Method and device for acquiring real-time underground oil saturation

Abstract

A method of measuring the oil saturation value of 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 to a formation, using a detector to detect neutrons and gamma ray signals in the formation, determining formation parameters including formation types and formation porosity, 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 are further determined, and oil saturation values are calculated using the determined parameters.

Description

Technical Field

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

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

[0003] Neutron porosity logging tools study formation porosity by measuring the ratio of the neutron count rates from near and far detectors after fast neutrons from an isotopic neutron source (e.g., Am-Be source) are slowed down by the tool's surroundings (e.g., wellbore fluids and formation) and scattered back to the detectors. 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 effects of changes in the near-wellbore environment (wellbore fluids, wellbore dimensions, etc.) on the porosity measurement.

[0004] Formation mineralogy can be obtained by pulsed neutron tools, which use a pulsed neutron source (e.g., a DT neutron generator) and one, two, or three detectors for detecting 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 fast neutron inelastic scattering and / or thermal neutron capture reactions, the elements can be identified and the relative percentage of gamma rays from each element in the formation, i.e., the element yield, can be obtained. The inelastic energy spectrum is the basis for measuring Mg, Fe, S, C, Al, Si, Ca, and O elements. The capture energy 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 relative concentrations of elements, they are expressed as 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. Elemental yield logs and cross sections of neutron inelastic scattering and neutron capture reactions of these elements can also be used to obtain the elemental content of the formation.

[0006] 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 when the formation salinity is high.

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

[0008] To accurately estimate oil saturation, one needs to know the formation type (e.g., limestone, sandstone, or dolomite), formation porosity, and wellbore environment (e.g., wellbore dimensions, wellbore fluids). For a particular formation type and wellbore environment, oil saturation can be a function of the C / O ratio and porosity. Figure 1A and 1B is an exemplary graph showing how oil saturation for near and far detectors depends on the C / O ratio in the limestone and the formation porosity. Figure 1C and 1D is an exemplary graph showing how oil saturation for near and far detectors depends on the C / O ratio in sandstone and the formation porosity. When porosity and oil saturation are the same, limestone has a higher C / O ratio than sandstone because limestone (CaCO3) contains carbon, while sandstone (SiO2) does not.

[0009] One method to obtain oil saturation is to use the calibration model to perform a series of simulations and obtain the C / O ratio of the near detector and the far detector at different formation porosities for a specific formation type (such as with known oil saturation) under specific wellbore conditions. Then develop an algorithm to relate the C / O ratio and formation porosity to oil saturation. In a field application, once the C / O ratio is obtained and the formation porosity is determined, the algorithm can be used to obtain oil saturation.

[0010] In most of these applications, neutrons and gamma rays are detected by their corresponding 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 the fast neutrons emitted from the 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), thereby ionizing the gas. Ions and electrons multiply and drift in the electric field to form electrical signals. 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 gamma rays into scintillation light. PMT converts scintillation light into electrons and amplifies them to form current signals.

[0011] Existing nuclear logging tools usually use single-function detectors to detect neutrons or gamma rays. For example, to obtain accurate oil saturation information, it is necessary to understand the mineralogy, such as sandstone (SiO2), limestone (CaCO3), dolomite (CaMg(CO3)2), formation porosity, and C / O ratio. Traditionally, since porosity, mineralogy, and C / O logging data are obtained by at least one neutron porosity tool and one pulsed neutron tool, respectively, oil saturation can only be estimated after all logging data are obtained, so it may not be possible to estimate oil saturation in real time.

[0012] There is a need to reduce the number of logging tools and logging operations, but still obtain various formation parameters required for drilling operations. For example, it is necessary to simultaneously measure the formation type, porosity, and C / O ratio in order to accurately estimate the formation oil saturation while drilling. The present disclosure provides a novel logging tool that combines a neutron source and a dual-function detector, which can more quickly evaluate the formation oil saturation and other formation parameters in real time by simultaneously measuring the formation mineralogy, porosity, and C / O ratio and determining the formation type and oil saturation. Summary of the invention

[0013] This section is intended to introduce some concepts in a simplified form, which will be further elaborated in the detailed description below. This section is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter.

