Coal bed gas reservoir gas saturation in-situ detection device and method

Abstract

The present application relates to coalbed methane reservoir gas saturation detection technical field, especially in kind detection device and method of coalbed methane reservoir gas saturation, device includes magnetic field generating mechanism, detection mechanism, acquisition control circuit and well digital signal processor, magnetic field generating mechanism includes two composite magnets, two composite magnets are connected through interval adjusting assembly, detection mechanism is arranged between two composite magnets, is used for heavy gathering pulse and receives stratum spin echo signal, acquisition control circuit is used for driving radio frequency coil and acquisition electric signal that receiving coil gathers, acquisition control circuit is connected with composite magnet simultaneously, is used for adjusting static magnetic field intensity, well digital signal processor and acquisition control circuit communication will gather nuclear magnetic signal calculation obtains gas saturation. Simultaneously discloses the method based on above-mentioned device, adopts above-mentioned in kind detection device and method of coalbed methane reservoir gas saturation, reduces interference influence, improves detection precision.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane reservoir gas saturation detection technology, and in particular to an in-situ detection device and method for coalbed methane reservoir gas saturation. Background Technology

[0002] Gas saturation in coalbed methane reservoirs is a core in-situ parameter for coalbed methane resource evaluation, fracturing and drainage scheme design, and reserve calculation. Traditional indoor core desorption methods and surface logging inversion methods have significant drawbacks: closed-loop pressure-maintaining coring is costly, has a long single-well operation cycle, and results in significant methane loss during the core extraction process, leading to large errors in saturation calculation; conventional wireline logging requires stopping drilling and retrieving instruments, causing drilling fluid to penetrate the coal seam pores for extended periods, disrupting the original reservoir fluid state and failing to obtain true in-situ gas-bearing characteristics. Nuclear magnetic resonance (NMR) logging while drilling can simultaneously measure formation fluids during drilling, acquiring reservoir fluid information before the formation is deeply penetrated by drilling fluid, making it the optimal in-situ detection method currently available. In existing technologies, in-situ detection is performed using NMR instruments while drilling. In existing technologies, a static magnetic field is provided by a permanent magnet. The permanent magnet is fixed and inconvenient to adjust, and the adjustment dimension is limited. Under high-temperature conditions, the magnetic flux of the permanent magnet decays and the asymmetric magnetic field distortion caused by magnetic impurities in the mud cannot be completely corrected by magnet displacement alone, and the effect of restoring magnetic field uniformity is limited. Summary of the Invention

[0003] The purpose of this invention is to provide an in-situ detection device and method for gas saturation in coalbed methane reservoirs, which solves the problems of existing logging-while-drilling nuclear magnetic resonance instruments, such as poor resistance to interference from ferromagnetic minerals in drilling mud, single magnetic field correction method, serious heat generation during continuous measurement, and failure to incorporate real-time magnetic field correction parameters in gas saturation calculation.

[0004] To achieve the above objectives, the present invention provides an in-situ detection device for gas saturation in coalbed methane reservoirs, comprising: The magnetic field generating mechanism includes two composite magnets sleeved on the mud channel, and the two composite magnets are connected by a spacing adjustment component. The detection mechanism is embedded in the drill pipe and positioned between two composite magnets. The detection mechanism includes a radio frequency coil and a receiving coil. The radio frequency coil is used to refocus the pulse, and the receiving coil is used to receive the formation spin echo signal. The acquisition control circuit is connected to the radio frequency coil to drive the radio frequency coil to emit refocusing pulses; the acquisition control circuit is also connected to the receiving coil to acquire the formation spin echo signal acquired by the receiving coil; at the same time, the acquisition control circuit is electrically connected to the composite magnet to adjust the static magnetic field strength. The wellhead digital signal processor communicates with the acquisition and control circuit and calculates the gas saturation based on the acquired formation spin echo signal.

[0005] Preferably, the composite magnet includes a main magnet and an auxiliary magnet that are fixedly connected in sequence. The main magnet is a permanent magnet and the auxiliary magnet is an electromagnet. The correction coil on the electromagnet is electrically connected to the acquisition and control circuit. The radial magnetic field gradient of the static magnetic field is corrected by adjusting the direction and magnitude of the current in the correction coil.

