Method and device for obtaining real-time stratum gas saturation

Abstract

A method for obtaining gas saturation values of a subterranean formation involves a tool comprising a plurality of dual function detectors for detecting neutrons and gamma rays. The method comprises the following steps: emitting neutrons to a stratum; detecting neutrons and gamma ray signals from the formation using a detector; determining formation parameters including formation types and formation porosity; parameters such as a ratio of thermal neutron count rates from at least two of the three detectors, a ratio of captured gamma ray count rates from at least two of the three detectors are also determined and a real-time gas saturation value is calculated using the determined parameters.

Description

Technical Field

[0001] The present disclosure provides methods and systems for nuclear logging and formation evaluation, particularly methods and systems for nuclear logging and data analysis using a nuclear logging tool with multiple dual-function detectors to obtain real-time formation gas saturation. Background Art

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

[0003] A neutron porosity logging tool studies formation porosity by measuring the ratio of the neutron count rates of a near detector and a far detector after fast neutrons from an isotopic neutron source (e.g., an Am-Be source) are slowed down and scattered back to the detector by the surrounding environment of the tool (e.g., wellbore fluid and formation). Then, based on a specific formation mineralogy (e.g., sandstone, limestone, or dolomite), this ratio is converted to porosity. Using the ratio of the count rates from two detectors can reduce the influence of changes in the near-wellbore environment (wellbore fluid, wellbore size, etc.) on porosity measurement.

[0004] A pulsed neutron tool uses a pulsed neutron source (e.g., a D-T neutron generator or a D-D neutron generator) and one, two, or three detectors that detect neutrons or neutron-induced gamma rays. The energy spectrum of the neutron-induced gamma rays from each element is unique. Therefore, by measuring the energy spectrum of the 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., the element yield, can be obtained. The inelastic energy spectrum is the basis for detecting elements such as Mg, Fe, S, C, Al, Si, Ca, and O. The capture energy spectrum provides information on other elements (e.g., Mg, S, Ti, Al, K, Ca, Si, Gd, Fe, Cl, and H elements).

[0005] Since element yield logging only provides the relative concentration of elements, they are usually 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. Element yield logging, as well as the reaction cross-sections of the neutron inelastic scattering and neutron capture reactions of these elements, can also be used to obtain the element concentration in the formation.

[0006] In addition, by measuring the thermal neutron time decay curve or the 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 the oil saturation when the formation salinity is high.

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

[0008] The value of the gas saturation in the formation can also be obtained. To accurately estimate the gas saturation, one needs to know the formation type (e.g., limestone, sandstone, or dolomite), the formation porosity, and the wellbore environment (e.g., wellbore size, wellbore fluid). The gas saturation value can be the ratio Rtn of the thermal neutron count rate (n) from the near detector to the thermal neutron count rate (f) from the far detector n / f and a function of porosity. The gas saturation value can also be the ratio Rg of the capture gamma ray count rate (n) from the near detector to the capture gamma ray count rate (f) from the far detector n / f and a function of porosity. Figure 1A and Figure 1B are exemplary charts that respectively show the correlation between the gas saturation in a limestone formation and Rtn n / f 、Rg n / f and the formation porosity. Figure 1C and Figure 1D are exemplary charts that respectively show how the gas saturation in a sandstone formation depends on Rtn n / f 、Rg n / f and the formation porosity.

[0009] One method for obtaining the gas saturation is to use a calibration model to perform a series of simulations and obtain Rtn at different formation porosities with known gas saturation for a specific formation type under specific wellbore conditions (e.g., wellbore size, casing size, wellbore fluid, etc.). n / f and / or Rg n / f . Then, an algorithm can be developed to relate Rtn n / f or Rg n / f and the formation porosity to the gas saturation. In field applications, after obtaining Rtn n / f or Rg n / f 、identifying the formation type and determining the formation porosity, this algorithm can be used to obtain the gas saturation.

[0010] In most of these applications, neutrons and gamma rays are detected by their respective detectors / sensors. For example, a He-3 gas detector is used to detect thermal neutrons. The He-3 isotope has a high thermal neutron absorption cross-section. After fast neutrons emitted from a neutron source are slowed down by the formation and scattered back to the detector, the neutrons are absorbed and produce other detectable ions, such as protons (p) and tritium (T), thereby ionizing the gas. Ions and electrons multiply and drift in an electric field to form an electrical signal. Various scintillation detectors, such as NaI, CsI, BGO, GSO, LaBr3, YAP scintillators, and photomultiplier tubes (PMT), can be used to detect gamma rays. These scintillators convert the accumulated energy of gamma rays into scintillation light. The PMT converts the scintillation light into electrons and amplifies them to form an electrical signal.

[0011] Existing nuclear logging tools typically use single-function detectors to detect neutrons or gamma rays. For example, to obtain accurate gas saturation information, one needs to know the formation mineral composition (e.g., sandstone (SiO2), limestone (CaCO3), dolomite (CaMg(CO3)2)), formation porosity, and Rtn n / f or Rg n / f . Traditionally, since porosity, mineral composition, and Rtn n / f or Rg n / f need to be obtained by at least one neutron porosity tool and one pulsed neutron tool respectively, the gas saturation can only be estimated after all logging data are acquired and processed. Thus, real-time gas saturation cannot be obtained.

