Coal body pore fracture evolution and damage quantitative evaluation method based on NMR and fractal theory

Abstract

The invention discloses a coal body pore fracture evolution and damage quantitative evaluation method based on an NMR and fractal theory, and the method comprises the following steps: S1, carrying out water saturation tests on a prepared coal rock sample under different conditions through a nuclear magnetic resonance imaging analysis system, and obtaining an accumulated nuclear magnetic signal spectrum after water saturation under different conditions and an NMRI internal pore fracture distribution form graph; s2, analyzing an accumulated nuclear magnetic signal spectrum obtained after water saturation under each condition and an NMRI internal pore and fracture distribution pattern diagram to obtain porosity, fractal dimension and pore and fracture area expansion increment, and comparing changes of parameters before and after water saturation under different conditions to obtain a porosity damage factor, a fractal dimension damage factor and a pore and fracture expansion damage factor. According to the method, the internal pore damage condition of the coal body is defined by utilizing the pore fracture area expansion increment, the fractal dimension and the porosity, the damage degree of the coal body can be quantitatively described by adopting multi-factor pore structure damage characterization, and the method provides a theoretical basis for exploring the microstructure damage rule of the coal body.

Description

Technical Field

[0001] This invention belongs to the field of deep gas thermal recovery and coal seam hydraulic fracturing technology, specifically involving a method for evaluating the evolution of coal body pore fractures and quantitative damage based on NMR and fractal theory. Background Technology

[0002] Engineering fields such as deep gas thermal recovery and coal seam hydraulic fracturing involve the thermo-mechanical coupling effect of coal and rock, especially the significant changes in the fracture structure and permeability of coal and rock under high or low temperatures. When temperature or other conditions change, the mismatched thermal expansion stress inside the coal and rock leads to the development of primary fractures, resulting in changes in coal and rock permeability. Currently, quantitative characterization methods for the development of internal cracks in coal and rock caused by temperature changes and other conditions are still lacking. Summary of the Invention

[0003] This invention proposes a method for evaluating the evolution and damage quantification of coal pores and fractures based on NMR and fractal theory. This method can quantify the degree of damage to the coal pore and fracture structure, which is beneficial for accurately characterizing the changes in coal pores and fractures during the temperature rise of low-permeability reservoirs and exploring the progressive damage law of coal microstructure.

[0004] Therefore, the technical solution adopted in this invention is: a method for evaluating the evolution of coal pore fractures and quantitative damage based on NMR and fractal theory, comprising the following steps:

[0005] S1: Prepare samples and conduct experiments; use an NMR imaging analysis system to conduct water saturation tests on the prepared coal and rock samples under different conditions, and obtain cumulative NMR signal spectra and NMRI internal pore and fracture distribution morphology maps after water saturation under different conditions.

[0006] S2: Analyze the cumulative NMR signal spectrum and NMRI internal pore and crack distribution morphology map obtained after water saturation under each condition to obtain porosity, fractal dimension and pore and crack area expansion ratio. Then compare the changes of parameters before and after water saturation under different conditions to obtain porosity damage factor, fractal dimension damage factor and pore and crack expansion damage factor.

[0007] S21: Based on the cumulative nuclear magnetic resonance signal spectrum, the unit volume signal amplitude measured by the coal sample is substituted into the calibration equation to obtain the porosity of the coal sample at each stage. Then, according to the porosity under different conditions, the calculation formula of the porosity damage factor is obtained.

[0008] S22: Based on the fractal geometric approximation calculation formula corresponding to the cumulative nuclear magnetic resonance signal spectrum, the fractal dimension of the pores and fractures at each stage of the coal sample can be obtained. Then, based on the fractal dimension of the pores and fractures under different conditions, the calculation formula of the pore and fracture fractal dimension damage factor can be obtained.

[0009] S23: Based on the internal pore and fracture distribution morphology map of NMRI, pore and fracture maps at the same cross-sectional location are extracted. The pore and fracture maps are binarized, and edge detection technology is used to perform contour detection on the binary images and calculate the pixel area of ​​the pore and fracture region and the coal body region. By changing the pore and fracture area difference of the coal and rock sample before and after changing the conditions, the pore and fracture area expansion amount of the cross section is defined, and the pore and fracture area expansion ratio under different conditions is obtained, so as to obtain the pore and fracture expansion damage factor of the same cross section.