[0006] Preferably, the spacing adjustment assembly includes a telescopic actuator fixed inside the drill pipe. The telescopic end of the telescopic actuator is connected to a first push ring via a first push rack. The first push ring is connected to one of the composite magnets. The first push rack meshes with a linkage gear inside the drill pipe. The linkage gear meshes with a second push rack. The second push rack is connected to a second push ring. The second push ring is connected to another composite magnet. Both the second push ring and the first push ring are slidably disposed inside the drill pipe. The telescopic actuator is electrically connected to the acquisition control circuit, and the distance between the two composite magnets is adjusted by adjusting the telescopic actuator's extension.

[0007] Preferably, a static magnetic field monitoring module is also provided, which includes multiple magnetic field sensors; the magnetic field sensors are electrically connected to the acquisition and control circuit, and the auxiliary magnet and spacing adjustment components are adjusted according to the actual magnetic field data.

[0008] Preferably, a cleaning mechanism is also provided, which includes two scraper rings slidably disposed on the outside of the drill pipe. The two scraper rings are connected by a connecting rod, and one of the scraper rings is connected to the reciprocating drive assembly through an extension sleeve.

[0009] Preferably, the reciprocating drive assembly includes a drive ring rotatably connected to the drill rod, the outer wall of the drive ring having a corrugated groove, a drive post being provided on the inner side of the extension sleeve, the drive post being disposed in the corrugated groove, the drive ring being driven to rotate by a ring motor, and the ring motor being connected to the acquisition and control circuit.

[0010] One method involves using the aforementioned in-situ detection device for coalbed methane reservoir gas saturation to detect the gas saturation of coalbed methane reservoirs. The specific steps are as follows: Step S1: Drilling stage; The drilling rig is drilling normally, and the magnetic field generating mechanism and the detection mechanism are in a non-working state, while the cleaning mechanism continues to work during the drilling process; Step S2: Reaching the target measurement point, reaching the static and stable stage; After reaching the target measuring point, drilling is stopped, the mud circulation discharge is reduced, the cleaning mechanism is delayed and shut down, and the static time is set according to the magnetic field data collected by the static magnetic field monitoring module. Step S3: Magnetic field adjustment stage; The auxiliary magnet and spacing adjustment components are adjusted based on the magnetic field data collected by the static magnetic field monitoring module. Step S4: The detection agency excites and acquires the formation spin echo signal; Step S5: Calculate the gas saturation based on the collected formation spin echo signal.

[0011] Preferably, in step S3, the magnetic field data collected by the static magnetic field monitoring module includes the magnetic field amplitude and the radial magnetic field gradient; The difference between the magnetic field amplitude and the standard magnetic field amplitude threshold, as well as the required detection depth, are related to the spacing between the composite magnets. The specific calculation formula is as follows: ; in, The distance between the two composite magnets The initial distance between the two composite magnets This is the magnetic field amplitude adjustment coefficient. For depth correction factor, The measured amplitude of the static magnetic field at the center point. The target standard magnetic field amplitude threshold, To preset the radial detection depth of the target, The baseline detection depth.

[0012] Preferably, in step S3, the corresponding auxiliary magnet is adjusted according to the radial magnetic field gradient data, and the adjustment formula is as follows: ; in, To compensate for the current in the correction coil of the auxiliary magnet, To provide the static bias current for the auxiliary magnet, This is the gradient compensation ratio coefficient. and These are the measured radial magnetic field gradient and the target standard radial magnetic field gradient, respectively.

[0013] Preferably, in step S4, the acquisition control circuit drives the radio frequency coil to emit 90° and 180° refocusing pulses, and acquires the formation spin echo signal through the receiving coil, and transmits the received formation spin echo signal to the surface digital signal processor, which solves the transverse relaxation time distribution spectrum through numerical algorithms. In step S5, the amplitudes of the microporous adsorbed methane signal, the amplitudes of the crack free methane signal, the amplitude of the water signal, and the amplitude of the total fluid are obtained based on the transverse relaxation time distribution spectrum. The total fluid amplitude is as follows: ; in, This represents the total fluid amplitude. and These represent the signal amplitudes of methane adsorbed in micropores and free methane released through cracks, respectively. This refers to the amplitude of the water signal. The formula for calculating gas saturation is as follows: ; in, For gas saturation, The magnetic field gradient correction coefficient is expressed as follows: ; in, The calibration constant for the coal seam block is taken as 0.6-1.2; The measured radial magnetic field gradient after correction for the auxiliary magnet.

