A method for evaluating reservoir pore structure

Abstract

The present invention provides a method for evaluating the pore structure of a reservoir, which relates to the technical field of oil and gas exploration. The method includes preparing a dried sample of a rock sample, reconstructing a three-dimensional digital X-ray core by X-ray tomography, measuring the nuclear magnetic resonance relaxation of the dried sample to obtain the nuclear magnetic resonance T2 spectrum, obtaining the nuclear magnetic resonance T2 spectrum and pore space distribution of the dried sample after saturation treatment, calibrating the three-dimensional digital core with porosity information to obtain an accurate pore image, and finally quantitatively evaluating the pore structure characteristics. The pore extraction of the three-dimensional digital X-ray core is constrained by the nuclear magnetic resonance relaxation signal, and the three-dimensional digital core of nuclear magnetic resonance imaging is used for verification to ensure the consistency and accuracy of pore structure identification. The present invention can significantly improve the evaluation accuracy and reliability, provide support for the calculation of physical property parameters and the evaluation of fluid properties of unconventional reservoirs, facilitate the optimization of reservoir development strategies, and improve the resource exploitation efficiency and economic benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration, and particularly relates to a method for evaluating the pore structure of a reservoir. Background Art

[0002] Unconventional reservoirs, such as shale gas and tight oil, pose great challenges to traditional oil and gas exploration technologies due to their complex geological compositions and irregular physical properties. The complexity and uncertainty of the pore structures in unconventional reservoirs lead to the inability of traditional evaluation methods, such as one-dimensional NMR and X-ray scanning, to accurately describe their porosity, pore size distribution, permeability, etc.

[0003] Nuclear magnetic resonance (NMR) technology performs well in the exploration of unconventional reservoirs. Especially one-dimensional NMR technology can calculate key parameters such as the porosity and pore size distribution of a reservoir by measuring the T2 spectrum. However, one-dimensional NMR technology can only provide macroscopic response characteristics and lacks spatial resolution, making it difficult to describe pore structure characteristics.

[0004] In terms of pore structure evaluation, nuclear magnetic resonance imaging (MRI) technology can capture the spatial distribution of hydrogen nuclei in a reservoir, is highly sensitive to fluids, and insensitive to other factors such as the rock skeleton, thus enabling the description of the spatial distribution of pores. In contrast, X-ray scanning imaging technology has a higher resolution than nuclear magnetic resonance imaging technology, capable of reaching a resolution of micrometers or even nanometers. However, in the case of low-density rock samples, it is difficult to accurately distinguish between the rock skeleton and pore fluids. Therefore, to accurately interpret pore space structure information, it is necessary to combine the high sensitivity of nuclear magnetic resonance to fluids with the high resolution of X-rays. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for evaluating the pore structure of a reservoir, which can integrate the advantages of X-ray scanning and nuclear magnetic resonance imaging technologies, provide a comprehensive and accurate method for evaluating the pore structure of a reservoir, and improve the exploration efficiency and resource utilization rate of unconventional reservoirs.

[0006] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0007] A method for evaluating the pore structure of a reservoir includes the following steps:

[0008] S1: Prepare a dried sample of the rock sample;

[0009] S2: Use X-rays to perform tomographic scanning on the dried sample of the rock sample and reconstruct a three-dimensional digital core of the X-rays;

[0010] S3: Use a nuclear magnetic resonance measuring device for the rock sample to measure the dried sample of the rock sample and obtain the nuclear magnetic resonance T2 spectrum of the dried sample of the rock sample;

[0011] S4: Saturate the dried sample of the rock sample to obtain a saturated sample of the rock sample;

[0012] S5: Perform nuclear magnetic resonance T2 spectrum measurement and nuclear magnetic resonance imaging scan on the saturated sample of the rock sample obtained in S4 to obtain the porosity information and pore space distribution characteristics of the core;

[0013] S6: Calibrate the three-dimensional digital core in S2 through the porosity information and pore space distribution characteristics obtained in S5 to obtain a calibrated three-dimensional digital core pore image;

[0014] S7: Quantitatively evaluate the pore structure characteristics of the reservoir based on the calibrated three-dimensional digital core pore image obtained in S6.

