Method for estimating net inelastic gamma ray count

Abstract

A method for estimating a net inelastic gamma ray count includes acquiring a burst gamma ray count measured during a neutron burst time interval, acquiring a capture gamma ray count measured during at least one neutron capture time interval, acquiring a neutron count during at least the neutron burst time interval, and estimating a net inelastic gamma ray count during the neutron burst time interval. And subtracting a portion of the trapping gamma ray count and a portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 378,273, filed on October 4, 2022, entitled "METHOD FOR ESTIMATING NET INELASTIC GAMMA RAY COUNTS", the disclosure of which is hereby incorporated by reference in its entirety. Background Art

[0003] Density logging measurements have been used in the oilfield industry for decades. These measurements have traditionally utilized 137 a 137 Cs gamma ray source to emit gamma rays into the wellbore. The emitted gamma rays are scattered back to the tool, where they are detected and processed to estimate formation density. In recent years, some density logging tools have replaced the 137 137 Cs gamma ray source with a neutron source (such as a pulsed neutron generator (PNG)). During logging operations, the emitted neutrons induce inelastic gamma rays (via inelastic scattering events) in the wellbore, and the gamma rays are scattered back to the tool, where they are detected and processed (e.g., via inversion or other algorithms) to estimate formation density.

[0004] Those of ordinary skill in the art know that the interactions of neutrons with the formation and other atomic nuclei are generally separated according to the energy of the neutrons. After a high - energy neutron (e.g., 14.1 MeV) is emitted by the source, it begins to lose energy through inelastic and elastic scattering processes. Inelastic scattering events typically occur when the neutron energy is above 1 - 2 MeV. In an inelastic interaction, the kinetic energy lost by the neutron excites the atomic nucleus, and the atomic nucleus emits characteristic "inelastic" gamma rays when de - exciting. In neutron - based density logging tools, these inelastic gamma rays can be the gamma ray source for formation density measurement. Thus, the determination of an accurate inelastic gamma ray count rate can be an important consideration in determining formation density. In an elastic interaction, kinetic energy is transferred from the neutron to the atomic nucleus without a corresponding gamma ray emission. When neutrons approach thermal energy (e.g., less than about 0.05 eV), they are typically absorbed by the atomic nuclei in the target, resulting in excited isotopes that emit characteristic "capture" gamma rays when de - exciting.

[0005] Inelastic interaction events and capture interaction events theoretically occur at different times (such that inelastic gamma rays and capture gamma rays are emitted at different times). The neutron energy drops very rapidly from 14 MeV to 1 MeV, for example, occurring in less than 1 microsecond. Capture events typically occur much later, for example, dozens or hundreds of microseconds after the neutrons leave the source. However, in actual logging applications, neutrons can be emitted from the neutron source in a series of short bursts. Thus, it should be understood that in practice, inelastic gamma rays and capture gamma rays can (and typically do) overlap in time, making it difficult to distinguish between them.

[0006] Various techniques are known for estimating inelastic gamma ray counts and capture gamma ray counts. For example, a portion of the gamma ray signal measured during the time interval between bursts can be subtracted from the gamma ray signal received during a burst. While this method is available, there is still room for further improvement. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] To more fully understand the disclosed subject matter and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

[0008] Figure 1 An example rig including the disclosed nuclear logging tool is depicted.

[0009] Figure 2 Is schematically depicted Figure 1 An example implementation of the nuclear logging tool shown.

[0010] Figure 3 An example PNG timing sequence that can be used by a pulsed neutron source to generate high energy neutrons (e.g., a burst of 14 MeV neutrons) is depicted.

[0011] Figure 4 An example graph of gamma ray count rate versus formation density is depicted.

[0012] Figure 5 An example graph of modeled neutron count rate versus formation density is depicted.

[0013] Figure 6A And Figure 6B (collectively referred to as FIG. 6) depict a flowchart of an example method implementation disclosed herein.

[0014] Figure 7A And Figure 7B (collectively referred to as FIG. 7) depict a graph of calculated net inelastic gamma ray count rate versus modeled net inelastic gamma ray count rate. DETAILED DESCRIPTION

[0015] Embodiments of the present disclosure include systems and methods for estimating net inelastic gamma ray counts. An example method includes: obtaining burst gamma ray counts measured during a neutron burst time interval, obtaining capture gamma ray counts measured during at least one neutron capture time interval, obtaining neutron counts during at least the neutron burst time interval, and subtracting a portion of the capture gamma ray counts and a portion of the neutron counts from the burst gamma ray counts to estimate the net inelastic gamma ray counts. The method may further include: processing the net inelastic gamma ray counts to estimate formation density.