[0014] According to one embodiment of the present disclosure, a method for measuring one or more parameters of an underground formation includes the following steps: deploying a nuclear logging tool into an underground 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 underground formation; receiving neutrons and gamma rays from the underground formation at three or more detectors and converting them into electrical signals; separating the electrical signals of neutrons and gamma rays for each of the three or more detectors; acquiring an energy spectrum of inelastic gamma rays and an energy spectrum of captured gamma rays; acquiring a single-element gamma-ray energy spectrum for each of a plurality of elements; calculating the element yield of each element to obtain the concentration of each element in the formation; and determining the formation type based on the element concentration in the formation.

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

[0016] The present disclosure further provides a method for obtaining the formation porosity of an underground formation, which includes the steps of: after obtaining the formation type, obtaining the count rate of total neutrons, fast neutrons and thermal neutrons of each detector among three or more detectors; calculating the neutron count rate ratio of every two detectors among the three or more detectors to obtain multiple neutron count rate ratios; and obtaining the formation porosity based on the multiple neutron count rate ratios and the formation type.

[0017] In some other embodiments, after determining the formation type and formation porosity, the oil saturation of the underground formation can be obtained using a method comprising the following steps: calculating the oil saturation using the formation type, formation porosity and a parameter selected from the C / O ratio of each of three or more detectors, the total neutron count rate of each of three or more detectors, and the thermal neutron count rate of each of three or more detectors.

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

[0019] The teachings of the present invention 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 The oil saturation of the formation in a specific wellbore environment for near and far detectors is shown to depend on the formation type, porosity and C / O ratio.

[0021] Figure 2A , Figure 2B , Figure 2C and Figure 2D Four exemplary configurations of a nuclear logging tool are shown, the tool having one neutron source S1 and three detectors D1 , D2 and D3 arranged in the longitudinal direction of the tool housing.

[0022] Figure 3A , Figure 3B and Figure 3C A cross-sectional view of an exemplary nuclear logging tool is shown having S1 , D1 , D2 , and D3 .

[0023] Figure 4A and Figure 4B 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, are shown.

[0024] Figure 5A An embodiment of a nuclear logging tool having 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] Fig. 6A Another exemplary embodiment of a nuclear logging tool having 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 may be implemented.

[0027] Figure 8 Schematic diagram showing the neutron pulse, the neutron count rate, and the inelastic and capture spectra of neutron-induced gamma rays.

[0028] Fig. 9 is a flow chart showing an exemplary method of obtaining oil saturation in a formation according to the present disclosure.

[0029] Fig.10 is another flow chart showing an exemplary method of obtaining oil saturation in a formation according to the present disclosure.

[0030] Throughout the drawings and detailed description, unless otherwise specified, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative sizes and descriptions of these elements may be exaggerated for clarity, display, and convenience. DETAILED DESCRIPTION

[0031] The following detailed description is provided to help readers gain a comprehensive understanding of the methods, devices and / or systems described herein. For details, reference may be made to the embodiments of the present disclosure, examples of which are shown in the accompanying drawings. Similar or similar reference numerals may be used in the drawings to indicate similar or similar elements.

[0032] The features described herein may be embodied in different forms and should not be construed as limitations on the embodiments described herein. Rather, the embodiments as described herein and depicted in the accompanying drawings have been provided so that this disclosure will be thorough and complete and will convey the full scope of the disclosure to those of ordinary skill in the art so that they can easily recognize from the following description that there are alternative embodiments without departing from the general principles of the disclosure.

