A method for classifying shale reservoir pore systems based on gas-water occurrence state

Abstract

The application discloses a method for classifying shale reservoir pore systems based on gas-water occurrence states, and belongs to the technical field of shale gas development. The method solves the problem that shale pore feature evaluation from a static perspective cannot meet the demand of shale gas effective development in the prior art, and specifically comprises the following steps: S1, shale reservoir pore gas occurrence state classification; S2, shale reservoir pore water occurrence state classification; S3, shale reservoir pore gas occurrence space size determination; S4, shale reservoir pore water occurrence space size determination; and S5, pore system classification. According to the shale reservoir pore fluid occurrence state, the corresponding pore system is established, which is helpful to understand the main pore space where free gas, adsorbed gas, movable water and bound water occur in the shale reservoir, and can better guide shale gas exploration and development.

Description

Technical Field

[0001] This invention relates to the field of shale gas development technology, and specifically to a method for classifying the pore system of shale reservoirs based on the gas-water occurrence state. Background Technology

[0002] In 1972, the International Union of Pure and Applied Chemistry (IUPAC) classified pores into micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) based on their absolute size. Loucks et al. (2012) proposed a three-terminal-member classification scheme for matrix porosity in shale reservoirs, dividing matrix porosity into three types: intergranular porosity, intragranular porosity, and organic matter porosity. Zhang et al. (2017) established a pore size classification scheme for shale oil reservoirs based on pore size: micropore-I (0–25 nm), micropore-II (25–100 nm), mesopores (100–1000 nm), and macropores (>1000 nm). These three mainstream classification methods are based on pore size or porosity origin as reference criteria, which cannot reflect the fluid transport characteristics in shale reservoirs. Therefore, evaluating shale porosity characteristics from a static perspective is insufficient to meet the needs of effective shale gas development.

[0003] To meet the needs of effective shale gas development, it is necessary to establish a corresponding pore system classification scheme from the perspective of the pore fluid occurrence state of shale reservoirs. This will better guide shale gas exploration and development. Shale gas reservoir pore systems not only contain a large amount of natural gas but also a significant amount of formation water. Therefore, classifying the shale reservoir pore system based on the gas and water occurrence state helps to understand the main pore spaces in shale reservoirs where free gas, adsorbed gas, mobile water, and bound water are located. This, in turn, guides the formulation of shale gas development plans and flowback measures, thereby enhancing the efficiency of shale gas well development. Summary of the Invention

[0004] To address the issue that evaluating shale pore characteristics from a static perspective is insufficient for the effective development of shale gas, this invention discloses a method for classifying shale reservoir pore systems based on gas and water occurrence states. This method helps to understand the main pore spaces where free gas, adsorbed gas, mobile water, and bound water occur in shale reservoirs, thus better guiding shale gas exploration and development.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for classifying the pore system of shale reservoirs based on gas-water occurrence states includes the following steps:

[0007] S1. Classification of Shale Reservoir Pore Gas Occurrence States: Based on the relationship between experimental pressure and the methane mass of different peaks in the saturated methane NMR T2 spectrum, the shale gas occurrence states are classified into free gas and adsorbed gas, including the following sub-steps:

[0008] S11. Prepare standard plunger samples from shale samples, dry them, and then test them.

[0009] S12. Measure the nuclear magnetic resonance signal of methane in a sample chamber with different pressures and known volume under laboratory conditions, and apply the corrected equation of state:

[0010]

[0011] In the formula, P represents fluid pressure, R represents gas constant, T represents fluid temperature, Vm represents fluid volume in the formation pore throat, parameter x represents the collision factor between fluid molecules, parameter B represents the correction term for molecular size, and parameter y represents the nanopore throat confinement effect parameter other than capillary force and adsorption force. Through the n=m / M conversion, the methane mass in the sample chamber under the action of the nanopore throat confinement effect can be calculated, thereby establishing the relationship between NMR signal intensity and methane mass.