[0015] Furthermore: The specific steps for preparing the sample of the rock sample in step S1 include:

[0016] S101: Prepare a core plug sample, including drilling, cutting, and grinding;

[0017] S102: Wash the plug sample with oil and salt to remove oil and salt;

[0018] S103: Dry the cleaned core to ensure that the sample does not contain fluid to obtain a dried sample of the rock sample.

[0019] Furthermore: In step S2, a three-dimensional digital core is formed using a reconstruction algorithm for the results of X-ray scanning.

[0020] Furthermore: In step S4, a vacuum pumping and pressurizing saturation device is used to saturate the dried sample of the rock sample.

[0021] Furthermore: The specific steps of S5 are: Put the saturated sample of the rock sample obtained in S4 into a nuclear magnetic resonance relaxation measurement device and a nuclear magnetic resonance imaging scan device for measurement. The porosity value of the saturated sample of the rock sample is obtained through nuclear magnetic resonance T2 relaxation measurement, and the pore space distribution characteristics of the saturated sample of the rock sample can be obtained through nuclear magnetic resonance imaging scan.

[0022] Even further: The calculation method for constructing a nuclear magnetic resonance three-dimensional digital core is as follows:

[0023]

[0024] Equation (1) is used to calculate the conversion relationship between the echo signal and the relaxation component, b i represents t i the i th T 2j echo amplitude measured at timej The transverse relaxation time of each component T 2 , f j is the signal volume when the transverse relaxation time is T 2j . Solving the corresponding f j process of each relaxation component is called T2 spectrum inversion;

[0025] Equations (2) - (3) represent the inversion of the T2 spectrum using the singular value decomposition method. b = [b 1 , b 2 , …, b m T is the echo amplitude vector; x = [f 1 , f 2 , …, f n T is the signal volume vector; A = [exp(- t i / T 2j )] m×n ; A is m×n an order matrix. When the number of rows m is greater than or equal to the number of columns n, there exist orthogonal matrices U m×m , non - negative diagonal matrix W m×n and orthogonal matrix V n×n , where W j ≥0 (1 ≤ j ≤ n ); U and V are orthogonal matrices;

[0026] Find the generalized inverse matrix of the coefficient matrix A and substitute it into Equation (4) to obtain the amplitude value of the T2 distribution. Integrating the amplitude value of the T2 spectrum can obtain the porosity of the reservoir core;

[0027] Use the Fourier transform in Equation (5) to convert the signal in the time domain to the frequency domain, and then obtain the image information of spatial encoding to form a nuclear magnetic resonance three - dimensional digital core.

[0028] Furthermore: In step S6, it specifically includes the following steps:

[0029] S601: Constrain the pore extraction method of the X - ray three - dimensional digital core by the porosity information obtained through nuclear magnetic resonance relaxation, so that the proportion of the pore volume extracted from the X - ray three - dimensional digital core is consistent with the porosity measured by nuclear magnetic resonance relaxation;

[0030] ​​S602: Extract the pore structure cross-sectional image according to the pore space distribution characteristics of the saturated sample in the nuclear magnetic resonance imaging obtained in S5; use the extracted pore structure cross-sectional image to verify the pore structure in the X-ray three-dimensional digital core; if the pore extraction of the X-ray three-dimensional digital core does not meet the verification of the pore structure cross-section, repeat step S601 until the pore structure characteristics extracted by both are consistent;

[0031] S603: Output the pore identification result of the corrected three-dimensional digital core.

[0032] Furthermore: In step S602, a similarity coefficient S is introduced to quantify the matching degree between the X-ray three-dimensional digital core and the pore structure cross-section. The closer the similarity coefficient is to 1, the stronger the correlation between the two.