[0016] Figure 1 Depicted is an oil or gas rig 20 that includes an example nuclear logging tool 50, such as a neutron-based density logging tool. In the depicted embodiment, a land rig 20 is positioned above an oil or gas formation (not shown). The rig may include a derrick and a hoisting device (not shown) for raising and lowering a drill string 30 that, as shown, extends into a borehole 40 and includes a drill bit 32 deployed at the lower end of a bottom hole assembly (BHA). The BHA further includes the example logging tool 50.

[0017] It should be understood that Figure 1 the deployment shown is merely exemplary. The drill string 30 may include substantially any suitable downhole tool components, e.g., including steering tools such as rotary steerable tools, downhole telemetry systems, and one or more additional MWD and / or LWD tools, including various sensors for sensing downhole properties of the borehole and surrounding formation. The disclosed embodiments are in no way limited to any particular drill string configuration.

[0018] It should further be understood that the disclosed embodiments are not limited to use with land rigs, but are equally well suited for use with onshore or offshore subterranean operations. Additionally, the disclosed embodiments are not limited to Figure 1 the logging-while-drilling embodiment shown. The disclosed embodiments are well suited for use with any nuclear logging tool, including wireline logging tools and slickline logging tools.

[0019] Figure 2Schematically depicts an example implementation of a nuclear logging tool 50. The tool 50 includes a neutron source 54 deployed within a tool housing 52 (or an inner mandrel). The tool housing 52 and the optional inner mandrel may be collectively referred to herein as the tool body. The neutron source 54 may advantageously include a pulsed neutron generator (PNG) that includes a power source utilizing, for example, a deuterium-tritium (D-T) nuclear reaction and / or a tritium-tritium (T-T) nuclear reaction. Such PNGs are widely used in the industry. The logging tool 50 further includes a gamma ray detector 56 deployed within the tool housing 52, for example, axially offset from the neutron source as depicted. The gamma ray detector 54 may include substantially any suitable gamma ray detector, for example, including a sodium iodide (NaI) scintillation crystal and a photomultiplier tube. Such gamma ray detectors are also widely used in the industry. The logging tool 50 further includes a neutron detector 58 deployed within the tool housing 52, for example, axially located between the neutron source 54 and the gamma ray detector 56 as depicted. The neutron detector 58 may include substantially any suitable neutron detector, for example, including a thermal neutron detector, an epithermal neutron detector, and / or a fast neutron detector. For example, the neutron detector 58 may include a conventional 3 3He proportional counter. In certain advantageous implementations, the neutron detector may include a thermal neutron detector or a detector sensitive to both thermal and epithermal neutrons. Such detectors typically provide a higher count rate and thus have better statistics and signal-to-noise ratio. Nevertheless, the disclosed implementations are not limited in this regard.

[0020] Continuing to refer Figure 2 , it should be understood that the disclosed implementations are not limited to tool implementations including axially spaced gamma ray and neutron detectors as depicted. For example, the gamma ray and neutron detectors may be deployed at the same axial location on the tool 50. Additionally, the disclosed implementations are not limited to tool implementations including different gamma ray and neutron detectors. In certain implementations, a combined detector sensitive to both neutrons and gamma rays, such as a cesium lithium yttrium chloride (e.g., Cs 2 LiYCl 6 ) scintillator coupled to a photodetector, may be employed. In such a detector, the shape of the light pulse induced by neutrons may be different from the shape of the light pulse induced by gamma rays, such that neutrons and gamma rays can be distinguished and counted separately.

[0021] Continuing to refer Figure 2, the logging tool 50 may further include an electronic controller 60, the electronic controller including one or more processors (e.g., a microprocessor) and an electronic memory. The controller 60 may include processor-executable instructions (e.g., stored in the memory), the processor-executable instructions being configured to cause the neutron source 54 (e.g., PNG) to emit neutrons in a predetermined emission sequence (e.g., in pulses having a predetermined pulse length and pulse interval). The controller may further be configured to receive electrical / electronic signals from the gamma ray detector 55 and be configured to process the signals to generate gamma ray counts. The controller 60 may still further be configured to receive electrical / electronic signals from the neutron detector 58 and be configured to process the signals to generate neutron counts. The controller 60 may include processor-executable instructions configured to perform the disclosed method steps described in more detail below (e.g., with respect to Figure 6A and Figure 6B ), e.g., configured to determine inelastic gamma ray counts by subtracting a portion of the detected neutron counts from the detected gamma ray counts. The controller may still further be configured to process the inelastic gamma ray counts to estimate formation density. Of course, it should be understood that the disclosed embodiments are not limited to the use or configuration of any particular controller hardware, firmware, and / or software.