[0012] S13. Place the prepared shale plunger sample into a sample chamber of known volume, and conduct nuclear magnetic resonance experiments of saturated methane under different pressures under laboratory conditions to obtain the T2 NMR spectra of saturated methane under different pressure conditions.

[0013] S14. Divide the obtained saturated methane NMR T2 spectrum into three peaks. Calculate the shale gas mass of different peaks using the established relationship between NMR signal intensity and methane mass. Determine the shale gas occurrence state of different peaks based on the relationship between experimental pressure and methane mass of different peaks.

[0014] S2. Classification of Pore Water Occurrence States in Shale Reservoirs: Using centrifugation and dry nuclear magnetic resonance methods, the occurrence states of water are classified into clay-bound fluids, capillary-bound water, and mobile water; specifically including the following sub-steps:

[0015] S21. After evacuating the plunger sample prepared in S11, saturate it with distilled water, and then measure the nuclear magnetic resonance of methane in the saturated water state to obtain the nuclear magnetic T2 spectrum of the sample in the saturated water state.

[0016] S22. After testing the NMR T2 spectrum of the saturated sample, centrifuge at different speeds and test the NMR T2 spectrum after each centrifugation to obtain the NMR T2 spectrum after centrifugation at different speeds.

[0017] S23. Plot the obtained NMR T2 spectra in saturated water and the NMR T2 spectra after centrifugation at different speeds into corresponding graphs to determine the maximum speed in bound water.

[0018] S24. The centrifuged samples were dried at different temperatures. The NMR T2 spectrum was measured after drying at each temperature, and the NMR porosity after different drying temperatures was statistically analyzed.

[0019] S25. Based on the Gibbs-Thomas equation and the slope difference of porosity with temperature, the relationship between pore volume and pore size is obtained. The logarithmic differential pore size distribution curve and the differential pore size distribution curve of the sample are calculated to determine the occurrence state of pore water.

[0020] S3. Determining the size of pore gas storage space in shale reservoirs: The size of the pore space for free gas and adsorbed gas is determined by converting the pore size between the centrifugal force of the centrifugal well and the corresponding NMR T2 time interval of the pores containing free and adsorbed gas. This includes the following sub-steps:

[0021] S31. Perform one-dimensional fractal analysis on the saturated water NMR T2 spectrum obtained in S21 to divide the shale into pore spaces of different sizes. Calculate the pore volume in different intervals and establish a correlation between the pore volume of these different spaces and the methane mass corresponding to different peaks in the saturated methane NMR T2 spectrum to determine the NMR T2 time intervals corresponding to the pores where free gas and adsorbed gas are located.

[0022] S32. By superimposing the NMR T2 spectrum of saturated water with the NMR T2 spectrum after centrifugation at the maximum centrifugation speed, the T2 time corresponding to the intersection point of the extended direction of the cumulative NMR signal intensity of the two is determined.

[0023] S33. Utilize the centrifugal force of the centrifugal well and the T2 time in S32 to convert the aperture and obtain the conversion coefficient C between T2 time and aperture;

[0024] S34. Determine the pore space size of free gas and adsorbed gas by using the C value and the T2 time interval corresponding to the occurrence of free gas and adsorbed gas.

[0025] S4. Determination of the size of pore water in shale reservoirs: The size of the pore space containing clay-bound fluid, capillary-bound water, and movable water is determined by converting the centrifugal force of the centrifugal well with the T2 cutoff value of capillary-bound water and clay-bound water.

[0026] S5. Pore system classification: Based on the size of the gas and water storage space in the shale reservoir pores determined in S3 and S4, the pores are classified into stagnant pores, bound pores, weakly bound pores, and large-diameter pores.

[0027] Specifically, in S14, the method for determining the occurrence state of shale gas with different spectral peaks is as follows: when the methane mass of the NMR peak has a logarithmic relationship with the experimental pressure, the shale gas corresponding to that peak is mainly adsorbed gas; when the methane mass of the NMR peak has a linear relationship with the experimental pressure, the shale gas corresponding to that peak is mainly free gas.