[0033] Furthermore: The specific formula of the similarity coefficient S is as follows:

[0034]

[0035] where and are the normalized gray values at the window coordinates ;

[0036] m and n are the unit width and unit height of the discrimination window;

[0037] is the mean of the normalized gray values in image A;

[0038] is the standard deviation of the normalized gray values in image A;

[0039] is the mean of the normalized gray values in image B;

[0040] is the standard deviation of the normalized gray values in image B;

[0041] is the covariance of the normalized gray values in image A and image B;

[0042] ; ;

[0043] is the pixel dynamic range;

[0044] , The default values are 0.01 and 0.03

[0045] Further: In step S7, the quantitative evaluation of the pore structure characteristics of the reservoir includes the porosity, pore distribution, and pore connectivity of the reservoir pore structure.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] First, the present invention combines X-ray scanning imaging and nuclear magnetic resonance imaging technologies to achieve a comprehensive and accurate evaluation of the pore structure of unconventional reservoirs, filling the gap in the quantitative analysis of pore structure by traditional single technologies, and significantly improving the accuracy and reliability of the evaluation.

[0048] Second, through the extraction of X-ray three-dimensional digital core pores under the constraint of nuclear magnetic resonance relaxation signals and the verification of nuclear magnetic resonance imaging three-dimensional digital cores, the consistency and accuracy of pore structure identification are ensured, providing a basis for the in-depth analysis of reservoir porosity, pore distribution, and connectivity.

[0049] Third, the method of the present invention provides support for the calculation of physical property parameters and the evaluation of fluid properties of unconventional reservoirs, which is beneficial to optimizing reservoir development strategies, improving resource extraction efficiency and economic benefits, and is conducive to the efficient exploration and sustainable development of unconventional oil and gas resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a flowchart of a method for evaluating the pore structure of a reservoir in one embodiment of the present invention;

[0051] Figure 2 is a reconstructed three-dimensional digital core map in one embodiment of the present invention;

[0052] Figure 3 is the nuclear magnetic resonance T2 spectrum of the dried sample obtained in one embodiment of the present invention;

[0053] Figure 4 is the nuclear magnetic resonance T2 spectrum of the saturated sample obtained in one embodiment of the present invention;

[0054] Figure 5 is an example of a pore structure cross-sectional image extracted by nuclear magnetic resonance imaging in one embodiment of the present invention;

[0055] Figure 6 is Figure 5 an example of an X-ray three-dimensional digital core slice map in the shown embodiment;

[0056] Figure 7 is a pore structure map after setting a threshold in combination with nuclear magnetic resonance scanning in one embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0057] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work shall fall within the scope of protection of the present invention.

[0058] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, the terms "first", "second", and "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0059] A method for evaluating the pore structure of a reservoir includes the following steps:

[0060] S1: Prepare a dried sample of the rock sample;

[0061] S2: Use X-rays to perform tomographic scanning on the dried sample of the rock sample and reconstruct a three-dimensional digital core of the X-rays;

[0062] S3: Use a nuclear magnetic resonance measuring device for the rock sample to measure the dried sample of the rock sample to obtain the nuclear magnetic resonance T2 spectrum of the dried sample of the rock sample;

[0063] S4: Saturate the dried sample of the rock sample to obtain a saturated sample of the rock sample;

[0064] S5: Perform nuclear magnetic resonance T2 spectrum measurement and nuclear magnetic resonance imaging scanning on the saturated sample of the rock sample obtained in S4 to obtain the porosity information and pore space distribution characteristics of the core;

[0065] S6: Calibrate the three-dimensional digital core in S2 through the porosity information and pore space distribution characteristics obtained in S5 to obtain a calibrated three-dimensional digital core pore image;

[0066] S7: Quantitatively evaluate the pore structure characteristics of the reservoir based on the calibrated three-dimensional digital core pore image obtained in S6.

[0067] As Figure 1 shown: In some other embodiments of the present invention, the specific steps for preparing the sample of the rock sample in step S1 include:

[0068] S101: Prepare a core plug sample, including drilling, cutting, and polishing;

[0069] S102: Wash the plunger sample with oil and salt to remove oil and salt.

[0070] S103: Dry the cleaned core to ensure that the sample does not contain fluid, and obtain the dried sample of the rock sample.