[0012] There is a current need to reduce the number of logging tools and the number of logging operations while still being able to obtain various formation parameters required for drilling operations. For example, it is necessary to obtain the formation type, porosity, Rtn n / f and / or Rg n / f simultaneously in order to accurately estimate the formation gas saturation during drilling. In addition, it is necessary to obtain the ratios of thermal neutrons and capture gamma rays (Rtn n / f and Rg n / f ) from near and far detectors and use them to obtain more accurate gas saturation. Additionally, it is also necessary to obtain the ratios of thermal neutrons and capture gamma rays (Rtn n / f , Rtn n / m , Rtn m / f and Rg n / f , Rg n / m , Rg m / f), and use them to automatically correct the influence of the near-wellbore environment, so that the measurement of gas saturation is independent of the influence of these environments. The present disclosure provides a novel logging tool that combines a neutron source and a dual-functional detector, which can achieve a faster and real-time evaluation of the gas saturation in a formation by measuring the formation mineral composition, the ratio of the thermal neutron count rates from at least two of the three detectors to the ratio of the capture gamma-ray count rates, and determining the formation type, porosity, and real-time gas saturation independent of the measurement environment. Summary of the Invention

[0013] The present invention provides an introduction to a series of concepts in a simplified form, which are further described in the following detailed description. It is not intended to identify the key features or essential features of the subject matter of the claims, nor is it intended to be used as an aid in determining the scope of the subject matter of the identified claims.

[0014] According to an embodiment of the present disclosure, a method for measuring one or more parameters of a subterranean formation includes the following steps: deploying a nuclear logging tool into the subterranean formation, the nuclear logging tool including one or more neutron sources and three or more dual-functional detectors configured to simultaneously detect neutrons and gamma rays; causing the one or more neutron sources to emit neutrons into the subterranean formation; receiving neutrons and gamma rays from the subterranean formation at the three or more detectors to form electrical signals; separating the electrical signals of neutrons and gamma rays from each of the three or more detectors; obtaining the detector count rates of total neutrons, fast neutrons, and thermal neutrons; obtaining the ratio of thermal neutrons and determining the formation porosity; obtaining the detector count rate of capture gamma rays, the energy spectrum from inelastic gamma rays, and the energy spectrum from capture gamma rays; obtaining the single-element gamma-ray energy spectrum of each of the plurality of elements; calculating the elemental yield of each element to obtain the concentration of each element in the formation; and determining the formation type based on the elemental concentration in the formation; using the formation type, porosity, the ratio of thermal neutrons, and the ratio of capture gamma rays to determine the gas saturation of the formation.

[0015] In some embodiments, the plurality of elements are selected from Mg, Fe, S, C, Al, Si, Ca, O, Ti, K, Gd, Cl, and H. Additionally, the fast neutron count rates and thermal neutron count rates, inelastic spectra, and capture spectra obtained at the three detectors are used to calculate the elemental concentrations.

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

[0017] In another embodiment, after determining the formation type and formation porosity, a method including the following steps can be used to obtain the gas saturation of a subterranean formation: Use the formation type, formation porosity, and a parameter selected from the Rtn n / f and / or Rg n / f ratios to calculate the gas saturation.

[0018] In certain embodiments, the calculating step further includes: using the formation type, formation porosity, and Rtn selected from any two of three detectors n / f 、Rtn n / m 、Rtn m / f and / or Rg n / f 、Rg n / m 、Rg m / f ratios to calculate multiple apparent gas saturation values; and using the multiple apparent gas saturation values to calculate a corrected gas saturation. 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 show the formation gas saturation in a particular wellbore environment depending on the formation type, porosity, and Rtn n / f or Rg n / f .

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

[0022] Figure 3A 、 Figure 3B and Figure 3C show a cross-sectional view of an exemplary nuclear logging tool 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, and D32) and six detectors (D1, D21, D22, D31, D32, and D33), respectively.

[0024] Figure 5Ais an embodiment of a nuclear logging tool having two neutron sources (S1, S2) and two detectors (D1, D2); Figure 5B and Figure 5C provides a cross-sectional view of an exemplary configuration of this embodiment.

[0025] Figure 6A is another exemplary embodiment of a nuclear logging tool having two neutron sources (S1, S2) and four detectors (D11, D12, D21, D22); Figure 6B provides a cross-sectional view of this embodiment.

[0026] Figure 7 is a block diagram of an exemplary drilling system implementing embodiments of the present disclosure.

[0027] Figure 8 is a schematic diagram showing neutron pulses, neutron count rates, and inelastic and capture energy spectra of neutron-induced gamma rays.

[0028] Figure 9 is a flowchart showing an exemplary method for obtaining gas saturation in a formation according to the present disclosure.