[0028] Specifically, the size of the pore gas storage space is divided as follows: less than 5nm is mainly adsorbed gas; 5-110nm is a mixture of adsorbed gas and free gas; and greater than 110nm is mainly free gas.

[0029] Specifically, in S4, determining the size of the pore water storage space includes determining the time corresponding to the T2 cutoff value of capillary bound water and clay bound water, and obtaining the corresponding pore size based on the C value and the T2 cutoff value.

[0030] Specifically, the pore water storage space is divided into the following categories: less than 5 nm mainly consists of clay-bound water; 5–25 nm mainly consists of capillary-bound water; and greater than 25 nm mainly consists of mobile water.

[0031] Specifically, in S5, the pore size of the retention pores is less than 5 nm; the pore size of the confined pores is 5–25 nm; the pore size of the weakly confined pores is 25–100 nm; and the pore size of the large-diameter pores is greater than 110 nm.

[0032] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0033] (1) The present invention provides a method for classifying the pore system of shale reservoirs based on the gas-water occurrence state. The pore system of shale gas reservoirs not only contains a large amount of natural gas, but also a large amount of formation water. The present invention classifies the pore system of shale reservoirs based on the gas-water occurrence state, classifying pores smaller than 5 nm as stagnant pores; free gas and adsorbed gas coexist in the 5-25 nm range, but the pore water is mainly capillary bound water, classifying this part of the pores as bound pores; free gas and adsorbed gas coexist in the 25-100 nm range, and the pore water is mobile water, classifying this part of the pores as weakly bound pores; and free gas and mobile water are the main components in the pores larger than 110 nm, classifying this part of the pores as large-diameter pores with a high degree of mobility.

[0034] (2) The present invention provides a method for classifying the pore system of shale reservoirs based on the gas and water occurrence state, which helps to understand the main pore spaces in shale reservoirs where free gas, adsorbed gas, movable water and bound water occur, thereby guiding the formulation of shale gas development plans and flowback measures, and enhancing the efficiency of shale gas well development. Attached Figure Description

[0035] Figure 1 T2 NMR spectra of saturated methane in shale samples under different pressures;

[0036] Figure 2 This is a graph showing the relationship between the mass of methane in the first peak and pressure.

[0037] Figure 3 This is a graph showing the relationship between the mass of methane in the second peak and pressure.

[0038] Figure 4 This is a graph showing the relationship between the mass of methane in the third peak and pressure.

[0039] Figure 5 The T2 NMR spectra are shown in the saturated water state and after centrifugation at different speeds.

[0040] Figure 6 The graph shows the variation of NMR porosity with temperature after different drying temperatures.

[0041] Figure 7 A diagram to distinguish the pore water occurrence state of shale based on centrifugation and dry nuclear magnetic resonance methods;

[0042] Figure 8 This is a fractal characteristic map of the T2 NMR spectrum of saturated water;

[0043] Figure 9 This is a superimposed image of the saturated NMR T2 spectrum and the NMR T2 spectrum after centrifugation at 12000 rpm;

[0044] Figure 10 Diagram showing the porosity division of the target section in well A;

[0045] Figure 11 This is a diagram showing the porosity division of the target section in well B. Detailed Implementation

[0046] To better understand the present invention, embodiments of the technical solution of the present invention are described in detail below. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore merely examples, and should not be used to limit the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the scope of protection of the present invention.

[0047] A method for classifying the pore system of shale reservoirs based on gas-water occurrence states specifically includes the following steps:

[0048] S1. Classification of gas occurrence states in shale reservoirs:

[0049] S11. Prepare standard plunger samples from shale samples, dry them, and then test them.