[0071] In some other embodiments of the present invention, in step S2, a reconstruction algorithm is used to form a three-dimensional digital core from the results of X-ray scanning. In this embodiment, the selected reconstruction algorithm is the filtered back-projection algorithm, which is a classic algorithm widely used in CT scan image reconstruction. This algorithm first filters the projection data and then performs back-projection to reconstruct the tomographic image. The FBP method is fast and easy to implement, and is suitable for the reconstruction of various tomographic scan data. As Figure 2 shown: Visualize the data structure of the X-ray scan, and convert the data measured at each point into pixel points in space. At this time, the digital core is a three-dimensional image constructed with default parameters, and it needs to be defined in combination with the pore structure obtained by nuclear magnetic resonance imaging scan.

[0072] As Figure 3 shown: In step S3, the dry sample signal measured by nuclear magnetic resonance is obtained by performing nuclear magnetic resonance measurement on the dried core, and the signal of non-free fluid in the sample can be obtained.

[0073] In some other embodiments of the present invention, in step S4, a vacuum pumping and pressure saturation device is used to saturate the dried sample of the rock sample.

[0074] In some other embodiments of the present invention, the specific steps of S5 are as follows: Put the saturated sample of the rock sample obtained in S4 into a nuclear magnetic resonance relaxation measurement device and a nuclear magnetic resonance imaging scan device for measurement. The porosity value of the saturated sample of the rock sample is obtained by nuclear magnetic resonance T2 relaxation measurement, and the pore space distribution characteristics of the saturated sample of the rock sample can be obtained by nuclear magnetic resonance imaging scan. As Figure 4 shown: The core is saturated with water, and the measurement is carried out after the pores are filled with fluid. The signal obtained minus Figure 4 the signal obtained can determine the total signal of the fluid in the pores, so as to determine the size of the porosity.

[0075] In one of the embodiments of the present invention: The calculation method for constructing the nuclear magnetic resonance three-dimensional digital core is as follows:

[0076]

[0077] Equation (1) is used to calculate the conversion relationship between the echo signal and the relaxation component. b i represents t i the amplitude of the i th echo measured at timeT 2j is the transverse relaxation time of the j th component, T 2 , f j is the signal volume when the transverse relaxation time is T 2j . The process of solving the corresponding f j relaxation components is called T2 spectrum inversion;

[0078] Equations (2) - (3) represent the inversion of the T2 spectrum using the singular value decomposition method, where b = [b 1 , b 2 , …, b m T is the echo amplitude vector; x = [f 1 , f 2 , …, f n T is the signal volume vector; A = [exp(- t i / T 2j )] m×n ; A is a m×n -order matrix. When the number of rows m is greater than or equal to the number of columns n, there exist orthogonal matrices U m×m , non-negative diagonal matrix W m×n and orthogonal matrix V n×n , where W j ≥0 (1 ≤ j ≤ n ); U and V are orthogonal matrices;

[0079] Find the generalized inverse matrix of the coefficient matrix A and substitute it into Equation (4) to obtain the amplitude value of the T2 distribution. Integrating the amplitude value of the T2 spectrum can obtain the porosity of the reservoir core;

[0080] Use the Fourier transform in Equation (5) to convert the signal in the time domain to the frequency domain, and then obtain the spatially encoded image information to form a nuclear magnetic resonance three-dimensional digital core.

[0081] In some other embodiments of the present invention, in step S6, it specifically includes the following steps:

[0082] S601: Constrain the pore extraction method of the X-ray three-dimensional digital core by the porosity information obtained from nuclear magnetic resonance relaxation, so that the proportion of the pore volume extracted from the X-ray three-dimensional digital core is consistent with the porosity measured by nuclear magnetic resonance relaxation;

[0083] ​​S602: Extract the pore structure cross-sectional image based on the pore space distribution characteristics of the saturated sample in the nuclear magnetic resonance imaging obtained in S5; use the extracted pore structure cross-sectional image to test the pore structure in the X-ray three-dimensional digital core; if the pore extraction of the X-ray three-dimensional digital core does not meet the test of the pore structure cross-section, repeat step S601 until the pore structure characteristics extracted by both are consistent;

[0084] S603: Output the corrected three-dimensional digital core pore identification result.