[0029] Figure 10 is another flowchart showing an exemplary method for obtaining gas saturation in a formation according to the present disclosure.

[0030] Throughout the drawings and the detailed description, unless otherwise specified, the same reference numerals will be understood to refer to the same elements, features, and structures. For clarity, illustration, and convenience, the relative sizes and descriptions of these elements may be exaggerated. Detailed Description

[0031] The following detailed embodiments provided can help the reader obtain a comprehensive understanding of the methods, apparatuses, and / or systems described herein. References to specific embodiments of the present disclosure are illustrated in the drawings. Similar or like reference numerals may be used in the drawings and may indicate similar or like elements.

[0032] The features described herein may be embodied in different forms and should not be construed as limiting the embodiments described herein. On the contrary, the embodiments described herein and depicted in the drawings have been provided so that the present disclosure will be thorough and complete and will convey the full scope of the present disclosure to those of ordinary skill in the art, enabling them to readily recognize from the following description that alternative embodiments exist without departing from the general principles of the present disclosure.

[0033] Therefore, the scope of the invention is defined not by the specific embodiments 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 neutrons and gamma rays. Such a detector uses a scintillation crystal (e.g., Cs2LiYCl6 (CLYC) or Cs2LiLaBr6 (CLLB)), and associated electronics (e.g., 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 Shows how formation gas saturation is related to formation type, porosity, and Rtn from near and far detectors for a specific wellbore environment. n / f or Rg n / f Ratios are related. Figure 1A and Figure 1B is a graph showing the ratio Rtn of the count rate of captured gamma rays from the near detector to the count rate of captured gamma rays from the far detector in limestone formations with different porosities. n / f and Rg n / f An example graph of the correlation between . Figure 1C and Figure 1D The Rtn in sandstone formations with different porosities is shown in Figure 2. n / f With Rg n / f An example graph of the correlation between Rtn and Rtn for limestone and sandstone at the same porosity and gas saturation. n / f and / or Rg n / f The ratios are different. Therefore, by using near and far detectors to measure Rtn n / f and / or Rg n / f The ratio and application of a correction algorithm for wellbore fluids can reduce the impact of the wellbore environment (e.g., wellbore fluids) on the measured formation gas saturation.

[0036] To obtain gas saturation, a series of simulations can also be performed using the calibration model to obtain Rtn from near and far detectors for different formation porosities of a specific formation type under specific wellbore conditions. n / f and / or Rg n / f。A calibrated model is a model that has been calibrated (i.e., adjusted) based on experimental data. The calibrated model has largely no systematic errors, but still has statistical errors. Then, algorithms can be developed to relate Rtn n / f and / or Rg n / f to the formation porosity and the gas saturation in a typical formation type and a specific wellbore environment (e.g., wellbore size, wellbore fluid, etc.).

[0037] In addition, algorithms can be developed to relate the ratios of thermal neutrons and capture gamma rays (Rtn n / f 、Rtn n / m 、Rtn m / f and Rg n / f 、Rg n / m 、Rg m / f ) from more than two detectors located at different positions, and use them to automatically correct the influence of the near-wellbore environment, so that the measurement of gas saturation is independent of these influences. The algorithm can be embedded in the firmware or software of the tool. In field applications, once the formation type and formation porosity are determined and the ratios are obtained, the algorithm can be used to obtain the gas saturation.

[0038] Figure 2A - 2D are schematic diagrams (not to scale) of four exemplary configurations of a cylindrical nuclear logging tool 200, which has a neutron source (S1) and three dual-function detectors (D1, D2, D3), and these three detectors are arranged along the housing of the logging tool suitable for logging-while-drilling (LWD) operations. A mud channel (MC) is arranged along the axis of the logging tool, and each detector is eccentrically arranged along the longitudinal direction of the logging tool. Figure 2A Also shown are: a high-voltage power supply (HV); electronic instruments (such as a controller) for sending instructions, receiving and processing data from the neutron source and each detector; 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, electronic instruments and telemetry are not shown in Figure 2B - 2D .

[0039] As shown in the figure, D1 is a near detector with the shortest distance from the neutron source in the longitudinal direction, D3 is a far detector with the longest longitudinal distance from the neutron source, and D2 is a middle detector with an intermediate longitudinal distance.

[0040] As Figure 2AAs 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 surface when the nuclear logging tool 200 is deployed downhole, while the distal side is the side away from the surface. The high-voltage power supply powers the detectors (D1, D2, D3) and the pulsed neutron source (S1). Signals from each detector are processed by the electronic instrument and the measurements / data are acquired and transmitted through the telemetry device. As Figure 2B , 2C and as shown in 2D, both the distal side and the proximal side of the neutron source have at least one detector disposed thereon.

[0041] In wireline logging, the tool can be installed in a probe that does not contain a mud channel. The detectors can be installed 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 through the cable.