[0050] S12. Measure the nuclear magnetic resonance signal of methane in a known-volume sample chamber under different pressures under laboratory conditions, and apply the corrected equation of state:

[0051]

[0052] In the formula, P represents fluid pressure, R represents gas constant, T represents fluid temperature, Vm represents fluid volume within the formation pore throat, parameter x characterizes the intermolecular collision factor, parameter B characterizes the molecular size correction term, parameter y characterizes the nanopore throat confinement effect parameter excluding capillary force and adsorption force factors. During the experiment, the experimental pressure and temperature are known parameters, n is Avogadro's constant, V is the sample chamber volume, n is the molar mass of methane in the sample chamber, and z is the compressibility coefficient of methane molecules. The molar mass of methane in the sample chamber is calculated from this, and thus the methane mass is calculated. Using the correlation analysis between NMR signals and methane mass under different pressure conditions, a relationship is established between NMR signal intensity (Q) and methane mass (m) (Q = Km), where K is the fitting slope between the NMR signal and methane mass.

[0053] S13. Place the shale plunger sample prepared in step S11 into a sample chamber of known volume. Conduct saturated methane NMR experiments under laboratory conditions at different pressures of 2.65 MPa, 4.63 MPa, 8.11 MPa, 10.21 MPa, 11.94 MPa, and 13.29 MPa to obtain the T2 NMR spectra of saturated methane under different pressure conditions, such as... Figure 1 As shown;

[0054] S14. Divide the obtained saturated methane NMR T2 spectrum into three peaks, and calculate the NMR signal intensity corresponding to the area of ​​each peak. Calculate the methane mass of different peaks based on the established relationship between NMR signal intensity and methane mass (Q = Km). Calculate the methane mass of each peak under different pressure conditions and plot the corresponding relationship in Excel (e.g., ...). Figures 2-4 The occurrence state of shale gas in different spectral peaks was determined based on the relationship between experimental pressure and the methane mass of different spectral peaks. According to the Langmuir equation, there is a logarithmic relationship between adsorbed methane and pressure. Therefore, when the methane mass of the NMR peak has a logarithmic relationship with the experimental pressure, the shale gas corresponding to that peak is mainly adsorbed gas; when the methane mass of the NMR peak has a linear relationship with the experimental pressure, the shale gas corresponding to that peak is mainly free gas.

[0055] S2. Classification of water occurrence in shale reservoirs:

[0056] S21. After evacuating the plunger sample prepared in step S11, saturate it with distilled water at 50 MPa, and then measure the nuclear magnetic resonance of methane in the saturated water state to obtain the nuclear magnetic T2 spectrum of the sample in the saturated water state.

[0057] S22. After testing the NMR T2 spectrum of the saturated sample, centrifuge at different speeds (4000 rpm, 6000 rpm, 8000 rpm, 10000 rpm, 12000 rpm) and test the NMR T2 spectrum after each centrifugation to obtain the NMR T2 spectrum after centrifugation at different speeds.

[0058] S23. Plot the obtained NMR T2 spectra in saturated water and the NMR T2 spectra after centrifugation at different speeds into corresponding plates, such as... Figure 5 As shown, the maximum rotation speed under the bound water state was determined to be 12,000 rpm (the current maximum rotation speed of the equipment; exceeding this speed would cause the core to break and make it impossible to obtain experimental results). The difference between this speed and the saturated NMR T2 spectrum is the mobile water.

[0059] S24. The centrifuged samples were subjected to drying experiments at different temperatures (40℃, 60℃, 80℃, 100℃, 120℃, 160℃, 200℃). NMR T2 spectra were measured after drying at each temperature, and the NMR porosity after different drying temperatures was statistically analyzed. Figure 6 As shown (4 samples);