[0085] As Figure 5 、 Figure 6 shown, Figure 5 is an example of the pore structure cross-sectional image extracted by nuclear magnetic resonance imaging, Figure 6 is an example of the X-ray three-dimensional digital core slice diagram: For the nuclear magnetic resonance imaging and the X-ray three-dimensional digital core slice diagram, calculate the similarity of the marked windows in the figure. If the similarity of the two figures is poor, it proves that the pore structures are not similar, and then the drawing parameters of the three-dimensional digital core need to be adjusted to make the similarity of the two images higher, so that the pore structures of the two can match.

[0086] In one embodiment of the present invention: In step S602, a similarity coefficient S is introduced to quantify the matching degree between the X-ray three-dimensional digital core and the pore structure cross-section. The closer the similarity coefficient is to 1, the stronger the correlation between the two. In addition, the porosity threshold needs to be continuously adjusted to make the porosity of the three-dimensional digital core consistent with the nuclear magnetic resonance porosity result. The schematic diagram of the pore structure diagram after setting the threshold in combination with the nuclear magnetic resonance scan is as Figure 6 shown.

[0087] In another embodiment of the present invention: The specific formula of the similarity coefficient S is as follows:

[0088]

[0089] where, and are the normalized gray values at the window coordinates ;

[0090] m and n are the unit width and unit height of the discrimination window;

[0091] is the mean value of the normalized gray values in image A;

[0092] is the standard deviation of the normalized gray values in image A;

[0093] is the mean value of the normalized gray values in image B;

[0094] is the standard deviation of the normalized gray values within image B;

[0095] is the covariance of the normalized gray values within image A and image B;

[0096] Image A is a sectional image of the pore structure extracted by nuclear magnetic resonance imaging, as shown in the above embodiments Figure 5 as shown;

[0097] Image B is a three-dimensional digital core slice map of X-ray, as shown in the above embodiments Figure 6 as shown;

[0098] ; ;

[0099] is the pixel dynamic range;

[0100] , with default values of 0.01 and 0.03;

[0101] In some other embodiments of the present invention, the quantitative evaluation of the pore structure characteristics of the reservoir in step S7 includes the porosity, pore distribution, and pore connectivity of the reservoir pore structure.

[0102] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent transformation or modification made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A reservoir pore structure evaluation method, characterized in that: The following steps are involved: S1: Drying sample for preparing rock sample; S2: Use X-rays to perform tomography on the dried rock sample and reconstruct it into an X-ray three-dimensional digital core; S3: using a rock sample nuclear magnetic resonance measuring device to measure the dried sample of the rock sample, and obtaining a nuclear magnetic resonance T2 spectrum of the dried sample of the rock sample; S4: performing saturation treatment on the dried sample of the rock sample to obtain a saturated sample of the rock sample; S5: Performing nuclear magnetic resonance T2 spectrum measurement and nuclear magnetic resonance imaging scanning on the saturated sample of the rock sample obtained in S4 to obtain the porosity information and pore space distribution characteristics of the core; S6: calibrating the three-dimensional digital core in S2 by using the porosity information and pore space distribution characteristics obtained in S5 to obtain a calibrated three-dimensional digital core pore image; Step S6 specifically includes the following steps: S601: constraining the pore extraction method of the X-ray three-dimensional digital core using the porosity information obtained by nuclear magnetic resonance relaxation, so that the pore volume ratio extracted from the X-ray three-dimensional digital core is consistent with the porosity measured by nuclear magnetic resonance relaxation measurement; S602: extracting a pore structure section image according to the pore space distribution characteristics of the saturated sample obtained by nuclear magnetic resonance imaging in S5; using the extracted pore structure section image to inspect the pore structure in the X-ray three-dimensional digital core; if the pore extraction of the X-ray three-dimensional digital core does not meet the inspection of the pore structure section, repeating step S601 until the pore structure characteristics extracted by both are consistent; S603: Outputting the calibrated three-dimensional digital core pore identification result; S7: Quantitatively evaluate the pore structure characteristics of the reservoir based on the calibrated three-dimensional digital core pore image obtained in S6.