[0010] As a preferred option among the above schemes, during the analysis process using the nuclear magnetic resonance imaging (NMR) analysis system, NMR technology utilizes the interaction mechanism between hydrogen-containing fluids and coal and rock samples, and combines the relaxation signals of hydrogen-containing fluids within the pores and fractures of the coal and rock samples to characterize the microstructure and fluid transport characteristics of the coal and rock samples, as detailed below:

[0011] The analysis of fluids in the pore fractures of coal and rock samples uses transverse relaxation time. It possesses three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation, which can be represented as:

[0012]

[0013] in, Represents the diffusion relaxation time. Represents the time of free relaxation. Representing the surface relaxation time, since the contributions of free relaxation and diffusion relaxation are much smaller than those of surface relaxation, the formula can be expressed as:

[0014]

[0015] in, The surface relaxation rate, This represents the volume of hydrogen-containing fluid within the pores and fractures of the coal and rock sample. Let be the pore surface area. Assuming the pores and fractures inside the coal and rock sample have a simple structure, the formula can be transformed into:

[0016]

[0017] in, For pore and fracture geometry factor, Let be the pore radius. Based on the above formula, the relationship between pore characteristic parameters and transverse relaxation time values ​​can be established.

[0018] Further optimization reveals that the calibration equation in S21 is obtained through the following method: Standard oil samples with known porosity are placed into the nuclear magnetic resonance imaging analysis system, and the corresponding nuclear magnetic resonance signal quantities are measured. The linear relationship between porosity and signal quantity is then obtained using nuclear magnetic resonance analysis software. This linear relationship is the calibration equation for porosity. ;

[0019] The porosity of the coal and rock sample was calculated based on the cumulative nuclear magnetic resonance (NMR) spectrum, as follows:

[0020]

[0021] in, The porosity of the coal and rock sample. For the integral area, For volume, the porosity damage factor is calculated as follows:

[0022]

[0023] in, Porosity damage factor , To determine the porosity of coal and rock samples before and after changing the conditions. The formula for calculating the porosity damage factor, obtained by varying the conditions multiple times, is as follows:

[0024]

[0025] in, , This represents the integral area of ​​the coal and rock samples before and after the change in conditions.

[0026] Further optimization yields the following fractal geometric approximation formula in S22:

[0027]

[0028] Taking the logarithm of both sides of the above equation, we get:

[0029]

[0030] in, For the lateral relaxation time, The maximum lateral relaxation time. For the lateral relaxation time to be less than The percentage of the cumulative pore volume to the total pore volume. The fractal dimension is obtained by calculation based on the cumulative NMR signal spectrum. and The value, and then thought x-axis, Establish a coordinate system for the vertical axis and connect the lines to obtain the coordinates. and The relationship curve is fitted to obtain the slope of the relationship curve. Thus, the fractal dimension is obtained as ;

[0031] The fractal dimension damage factor is calculated as follows:

[0032]

[0033] in, , To determine the fractal dimension of coal and rock samples before and after changing conditions. To determine the number of times the conditions are changed, the formula for calculating the fractal dimension damage factor can be obtained as follows:

[0034]

[0035] in, , To determine the corresponding coal and rock samples before and after changing conditions and The slope of the relationship curve.

[0036] Further optimization, in fitting and When using a relationship curve, according to The size is divided into regions, and piecewise linear fitting is performed on each region. The slope of the entire relationship curve is the mean of the slopes of the regions.

[0037] Further optimization, in S23, by using edge detection technology to perform contour detection on the binary image and calculating the pixel area of ​​the pore and fracture region and the coal body region, the proportion of pores and fractures in the cross-section can be obtained. The calculation of the pore fracture area expansion is as follows:

[0038]

[0039] in, , The initial pore area ratio and the pore and fracture area ratio during the damage process are respectively defined as the proportion of pore and fracture area expansion in the coal body before and after the change of conditions to the initial coal body. The pore and fracture area expansion ratio is defined as the ratio of pore and fracture area expansion before and after the change of conditions to the initial coal body. The calculation is as follows:

[0040] ;

[0041] Simultaneously, a pore crack propagation damage factor was proposed. The calculation is as follows:

[0042]

[0043] in, , Let the increase in pore fracture area before and after the change in conditions during the damage process of the same cross-section be represented by the following formula for calculating the pore fracture propagation damage factor:

[0044]

[0045] in , These represent the percentage of pore and crack area before and after the change of conditions during the damage process of the same cross section.

[0046] Further preferred, in S1, different conditions include any one of the following changes: temperature, confining pressure, and injection pressure, or a combination of two or more.