[0014] Therefore, the present invention, employing the above-mentioned in-situ detection device and method for gas saturation in coalbed methane reservoirs, has the following beneficial effects: (1) The composite magnet structure is adopted. On the one hand, the spacing between the two composite magnets is adjusted by the spacing adjustment component to achieve large-scale magnetic field gradient adjustment. On the other hand, the local asymmetric magnetic field distortion is offset by the correction coil current magnitude and polarity fine adjustment in the electromagnet. The magnetic field uniformity is improved, which can compensate for various magnetic field drifts caused by high temperature demagnetization and mud impurity magnetism, and improve detection accuracy.

[0015] (2) During the drilling stage, the ring motor is turned on to drive the scraper ring to slide back and forth, continuously scraping off the mud cake on the well wall, breaking up the ferromagnetic debris and iron-containing rock fragments that are aggregated in the annular mud. The scraper ring moves upward with the mud returning to the surface, reducing the amount of magnetic solid phase enrichment in the nuclear magnetic sensitive detection area from the source. After reaching the detection point, the cleaning mechanism is turned off after a delay. Combined with the process of reducing the discharge rate and allowing the ferromagnetic particles to settle away from the radio frequency detection window, the degree of dynamic distortion of the static magnetic field of the ferromagnetic particles is greatly reduced, and the relaxation spectrum distortion is reduced.

[0016] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0017] Figure 1 This is a cross-sectional view of an in-situ detection device for gas saturation in coalbed methane reservoirs according to the present invention. Figure 2 This is a schematic diagram of the magnetic field generating mechanism of the present invention; Figure 3 This is a schematic diagram of the cleaning mechanism structure of the present invention; Figure 4 This is a flowchart of the method of the present invention.

[0018] Figure Labels 1. Magnetic field generating mechanism; 11. Composite magnet; 111. Main magnet; 112. Auxiliary magnet; 12. Gap adjustment assembly; 121. Telescopic actuator; 122. First push ring; 123. First push rack; 124. Linkage gear; 125. Second push rack; 126. Second push ring; 2. Detection mechanism; 3. Acquisition and control circuit; 4. Wellhead digital signal processor; 5. Magnetic field sensor; 6. Cleaning mechanism; 61. Scraper ring; 62. Connecting rod; 63. Reciprocating drive assembly; 631. Drive ring; 632. Corrugated groove; 633. Ring motor; 634. Extension sleeve; 635. Drive column; 7. Drill pipe; 8. Mud channel. Detailed Implementation

[0019] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product is in use. They are used only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," and "connect" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0020] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0021] Example 1 like Figure 1 As shown, an in-situ detection device for gas saturation in a coalbed methane reservoir includes a magnetic field generating mechanism 1, a detection mechanism 2, an acquisition and control circuit 3, and a surface digital signal processor 4. The surface digital signal processor 4 communicates with the acquisition and control circuit 3 and calculates the gas saturation from the acquired formation spin echo signal. The surface digital signal processor 4 and the acquisition and control circuit 3 use existing circuit modules.

[0022] The magnetic field generating mechanism 1 includes two composite magnets 11 mounted on the mud channel 8. Each composite magnet 11 comprises a main magnet 111 and an auxiliary magnet 112, which are fixedly connected in sequence. The main magnet 111 is a permanent magnet, and the auxiliary magnet 112 is an electromagnet. A correction coil on the electromagnet is electrically connected to the acquisition and control circuit 3. The radial magnetic field gradient of the static magnetic field is corrected by adjusting the direction and magnitude of the current in the correction coil. The two composite magnets 11 are connected by a spacing adjustment assembly 12, such as... Figure 2 As shown, the spacing adjustment assembly 12 includes a telescopic driver 121 fixed inside the drill rod 7. The telescopic end of the telescopic driver 121 is connected to a first push ring 122 via a first push rack 123. The first push ring 122 is connected to one of the composite magnets 11. The first push rack 123 meshes with a linkage gear 124 inside the drill rod 7. The linkage gear 124 meshes with a second push rack 125. The second push rack 125 is connected to a second push ring 126. The second push ring 126 is connected to another composite magnet 11. Both the second push ring 126 and the first push ring 122 are slidably disposed inside the drill rod 7. The telescopic driver 121 is electrically connected to the acquisition control circuit 3. The spacing between the two composite magnets 11 is adjusted by adjusting the telescopic amount of the telescopic driver 121.