[0042] In Figure 2A - 2D the neutron source S1 in each logging tool depicted is a pulsed neutron generator. However, an isotope neutron source can also be used. The pulsed neutron source can be a deuterium-tritium (D-T) pulsed neutron generator, which can operate in a pulsed mode with various pulse principles (e.g., frequency, pulse duration). For example, the frequency of the neutron pulse can be approximately 10 kHz (period of 100 μs), and the duration of the neutron pulse can be approximately 20 μs. In another embodiment, the frequency of the neutron pulse can be approximately 1 kHz (period of 1000 μs), and the neutron duration can be 50 μs. Depending on the method and measurement, the D-T neutron generator can also operate in a continuous mode. In this case, the start-up frequency of the neutron generator is high enough such that neutrons are continuously emitted. Neutrons from the D-T neutron generator have an initial energy of approximately 14.1 MeV.

[0043] Depending on the target formation parameters and measurement method, an isotope neutron source (e.g.: Am-Be, Pu-Be, Cf-252) can also be used instead of the pulsed neutron source. Neutrons from these isotope neutron sources have different energy spectra. For example, the energy of neutrons emitted from the Am-Be source ranges from 0 MeV to approximately 10 MeV and has an average energy of approximately 4.2 MeV. However, due to the lower neutron energy, gamma-ray signals generated from fast inelastic scattering from carbon and oxygen are much lower when using an isotope neutron source than those triggered by the D-T neutron generator.

[0044] In Figure 1A - 1D the neutron source S1 and the detectors D1, D2 and D3 only indicate their relative positions along the longitudinal direction of the housing of the tool 200, and do not indicate their positions in the radial direction in the cross-section of the tool housing.

[0045] In some embodiments, S1, D1, D2, and D3 may be arranged in the same radial direction or different radial directions, i.e., having the same or different measurement azimuth angles when deployed in a formation. Figure 3A , 3B and 3C show exemplary cross-sectional views in the Figure 2A - 2D directions A-A, B-B, C-C, and D-D shown.

[0046] Figure 3A In, S1, D1, D2, and D3 are arranged at the same azimuth angle. However, in Figure 3B , S1, D1, and D3 have the same azimuth angle, while D2 is at a different azimuth angle. In Figure 3C , S1 and D1 have the same azimuth angle, while each of D2 and D3 has a different azimuth angle.

[0047] Other embodiments of the logging tool may have more than three detectors. For example, Figure 4A depicts Figure 2A a variant of the logging tool in, which has four detectors, namely D1, D2, D31, and D32. D31 and D32 are approximately the same distance from S1, but are arranged at two different azimuth angles. Similarly, Figure 3B depicts Figure 2A another variant of the tool in, which has six detectors, namely D1, D21, D22, D31, D32, and D33. In this embodiment, D21 and D22 are arranged 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 arranged at intervals of 120° in the cross-section of the logging tool, i.e., the difference in azimuth angles between any two of D31, D32, and D33 is 120°. Having different azimuth angles 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.

[0048] Additionally, in Figure 4A , D31 and D32 are substantially the same distance from S1. In Figure 4BAmong them, the distances between the middle detectors D21 and D22 and S1 are substantially the same, and the distances between the far detectors D31, D32, and D33 and S1 are substantially the same. "Substantially the same distance" means that the distances from S1 to the scintillator centers of the detectors (e.g., D31 and D32) are approximately the same. For example, the difference is less than one-half or one-quarter of an inch. With this layout, the middle detectors as a whole and the far detectors as a whole have a higher counting rate than when only one middle detector or only one far detector is used. Therefore, the neutron generator S1 can be a source with lower power, and the constraints on it can be less stringent than those on a more powerful neutron source. In addition, the counting rates of individual detectors can be recorded and processed separately. The differences in distance and azimuth angle of various detectors can be used to obtain formation information in a specific azimuth angle direction.

[0049] 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 in the logging tool and each detector so that the detectors receive neutrons and gamma rays from the formation rather than those passing through the logging tool body. Alternatively, the detectors can also be partially shielded by a shielding material capable of absorbing neutrons and gamma rays from certain directions.

[0050] The shields are made of or contain one or more materials that can effectively attenuate thermal neutrons and gamma rays. The shielding material can include materials selected from heavy elements with 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 combined with other heavy metals (such as Fe, Pb, or W), or boron-containing materials (such as tungsten boride (WB, WB2, etc.)).

[0051] The shields can be independent metal pieces inserted into the logging tool or an integral part of the detector housing. For example, the part of the detector housing facing inward towards the logging tool can be made of a shielding material, and the part 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. Thus, neutrons and gamma rays from certain incident angles can be absorbed by the shielding material, while those passing through the window are received by the detectors. Therefore, by adjusting the size and direction of the window in the detector housing, the detectors can be more sensitive to certain incident angles. During the operation, the data collected by various detectors may yield formation properties in a specific direction, which can be used to guide the drilling direction.