[0047] Further preferably, the coal and rock sample is a cylindrical coal and rock sample with a diameter of 25 mm and a height of 100 mm, and the types of coal and rock samples include lignite, bituminous coal and anthracite.

[0048] The beneficial effects of this invention are as follows: Based on the damage mechanics theory, damage factors are defined. Based on the changes in pores and cracks in the same coal and rock sample, the internal pore damage of the coal body is defined by the pore area expansion ratio, fractal dimension, and porosity. The multi-factor pore structure damage characterization method can quantitatively describe the degree of coal body damage. This method provides a theoretical basis for exploring the damage law of the microstructure of coal body. Attached Figure Description

[0049] Figure 1 This is a flowchart of the present invention.

[0050] Figure 2 This is the cumulative NMR signal spectrum obtained during the detection of various coal rocks in this embodiment.

[0051] Figure 3 This is a NMRI image showing the distribution of internal pores and fractures obtained during coal and rock testing in this embodiment.

[0052] Figure 4 This is a coordinate diagram of the damage factors obtained by this method for each coal and rock in this embodiment. Detailed Implementation

[0053] The present invention will be further described below with reference to the embodiments and accompanying drawings:

[0054] like Figure 1 As shown, a method for evaluating the evolution of coal pore fractures and quantitative damage based on NMR (nuclear magnetic resonance) and fractal theory includes the following steps:

[0055] Step 1: Sample preparation and testing. Using an NMR (nuclear magnetic resonance imaging) analysis system, the prepared coal and rock samples were subjected to water saturation tests under different conditions. The cumulative NMR signal spectra and NMR (nuclear magnetic resonance imaging) morphology maps of internal pore and fracture distribution were obtained after water saturation under different conditions. These different conditions can be any change in temperature, confining pressure, or injection pressure, or a combination of two or more of these conditions.

[0056] During the analysis using the nuclear magnetic resonance imaging system, NMR technology utilizes the interaction mechanism between hydrogen-containing fluids and coal and rock samples, and combines the relaxation signals of hydrogen-containing fluids within the pores and fissures of the coal and rock samples to characterize the microstructure and fluid transport characteristics of the coal and rock samples.

[0057] The analysis of fluids in the pore fractures of coal and rock samples uses transverse relaxation time. It exhibits three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation. This can be represented as:

[0058]

[0059] in, Represents the diffusion relaxation time. Represents the time of free relaxation. Representing the surface relaxation time, since the contributions of free relaxation and diffusion relaxation are much smaller than those of surface relaxation, the formula can be expressed as:

[0060]

[0061] in, The surface relaxation rate (a fixed value for the same type of coal and rock sample). This represents the volume of hydrogen-containing fluid within the pores and fractures of the coal and rock sample. Let be the pore surface area. Assuming the pores and fractures inside the coal and rock sample have a simple structure, the formula can be transformed into:

[0062]

[0063] in, This refers to the pore / fracture geometry factor (its value is closely related to the pore / fracture geometry; for spherical pores / fractures...). A value of 3 indicates a columnar pore fracture. The value is 2). Let be the pore radius. Based on the above formula, the relationship between pore characteristic parameters and transverse relaxation time values ​​can be established.

[0064] The second step involves analyzing the cumulative NMR signal spectrum and NMRI internal pore and fissure distribution morphology obtained after saturation under each condition to obtain porosity, fractal dimension, and pore and fissure area expansion ratio. Then, by comparing the changes in parameters before and after saturation under different conditions, porosity damage factor, fractal dimension damage factor, and pore and fissure expansion damage factor are obtained.

[0065] 1) Porosity. Based on the cumulative nuclear magnetic resonance signal spectrum, the signal amplitude per unit volume of the coal sample is substituted into the calibration equation to obtain the porosity of the coal sample at each stage. Then, according to the porosity under different conditions, the calculation formula of the porosity damage factor is obtained.

[0066] Specifically as follows:

[0067] The calibration equation is obtained as follows: Standard oil samples with known porosity are placed into the nuclear magnetic resonance imaging analysis system, and the corresponding nuclear magnetic resonance signal quantities are measured. The linear relationship between porosity and signal quantity is obtained using nuclear magnetic resonance analysis software. This linear relationship is the calibration equation for porosity. The porosity of the coal and rock samples was calculated based on the cumulative nuclear magnetic resonance (NMR) spectrum, as follows:

[0068]

[0069] in, The porosity of the coal and rock sample. For the integral area, For volume, the porosity damage factor is calculated as follows:

[0070]

[0071] in, Porosity damage factor , To determine the porosity of coal and rock samples before and after changing the conditions. The formula for calculating the porosity damage factor, obtained by varying the conditions multiple times, is as follows:

[0072]

[0073] in, , This represents the integral area of ​​the coal and rock samples before and after the change in conditions.