[0022] Figure 3Depicts an example PNG timing sequence 70 that can be used by a neutron source 54 to generate high-energy neutrons (e.g., a burst of 14 MeV neutrons). The depicted example timing sequence includes a series of short-duration neutron bursts 72 (e.g., each burst having a duration of 10 microseconds). Each burst 72 can be followed by a corresponding short capture interval 74 during which no neutrons are generated (e.g., having a duration of 25 microseconds). Additionally, as depicted, the capture interval 74 can be divided into an early capture interval 75 and a late capture interval 76 having corresponding durations of, for example, 5 microseconds and 20 microseconds. This burst packet sequence 80 (the sequence of neutron bursts 72 and capture intervals 74) can be repeated essentially any suitable number of times, e.g., 32 times in the depicted example, and can then be followed by a longer capture interval 82 (e.g., a sigma decay interval having a duration of 380 microseconds). This sigma packet sequence 86 (the repeated burst packet sequence 80 and sigma decay 82) can also be repeated essentially any suitable number of times, e.g., 62 times in the depicted example, and can then be followed by a longer capture interval or background interval 88 (e.g., having a duration of 7 milliseconds). In this example, the PNG timing sequence 70 has a duration of 100 milliseconds. It should be understood that the depicted PNG timing sequence 70 can be repeated essentially any number of times (e.g., at 100 millisecond intervals) during a logging operation. Additionally, one of ordinary skill in the art will readily understand that the PNG timing sequence 70 is merely an example. The disclosed embodiments are in no way limited to any particular PNG timing sequence configuration.

[0023] During a logging operation, high-energy (e.g., 14 MeV) neutrons can be emitted during the neutron burst portion of the PNG timing sequence (e.g., during burst 72 in Figure 3 . The emitted neutrons lose energy via inelastic and elastic scattering with atomic nuclei in the surrounding environment. Inelastic scattering events (and the corresponding emission of inelastic gamma rays) typically occur within about 1 microsecond after neutron emission, i.e., as the neutron energy decreases from 14 MeV to about 1 MeV. Neutron capture events (and the corresponding emission of capture gamma rays) typically occur much later (within the lifetime of a single neutron) than inelastic scattering events, e.g., dozens or hundreds of microseconds after neutron emission.

[0024] However, as described above, in actual logging applications, neutrons are emitted from the neutron source in a series of short bursts and capture intervals. Inelastic gamma rays may be generated during a neutron burst (e.g., within 1 microsecond of neutron generation). Capture gamma rays may be generated during a later neutron burst or capture interval (e.g., tens or hundreds of microseconds after neutron generation). Thus, the emission of inelastic gamma rays and capture gamma rays typically overlaps in time. In particular, inelastic gamma rays and capture gamma rays can be detected during a neutron burst. Inelastic gamma rays are typically not detected during individual capture intervals, such that capture gamma rays can be primarily detected during these intervals (during which no neutrons are generated).

[0025] During common nuclear (e.g., density) logging operations, gamma rays can be accumulated during a neutron burst and during one or more of the capture intervals or some combination thereof within the capture intervals to generate burst counts and capture counts. The inelastic gamma ray count (or count rate depending on the operational objective) is typically obtained by subtracting the measured capture gamma rays (or a fraction or multiple of the capture gamma rays) from the burst gamma rays, e.g., as shown below:

[0026] I 净 = B - α·C (1)

[0027] where I 净 represents the net inelastic gamma ray count, B represents the measured burst interval gamma ray count, C represents the measured capture interval gamma ray count, and α represents a fractional coefficient that can depend on the acquisition time, dead time, and specific capture interval used to measure the capture gamma rays, as well as other factors such as the borehole and formation thermal neutron capture cross section (Sigma), and the epithermal neutron moderation time (the time for epithermal neutrons to moderate to thermal energy), and / or other metrics of the thermal and epithermal neutron fluxes. It should be understood that α can be a constant value or can vary with the borehole and formation environment during the logging operation. Further, it should be understood that although not shown in Equation (1), background gamma rays (e.g., activation and natural gamma rays) can also be subtracted. The contribution of these background gamma rays is typically relatively small and is thus ignored in Equation (1).

[0028] Although the above method for obtaining the net inelastic gamma ray count has been used in commercial logging operations (e.g., in density logging measurements), there is still room for further improvement. For example, the observed net inelastic gamma ray count rate sometimes does not match the modeled count rate (e.g., obtained via Monte Carlo simulation). In particular, at high formation densities (e.g., about 2.4 g / cm 3 or above), the experimental net inelastic gamma ray count rate can significantly exceed the inelastic gamma ray count rate estimated via Monte Carlo simulation.