[0052] The nuclear logging tool may have more than one neutron source. Figure 5AAnother embodiment of a logging tool with two neutron sources (S1, S2) is shown, one neutron source at the proximal end and the other at the distal end, while two detectors (D1 and D2) are arranged between S1 and S2. Alternatively, depending on engineering considerations, S1 and S2 can be arranged in series and arranged near one end of the logging tool, while D1 and D2 are arranged in series near the other end of the logging tool. In these two embodiments, the distance between S1 and D1 is d1, the distance between S1 and D2 is d2, the distance between S2 and D2 is d3, and the distance between S2 and D1 is d4. When both S1 and S2 are pulsed neutron generators, they can be turned on or off alternately, thus alternately inducing neutrons and gamma rays from the formation to be received by D1 and D2. Due to the existence of four different source-to-detector distances (d1 - d4), the data generated in D1 and D2 can better compensate for the near-wellbore 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. Ultimately, the obtained formation parameters can be more accurate.

[0053] Figure 5B and Figure 5C Two exemplary embodiments of the design are shown, where two sources and two detectors are arranged at the same azimuth angle or different azimuth angles. As Figure 5B shown, when the sources and detectors have the same azimuth angle, the measurement covers the same sector in the formation at any given time. As Figure 5C shown, when the sources and detectors have different azimuth angles, the data generated in D1 and D2 reflects different sectors of the formation, so the differences between different formation sectors at any given time can be revealed by comparing the measurements of D1 and D2.

[0054] Figure 6A and 6B A logging tool with four detectors (D11, D12, D21, D22) and two neutron sources (S1 and S2) is shown. It should be noted that the detector pairs D11 and D12 (and D21 and D22) are set at substantially the same distance from S1 or S2. As mentioned above, having more than one detector at a specific distance increases the counting rate at that distance, so a less powerful neutron source can be used. The counting rate of a single far detector may be too low to provide reliable measurement data. By using two or more far detectors, the counting rate can be significantly increased, and thus reliable measurement results can be obtained by processing the data from multiple far detectors as a whole.

[0055] In some embodiments, S1 and S2 can be turned on or off simultaneously. Doing so increases the counting rate of D1 and D2, thus reducing the uncertainty of statistical measurements.

[0056] In another embodiment, S1 and S2 can be neutron generators of the same or different types. For example, both S1 and S2 can be D-T neutron generators or D-D neutron generators, or S1 is a D-T neutron generator while S2 is a D-D neutron generator.

[0057] In yet another embodiment, both S1 and S2 are isotope neutron sources. Compared with pulsed neutron sources, isotope neutron sources do not require a power supply, making the logging tool more compact. In addition, isotope neutron sources have a longer lifespan and are more reliable. For example, the isotope Am-Be neutron source has a half-life of 432 years, which is much longer than the average tube life of neutron generators with a lifespan of 500 hours to 4000 hours.

[0058] In still another embodiment, S1 and S2 can be two different types of neutron sources. For example, S1 can be a D-T neutron generator or a D-D neutron generator, while S2 can be an Am-Be neutron source. In the field, the D-T neutron generator or the D-D neutron generator can be turned off, allowing the Am-Be neutron source to operate on its own to perform neutron porosity logging. Alternatively, the Am-Be neutron source can be removed from the logging tool so that the D-T neutron generator or the D-D neutron generator can emit neutron pulses into the surrounding formation alone. In this case, a D-T neutron generator or a D-D neutron generator can be used to obtain neutron porosity logging and other measurements (density, oil and gas saturation, etc.).

[0059] The porosity logging obtained using the Am-Be source and the D-T source is slightly different. By comparing these porosity logs for the same well obtained using two different neutron sources, the correlation between the two logs can be obtained. Since porosity logging has historically been mainly obtained using isotope neutron sources, this correlation will help update past porosity logs to make them comparable to new logs obtained using pulsed neutron sources. Similarly, new pulsed neutron porosity logs can be converted into past porosity logs to continuously apply the reservoir models built using past logs in production forecasting.

[0060] The logging tool 200 can be part of a wireline logging tool or be included in downhole equipment as an LWD logging tool in a drilling operation. Figure 7FIG. 0 is a schematic view of an oil drilling system 10 applied in the 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 drawworks 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. The drill string 100 includes a plurality of drill pipes 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 drawworks 18 is used to lower the drill string 100 into the wellbore 16 and apply a controllable axial compressive load. The downhole drill assembly 150 is disposed at the distal end of the drill string 100.

[0061] The drilling fluid (also known as mud) is typically stored in a mud pit or mud tank 46 and is conveyed using a mud pump 38, which forces the drilling fluid to flow through a surge suppressor 40, then through a kelly hose 42, and through the swivel 24, thereby entering 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 flows into the downhole drill assembly 150. Then, the drilling fluid 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 conveyed back to the mud tank 46 through a mud return line 44.

[0062] The pressure required to maintain the drilling fluid circulation 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 pulse generated by a pulse generator. The amplitude of the pressure wave from the pulse generator can reach 500 psi or higher. 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 the sensor cable 50 to a decoder, which decodes the electrical signal and transmits the decoded signal to the surface computer 52, and the surface computer 52 displays the data on a display screen.