[0060] S25. According to the Gibbs-Thomas equation, the forces required to overcome for water to escape from a core sample in a porous medium vary. The correlation between the amount of water escaping from the core and temperature can be used to classify the water's state of occurrence. After drying at different temperatures, water will escape from the core. Correlation analysis of porosity and temperature measured by nuclear magnetic resonance in Excel can be performed, and the state of pore water can be determined based on the slope. Figure 6 The relationship between nuclear magnetic resonance porosity and temperature under different drying conditions exhibits a three-segment pattern. Since clay-bound water is the most difficult to escape from the core, the point of drying at 120℃ is used as the node to delineate clay-bound water. Figure 7 As shown, combining centrifuged and dried NMR, the cumulative signal quantities of the saturated, centrifuged at 12000 rpm, and dried at 120℃ NMR T2 spectra were plotted. The difference between the saturated cumulative curve and the cumulative curve at 12000 rpm represents the movable water volume; the difference between the cumulative curve at 12000 rpm and the cumulative curve at 120℃ represents the capillary bound water volume; and the cumulative curve at 120℃ represents the clay bound water volume. Figure 7 The T2 time corresponding to the intersection of the cumulative curves is the T2 cutoff value. Figure 9 The relationship between pore volume and pore size is obtained, and the logarithmic differential pore size distribution curve and differential pore size distribution curve of the sample are calculated to determine the occurrence state of pore water.

[0061] S3. Determination of the size of gas storage space in shale reservoirs:

[0062] S31, such as Figure 6As shown, the saturated water NMR T2 spectrum obtained in step S21 is subjected to one-dimensional fractal analysis to divide the shale into pore spaces of different sizes. The pore volume within different intervals is statistically analyzed using the measured NMR T2 spectra. Figure 8 Correlation analysis was performed in Excel on the pore volumes of these different spaces and the methane masses corresponding to different peaks in the T2 NMR spectrum of saturated methane. Generally, the smaller pore volume correlates better with the methane mass of the adsorbed peak; the free methane mass correlates better with the pore volume of the larger pore size. This allows us to determine the NMR T2 time intervals corresponding to the pores where free gas and adsorbed gas are located.

[0063] S32, such as Figure 7 As shown, the T2 time corresponding to the intersection of the direction extension of the cumulative NMR signal intensity of the two is determined by superimposing the NMR T2 spectrum of saturated water with the NMR T2 spectrum after centrifugation at the maximum centrifugation speed of 1200 rpm.

[0064] S33. Utilize the conversion between centrifugal force and time T2 to obtain the conversion coefficient C between time T2 and pore size. The specific conversion process is as follows: Pore radius r = CT2; Centrifugal force F = Capillary pressure f; i.e., mω 2 R = σcosθ / r; In the above formula, only r and C are unknown parameters, and the rest are known parameters;

[0065] S34. Determine the pore space size of free gas and adsorbed gas by using the C value and the T2 time interval corresponding to the occurrence of free gas and adsorbed gas (i.e., determined by the formula r = CT2); less than 5nm is dominated by adsorbed gas, 5~110nm is dominated by both adsorbed gas and free gas; greater than 110nm is dominated by free gas.

[0066] S4. Determining the size of water storage space in shale reservoirs:

[0067] Determine the time corresponding to the T2 cutoff value for capillary bound water and clay bound water, and obtain the corresponding pore size based on the C value and the T2 cutoff value (refer to step S25 for the calculation method of the T2 cutoff value); less than 5nm is mainly clay bound water, 5~25nm is mainly capillary bound water; greater than 25nm is mainly mobile water.

[0068] S5. Pore System Classification: As shown in Table 1 below, since adsorbed gas and clay-bound water are difficult to utilize in the pore system, pores smaller than 5 nm are called stagnant pores; free gas and adsorbed gas coexist in the range of 5–25 nm, but the pore water is mainly capillary-bound water, so this part of the pores is called bound pores; free gas and adsorbed gas coexist in the range of 25–100 nm, and the pore water is mobile water, so this part is called weakly bound pores; pores larger than 110 nm are dominated by free gas and mobile water, so this part is called large-diameter pores with a high degree of utilization.