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

[0034] In this disclosure, unless otherwise specified, a detector refers to a dual-function detector that can detect both neutrons and gamma rays. Such a detector uses a scintillation crystal, such as Cs2LiYCl6 (CLYC) or Cs2LiLaBr6 (CLLB), and associated 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 a 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 The correlation between formation oil saturation and formation type, porosity and C / O ratio of near and far detectors under specific wellbore environment is shown. Figure 1A and 1B are exemplary graphs showing the correlation between C / O ratio and formation porosity in limestone for near and far detectors, respectively. Figure 1C and 1D are exemplary graphs showing the correlation between C / O ratio and formation porosity for near and far detectors in sandstone, respectively. At the same porosity and oil saturation, limestone has a higher C / O ratio than sandstone, which may be because limestone (CaCO3) contains carbon while sandstone (SiO2) does not. In addition, the C / O ratios of the near and far detectors are 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 impact 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 calibration model to obtain the C / O ratio of the near detector and the far detector at different formation porosities for a specific formation type under specific wellbore conditions. A calibration model is a model that has been calibrated (i.e., adjusted) based on experimental data. The calibrated model eliminates systematic errors to a large extent, but statistical errors still exist. An algorithm can then be developed to correlate the C / O ratio and formation porosity to oil saturation for typical formation types and specific wellbore environments (such as wellbore dimensions, wellbore fluids, 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 oil saturation.

[0037] Figures 2A-2D Schematic diagrams (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 a housing of the logging tool suitable for logging while drilling (LWD) operations. A mud channel (MC) is disposed along the axis of the logging tool, and the detectors are disposed eccentrically along the longitudinal direction of the logging tool. Figure 2A 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 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 supplies, electronic instruments, and telemetry are not shown. Figure 2B-2D Shown in.

[0038] As shown in the figure, D1 is a near detector having the shortest longitudinal distance from the neutron source, D3 is a far detector having the longest longitudinal distance from the neutron source, and D2 is an intermediate detector having a longitudinal distance in the middle.

[0039] like Figure 2A As shown, all three detectors are located on one side of the neutron source along the logging tool 200. This side can be the proximal side or the distal side of the neutron source. The proximal side is the side closer to the ground when the nuclear logging tool 200 is deployed downhole, and the distal side is the side away from the ground. A high voltage power supply powers the detectors D1, D2, D3 and the pulsed neutron source S1. The signals from each detector are processed by electronic instruments, and the measurements / data are collected and transmitted through telemetry. As shown in FIG. Figure 2B , 2C As shown in FIG2D , both the distal side and the proximal side of the neutron source have at least one detector disposed thereon.

[0040] In wireline logging, the tool can be mounted 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 cable.

[0041] exist Figures 2A-2D The neutron source S1 in each logging tool depicted in 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 may 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 pulse may be about 10 kHz (period of 100 μs), and the duration of the neutron pulse may be about 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 start-up 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 about 14.1 MeV.

[0042] Depending on the target formation parameters and the measurement method, isotope neutron sources (such as Am-Be, Pu-Be, Cf-252) can also be used instead of pulsed neutron sources. The neutrons from these isotope neutron sources have different energy spectra. For example, the neutron energy emitted by the Am-Be source ranges from 0 MeV to about 10 MeV, with an average energy of about 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 some 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 In the example, S1, D1, D2, and D3 are set at the same azimuth angle. 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 may have more than three detectors. For example, Figure 4A Depicted Figure 2A A variation of the logging tool in has four detectors, namely D1, D2, D31 and D32. D31 and D32 are at approximately the same distance from S1, but are set at two different azimuths. Similarly, Figure 4B Depicted in Figure 2A Another variation of the tool in, which has six detectors, namely D1, D21, D22, D31, D32 and D33. In this embodiment, D21 and D22 are arranged relative to each other on the cross section of the logging tool, that is, the azimuths of D21 and D22 are 0° and 180° respectively. D31, D32, D33 are arranged at intervals of 120° on the cross section of the logging tool, that is, the difference in azimuth between any two of D31, D32 and D33 is 120°. Having different azimuths allows the detectors to preferentially receive neutrons and gamma rays at specific incident angles from the formation. This embodiment also improves the detection efficiency of neutrons and gamma rays by increasing the total count rate of all detectors.