[0063] As described above, the lower portion (“distal portion”) of the drill string 100 includes a bottom hole 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 LWD instruments, a downhole motor 170, a near-bit measurement sub 175, and a drill bit 180 having drilling nozzles (not shown). Drilling fluid flows through the drill string 100 and exits through the drilling nozzles of the drill bit 180. During a drilling operation, the drilling system 10 can operate in a rotary mode, in which the drill string 100 is rotated from the surface by an electric motor (i.e., a top drive) in the rotary table 28 or the traveling block 20. The drilling system 10 can also operate in a sliding mode, in which the drill string 100 is not rotated from 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 and is injected into the annulus between the drill string 100 and the wall of the wellbore 16. The drilling fluid carries cuttings from the wellbore 16 to the surface.

[0064] 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 storage 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 communicate 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 such that the pulse generator 300 generates pressure pulses.

[0065] The non-magnetic drill collar houses the MWD system 160, which includes a set of instruments for measuring inclination, azimuth, well trajectory (borehole trajectory), etc. A nuclear logging tool 200 and associated electronics can be located in the LWD tool sub 165. The nuclear logging tool 200 and other logging instruments can be electrically or wirelessly coupled together and powered by a battery pack or a drill fluid-driven generator. All of the information collected is transmitted to the surface in the form of pressure pulses generated by the pulse generator 300 through the mud column in the drill string.

[0066] The 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 bottom hole assembly 150.

[0067] In one embodiment of the present disclosure, a logging tool having a D-T neutron generator and three dual-function detectors is used to obtain various formation parameters.Figure 8 A schematic diagram showing a neutron pulse, neutron count rate, and inelastic and capture energy spectra of neutron-induced gamma rays. The frequency of the neutron pulse is 10 kHz (period 100 μs), and the neutron working time is 20 μs, as shown in subfigure (b) of Figure 8 as shown.

[0068] The neutron count rate measured by each of the three detectors is as shown in subfigure (a) of Figure 8 as shown, for obtaining formation porosity. The neutrons from the three detectors can be further separated according to whether the neutron pulse is on or off, so as to be coincidence or anti-coincidence signals of the neutrons from the three detectors, so that neutrons are mainly recorded as fast neutrons during the neutron pulse (the neutron pulse is on). Between neutron pulses (the neutron pulse is off), 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 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.

[0069] The gamma rays from the three detectors can be further separated according to whether the neutron pulse is on or off, so as to be coincidence or anti-coincidence signals of the gamma rays from the three detectors, so that gamma rays are mainly recorded as inelastic energy spectra during the neutron pulse (the neutron pulse is on), as shown in subfigure (c) of Figure 8 as shown. Between neutron pulses (the neutron pulse is off), gamma rays are recorded as capture energy spectra, as shown in subfigure (d) of Figure 8 as shown. A suitable time window is selected so that the gamma rays measured in the capture time window all come from thermal neutron capture reactions, and most of the gamma rays measured in the inelastic time window come from fast neutron inelastic scattering.

[0070] The background noise of various detectors 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 part of the neutrons measured between neutron pulses to obtain "pure" fast neutrons. Similarly, the capture gamma rays measured during the neutron pulse can be further subtracted by using a small part of the capture energy spectra measured between neutron pulses to obtain a "pure" inelastic energy spectrum.

[0071] The gamma rays detected by each detector can also be recorded in an energy spectrum (e.g., a total energy spectrum), whether they are induced by neutron inelastic scattering or neutron capture reactions. In this way, several formation measurements such as formation porosity, elemental concentration, and formation oil / gas saturation are feasible, but formation density may not be obtained because for a measurement system based on a D-T pulsed neutron generator, an inelastic gamma ray energy spectrum is required to obtain formation density.

[0072] Figure 9 is an exemplary workflow showing the steps for processing data from a logging tool 200 to obtain real-time gas saturation in a formation. The logging tool 200 has a D-T neutron generator and three detectors, namely a near detector, a mid detector, and a far detector. In step 1001, the D-T neutron generator emits neutron pulses into the formation surrounding the measurement tool. In step 1002, the fast neutrons are slowed down to thermal neutrons. Inelastic gamma rays and capture gamma rays are generated.

[0073] In step 1003, neutrons and neutron-induced gamma rays are detected by the three detectors. In step 1004, pulse shape discrimination (PSD) technology is used to distinguish signals from neutrons and neutron-induced gamma rays. Then, in step 1010, the neutron signals from the three detectors are used to obtain the total count rate (CRN n 、CRN m 、CRN f ), the fast neutron count rate (CRFN n 、CRFN m 、CRFN f ), the thermal neutron count rate (CRTN n 、CRTN m 、CRTN f ). Also, these count rates are used to obtain elemental concentration in step 1008 and neutron porosity in step 1013 by using the total neutron ratio (Rn m / f 、Rn n / f 、Rn n / m ) or the thermal neutron ratio (Rtn m / f 、Rtn n / f 、Rtn n / m ) obtained in step 1011 and the formation type obtained in step 1009.