[0069] Table 1. Shale Reservoir Pore System Classification Scheme

[0070] Retention-type pores <5 Adsorbed gas Clay binds water Basically no use Constrained pores 5～25 Adsorbed gas, free gas Binding water Immersion Weakly bound pores 25～110 Adsorbed gas, free gas movable water Analysis of the action Large pore size >110 Free gas movable water Basic usable

[0071] This invention provides a method for classifying the pore system of shale reservoirs based on the gas-water occurrence state. The method classifies the pore system according to the gas-water occurrence state, classifying pores smaller than 5 nm as stagnant pores; pores between 5 and 25 nm, where free gas and adsorbed gas coexist but pore water is mainly capillary bound water, are classified as bound pores; pores between 25 and 100 nm, where free gas and adsorbed gas coexist and pore water is mobile water, are classified as weakly bound pores; and pores larger than 110 nm, dominated by free gas and mobile water, are classified as large-diameter pores with high utilization. This method helps to understand the main pore spaces in shale reservoirs where free gas, adsorbed gas, mobile water, and bound water occur, thereby guiding the formulation of shale gas development plans and flowback measures, and enhancing the efficiency of shale gas well development.

[0072] Reference Figure 10-11 Analysis of the shale reservoirs in the target sections of two wells reveals the following: Well A's target shale reservoir exhibits a high proportion of weakly confined pores and large-diameter pores, indicating good shale gas well production, with an average daily gas production of 100,000 cubic meters per day in the first three months. Well B's target shale reservoir has a high proportion of stagnant pores, resulting in poor shale gas well production with minimal output. Therefore, the pore segmentation scheme of this invention can effectively guide the development of shale gas wells.

[0073] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for classifying the pore system of shale reservoirs based on the gas-water occurrence state, characterized in that, Includes the following steps: S1. Classification of Shale Reservoir Pore Gas Occurrence States: Based on the relationship between experimental pressure and the methane mass of different peaks in the saturated methane NMR T2 spectrum, the shale gas occurrence states are classified into free gas and adsorbed gas, including the following sub-steps: S11. Prepare standard plunger samples from shale samples, dry them, and then test them. S12. Measure the nuclear magnetic resonance signal of methane in a sample chamber with different pressures and known volume under laboratory conditions, and apply the corrected equation of state: In the formula, P represents fluid pressure, R represents gas constant, T represents fluid temperature, Vm represents fluid volume in the formation pore throat, parameter x represents the collision factor between fluid molecules, parameter B represents the correction term for molecular size, and parameter y represents the nanopore throat confinement effect parameter other than capillary force and adsorption force. Through the n=m / M conversion, the methane mass in the sample chamber under the action of the nanopore throat confinement effect can be calculated, thereby establishing the relationship between NMR signal intensity and methane mass. S13. Place the prepared shale plunger sample into a sample chamber of known volume, and conduct nuclear magnetic resonance experiments of saturated methane under different pressures under laboratory conditions to obtain the T2 NMR spectra of saturated methane under different pressure conditions. S14. Divide the obtained saturated methane NMR T2 spectrum into three peaks. Calculate the shale gas mass of different peaks using the established relationship between NMR signal intensity and methane mass. Determine the shale gas occurrence state of different peaks based on the relationship between experimental pressure and methane mass of different peaks. S2. Classification of Pore Water Occurrence States in Shale Reservoirs: Using centrifugation and dry nuclear magnetic resonance methods, the occurrence states of pore water are classified into clay-bound fluids, capillary-bound water, and mobile water; specifically including the following sub-steps: S21. After evacuating the plunger sample prepared in S11, saturate it with distilled water, and then measure the nuclear magnetic resonance of methane in the saturated water state to obtain the nuclear magnetic T2 spectrum of the sample in the saturated water state. S22. After testing the NMR T2 spectrum of the saturated sample, centrifuge at different speeds and test the NMR T2 spectrum after each centrifugation to obtain the NMR T2 spectrum after centrifugation at different speeds. S23. Plot the obtained NMR T2 spectra in saturated water and the NMR T2 spectra after centrifugation at different speeds into corresponding graphs to determine the maximum speed in bound water. S24. The centrifuged samples were dried at different temperatures. The NMR T2 spectrum was measured after drying at each temperature, and the NMR porosity after different drying temperatures was statistically analyzed. S25. Based on the Gibbs-Thomas equation and the slope difference of porosity with temperature, the relationship between pore volume and pore size is obtained. The logarithmic differential pore size distribution curve and the differential pore size distribution curve of the sample are calculated to determine the occurrence state of pore water. S3. Determining the size of pore gas storage space in shale reservoirs: The size of the pore space for free gas and adsorbed gas is determined by converting the pore size between the centrifugal force of the centrifugal well and the corresponding NMR T2 time interval of the pores containing free and adsorbed gas. This includes the following sub-steps: S31. Perform one-dimensional fractal analysis on the saturated water NMR T2 spectrum obtained in S21 to divide the shale into pore spaces of different sizes. Calculate the pore volume in different intervals and establish a correlation between the pore volume of these different spaces and the methane mass corresponding to different peaks in the saturated methane NMR T2 spectrum to determine the NMR T2 time intervals corresponding to the pores where free gas and adsorbed gas are located. S32. By superimposing the NMR T2 spectrum of saturated water with the NMR T2 spectrum after centrifugation at the maximum centrifugation speed, the T2 time corresponding to the intersection point of the extended direction of the cumulative NMR signal intensity of the two is determined. S33. Utilize the centrifugal force of the centrifugal well and the T2 time in S32 to convert the aperture and obtain the conversion coefficient C between T2 time and aperture; S34. Determine the pore space size of free gas and adsorbed gas by using the C value and the T2 time interval corresponding to the occurrence of free gas and adsorbed gas. S4. Determination of the size of pore water in shale reservoirs: The size of the pore space containing clay-bound fluid, capillary-bound water, and movable water is determined by converting the centrifugal force of the centrifugal well with the T2 cutoff value of capillary-bound water and clay-bound water. S5. Pore system classification: Based on the size of the gas and water storage space in the shale reservoir pores determined in S3 and S4, the pores are classified into stagnant pores, bound pores, weakly bound pores, and large-diameter pores.