[0046] In addition, Figure 4A In , the distances between D31 and D32 and S1 are basically the same. Figure 4B In the embodiment, the intermediate detectors D21 and D22 are at substantially the same distance from S1, while the distances of the remote detectors D31, D32 and D33 from S1 are substantially the same. "Substantially the same distance" means that the distances from S1 to the center of the scintillator of the detectors (e.g., D31 and D32) are substantially the same. For example, the difference is less than one-half or one-quarter inch. With this arrangement, the intermediate detectors as a whole or the remote detectors as a whole have a higher count rate than when only one intermediate detector or only one remote detector is used. Therefore, the neutron generator S1 can be a less powerful source that is not subject to the strict regulations of more powerful neutron sources. In addition, the count rates of individual detectors can be recorded and processed separately. The differences in distance and azimuth between the various detectors can be used to obtain stratigraphic information in a specific azimuth direction.

[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 each detector in the logging tool so that the detector receives neutrons and gamma rays from the formation instead of neutrons and gamma rays that pass through the logging tool itself. Alternatively, the detectors can also be partially shielded by shielding materials that can absorb neutrons and gamma rays from certain directions.

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

[0049] The shielding member can be a separate metal member 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 a shielding material, while the portion facing the formation can be made of a material transparent to neutrons and gamma rays to form a window 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 that pass through the window are received by the detector. Therefore, by adjusting the size and direction of the window in the detector housing, the detector can be made more sensitive to certain incident angles. During operation, the data collected by the various detectors may produce formation properties in a specific direction, which can be used to guide the drilling direction.

[0050] A nuclear logging tool may have more than one neutron source. Figure 5A Another embodiment of a logging tool having two neutron sources S1 and S2 is shown, wherein one neutron source is located at a proximal end and the other is located at a distal end, and two detectors D1 and D2 are arranged between S1 and S2. Alternatively, as required by engineering considerations, S1 and S2 may be arranged in series and located near one end of the logging tool, and D1 and D2 may also be arranged in series and located 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 may be alternately turned on or off, thereby alternately inducing neutrons and gamma rays from the formation, which are received by D1 and D2. Since there are four different source-to-detector distances (d1 to d4), the data generated by D1 and D2 will better compensate for near-hole effects such as wellbore size, tool spacing, mud weight and / or salinity, casing size, cement thickness, etc. than tools with only two or three source-to-detector distances. Therefore, the formation parameters obtained will be more accurate.

[0051] Figure 5B and Figure 5C Two exemplary embodiments of logging tools are shown, wherein two sources and two detectors are arranged at the same tool face angle or at different tool face angles. Figure 5BAs shown in , when the source and detector have the same tool face angle, the measurements will cover the same sector in the formation at any given time. Figure 5C As shown, 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] Fig. 6A and 6B A logging tool having four detectors D11, D12, D21, D22 and two neutron sources S1 and S2 is shown. It should be noted that a pair of detectors D11 and D12 (and D21 and D22) are arranged at substantially the same distance from S1 or S2. As previously mentioned, arranging more than one detector at a certain distance can increase the count rate at that distance, thereby allowing the use of a less powerful neutron source. The count rate of one 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, thereby enabling reliable measurement results to be obtained by processing the overall data from multiple remote detectors.

[0053] In some embodiments, S1 and S2 can be turned on or off at the same time, which can increase the count rate of D1 and D2, thereby reducing the uncertainty of statistical measurement.

[0054] In another embodiment, both S1 and S2 are isotope neutron sources. Compared with pulsed neutron sources, isotope neutron sources do not require power, so the logging tool can be more compact. In addition, isotope neutron sources have a longer life and are more reliable. For example, the half-life of an isotope Am-Be neutron source is 432 years, which is much longer than the average tube life of a neutron generator of 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 to the surrounding formation alone. In this case, the DT neutron generator can be used to obtain neutron porosity logs as well as other measurements (density, oil and gas saturation, etc.)