[0029] Figure 4 Depicts an example graph 120 of gamma ray count rate in counts per second versus formation density. The modeled inelastic gamma ray count rate for the example is plotted at 122. Note that in the depicted example, the modeled inelastic gamma ray count rate is not strongly affected by formation density and monotonically decreases from a high value of about 170 at a formation density of 1.1 to a low value of about 130 at a formation density of 2.2 and then monotonically increases to about 145 at a formation density of 2.7. The example measured burst count rate (obtained during burst 72 in Figure 3 is plotted at 124. The example measured early and late capture count rates (obtained during early capture interval 75 and late capture interval 76 in Figure 3 are plotted at 126 and 128.

[0030] Continuing to refer to Figure 4 , note that the relationship between the measured burst and capture gamma ray count rates 124, 126, and 128 and formation density is similar to the relationship of the modeled inelastic gamma ray count rate at low formation densities (e.g., less than about 2.4). However, at higher formation densities (e.g., greater than about 2.4), the measured burst and capture gamma ray count rates 124, 126, and 128 increase exponentially with increasing formation density, generally as depicted at 130. Thus, it is difficult to accurately reproduce the modeled inelastic gamma ray count rate (e.g., graph 122) using a linear combination (e.g., as shown in formula (1)) of the burst count rate (e.g., graph 124) and one or more capture count rates (e.g., graphs 126 and 128). When the coefficients of the capture count rates are chosen to accurately reproduce the modeled inelastic gamma ray count rate at low formation densities, a poor reproduction is achieved at high formation densities. Similarly, when the coefficients are chosen to accurately reproduce the modeled inelastic gamma ray count rate at high formation densities, a poor reproduction is achieved at low formation densities.

[0031] One aspect of the disclosed embodiments is the recognition that the measured and modeled neutron count rates show a similar increase with increasing formation density. Figure 5Depicts an example graph of modeled neutron count rates (thermal and epithermal) versus formation density. Note that the depicted neutron count rate increases with increasing formation density, particularly at formation densities above about 2.4, where the count rate increases rapidly with density. A further implementation of the disclosed embodiments is that the modeled inelastic gamma ray count rate can be better reproduced by subtracting both the captured gamma rays and the measured neutrons from the prompt gamma rays or gamma ray count rate. For example, it is recognized that the modeled inelastic gamma ray count rate can be better reproduced by subtracting a portion of the captured gamma rays obtained within at least one capture interval and a portion of the measured neutrons or a portion of a mathematical function of the measured neutrons from the prompt gamma rays.

[0032] Now turning to Figure 6A and Figure 6B (collectively referred to as Figure 6), depicts flowcharts of example method embodiments 200 and 220 for estimating net inelastic gamma ray counts. It should be understood that gamma ray count refers to the gamma ray count (number of gamma rays) and / or the gamma ray count rate (number of gamma rays per unit time). In Figure 6A , method 200 includes obtaining a prompt gamma ray count at 202 and obtaining a captured gamma ray count at 204. A neutron count is also obtained at 206. Then the obtained prompt gamma ray count, captured gamma ray count, and neutron count are processed at 208 to estimate the net inelastic gamma ray count. Method 200 may further optionally include processing the net inelastic gamma ray count (or count rate) at 210 to estimate the formation density. For example, the formation density can be estimated as disclosed in U.S. Patents 5,608,215 and 5,804,820.

[0033] Continuing to refer to Figure 6A , the net inelastic gamma ray count can be estimated at 208, for example, by subtracting a portion of the captured gamma ray count and a portion of the neutron count from the prompt gamma ray count. In another example, the processing at 208 can include subtracting a function of the obtained neutrons from the prompt gamma rays, such as a logarithmic function or a polynomial function of the obtained neutrons. The processing at 208 can be represented mathematically, for example, according to one of the following formulas:

[0034] I 净 = α·C - γ·N (2)

[0035] I 净 = B - α 1 ·C 1 - α 2 ·C 2 - γ·N (3)

[0036] where I净 represents the net inelastic gamma ray count, B represents the acquired burst gamma ray count, C represents the acquired capture interval gamma ray count, and N represents the acquired neutron count. In Equation (3), C 1 and C 2 represent the capture interval gamma ray counts acquired corresponding to a first capture interval and a second different or overlapping capture interval (such as the early capture interval 75 and the late capture interval 76 described above with respect to Figure 3 ). The coefficients α, α 1 , α 2 and γ can be fractions with values less than 1 (the fractions can be the same or different), and can be related to various operating parameters (such as acquisition time, dead time, and specific capture intervals, as well as other factors such as borehole and formation thermal neutron capture cross-section (as described above with respect to Equation (1))). Additionally, as described above, the processing can include subtracting a function of the acquired neutrons, for example, as shown below:

[0037] I 净 = B - α·Cf(N) (4)

[0038] I 净 = B - α 1 ·C 1 - α 2 ·C 2 - f(N) (5)

[0039] where f(N) represents a function of the acquired neutron count, such as a logarithmic function or a polynomial function of the acquired neutron count. In such an embodiment, the acquired neutron count is first processed according to the function (for example, taking the logarithm of the neutron count) to obtain a corresponding functional neutron quantity. Then the functional neutron quantity is subtracted, for example, as shown in Equations (4) and (5). It should be understood that f(N) is typically less than N such that the functional neutron quantity represents a portion of the measured neutron count. Additionally, although not shown in Equations (2)-(5), it should be understood that B can also be multiplied by a corresponding fractional coefficient such that the following quantities are subtracted from the burst count B or a fraction (or portion) thereof. The disclosed embodiments are not limited in this regard.

[0040] In Figure 6B , method 220 includes deploying a logging tool (such as logging tool 50 ( Figure 2 )) in a wellbore penetrating a subterranean formation at 222. In certain advantageous embodiments, the logging tool can include a PNG neutron source, a gamma ray detector, and a neutron detector as described above. At 224, a neutron source (such as a PNG) emits neutrons into the wellbore. The neutrons can be emitted in a series of neutron burst intervals, each followed by a corresponding capture interval, as described above with respect to Figure 3As described above. The emitted neutrons typically cause various atomic nuclei in the wellbore environment to emit gamma rays. Prompt gamma rays (gamma ray counts) can be measured using a gamma ray detector during one or more prompt intervals at 226, while capture gamma rays (gamma ray counts) can be measured using a gamma ray detector during one or more capture intervals at 228. For example, the measured prompt and capture gamma rays can include total prompt and capture gamma ray counts or prompt and capture gamma ray count rates (e.g., counts per second or per minute). These different measurements are collectively referred to herein as gamma ray counts. Neutrons can be measured using a neutron detector during at least one prompt interval at 230 (neutron counts). The acquired neutrons can include substantially any detected neutrons, such as including thermal neutrons, epithermal neutrons, and / or fast neutrons. In certain advantageous embodiments, the acquired neutrons can include at least thermal neutrons. Neutron count also means neutron count rate. The acquired prompt gamma ray counts, capture gamma ray counts, and neutron counts are then processed at 232 to estimate the net inelastic gamma ray counts, e.g., as described above with respect to Figure 6A and formulas (2), (3), (4), and (5). Method 220 can further optionally include processing the net inelastic gamma ray counts (or count rates) at 234 to estimate the formation density. It should be understood that the processing can be performed in real time during logging by a downhole processor deployed in the logging tool, or during the processing of the data after the logging operation is completed. The disclosed embodiments are not limited in this regard.

[0041] Figure 7A and Figure 7B (collectively referred to as FIG. 7) depict a graph of the calculated net inelastic gamma ray count rate on the vertical axis versus the modeled net inelastic gamma ray count rate obtained using Monte Carlo simulation on the horizontal axis. The individual data points in the graph represent measurements made in a series of environments and lithologies, including anhydrite, dolomite, lignite, limestone, gypsum, sandstone, shale, water, and diesel. In Figure 7A , the net inelastic gamma ray count rate is calculated by subtracting a portion of the capture gamma ray count rate plotted on the vertical axis from the prompt gamma ray count rate, as in formula (1). Note that the calculated net inelastic gamma ray count rate shows significant scatter around the modeled gamma ray count rate at low count rates (e.g., less than about 300 in this example), while deviating from the modeled count rate at higher count rates. In Figure 7BIn [the reference], the net inelastic gamma-ray count rate on the vertical axis is calculated by subtracting a portion of the capture gamma-ray count rate and a portion of the function of the neutron count rate from the burst gamma-ray count rate, as shown in Equation (5). Note that there is excellent agreement between the calculated net inelastic gamma-ray count rate and the modeled net inelastic gamma-ray count rate over the entire count rate and lithology range. In this particular example, f(N) in Equation (5) is 33 log(N) + 235. It should be understood that the same result can be obtained using a polynomial function (such as a second-order, third-order, or fourth-order polynomial) that gives a similar amount of function for neutrons within the depicted neutron count rate range.