[0023] The detection mechanism 2 is embedded in the drill pipe 7 and positioned between the two composite magnets 11. The detection mechanism 2 includes a radio frequency coil and a receiving coil. The radio frequency coil is used to transmit refocusing pulses, and the receiving coil is used to receive formation spin echo signals. The acquisition control circuit 3 is connected to the radio frequency coil and is used to drive the radio frequency coil to emit refocusing pulses; the acquisition control circuit 3 is connected to the receiving coil and is used to acquire the formation spin echo signal acquired by the receiving coil; at the same time, the acquisition control circuit 3 is electrically connected to the composite magnet 11 and is used to adjust the static magnetic field strength. In order to adjust the corresponding components according to the actual static magnetic field, a static magnetic field monitoring module is also provided. The static magnetic field monitoring module includes multiple magnetic field sensors 5, which are arranged at multiple points to collect the static magnetic field at the center point and the radial magnetic field gradient. The magnetic field sensors 5 are electrically connected to the acquisition control circuit 3, and the auxiliary magnet 112 and the spacing adjustment component 12 are adjusted according to the actual magnetic field data.

[0024] Example 2 The difference between this embodiment and Embodiment 1 is that, in order to reduce the influence of magnetic materials on the detection, a cleaning mechanism 6 is also provided, such as... Figure 3As shown, the cleaning mechanism 6 includes two scraper rings 61 slidably disposed on the outside of the drill rod 7. The two scraper rings 61 are connected by a connecting rod 62, and one of the scraper rings 61 is connected to the reciprocating drive assembly 63 via an extension sleeve 634. The reciprocating drive assembly 63 includes a drive ring 631 rotatably connected to the drill rod 7. The outer wall of the drive ring 631 has a wavy groove 632. A drive post 635 is disposed on the inner side of the extension sleeve 634 and is disposed within the wavy groove 632. The drive ring 631 is driven to rotate by a ring motor 633, which is connected to the acquisition and control circuit 3.

[0025] Example 3 like Figure 4 As shown, one method uses the in-situ detection device for coalbed methane reservoir gas saturation described in Example 2 to detect the gas saturation of coalbed methane reservoirs. The specific steps are as follows: Step S1: Drilling stage; The drilling rig is drilling normally, and the magnetic field generating mechanism 1 and the detection mechanism 2 are in a non-working state, while the cleaning mechanism 6 continues to work during the drilling process.

[0026] Step S2: Reaching the target measurement point, reaching the static and stable stage; Upon reaching the target measuring point, drilling is stopped, the mud circulation rate is reduced, and the cleaning mechanism 6 shuts off after a delay. The set rest time is based on the magnetic field data collected by the static magnetic field monitoring module. The magnetic field data collected by the static magnetic field monitoring module includes the magnetic field amplitude and radial magnetic field gradient; when the magnetic field amplitude is not greater than the set threshold, the rest time is 15 seconds; when the magnetic field amplitude is greater than the set threshold, the rest time is 30-60 seconds. The drill string descends to the pre-set longitudinal measurement step distance (1m interval for main coal seam sections, 2-3m interval for homogeneous mudstone interlayers) to achieve intermittent detection.

[0027] Step S3: Magnetic field adjustment stage; The auxiliary magnet 112 and the spacing adjustment component 12 are adjusted based on the magnetic field data collected by the static magnetic field monitoring module. The difference between the magnetic field amplitude and the standard magnetic field amplitude threshold, as well as the required detection depth, are related to the spacing of the composite magnet 11. The specific calculation formula is as follows: ; in, The distance between the two composite magnets The initial distance between the two composite magnets This is the magnetic field amplitude adjustment coefficient. For depth correction factor, The measured amplitude of the static magnetic field at the center point. The target standard magnetic field amplitude threshold, To preset the radial detection depth of the target, The baseline detection depth.