[0056] Porosity logs obtained using Am-Be neutron sources and DT neutron sources are slightly different. By comparing the porosity logs obtained using two different neutron sources for the same well, a correlation between the two logs can be obtained. Since historical porosity logs are primarily obtained using isotopic neutron sources, this correlation may help update historical porosity logs to make them comparable to new logs obtained using pulsed neutron sources. Similarly, new pulsed neutron porosity logs can also be converted to match historical porosity logs so that reservoir models built using historical logs can continue to be used in production predictions.

[0057] The logging tool 200 may be part of a wireline logging tool or included in downhole equipment as a LWD logging tool in drilling operations. Figure 7 1 is a schematic diagram of an oil drilling system 10 for use in directional drilling of a wellbore 16. The oil drilling system 10 can be used for drilling on land as well as underwater. A rotary drilling rig including a derrick 12, a drill floor 14, a winch 18, a traveling block 20, a hook 22, a swivel 24, a kelly joint 26, and a rotary table 28 is used to drill a wellbore 16 in a formation. A drill string 100 includes a plurality of drill rods connected in series and fixed to the bottom of the kelly joint 26 at the ground. 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 controllable axial compressive load. A downhole drilling tool assembly 150 is disposed at the distal end of the drill string 100.

[0058] Drilling fluid (also referred to as mud) is typically stored in a mud pit or mud tank 46 and is delivered using a mud pump 38, which forces the drilling fluid through a surge suppressor 40, then through a kelly hose 42, and through a swivel 24, into the top of the drill string 100. The drilling fluid flows through the drill string 100 at a rate of about 150 gallons per minute to about 600 gallons per minute and into the downhole drilling 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 delivered back to the mud tank 46 through a mud return line 44.

[0059] The pressure required to keep the drilling fluid circulating is measured by a pressure sensitive sensor 48 on the kelly hose 42. The pressure sensitive sensor detects the pressure change caused by the pressure pulses generated by the pulse generator. The amplitude of the pressure wave from the pulse generator can reach 500 psi or more. The measured pressure is transmitted as an electrical signal through the sensor cable 50 to the surface computer 52, which decodes and displays the transmitted information. Alternatively, the measured pressure is transmitted as an electrical signal through the sensor cable 50 to a decoder, which decodes the electrical signal and transmits the decoded signal to the 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 downhole drilling assembly (BHA) 150, which includes a non-magnetic drill collar in which an MWD system (MWD equipment or MWD tool) 160 is installed, a logging while drilling (LWD) tool sub 165 containing an LWD tool, a downhole motor 170, a near-bit measurement sub 175, and a drill bit 180 having a drilling nozzle (not shown). The drilling fluid flows through the drill string 100 and flows out through the drilling nozzle of the drill bit 180. During the drilling operation, the drilling system 10 can be operated in a rotating mode, in which the drill string 100 is rotated relative to the ground by a motor (i.e., a 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 does not rotate relative to the ground, but the drill bit 180 is driven to rotate by the downhole motor 170. The drilling fluid is pumped from the surface through the drill string 100 to the drill bit 180 and is injected into the annular space between the drill string 100 and the wall of the wellbore 16. The drilling fluid carries cuttings from the wellbore 16 to the surface.

[0061] In one or more embodiments, the MWD system 160 may include a pulser sub, a pulser drive sub, a battery sub, a central storage unit, a main board, a power sub, a directional module sub, 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 pulser sub and the pulser drive sub may communicate with the pulser 300, which may be located below the MWD system 160. The MWD system 160 may transmit data to the pulser 300 so that the pulser 300 generates pressure pulses.