[0042] Continuing to refer Figure 7B , it should be understood that the coefficients in Equations (2), (3), (4), and / or (5) can be determined, for example, by fitting a set of calibration data to the Monte Carlo model net inelastic gamma-ray count rate prediction. For example, the data can include the burst gamma-ray count rate, capture gamma-ray count rate, and neutron count rate of multiple different formations with a certain density range. The calibration model (one or more of Equations (2), (3), (4), and (5)) can be used to iteratively process the data to obtain the corresponding net inelastic gamma-ray count rate. The coefficients can be selected, for example, based on the least squares best fit of a set of calibration data to the Monte Carlo simulation.

[0043] It should be understood that the present disclosure includes numerous embodiments. These embodiments include, but are not limited to, the following embodiments.

[0044] In a first embodiment, a method for estimating a net inelastic gamma-ray count includes: obtaining a burst gamma-ray count measured during a neutron burst time interval; obtaining a capture gamma-ray count measured during at least one neutron capture time interval; obtaining a neutron count during at least the neutron burst time interval; and subtracting a portion of the capture gamma-ray count and a portion of the neutron count from the burst gamma-ray count to estimate the net inelastic gamma-ray count.

[0045] A second embodiment may include the first embodiment, and the method further includes: deploying a logging tool in a subterranean wellbore, the logging tool including a pulsed neutron generator (PNG), a gamma-ray detector, and a neutron detector; and causing the PNG to emit neutrons into the subterranean wellbore during the neutron burst time interval.

[0046] The third embodiment may include the second embodiment, wherein obtaining the burst gamma-ray count includes causing the gamma-ray detector to accumulate gamma rays during the neutron burst time interval; obtaining the capture gamma-ray count includes causing the gamma-ray detector to accumulate gamma rays during the at least one neutron capture time interval; and obtaining the neutron count includes causing the neutron detector to accumulate neutrons during at least the neutron burst time interval.

[0047] The fourth embodiment may include the third embodiment, wherein obtaining the neutron count includes causing the neutron detector to accumulate neutrons during both the neutron burst time interval and one or more of the at least one neutron capture time intervals.

[0048] The fifth embodiment may include any one of the first to fourth embodiments, wherein the obtained burst gamma-ray count includes a burst gamma-ray count rate; the obtained capture gamma-ray count includes a capture gamma-ray count rate; and the obtained neutron count includes a neutron count rate.

[0049] The sixth embodiment may include any one of the first to fifth embodiments, wherein obtaining the burst gamma-ray count includes obtaining the burst gamma-ray count during a plurality of neutron burst time intervals; obtaining the capture gamma-ray count includes obtaining the capture gamma-ray count during a plurality of neutron capture time intervals; and obtaining the neutron count includes obtaining the neutron count during the plurality of neutron burst intervals and the plurality of neutron capture intervals.

[0050] The seventh embodiment may include any one of the first to sixth embodiments, wherein the obtained neutron count includes at least thermal neutrons.

[0051] The eighth embodiment may include any one of the first to seventh embodiments, wherein the subtracting further includes: multiplying the capture gamma-ray count by a first coefficient to calculate the portion of the capture gamma-ray count; multiplying the neutron count by a second coefficient to calculate the portion of the neutron count; and subtracting the portion of the capture gamma-ray count and the portion of the neutron count from the burst gamma-ray count to estimate the net inelastic gamma-ray count.

[0052] The ninth embodiment may include any one of the first to eighth embodiments, wherein the subtracting further includes: multiplying the captured gamma ray count by a first coefficient to calculate the portion of the captured gamma ray count; processing the neutron count using a function to calculate the portion of the neutron count; and subtracting the portion of the captured gamma ray count and the portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

[0053] The tenth embodiment may include the ninth embodiment, wherein the function is a logarithmic function or a polynomial function.

[0054] In an eleventh embodiment, a nuclear logging tool includes: a pulsed neutron generator (PNG) deployed in a logging tool body; a gamma ray detector deployed in the logging tool body; a neutron detector deployed in the logging tool body; and an electronic controller deployed in the logging tool body, the electronic controller including a processor, the processor including instructions configured to: cause the PNG to emit neutrons in a series of burst intervals separated by corresponding capture intervals; cause the gamma ray detector to acquire a burst gamma ray count during the burst intervals; cause the gamma ray detector to acquire a captured gamma ray count during at least one of the capture intervals; cause the neutron detector to acquire a neutron count during at least the burst intervals; and subtract a portion of the captured gamma ray count and a portion of the neutron count from the burst gamma ray count to estimate a net inelastic gamma ray count.

[0055] The twelfth embodiment may include the eleventh embodiment, wherein the instructions are configured to cause the neutron detector to acquire the neutron count during both the burst intervals and the capture intervals.

