two porosities obtained from neutrons and capture gammas. In other methods, formation porosity Φ n can be obtained directly from three ratios of neutrons and three ratios of capture gammas in other ways.

[0083] Formation type can be obtained by measuring the energy spectrum of the gamma rays from the neutron inelastic scattering and the neutron capture reaction with the same tool. The timing of the neutron pulse from the D-T neutron generator is shown in Figure 1. Next, after separation from the neutron signal, the neutron-induced gamma ray signal from the three detectors is further separated into the gamma ray signal from the thermal neutron capture reaction and the gamma ray signal from the fast neutron inelastic scattering. Both of these signals can be used for elemental analysis of the formation, such as hydrogen (H), silicon (Si), calcium (Ca), iron (Fe), sulfur (S), and chlorine (Cl). Figure 8

[0084] ​In some embodiments, the gamma rays detected by each detector can be recorded in two separate energy spectra (inelastic and capture energy spectra) or in one energy spectrum (total energy spectrum). In both cases, the elements can be identified and the relative yields of the characteristic gamma rays of these elements can be obtained, thereby determining the element concentrations.

[0085] Since the three detectors in the logging tool 200 detect neutrons and gamma rays simultaneously at three different locations, the neutron count rates from the three detectors can be used to obtain more accurate neutron spatial distributions (including fast and thermal neutron spatial distributions). The measured neutron spatial distributions can be used to more accurately calculate the concentrations of elements such as C, O, H, Cl, Si, etc.

[0086] The inelastic energy spectra from the three detectors are fitted to the standard gamma ray energy spectra of the elements in the formation (including C and O), so that the gamma ray signals from O and C can be separated from the inelastic energy spectra. Thus, the ratio of the total inelastic gamma ray count rates from C to O can be used to obtain the inelastic gamma ray count rate ratio from C and O (C / O ratio). The C / O ratio can also be obtained from the ratio of the inelastic gamma ray count rates under the C or O peaks in a selected energy window. On the other hand, the formation element concentrations can be used to determine the formation type. The formation type can also be used to obtain the formation porosity. With the knowledge of the C / O ratio, the formation type, and the porosity, the oil saturation can be estimated for a specific wellbore environment (such as the borehole size, borehole fluid, etc.).

[0087] An example algorithm calculates the apparent oil saturations (So n , So m , So f ) for the near, middle, and far detectors for a known wellbore environment (borehole size, wellbore fluid, casing, etc.) by Equations 16-18, respectively. After obtaining the three apparent oil saturations, a corrected oil saturation So n can be calculated, for example, by a weighted average of the apparent oil saturations or a multinormal function of So m , So f with different coefficients. The calculation of So n , So m , So f can be represented by Equation 19 as follows:

[0088] So n = f1(Rg n,c / o , Φ, formation type) (16)

[0089] So m = f2(Rgm,c / o , Φ, formation type) (17)

[0090]

[0091] So = f4(So n , So m , So f ) (19)

[0092] Rg n,c / o , Rg m,c / o , and Rg f,c / o are the C / O ratio values derived from the non-elastic gamma-ray count rates of carbon and oxygen detected at the near, intermediate, and far detectors, respectively. The corrected oil saturation can also be obtained by using the C / O ratio values (Rg n,c / o , Rg m,c / o , and Rg f,c / o ), formation type, formation porosity (Φ), and fast neutron ratio (Rfn f / m , Rfn f / n , Rfn m / n ) from the three detectors, as shown in Equation 20:

[0093] So = f5(Rg n,c / o , Rg m,c / o , Rg f,c / o , Φ, formation type, Rfn m / f , Rfn n / f , Rfn n / m ) (20)

[0094] In another embodiment, the corrected oil saturation can also be obtained by using the C / O ratio values (Rg n,c / o , Rg m,c / o , and Rg f,c / o ), formation type, and one of the thermal neutron ratio (Rtn f / m , Rtn f / n , Rtn m / n ) from two of the three detectors (i.e., near and far detectors), as shown in Equation 21:

[0095] So = f6(Rg n,c / o , Rg m,c / o , Rg f,c / o , formation type, Rtn n / f ) (21)

[0096] In another embodiment, the corrected oil saturation can also be obtained by using the C / O ratio values (Rg n,c / o , Rg m,c / oand Rg f,c / o ), formation type, one of the thermal neutron ratios (Rtn f / m , Rtn f / n , Rtn m / n ) from two of the three detectors (i.e., the near and far detectors), and the fast neutron ratios (Rfn f / m , Rfn f / n , Rfn m / n ) from the three detectors, as shown in Equation 22:

[0097] So = f7(Rg n,c / o , Rg m,c / o , Rg f,c / o , formation type, Rtn n / f , Rfn m / f , Rfn n / f , Rfn n / m ) (22)

[0098] It should be noted that Equations 20 through 22 represent one suitable algorithm, with the variables in parentheses. The algorithm can be a multinormal function of So n , So m , So f with theoretical or empirical coefficients.

[0099] Alternatively, the total energy spectrum of inelastic gamma rays and capture gamma rays can be used instead of the inelastic energy spectrum to obtain the C / O ratio. In this case, the total energy spectrum is fitted using standard gamma ray total energy spectra for each element in the formation (including both inelastic gamma rays and capture gamma rays) to separate out the gamma rays of C and O. This fitting is more complex than fitting only the inelastic energy spectrum, and the results can be less accurate. Also, the ratio of the total gamma ray count rate under the C or O peak in a selected energy window can be used to obtain the C / O ratio. However, this approach allows the D-T neutron generator to be operated in continuous mode, or an isotopic neutron source to be used, such as Am-Be, Pu-Be, Cf-252, etc.