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

[0063] The near-bit measurement sub 175 can be disposed between the downhole motor 170 and the drill bit 180. A 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. The data can be transmitted to the MWD system 160 in the downhole drilling tool assembly 150 via a cable embedded in the downhole motor 170.

[0064] In one embodiment of the present disclosure, a logging tool having a DT neutron generator and three dual-function detectors is used to obtain various formation parameters. Figure 8 The schematic diagram of neutron pulse, neutron count rate, and inelastic energy spectrum and capture energy spectrum of neutron-induced gamma rays is shown. The frequency of neutron pulse is 10kHz (period is 100μs), and the neutron working time is 20μs. Figure 8 As shown in sub-figure (b) in .

[0065] like Figure 8 The neutron count rate measured by each of the three detectors shown in sub-figure (a) in is used to obtain the 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 anti-coincidence signal of the neutrons of the three detectors so that the neutrons are mainly recorded as fast neutrons during the neutron pulse (when the neutron pulse is on). Between the neutron pulses (when the neutron pulse is off), the neutrons are recorded as thermal neutrons. The fast neutrons and thermal neutrons recorded by the three detectors can be used to obtain the fast neutron spatial distribution and the thermal neutron spatial distribution. The neutrons from each detector can also be recorded together. In this case, all neutrons (from thermal neutrons to fast neutrons) are used to obtain the 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 anti-coincidence signal for the gamma rays from the three detectors, so that during the neutron pulse (neutron pulse on), the gamma rays are mainly recorded as the inelastic energy spectrum caused by inelastic scattered fast neutrons, such as Figure 8 In between neutron pulses (neutron pulse is off), gamma rays are recorded as the capture spectrum induced by thermal neutrons, as shown in sub-figure (c) of Figure 1. Figure 8The appropriate time window is selected so that the gamma rays measured in the capture time window are all from thermal neutron capture reactions, and most of the gamma rays measured in the inelastic time window are from fast neutron inelastic scattering.

[0067] The background noise in each detector can be measured when the neutron generator is turned off for a period of time and can be subtracted from the total signal of neutrons or gamma rays. The neutron background measured during the neutron pulse can be further subtracted by using a small portion of the neutrons measured between neutron pulses as the background to obtain a "pure" fast neutron. Similarly, the captured gamma rays measured during the neutron pulse can be further subtracted by using a small portion of the capture spectrum measured between neutron pulses as the background to obtain a "pure" inelastic spectrum.

[0068] The gamma rays detected by each detector can also be recorded in an energy spectrum (such as a total energy spectrum), regardless of whether they originate from neutron inelastic 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, because for measurement systems based on DT pulsed neutron generators, the inelastic gamma ray spectrum is required to obtain formation density.

[0069] Fig. 9 1 is an exemplary workflow showing the steps of processing data from a logging tool 200 having a DT neutron generator and three detectors (near detector, intermediate detector, and far detector) for obtaining real-time oil saturation in a formation. In step 1001, the DT neutron generator emits neutron pulses into the formation surrounding the measurement tool. In step 1002, fast neutrons are decelerated to become thermal neutrons. Inelastic gamma rays and capture gamma rays are generated.

[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 a 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), which are then used in step 1010 by calculating the total neutron ratio (Rn f / m , Rn f / n , Rn m / n ) or the ratio of thermal neutrons (Rtn f / m 、Rtn f / n 、Rtn m / n ) to obtain the neutron porosity.

[0071] On the other hand, in step 1005, the total energy spectrum of the inelastic gamma rays and the total energy spectrum of the captured 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 single elements (such as C, O, Fe, etc.). Therefore, the C / O ratio can be obtained in step 1007.

[0072] In addition, in step 1011, the formation element concentration is calculated using the fast neutron count rates and thermal neutron count rates of the three detectors in step 1010 and the inelastic energy spectrum and capture energy spectrum obtained in step 1006. Once the formation element concentration is known, the formation type can be determined in step 1008. With the formation type in step 1008 and the count rate in step 1010, the formation porosity can be obtained in step 1012.