[0028] The auxiliary magnet is adjusted based on the radial magnetic field gradient data, and the adjustment formula is as follows: ; in, To compensate for the current in the correction coil of the auxiliary magnet, To provide the static bias current for the auxiliary magnet, This is the gradient compensation ratio coefficient. and These are the measured radial magnetic field gradient and the target standard radial magnetic field gradient, respectively.

[0029] Step S4: Detection unit 2 excites and collects formation spin echo signals.

[0030] The acquisition control circuit 3 drives the radio frequency coil to emit refocusing pulses at 90° and 180°. Under pulse excitation, the hydrogen nuclei contained in methane and bound water in the formation pores generate spin resonance relaxation and release weak alternating electromagnetic signals. The weak alternating electromagnetic signals (formation spin echo signals) are acquired by the receiving coil and transmitted to the surface digital signal processor 4. The surface digital signal processor 4 solves the transverse relaxation time distribution spectrum through numerical algorithms.

[0031] Step S5: Calculate the gas saturation based on the collected formation spin echo signal.

[0032] The amplitudes of adsorbed methane in micropores, free methane in cracks, water, and total fluid were obtained based on the transverse relaxation time distribution spectrum.

[0033] The total fluid amplitude is as follows: ; in, This represents the total fluid amplitude. and These represent the signal amplitudes of methane adsorbed in micropores and free methane released through cracks, respectively. This represents the amplitude of the water signal.

[0034] The formula for calculating gas saturation is as follows: ; in, For gas saturation, The magnetic field gradient correction coefficient is expressed as follows: ; in, The calibration constant for the coal seam block is taken as 0.6-1.2; The measured radial magnetic field gradient after correction for the auxiliary magnet.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An in-situ detection device for gas saturation in coalbed methane reservoirs, characterized in that, include: The magnetic field generating mechanism includes two composite magnets sleeved on the mud channel, and the two composite magnets are connected by a spacing adjustment component. The detection mechanism is embedded in the drill pipe and positioned between two composite magnets. The composite magnets include a main magnet and an auxiliary magnet that are fixedly connected in sequence. The main magnet is a permanent magnet and the auxiliary magnet is an electromagnet. The detection mechanism includes a radio frequency coil and a receiving coil. The radio frequency coil is used to transmit refocusing pulses, and the receiving coil is used to receive formation spin echo signals. The acquisition control circuit is connected to the radio frequency coil to drive the radio frequency coil to emit refocusing pulses; the acquisition control circuit is also connected to the receiving coil to acquire the formation spin echo signal acquired by the receiving coil; at the same time, the acquisition control circuit is electrically connected to the composite magnet to adjust the static magnetic field strength. The wellhead digital signal processor communicates with the acquisition and control circuit and calculates the gas saturation based on the acquired formation spin echo signal.

2. The in-situ detection device for gas saturation in coalbed methane reservoirs according to claim 1, characterized in that: The correction coil on the electromagnet is electrically connected to the acquisition and control circuit. The radial magnetic field gradient of the static magnetic field is corrected by adjusting the direction and magnitude of the current in the correction coil.

3. The in-situ detection device for gas saturation in coalbed methane reservoirs according to claim 2, characterized in that: The spacing adjustment assembly includes a telescopic actuator fixed inside the drill pipe. The telescopic end of the telescopic actuator is connected to a first push ring via a first push rack. The first push ring is connected to one of the composite magnets. The first push rack meshes with a linkage gear inside the drill pipe. The linkage gear meshes with a second push rack. The second push rack is connected to a second push ring. The second push ring is connected to another composite magnet. Both the second push ring and the first push ring are slidably disposed inside the drill pipe. The telescopic actuator is electrically connected to the acquisition control circuit, and the distance between the two composite magnets is adjusted by adjusting the telescopic actuator's extension.