[0074] On the other hand, in step 1005, after the discrimination in step 1004, the total energy spectra from inelastic gamma rays and the total energy spectra from capture gamma rays are obtained. In step 1006, the total energy spectra in step 1005 are deconvoluted using the standard energy spectra of single elements such as Mg, Fe, S, C, Al, Si, Ca, O, Ti, K, Gd, Cl, H, etc. Then, all the elemental concentrations obtained in step 1008 are used to determine the formation type mainly based on the concentrations of elements such as C, O, Mg, Si, Ca, etc. in step 1009. Thus, Rg can be obtained in step 1007 n / f .

[0075] In addition, in step 1013, the formation elemental concentrations are calculated using the fast neutron count rates and thermal neutron count rates at the three detectors in step 1011 and the inelastic spectra and capture spectra obtained in step 1006. Once the formation elemental concentrations are known, the formation type can be determined in step 1009. Combining the formation type from step 1009 and the count rate ratio of thermal neutrons from step 1011, the formation porosity can be obtained in step 1013. The ratio (Rtn n / f ) of the thermal neutron count rate from the near detector to the thermal neutron count rate from the far detector can be obtained in step 1012

[0076] Finally, in step 1014, the formation gas saturation can be determined using the information on the formation type, the thermal neutron ratio Rtn n / f , the capture gamma ray ratio Rg n / f , and the formation porosity Ф, as shown in Figure 10 and the following description. It should be noted that most of the neutrons detected by the detector are thermal neutrons, but some epithermal neutrons are also detected

[0077] Figure 10 Another embodiment of the present disclosure is shown using mathematical symbols and equations, which is performed by an exemplary logging tool 200 (e.g., Figure 2A - Figure 2D ). Using the neutron (CRN) count rates (CRN n , CRN m , CRN f ) measured from the near detector, the middle detector, and the far detector during and between neutron pulses, the formation porosity (Φ) is obtained by using the ratio of the count rates

[0078] The far - middle ratio (Rn m / f ), the near - far ratio (Rn n / f ), and the near - middle ratio (Rn n / m are obtained through formulas 1, 2, and 3 respectively)。Since the three detectors are set at different distances from the neutron source, they have different detection depths. Therefore, the near-wellbore environment (such as wellbore fluid, cement, etc.) has different effects on these three ratios. Rn m / f is more sensitive to the formation, and Rn n / m is more sensitive to near-wellbore changes.

[0079]

[0080] The formation porosity Φ n can be obtained as follows: First, use Rn n / m and / or Rn n / f to correct Rn m / f , and then use the corrected medium-far ratio Rnc m / f to obtain the formation porosity of a specific formation (such as sandstone, limestone or dolomite). Formulas (4)-(6) illustrate this algorithm, where ΔR is the correction value.

[0081] Rnc m / f = Rn m / f + ΔR (4)

[0082] ΔR = f1(Rn m / f , Rn n / f , Rn n / m ) (5)

[0083] Φ n = f2(Rnc m / f ) (6)

[0084] In addition, according to the algorithm similar to that described in Formulas 1-6, the formation porosity Φ n can also be obtained by using the ratio of the three count rates of the capture gamma-ray count rates obtained by the three detectors.

[0085] The formation porosity Φ n can also be obtained by combining two porosities obtained separately by neutrons and capture gamma rays. In other methods, the formation porosity Φ n is directly obtained from the three ratios of neutrons and the three ratios of capture gamma rays using other methods.

[0086] The formation type can be obtained by using the same tool to measure the gamma-ray energy spectra from neutron inelastic scattering and neutron capture reactions. Neutron pulses from a D-T neutron generator are as Figure 8Timing is carried out as described. After being separated from the neutron signal, the neutron-induced gamma-ray signals from the three detectors are further separated into gamma-ray signals from thermal neutron capture reactions and gamma-ray signals from fast neutron inelastic scattering. The inelastic energy spectrum is the basis for detecting elements such as Mg, Fe, S, C, Al, Si, Ca, and O. The capture energy spectrum provides information about other elements (such as Mg, S, Ti, Al, K, Ca, Si, Gd, Fe, Cl, H elements).

[0087] In some embodiments, the gamma rays detected by each detector can be recorded in two separate energy spectra (inelastic energy spectrum and capture energy spectrum), or can be recorded in one energy spectrum (total energy spectrum). In either case, the elements can be identified, and the relative yields of characteristic gamma rays and element concentrations can be obtained from these elements.

[0088] Since the three detectors in the logging tool 200 detect neutrons and gamma rays simultaneously at three different positions, the neutron count rates from the three detectors can be used to obtain a more accurate neutron spatial distribution (fast neutron spatial distribution and thermal neutron spatial distribution). Then, the measured neutron spatial distribution is used to obtain a more accurate calculation of the element concentration.

[0089] In this embodiment, for example, the capture gamma rays and thermal neutrons obtained in steps 1005 and 1010 in Figure 9 are used to calculate the ratio between any two of the near, middle, and far detectors (such as Rg n / f , Rg n / m or Rg m / f ). Similarly, the ratio of the thermal neutron count rates between any two of the near, middle, and far detectors (such as Rtn n / f , Rtn n / m and Rtn m / f ) can also be obtained. The following discussion uses the ratios obtained between the near and far detectors (Rg n / f and Rtn n / f ) as an example for illustration.