[0073] Finally, in step 1013, information about the formation type, C / O ratio, formation porosity, or thermal neutron count rate ratio may be used to determine the formation oil saturation. Fig.10 and explained below. It should be noted that most of the neutrons detected by the detector are thermal neutrons, but some surface neutrons are also detected.

[0074] Fig.10 Another embodiment of the present disclosure is described using mathematical symbols and formulas, which uses an exemplary well logging tool 200 (e.g., Figures 2A to 2D The neutron count rate CRN measured during and between neutron pulses from the near detector, the middle detector, and the far detector, i.e., CRN n ,CRN m ,CRN f , the count rate ratio can be used to obtain the formation porosity (Φ).

[0075] Middle-to-far ratio (Rn m / f ), near-far ratio (Rn n / f ) and near-medial ratio (Rn n / m ) can be obtained by formulas 1, 2 and 3 respectively. Since the three detectors are set at different distances from the neutron source, their detection depths are also different. Therefore, the near-wellbore environment (such as wellbore fluid, cement, etc.) has different effects on the three ratios.m / f More sensitive to the formation, Rn n / m More sensitive to changes near the wellbore, while Rn n / f Sensitive to both.

[0076]

[0077] Formation porosity Φ n It can be obtained by the following method: First use Rn n / m and / or Rn n / f To correct Rn m / f , and then use the corrected far-to-center ratio Rnc m / f To obtain the formation porosity of a specific formation (such as sandstone, limestone or dolomite), Equations 4 to 6 illustrate this algorithm, where ΔR is the correction value.

[0078] R 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, the formation porosity Φ n It is also possible to use three ratios of the capture gamma-ray count rates obtained by the three detectors, according to an algorithm similar to that described in equations 1 to 6.

[0082] Formation porosity Φ n It can also be obtained by combining the two porosities obtained from neutrons and captured gamma rays. In some other methods, the formation porosity Φ n It can be obtained directly from the three ratios of neutrons and the three ratios of captured gamma rays by other means.

[0083] By measuring the energy spectrum of gamma rays from neutron inelastic scattering and neutron capture reactions with the same tool, the formation type can be obtained. The timing of the neutron pulses emitted by the DT neutron generator is as follows: Figure 8 As shown. Then, after being separated from the neutron signal, the neutron-induced gamma-ray signals of the three detectors are further separated into gamma-ray signals produced by thermal neutron capture reactions and gamma-ray signals produced by inelastic scattering of fast neutrons. Both signals can be used to perform elemental analysis of the formation, such as hydrogen (H), silicon (Si), calcium (Ca), iron (Fe), sulfur (S) and chlorine (Cl).

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

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

[0086] The inelastic energy spectra of the three detectors are fitted to the standard gamma-ray energy spectra of each element (including C and O) in the formation, so that the gamma-ray signal from O and the gamma-ray signal from C can be separated from the inelastic energy spectrum. Therefore, the ratio of the inelastic gamma-ray count rates from C and O (C / O ratio) can be used to obtain the ratio of the inelastic gamma-ray count rates from C and O. The C / O ratio can also be obtained by the ratio of the inelastic gamma-ray count rates under the C or O peak in the selected energy window. On the other hand, the formation element concentration can be used to determine the formation type. The formation type can also be used to obtain the formation porosity. After knowing the C / O ratio, formation type and porosity, the oil saturation can be estimated for a specific wellbore environment (such as wellbore size, wellbore fluid, etc.).