2. The method for classifying the pore system of shale reservoirs based on gas-water occurrence state according to claim 1, characterized in that, In S14, the method for determining the occurrence state of shale gas with different spectral peaks is as follows: when the methane mass of the nuclear magnetic resonance peak has a logarithmic relationship with the experimental pressure, the shale gas corresponding to that peak is mainly adsorbed gas; when the methane mass of the nuclear magnetic resonance peak has a linear relationship with the experimental pressure, the shale gas corresponding to that peak is mainly free gas.

3. The method for classifying the pore system of shale reservoirs based on gas-water occurrence state according to claim 1, characterized in that, The specific size of the pore gas storage space is divided as follows: less than 5nm is mainly adsorbed gas; 5-110nm is a mixture of adsorbed gas and free gas; greater than 110nm is mainly free gas.

4. The method for classifying the pore system of shale reservoirs based on gas-water occurrence state according to claim 1, characterized in that, In S4, determining the size of the pore water storage space specifically includes determining the time corresponding to the T2 cutoff value of capillary bound water and clay bound water, and obtaining the corresponding pore size based on the C value and the T2 cutoff value.

5. The method for classifying the pore system of shale reservoirs based on gas-water occurrence state according to claim 1, characterized in that, The specific size of the pore water storage space is divided as follows: less than 5nm is mainly clay-bound water, 5-25nm is mainly capillary-bound water, and greater than 25nm is mainly mobile water.

6. The method for classifying the pore system of shale reservoirs based on gas-water occurrence state according to claim 1, characterized in that, In S5, the pore size of the retention pores is less than 5 nm; the pore size of the confined pores is 5–25 nm; the pore size of the weakly confined pores is 25–100 nm; and the pore size of the large-diameter pores is greater than 110 nm.

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