[0090] An exemplary algorithm for calculating the gas saturation in a known wellbore environment (hole size, wellbore fluid, casing, etc.) is shown by using Equation 7-9.

[0091] Sg n = f1(Rg n / f , Φ, formation type) (7)

[0092] Sg g = f2(Rg n / f , Φ, formation type) (8)

[0093] Sg=f4(Sg n ,Sg g ) (9)

[0094] Here, the formation type can be sandstone, limestone, or dolomite, which can be determined in real time during logging while drilling. The formation porosity Φ can also be obtained as shown in Figure 9 and Figure 10 according to an algorithm (e.g., as shown in Equation 6). The wellbore fluid is the fluid filling the wellbore, which can be water, oil, gas, drilling mud, or a mixture thereof, which is known in drilling operations.

[0095] Sg n is the formation gas saturation estimated based on thermal neutron data, Sg g is the formation gas saturation estimated based on captured gamma ray data, and Sg is the gas saturation estimated by combining Sg n and Sg g according to an empirical or theoretical relationship based on historical data. Therefore, for a specific well filled with a specific fluid, two or more dual-functional detectors located at different positions can be used to calculate the gas saturation for various formation types in real time.

[0096] Although the above embodiments use the ratio of the thermal neutron count rate and the captured gamma ray count rate between the near and far detectors, the ratio between the near and middle detectors or the middle and far detectors can also be used. If there are more than three dual-functional detectors, more ratios can be obtained. Therefore, multiple estimated gas saturation values can be obtained. These values can be integrated according to an algorithm to obtain another gas saturation value.

[0097] Although the present disclosure has been described in connection with certain of its preferred embodiments in the foregoing specification and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the present disclosure is susceptible to modification and that certain other details described herein can vary significantly 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 one of the embodiments herein can also be used in other embodiments.

Claims

1. A method for evaluating the gas saturation of a subterranean formation, comprising the steps of: S1: Deploy the nuclear logging tool into the underground formation, where The nuclear logging tool includes one or more neutron sources and three or more dual-function detectors configured to simultaneously detect neutrons and gamma rays; S2: Cause the one or more neutron sources to emit neutrons into the subterranean formation; S3: Detect neutrons and gamma rays from the subterranean formation at the three or more detectors to form electrical signals; S4: Separate the electrical signals of neutrons and gamma rays from each of the three or more detectors; S5: Obtain the detector count rates of total neutrons, fast neutrons, and thermal neutrons; S6: Obtain the ratio of thermal neutrons and determine the formation porosity; S7: Obtain the detector count rate of captured gamma rays, the energy spectrum from inelastic gamma rays, and the energy spectrum of captured gamma rays; S8: Determine the formation type and obtain the ratio of captured gamma rays from at least two detectors; S9: Use the formation type, porosity, the ratio of the thermal neutrons, and the ratio of the captured gamma rays to determine the gas saturation of the formation.

2. The method according to claim 1, wherein The neutron source is used to generate neutrons in pulsed mode or continuous mode.

3. 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.

4. The method according to claim 3, characterized in that, Obtain the single-element gamma ray energy spectrum of each of the plurality of elements.

5. The method according to claim 4, wherein Use the elemental yield of each element to obtain the concentration of each element in the formation.

6. The method according to claim 5, wherein Jointly use the fast neutrons and the thermal neutrons obtained at different detectors with the inelastic energy spectrum and the capture energy spectrum obtained at each detector to calculate the elemental concentration and determine the formation type.

7. The method according to claim 1, characterized in that Obtaining the formation porosity of a subterranean formation includes the steps of: Determine the formation type; S10: Obtain the count rates of total neutrons, fast neutrons, and thermal neutrons of each of the three or more detectors; S11: Calculate 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 ratios; and S12: Obtain the formation porosity based on the plurality of neutron count rate ratios and the formation type.

8. The method according to claim 7, characterized in that, In step S11, obtain a first ratio, a second ratio, and a third ratio of the count rates, and step S12 further includes: Use the second neutron count rate ratio and the third neutron count rate ratio to correct the first neutron count rate ratio under the influence of the near-wellbore; Obtain the correlation between the neutron count rate ratio of the formation type and the formation porosity; and Input the corrected first neutron count rate ratio into the correlation to obtain the porosity of the formation.

9. The method according to claim 1, wherein Measuring the gas saturation of a subterranean formation includes: Obtain the formation type and the formation porosity; and Use the formation type, the formation porosity, the ratio of the captured gamma ray count rates from two or more detectors, and / or the ratio of the thermal neutron count rates from two or more detectors to calculate the gas saturation.

10. The method according to claim 9, characterized in that, The calculation step further includes: Calculate one or more gas saturation values using the formation type, the formation porosity, the ratio of one or more captured gamma ray count rates, and the ratio of one or more thermal neutron count rates; and Calculate a gas saturation value using the one or more gas saturation values.

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