[0056] The thirteenth embodiment may include any one of the eleventh to twelfth embodiments, wherein the subtracting further includes: multiplying the captured gamma ray count by a first coefficient to calculate the portion of the captured gamma ray count; multiplying the neutron count by a second coefficient to calculate the portion of the neutron count; and subtracting the portion of the captured gamma ray count and the portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

[0057] The fourteenth embodiment may include any one of the eleventh to the thirteenth embodiments, wherein the subtracting further includes: multiplying the captured gamma-ray count by a first coefficient to calculate the portion of the captured gamma-ray count; processing the neutron count using a function to calculate the portion of the neutron count; and subtracting the portion of the captured gamma-ray count and the portion of the neutron count from the prompt gamma-ray count to estimate the net inelastic gamma-ray count.

[0058] The fifteenth embodiment may include the fourteenth embodiment, wherein the function is a logarithmic function or a polynomial function.

[0059] In a sixteenth embodiment, a method for estimating a net inelastic gamma-ray count includes: causing a pulsed neutron generator to emit neutrons in a wellbore, the neutrons being emitted in a series of burst intervals interleaved with corresponding capture intervals; causing a gamma-ray detector to acquire a prompt gamma-ray count during a plurality of the burst intervals; causing the gamma-ray detector to acquire a captured gamma-ray count during a plurality of the capture intervals; causing a neutron detector to acquire a neutron count during at least the plurality of burst intervals; processing a function of the neutron count to calculate a functional neutron amount; and subtracting a portion of the captured gamma-ray count and the functional neutron amount from the prompt gamma-ray count to estimate the net inelastic gamma-ray count.

[0060] The seventeenth embodiment may include the sixteenth embodiment, wherein the function is a logarithmic function or a polynomial function.

[0061] The eighteenth embodiment may include any one of the sixteenth to the seventeenth embodiments, wherein: causing the gamma-ray detector to acquire the captured gamma-ray count includes causing the gamma-ray detector to acquire a gamma-ray count during a plurality of early capture intervals and a captured gamma-ray count during a plurality of late capture intervals, wherein each of the plurality of late capture intervals temporally follows a corresponding one of the plurality of early capture intervals; and the subtracting includes subtracting a portion of the early captured gamma-ray count, a portion of the late captured gamma-ray count, and the functional neutron amount from the prompt gamma-ray count to estimate the net inelastic gamma-ray count.

[0062] The nineteenth embodiment may include any one of the sixteenth to the eighteenth embodiments, wherein the prompt gamma-ray count includes a prompt gamma-ray count rate; the captured gamma-ray count includes a captured gamma-ray count rate; and the neutron count includes a neutron count rate.

[0063] The twentieth embodiment may include any one of the sixteenth embodiment to the nineteenth embodiment, and the method further includes: processing the net inelastic gamma ray count to estimate the density of the formation penetrated by the wellbore.

[0064] Although the estimation of the net inelastic gamma ray count has been described in detail, it should be understood that various changes, substitutions, and alterations may be made herein without departing from the spirit and scope of the present disclosure as defined by the appended claims.

Claims

1. A method for estimating net inelastic gamma ray counts, the method comprises: obtaining burst gamma ray counts measured during a neutron burst time interval; obtaining capture gamma ray counts measured during at least one neutron capture time interval; obtaining neutron counts during at least the neutron burst time interval; and subtracting a portion of the capture gamma ray counts and a portion of the neutron counts from the burst gamma ray counts to estimate the net inelastic gamma ray counts.

2. The method according to claim 1, the method further comprises: deploying a logging tool in a subterranean wellbore, the logging tool including a pulsed neutron generator (PNG), a gamma ray detector, and a neutron detector; and causing the PNG to emit neutrons into the subterranean wellbore during the neutron burst time interval.

3. The method according to claim 2, wherein: the obtaining the burst gamma ray counts comprises causing the gamma ray detector to accumulate gamma rays during the neutron burst time interval; the obtaining the capture gamma ray counts comprises causing the gamma ray detector to accumulate gamma rays during the at least one neutron capture time interval; and the obtaining the neutron counts comprises causing the neutron detector to accumulate neutrons during at least the neutron burst time interval.

4. The method according to claim 3, wherein the obtaining the neutron counts comprises causing the neutron detector to accumulate neutrons during both the neutron burst time interval and one or more of the at least one neutron capture time intervals.

5. The method according to claim 1, wherein: the obtained burst gamma ray counts comprise a burst gamma ray count rate; the obtained capture gamma ray counts comprise a capture gamma ray count rate; and the obtained neutron counts comprise a neutron count rate.