[0074] 2) Fractal Dimension. Based on the fractal geometric approximation formula corresponding to the cumulative NMR signal spectrum, the fractal dimension of pores and fractures at each stage of the coal sample can be obtained. Then, based on the fractal dimension of pores and fractures under different conditions, the calculation formula for the pore and fracture fractal dimension damage factor is obtained. Specifically:

[0075] The fractal geometric approximation formula is:

[0076]

[0077] Taking the logarithm of both sides of the above equation, we get:

[0078]

[0079] in, For the lateral relaxation time, The maximum lateral relaxation time. For the lateral relaxation time to be less than The percentage of the cumulative pore volume to the total pore volume. The fractal dimension can be calculated based on the cumulative NMR spectrum. and The value, and then thought x-axis, Establish a coordinate system for the vertical axis and connect the lines to obtain the coordinates. and The relationship curve is fitted to obtain the slope of the relationship curve. Thus, the fractal dimension is obtained as .

[0080] The fractal dimension damage factor is calculated as follows:

[0081]

[0082] in, , To determine the fractal dimension of coal and rock samples before and after changing conditions. To determine the number of times the conditions are changed, the formula for calculating the fractal dimension damage factor can be obtained as follows:

[0083]

[0084] in, , To determine the corresponding coal and rock samples before and after changing conditions and The slope of the relationship curve.

[0085] 3) Pore and fracture area expansion ratio. Based on the pore and fracture distribution morphology map inside NMRI, pore and fracture maps at the same cross-sectional location are extracted. The pore and fracture maps are binarized, and edge detection technology is used to perform contour detection on the binary images and calculate the pixel area of ​​the pore and fracture region and the coal body region (usually white represents pores and fractures, and black represents coal body). The pore and fracture area expansion ratio of the cross-section is defined by the difference in pore and fracture area of ​​the coal and rock samples before and after changing the conditions. The pore and fracture area expansion ratio under different conditions is obtained, thus obtaining the pore and fracture expansion damage factor of the same cross-section.

[0086] In the specific processing, the captured image is first converted to the HSV (Hue, Saturation, Value) color space to accurately distinguish different regions in the image. Then, during binarization, a threshold range needs to be set. Specifically:

[0087] By using edge detection technology to perform contour detection on a binary image and calculating the pixel area of ​​the pore and fracture region and the coal body region (i.e., calculating the pixel area of ​​the white region and the black region respectively), the proportion of pores and fractures in the cross-section can be obtained. The difference in pore and fracture area during coal damage is defined as the pore and fracture area expansion. The calculation of the pore and fracture area expansion is as follows:

[0088]

[0089] in, , The initial pore area ratio and the pore and fracture area ratio during the damage process are respectively defined as the proportion of pore and fracture area expansion in the coal body before and after the change of conditions to the initial coal body. The pore and fracture area expansion ratio is defined as the ratio of pore and fracture area expansion before and after the change of conditions to the initial coal body. The calculation is as follows:

[0090] ;

[0091] Simultaneously, a pore crack propagation damage factor was proposed. The calculation is as follows:

[0092]

[0093] in, , Let the increase in pore fracture area before and after the change in conditions during the damage process of the same cross-section be represented by the following formula for calculating the pore fracture propagation damage factor:

[0094]

[0095] in , These represent the percentage of pore and crack area before and after the change of conditions during the damage process of the same cross section.

[0096] In summary, the evolution and damage of coal pores and fractures can be quantified using the following formula.

[0097] Specific Implementation

[0098] Multiple standard cylindrical coal samples with a diameter of 25 mm and a height of 100 mm were prepared. Each sample was tested using a coal porous flow nuclear magnetic resonance imaging (NMR) system (Suzhou Newmax Analytical Instruments Co., Ltd.'s high-temperature, high-pressure NMR online flow analysis and imaging system). The coal samples included lignite H1, bituminous coal Y1, and anthracite WY1. With only temperature variations (as shown in Table 1 below, which lists the test parameters for each coal sample), cumulative NMR signal spectra and NMR morphology maps of internal pores and fractures were obtained after water saturation at different temperatures. See attached table for details. Figure 2 and attached Figure 3 , attached Figure 2In the middle (a), (b), and (c), the cumulative NMR spectrum of lignite H1, bituminous coal Y1, and anthracite WY1 are shown respectively.