4. The in-situ detection device for gas saturation in coalbed methane reservoirs according to claim 3, characterized in that: It is also equipped with a static magnetic field monitoring module, which includes multiple magnetic field sensors. The magnetic field sensors are electrically connected to the acquisition and control circuit, and the auxiliary magnet and spacing adjustment components are adjusted according to the actual magnetic field data.

5. The in-situ detection device for gas saturation in coalbed methane reservoirs according to claim 4, characterized in that: It is also equipped with a cleaning mechanism, which includes two scraper rings that are slidably disposed on the outside of the drill pipe. The two scraper rings are connected by a connecting rod, and one of the scraper rings is connected to the reciprocating drive assembly through an extension sleeve.

6. The in-situ detection device for gas saturation in coalbed methane reservoirs according to claim 5, characterized in that: The reciprocating drive assembly includes a drive ring rotatably connected to the drill pipe. The outer wall of the drive ring has a corrugated groove, and a drive post is provided on the inner side of the extension sleeve. The drive post is located in the corrugated groove. The drive ring is driven to rotate by a ring motor, which is connected to the acquisition and control circuit.

7. A detection method, comprising using the in-situ detection device for gas saturation of coalbed methane reservoirs as described in claim 6 to detect the gas saturation of coalbed methane reservoirs, characterized in that, The specific steps are as follows: Step S1: Drilling stage; The drilling rig is drilling normally, and the magnetic field generating mechanism and the detection mechanism are in a non-working state, while the cleaning mechanism continues to work during the drilling process; Step S2: Reaching the target measurement point, reaching the static and stable stage; After reaching the target measuring point, drilling is stopped, the mud circulation discharge is reduced, the cleaning mechanism is delayed and shut down, and the static time is set according to the magnetic field data collected by the static magnetic field monitoring module. Step S3: Magnetic field adjustment stage; The auxiliary magnet and spacing adjustment components are adjusted based on the magnetic field data collected by the static magnetic field monitoring module. Step S4: The detection agency excites and acquires the formation spin echo signal; Step S5: Calculate the gas saturation based on the collected formation spin echo signal.

8. The detection method according to claim 7, characterized in that: In step S3, the magnetic field data collected by the static magnetic field monitoring module includes the magnetic field amplitude and the radial magnetic field gradient; The difference between the magnetic field amplitude and the standard magnetic field amplitude threshold, as well as the required detection depth, are related to the spacing between the composite magnets. The specific calculation formula is as follows: ； in, The distance between the two composite magnets The initial distance between the two composite magnets This is the magnetic field amplitude adjustment coefficient. For depth correction factor, The measured amplitude of the static magnetic field at the center point. The target standard magnetic field amplitude threshold, To preset the radial detection depth of the target, The baseline detection depth.

9. The detection method according to claim 8, characterized in that: In step S3, the corresponding auxiliary magnet is adjusted based on the radial magnetic field gradient data, and the adjustment formula is as follows: ； in, To compensate for the current in the correction coil of the auxiliary magnet, To provide the static bias current for the auxiliary magnet, This is the gradient compensation ratio coefficient. and These are the measured radial magnetic field gradient and the target standard radial magnetic field gradient, respectively.

10. The detection method according to claim 9, characterized in that: In step S4, the acquisition control circuit drives the radio frequency coil to emit 90° and 180° refocusing pulses, and acquires the formation spin echo signal through the receiving coil. The received formation spin echo signal is then transmitted to the surface digital signal processor, which solves the transverse relaxation time distribution spectrum using a numerical algorithm. In step S5, the amplitudes of the microporous adsorbed methane signal, the amplitudes of the crack free methane signal, the amplitude of the water signal, and the amplitude of the total fluid are obtained based on the transverse relaxation time distribution spectrum. The total fluid amplitude is as follows: ； in, This represents the total fluid amplitude. and These represent the signal amplitudes of methane adsorbed in micropores and free methane released through cracks, respectively. This refers to the amplitude of the water signal. The formula for calculating gas saturation is as follows: ； in, For gas saturation, The magnetic field gradient correction coefficient is expressed as follows: ； in, The calibration constant for the coal seam block is taken as 0.6-1.2; The measured radial magnetic field gradient after correction for the auxiliary magnet.

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