[0100] While the disclosure has been described in connection with certain preferred embodiments and with many details set forth in the specification, it will be apparent to those skilled in the art from this disclosure that variations in form, and in substitutions of equivalents, can be made, and that certain other details described herein can also be varied greatly without departing from the spirit of the disclosure. Furthermore, it should be understood that structural features or methods shown or described in any one embodiment herein can be used in other embodiments as well.

Claims

1. A method for measuring parameters of an underground formation, comprising the steps of: S1: deploying a nuclear logging tool into the underground formation, wherein the nuclear logging tool comprises one or more neutron sources and three or more detectors configured to simultaneously detect neutrons and gamma rays; S2: causing the one or more neutron sources to emit neutrons toward the underground formation; S3: receiving neutrons and gamma rays from the underground formation at the three or more detectors to generate electrical signals; S4: separating an electrical signal from neutrons and gamma rays for each of the three or more detectors; S5: obtaining the inelastic energy spectrum of the inelastic gamma rays and the captured energy spectrum of the captured gamma rays; S6: obtaining a single element gamma-ray spectrum for each of the plurality of elements; S7: calculating the element yield of each element to obtain the concentration of each element in the formation; S8: determining the formation type according to the concentration of elements in the formation; S9: Obtaining a count rate of total neutrons, fast neutrons, and thermal neutrons for each of the three or more detectors; S10: calculating a ratio of neutron count rates of every two detectors among the three or more detectors to obtain a plurality of neutron count rate ratio values; as well as S11: Obtaining formation porosity according to the multiple neutron count rate ratios and the formation type.

2. The method according to claim 1, wherein The plurality of elements are selected from Mg, Fe, S, C, Al, Si, Ca, O, Ti, K, Gd, Cl and H.

3. The method according to claim 1, wherein Element concentrations are calculated using the fast neutron count rates and thermal neutron count rates of the three or more detectors and the obtained inelastic energy spectrum and capture energy spectrum.

4. The method according to claim 1, wherein In step S11, the first neutron count rate ratio, the second neutron count rate ratio, and the third neutron count rate ratio are obtained. The method further includes step S12: using the second neutron count rate ratio and the third neutron count rate ratio to correct the first neutron count rate ratio for near-wellbore effects, Obtaining a correlation between a neutron count rate ratio and a formation porosity of the formation type; as well as The corrected first neutron count rate ratio is input into the correlation to obtain the formation porosity.

5. A method for measuring oil saturation of an underground formation, comprising the steps of: Obtaining formation type and formation porosity according to the method of claim 1; as well as Oil saturation is calculated using the formation type, formation porosity, and a parameter selected from the group consisting of a C / O ratio of each of the three or more detectors, a total neutron count rate of each of the three or more detectors, a fast neutron count rate of each of the three or more detectors, and a thermal neutron count rate of each of the three or more detectors.

6. The method according to claim 5, wherein: The step of calculating further comprises: calculating a plurality of apparent oil saturation values ​​using the formation type, formation porosity, and each C / O ratio value of each of the three or more detectors; and A corrected oil saturation is calculated using the plurality of apparent oil saturation values. 7 . The method according to claim 6 , wherein the nuclear logging tool has a near detector, a middle detector, and a far detector, and three apparent oil saturation values ​​of the near detector, the middle detector, and the far detector are calculated respectively.

8. The method of claim 7, further comprising calculating a corrected oil saturation value using the three apparent oil saturation values.

9. The method according to claim 5, wherein: The oil saturation value is calculated using the formation type, formation porosity, at least one C / O ratio of one of the one or more detectors, and at least one fast neutron count rate ratio of one of the one or more detectors.

10. The method according to claim 9, wherein: The nuclear logging tool has a near detector, an intermediate detector, and a far detector, and calculates the oil saturation value using the formation type, formation porosity, the C / O ratio and fast neutron count rate ratio of the near detector, the C / O ratio and fast neutron count rate ratio of the intermediate detector, and the C / O ratio and fast neutron count rate ratio of the far detector.

11. The method according to claim 5, wherein: The oil saturation value is calculated using the formation type, at least one C / O ratio of one of the one or more detectors, and a thermal neutron count rate ratio of one of the one or more detectors.

12. The method according to claim 11, wherein The nuclear logging tool has a near detector, a middle detector, and a far detector, and calculates the oil saturation value using the formation type, the C / O ratio of the near detector, the C / O ratio of the middle detector, the C / O ratio of the far detector, and a thermal neutron count rate ratio between two detectors among the near detector, the middle detector, and the far detector.

13. The method according to claim 5, wherein: The oil saturation value is calculated using the formation type, at least one C / O ratio of one of the one or more detectors, a thermal neutron count rate ratio of one of the one or more detectors, and at least one fast neutron count rate ratio of one of the one or more detectors.

14. The method according to claim 13, wherein The nuclear logging tool has a near detector, a middle detector, and a far detector, and calculates the oil saturation value using the formation type, the C / O ratio of the near detector, the C / O ratio of the middle detector, the C / O ratio of the far detector, and a thermal neutron count rate ratio between two detectors among the near detector, the middle detector, and the far detector.

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