[0099] Table 1 Parameter Setting Table

[0100] coal sample D / mm H / mm Temperature / °C Injection pressure / MPa Confining pressure / MPa Lignite H1 25 50 25、50、75、100 3 5 Bituminous coal Y1 25 50 25、50、75、100 3 5 Anthracite WY1 25 50 25、50、75、100 3 5

[0101] In the porosity calculation, the porosity mark was measured using a standard oil sample provided with the equipment. The formula for calculating the porosity damage factor. The value is 3.53. (This is in the context of establishing...) and After obtaining the relationship curve, it was found that the slope of the entire fitted curve varied significantly with different pore radii. To better obtain the slope of the relationship curve, [further details are needed]. The size is further divided into microporous fractal regions ( ), small aperture fractal region ( ), Mesopore fractal region ( ), fractal regions of macropores and microcracks ( Four fractal regions were defined, and piecewise linear fitting was performed on each region. The slope of the entire relationship curve was the mean of the slopes of the regions.

[0102] In the calculation of the increase in pore and fracture area, the set threshold ranges are: hue 100-140, saturation 50-255, and brightness 50-255. Based on the above calculations, the damage factors for each coal and rock sample are shown in the appendix. Figure 4 As shown, the damage factors of coal samples of different coal grades all show an increasing trend with increasing temperature. Among them, lignite H1 and anthracite WY1 have a linear growth relationship, while bituminous coal Y1 has an exponential growth relationship.

Claims

1. A method for quantitative evaluation of coal pore fracture evolution and damage based on NMR and fractal theory, characterized in that, Includes the following steps: S1: Prepare samples and conduct experiments; use an NMR imaging analysis system to conduct water saturation tests on the prepared coal and rock samples under different conditions, and obtain cumulative NMR signal spectra and NMRI internal pore and fracture distribution morphology maps after water saturation under different conditions. S2: Analyze the cumulative NMR signal spectrum and NMRI internal pore and crack distribution morphology map obtained after water saturation under each condition to obtain porosity, fractal dimension and pore and crack area expansion ratio. Then compare the changes of parameters before and after water saturation under different conditions to obtain porosity damage factor, fractal dimension damage factor and pore and crack expansion damage factor. S21: Based on the cumulative nuclear magnetic resonance signal spectrum, the unit volume signal amplitude measured by the coal sample is substituted into the calibration equation to obtain the porosity of the coal sample at each stage. Then, according to the porosity under different conditions, the calculation formula of the porosity damage factor is obtained. S22: Based on the fractal geometric approximation calculation formula corresponding to the cumulative nuclear magnetic resonance signal spectrum, the fractal dimension of the pores and fractures at each stage of the coal sample can be obtained. Then, based on the fractal dimension of the pores and fractures under different conditions, the calculation formula of the pore and fracture fractal dimension damage factor can be obtained. S23: Based on the internal pore and fracture distribution morphology map of NMRI, pore and fracture maps at the same cross-sectional location are extracted. The pore and fracture maps are binarized, and edge detection technology is used to perform contour detection on the binary images and calculate the pixel area of ​​the pore and fracture region and the coal body region. By changing the pore and fracture area difference of the coal and rock sample before and after changing the conditions, the pore and fracture area expansion amount of the cross section is defined, and the pore and fracture area expansion ratio under different conditions is obtained, so as to obtain the pore and fracture expansion damage factor of the same cross section.

2. The method for evaluating the evolution of coal pore fractures and quantitative damage based on NMR and fractal theory as described in claim 1, characterized in that, During the analysis using the nuclear magnetic resonance imaging (NMR) system, NMR technology utilizes the interaction mechanism between hydrogen-containing fluids and coal samples, and combines this with relaxation signals of the hydrogen-containing fluids within the pores and fractures of the coal samples to characterize the microstructure and fluid transport features of the coal samples, as detailed below: The analysis of fluids in the pore fractures of coal and rock samples uses transverse relaxation time. It possesses three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation, which can be represented as: in, Represents the diffusion relaxation time. Represents the time of free relaxation. Representing the surface relaxation time, since the contributions of free relaxation and diffusion relaxation are much smaller than those of surface relaxation, the formula can be expressed as: ,in, The surface relaxation rate, This represents the volume of hydrogen-containing fluid within the pores and fractures of the coal and rock sample. Let be the pore surface area. Assuming the pores and fractures inside the coal and rock sample have a simple structure, the formula can be transformed into: in, For pore and fracture geometry factor, Let be the pore radius. Based on the above formula, the relationship between pore characteristic parameters and transverse relaxation time values ​​can be established.