6. The method according to claim 1, wherein: the obtaining the burst gamma ray counts comprises obtaining the burst gamma ray counts during a plurality of neutron burst time intervals; the obtaining the capture gamma ray counts comprises obtaining the capture gamma ray counts during a plurality of neutron capture time intervals; and the obtaining the neutron counts comprises obtaining the neutron counts during the plurality of neutron burst intervals and the plurality of neutron capture intervals.

7. The method according to claim 1, wherein the obtained neutron counts comprise at least thermal neutrons.

8. The method according to claim 1, wherein the subtracting further comprises: multiplying the capture gamma ray counts by a first coefficient to calculate the portion of the capture gamma ray counts; multiplying the neutron counts by a second coefficient to calculate the portion of the neutron counts; and subtracting the portion of the capture gamma ray counts and the portion of the neutron counts from the burst gamma ray counts to estimate the net inelastic gamma ray counts.

9. The method according to claim 1, wherein the subtracting further comprises: multiplying the capture gamma ray counts by a first coefficient to calculate the portion of the capture gamma ray counts; Process the neutron count using a function to calculate the portion of the neutron count; and Subtract the portion of the capture gamma ray count and the portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

10. The method according to claim 9, wherein the function is a logarithmic function or a polynomial function.

11. A nuclear logging tool, the nuclear logging tool comprising: A pulsed neutron generator (PNG) deployed in a logging tool body; A gamma ray detector deployed in the logging tool body; A neutron detector deployed in the logging tool body; and An electronic controller deployed in the logging tool body, the electronic controller including a processor, the processor including instructions configured to: Cause the PNG to emit neutrons in a series of burst intervals separated by corresponding capture intervals; Cause the gamma ray detector to acquire a burst gamma ray count during the burst interval; Cause the gamma ray detector to acquire a capture gamma ray count during at least one of the capture intervals; Cause the neutron detector to acquire a neutron count during at least the burst interval; and Subtract a portion of the capture gamma ray count and a portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

12. The nuclear logging tool according to claim 11, wherein the instructions are configured to cause the neutron detector to acquire the neutron count during both the burst interval and the capture interval.

13. The nuclear logging tool according to claim 11, wherein the subtracting further comprises: Multiply the capture gamma ray count by a first coefficient to calculate the portion of the capture gamma ray count; Multiply the neutron count by a second coefficient to calculate the portion of the neutron count; and Subtract the portion of the capture gamma ray count and the portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

14. The nuclear logging tool according to claim 11, wherein the subtracting further comprises: Multiply the capture gamma ray count by a first coefficient to calculate the portion of the capture gamma ray count; Process the neutron count using a function to calculate the portion of the neutron count; and Subtract the portion of the capture gamma ray count and the portion of the neutron count from the burst gamma ray count to estimate the net inelastic gamma ray count.

15. The nuclear logging tool according to claim 14, wherein the function is a logarithmic function or a polynomial function.

16. A method for estimating a net inelastic gamma ray count, the method comprising: Cause a pulsed neutron generator to emit neutrons in a wellbore, the neutrons being emitted in a series of burst intervals interspersed with corresponding capture intervals; Cause a gamma ray detector to acquire a burst gamma ray count during a plurality of the burst intervals; Cause the gamma ray detector to acquire a capture gamma ray count during a plurality of the capture intervals; Cause a neutron detector to obtain neutron counts during at least the plurality of burst intervals; Process a function of the neutron counts to calculate a function neutron amount; and Subtract a portion of the capture gamma ray counts and the function neutron amount from the burst gamma ray counts to estimate the net inelastic gamma ray counts.

17. The method according to claim 16, wherein the function is a logarithmic function or a polynomial function.

18. The method according to claim 16, wherein: The causing the gamma ray detector to obtain the capture gamma ray counts includes causing the gamma ray detector to obtain gamma ray counts during a plurality of early capture intervals and capture gamma ray counts during a plurality of late capture intervals, wherein each of the plurality of late capture intervals temporally follows a corresponding one of the plurality of early capture intervals; and The subtracting includes subtracting a portion of the early capture gamma ray counts, a portion of the late capture gamma ray counts, and the function neutron amount from the burst gamma ray counts to estimate the net inelastic gamma ray counts.

19. The method according to claim 16, wherein The burst gamma ray counts include a burst gamma ray count rate; The capture gamma ray counts include a capture gamma ray count rate; and The neutron counts include a neutron count rate.

20. The method according to claim 16, the method further comprises: Processing the net inelastic gamma ray counts to estimate the density of the formation penetrated by the wellbore.

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