[0087] The exemplary algorithm calculates the apparent oil saturation (So) of the near, middle, and far detectors for a known wellbore environment (wellbore size, wellbore fluid, casing, etc.) using equations 16 to 18, respectively. n So m So f ). After obtaining the three apparent oil saturations, the corrected oil saturation So can be calculated, for example, by weighted averaging the apparent oil saturations or So with different coefficients n So m So f The multinormal function of n So m So f The calculation of and So can be expressed by formula 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] R n,c / o , Rg m,c / o and Rg f,c / o The C / O ratios are based on the inelastic gamma-ray count rates of carbon and oxygen detected at the near, middle, and far detectors, respectively. The corrected oil saturation can also be calculated using the C / O ratios from the three detectors (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 ) to obtain, as shown in Formula 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 calculated by using the C / O ratio (Rg n,c / o , Rg m,c / o and Rg f,c / o ), formation type, and the thermal neutron ratio (Rtn) from two of the three detectors (i.e., the near and far detectors) f / m 、Rtn f / n 、Rtn m / n ) is obtained by one of the following equations, as shown in Formula 21:

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

[0096] In another embodiment, the corrected oil saturation can also be calculated by using the C / O ratio (Rg n,c / o , Rg m,c / oand Rg f,c / o ), formation type, thermal neutron ratio (Rtn) from two of the three detectors (i.e., near and far detectors) f / m 、Rtn f / n 、Rtn m / n ) and the fast neutron ratio (Rfn f / m 、Rfn f / n 、Rfn m / n ) to obtain, as shown in Formula 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 )(twenty two)

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

[0099] Alternatively, the total energy spectrum of inelastic gamma rays and captured gamma rays can be used instead of the inelastic energy spectrum to obtain the C / O ratio. In this case, the standard total gamma ray energy spectrum of each element in the formation (including inelastic gamma rays and captured gamma rays) is used to fit the total energy spectrum to separate the gamma rays of C and O. This fitting is more complicated than fitting only the inelastic energy spectrum, and the results may be less accurate. Similarly, the C / O ratio can be obtained by using the ratio of the total gamma ray count rate under the C or O peak in a selected energy window. However, this method allows the DT neutron generator to be operated in continuous mode, or the use of isotope neutron sources, such as Am-Be, Pu-Be, Cf-252, etc. as neutron sources.

[0100] Although the present disclosure has been described in conjunction with certain preferred embodiments in the above description, and many details are listed for the purpose of illustration, it is obvious to those skilled in the art that the present disclosure can be changed, and certain other details described herein can also be greatly changed without departing from the basic principles of the present disclosure. In addition, it should be understood that the structural features or methods shown or described in any embodiment herein can also be used in other embodiments.

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 and converting them into electrical signals; S4: separating electrical signals from neutrons and gamma rays for each of the three or more detectors; S5: Obtain the energy spectrum of inelastic gamma rays and the energy spectrum of 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; and S8: Determine the type of formation according to the element concentration in the formation.

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: The fast and thermal neutron count rates of the three detectors and the obtained inelastic and capture spectra are used to calculate the element concentrations.

4. A method for obtaining the formation porosity of an underground formation, comprising the steps of: Determining the type of formation using the method according to claim 1; S9: Obtaining the count rate of total neutrons, fast neutrons, and thermal neutrons of each of the three or more detectors; S10: calculating the ratio of the neutron count rates of every two detectors among the three or more detectors to obtain a plurality of neutron count rate ratio values; and S11: Obtaining formation porosity according to the multiple neutron count rate ratios and the formation type.

5. The method according to claim 4, 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, and 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.

6. A method for measuring the oil saturation of an underground formation, comprising the steps of: Obtaining formation type and formation porosity according to the method of claim 4; as well as The oil saturation is calculated using the formation type, formation porosity and a parameter, wherein the parameter is 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.

7. The method according to claim 6, 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 The plurality of apparent oil saturation values ​​are used to calculate a corrected oil saturation.

8. The method according to claim 7, 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.

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

10. The method according to claim 6, 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.

11. The method according to claim 10, wherein: The nuclear logging tool has a near detector, an intermediate detector and a far detector, and uses 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 to calculate the oil saturation value.

12. The method according to claim 6, 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.

13. The method according to claim 12, 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.

14. The method according to claim 6, 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.

15. The method according to claim 14, 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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