3. The method for evaluating the evolution of coal pore fractures and quantitative damage based on NMR and fractal theory as described in claim 1, characterized in that, The calibration equation in S21 is obtained as follows: Standard oil samples with known porosity are placed into the nuclear magnetic resonance imaging analysis system, and the corresponding nuclear magnetic resonance signal quantities are measured. The linear relationship between porosity and signal quantity is obtained using nuclear magnetic resonance analysis software. This linear relationship is the calibration equation for porosity. ; The porosity of the coal and rock samples was calculated based on the cumulative nuclear magnetic resonance (NMR) spectrum, as follows: in, Porosity of the coal and rock sample. For the integral area, For volume, the porosity damage factor is calculated as follows: in, Porosity damage factor , To determine the porosity of coal and rock samples before and after changing the conditions. The formula for calculating the porosity damage factor, obtained by varying the conditions multiple times, is as follows: ,in, , This represents the integral area of ​​the coal and rock samples before and after the change in conditions.

4. The method for evaluating the evolution of coal pore fractures and the quantitative assessment of damage based on NMR and fractal theory as described in claim 1, characterized in that: The fractal geometric approximation formula in S22 is: Taking the logarithm of both sides of the above equation, we get: in, For the lateral relaxation time, The maximum lateral relaxation time. For the lateral relaxation time to be less than The percentage of the cumulative pore volume to the total pore volume. The fractal dimension is obtained by calculation based on the cumulative NMR signal spectrum. and The value, and then thought x-axis, Establish a coordinate system for the vertical axis and connect the lines to obtain the coordinates. and The relationship curve is fitted to obtain the slope of the relationship curve. Thus, the fractal dimension is obtained as ; The fractal dimension damage factor is calculated as follows: in, , To determine the fractal dimension of coal and rock samples before and after changing conditions. To determine the number of times the conditions are changed, the formula for calculating the fractal dimension damage factor can be obtained as follows: ,in, , To determine the corresponding coal and rock samples before and after changing conditions and The slope of the relationship curve.

5. The method for evaluating the evolution of coal pore fractures and the quantitative assessment of damage based on NMR and fractal theory as described in claim 4, characterized in that: In fitting and When using a relationship curve, according to The size is divided into regions, and piecewise linear fitting is performed on each region. The slope of the entire relationship curve is the mean of the slopes of the regions.

6. The method for evaluating the evolution of coal pore fractures and the quantitative assessment of damage based on NMR and fractal theory as described in claim 1, characterized in that: In S23, by using edge detection technology to perform contour detection on the binary image and calculating the pixel area of ​​the pore and fracture region and the coal body region, the proportion of pores and fractures in the cross-section can be obtained. The calculation of the pore fracture area expansion is as follows: in, , The initial pore area ratio and the pore and fracture area ratio during the damage process are respectively defined as the proportion of pore and fracture area expansion in the coal body before and after the change of conditions to the initial coal body. The pore and fracture area expansion ratio is defined as the ratio of pore and fracture area expansion before and after the change of conditions to the initial coal body. The calculation is as follows: ； Simultaneously, a pore crack propagation damage factor was proposed. The calculation is as follows: in, , Let the increase in pore fracture area before and after the change in conditions during the damage process of the same cross-section be represented by the following formula for calculating the pore fracture propagation damage factor: ,in , These represent the percentage of pore and crack area before and after the change of conditions during the damage process of the same cross section.

7. The method for evaluating the evolution of coal pore fractures and the quantitative assessment of damage based on NMR and fractal theory as described in claim 1, characterized in that: In S1, different conditions include any one of the following: changes in temperature, changes in confining pressure, and changes in injection pressure, or a combination of two or more of these.

8. The method for evaluating the evolution of coal pore fractures and the quantitative assessment of damage based on NMR and fractal theory as described in claim 1, characterized in that: The coal and rock samples used are cylindrical coal and rock samples with a diameter of 25 mm and a height of 100 mm. The types of coal and rock samples include lignite, bituminous coal and anthracite.

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