2. A reservoir pore structure evaluation method according to claim 1, characterized in that: Step S1: The specific steps of preparing the rock sample include: S101: Prepare core plug samples, including drilling, cutting and grinding; S102: washing the plunger sample with oil and salt to remove oil and salt; S103: Drying the core after washing the oil and salt to ensure that the sample does not contain fluid, and obtaining a dried sample of the rock sample.

3. A reservoir pore structure evaluation method according to claim 1, characterized in that: In step S2, a reconstruction algorithm is used to convert the result of the X-ray scanning into a three-dimensional digital core.

4. A reservoir pore structure evaluation method according to claim 1, characterized in that: In step S4, a vacuum pressurized saturation device is used to perform saturation treatment on the dried rock sample.

5. A reservoir pore structure evaluation method according to claim 1, characterized in that: The specific steps of S5 are: putting the saturated sample of the rock sample obtained in S4 into a nuclear magnetic resonance relaxation measurement device and a nuclear magnetic resonance imaging scanning device for measurement, obtaining the porosity value of the saturated sample of the rock sample by nuclear magnetic resonance T2 relaxation measurement, and obtaining the pore space distribution characteristics of the saturated sample of the rock sample by nuclear magnetic resonance imaging scanning.

6. A reservoir pore structure evaluation method according to claim 5, characterized in that: The calculation method for constructing the three-dimensional digital core of nuclear magnetic resonance is as follows: Formula (1) is used to calculate the conversion relationship between the echo signal and the relaxation component. b i express t i The time measurement i The echo amplitude, T 2j For the j transverse relaxation time of the relaxation component T 2, f j The transverse relaxation time is T 2j The signal volume at the time; solve the corresponding relaxation components f j The process is called T2 spectrum inversion; Formula (2)-Formula (3) represents the inversion of T2 spectrum using singular value decomposition method, b=[b1, b2, …, b m ] T is the echo amplitude vector; x=[f1, f2, …, f n ] T is the semaphore vector; A=[exp(- t i / T 2j )] m×n ; A is m×n When the number of rows m is greater than or equal to the number of columns n, there exists an orthogonal matrix U m×m , a non-negative diagonal matrix W m×n And the orthogonal matrix V n×n , where W j ≥0(1≤ j≤n ); U and V are orthogonal matrices; The generalized inverse matrix of the coefficient matrix A is obtained and substituted into equation (4) to obtain the amplitude value of the T2 distribution; the porosity of the reservoir core can be obtained by integrating the amplitude value of the T2 spectrum; The Fourier transform in equation (5) is used to convert the time domain signal into the frequency domain, and then the spatially encoded image information is obtained to form the three-dimensional digital core of nuclear magnetic resonance.

7. A reservoir pore structure evaluation method according to claim 1, characterized in that: In step S602, a similarity coefficient S is introduced to quantify the matching degree between the X-ray three-dimensional digital core and the pore structure section. The closer the similarity coefficient is to 1, the stronger the correlation between the two.

8. A reservoir pore structure evaluation method according to claim 7, characterized in that: The similarity coefficient S formula is as follows: in, and is the window coordinate The gray value after standardization; m, n are the unit width and unit height of the discrimination window; is the mean of the standardized grayscale values ​​in image A; is the standard deviation of the grayscale value after normalization in image A; is the mean of the standardized grayscale values ​​in image B; is the standard deviation of the grayscale values ​​after normalization in image B; is the covariance of the standardized grayscale values ​​in image A and image B; Image A is a cross-sectional image of the pore structure extracted by MRI; Image B is an X-ray three-dimensional digital core slice; ； ； is the pixel dynamic range; , The default values ​​are 0.01 and 0.

03.

9. A reservoir pore structure evaluation method according to claim 1, characterized in that: The pore structure characteristics of the reservoir quantitatively evaluated in step S7 include the porosity, pore distribution and pore connectivity of the reservoir pore structure.

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