Method for determining pore diameter lower limit and effective pore volume ratio of movable fluid

Abstract

The invention provides a method for determining the pore diameter lower limit and the effective pore volume ratio of movable fluid. The method comprises the steps that the temperature of each sample under the stratum in-situ condition is calculated according to the ground temperature gradient and the burial depth; correcting the surface tension coefficient of the water by using the calculated temperature; determining an average contact angle of water and shale; calculating capillary force, enabling the capillary force to be equal to reservoir fluid pressure, and calculating the lower limit of the aperture of the movable water; carrying out carbon dioxide adsorption, liquid nitrogen adsorption and mercury injection experiments on the sample to obtain connectivity pore volume-pore size distribution, and calculating effective pore volume in combination with a movable water pore size lower limit; carrying out a low-field nuclear magnetic resonance experiment, calculating the total pore volume of macropores, obtaining the total pore volume of small pores in combination with a carbon dioxide experiment, and finally calculating the total pore volume; and calculating the pore volume ratio of the effective pores by using the pore volume of the effective pores and the total pore volume. According to the method, the evaluation precision of the lower limit of the pore diameter of the shale reservoir movable fluid and the pore volume ratio of the effective pores is effectively improved, and the test cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of shale gas development geological technology, and in particular to a method for determining the lower limit of movable fluid pore diameter and the proportion of effective pore volume. Background Art

[0002] As a clean energy source, shale gas has received increasing national attention. In North America and China, shale gas exploration and development have been commercialized to varying degrees. During the development and production process, determining the connectivity and effectiveness of shale reservoir pores is crucial for efficient shale gas production. Gas in connected pores within shale can efficiently migrate to artificial fractures, while capillary forces hinder gas expulsion. Only when reservoir fluid pressure exceeds capillary forces can non-wetting gas expel water and migrate to artificial fractures.

[0003] Currently, research on pore effectiveness, both domestically and internationally, has focused solely on pore connectivity or water mobility, with evaluation metrics primarily based on the connected pore volume fraction or the effective pore volume fraction. There is no calculation method for the effective pore volume that simultaneously considers both pore connectivity and water mobility. In reality, determining the effective pore volume fraction is based on determining the lower limit of the pore size for movable fluids. Current methods for determining this lower limit rely primarily on laboratory experiments using centrifugation and nuclear magnetic resonance (NMR) techniques, calculating the lower limit based on the centrifugal force and capillary force equations. However, the results of commonly used centrifugation and low-field NMR methods are severely constrained by the direction of pore development, and the centrifugation speed has an upper limit. The applied centrifugal force falls far short of the pore fluid pressure in shale reservoirs, particularly in deep shale gas reservoirs, where overpressure is prevalent, the reservoir fluid pressure coefficient is extremely high, and the pore fluid pressure is enormous. This results in discrepancies between previous calculations and the lower limit of the movable fluid pore size and the effective pore volume fraction in actual shale reservoirs. In addition, current laboratory conditions cannot restore the true reservoir temperature, and temperature is an important factor that has a significant impact on the water surface tension coefficient, which in turn seriously affects the calculation of capillary force, which further increases the error in the calculation of the lower limit of the movable fluid pore size and the effective pore volume. Summary of the Invention

[0004] The present invention aims to provide a method for determining the lower limit of movable fluid pore size and the proportion of effective pore volume, so as to solve the problem that it is impossible to restore the actual temperature and pressure of the reservoir under laboratory conditions, and it is impossible to measure the lower limit of movable fluid under real reservoir conditions, as well as the current problem that it is impossible to calculate the volume of effective pores that are connected and from which water can be discharged, so as to more accurately evaluate the lower limit of movable fluid pore size and the proportion of effective pore volume in shale reservoirs.

[0005] The present invention provides a method for determining the lower limit of the pore size of a movable fluid and the effective pore volume ratio, comprising the following steps:

[0006] Step 1: Calculate the temperature of each sample under in-situ conditions in the formation based on the geothermal gradient and the burial depth of each sample;

[0007] Step 2: using the temperature of each sample under the in-situ formation conditions to correct the surface tension coefficient of water in each sample under the in-situ reservoir conditions;

[0008] Step 3: Determine the average contact angle between water and shale;

[0009] Step 4: Calculate the capillary force under the reservoir in-situ temperature and pressure conditions, make the capillary force equal to the reservoir fluid pressure, and calculate the lower limit of the movable water pore diameter under the reservoir in-situ temperature and pressure conditions;

[0010] Step 5: Conduct low-temperature carbon dioxide adsorption experiments, low-temperature liquid nitrogen adsorption experiments, and high-pressure mercury injection experiments on the collected samples. By calibrating and combining the three experiments, the full-scale connected pore volume-pore size distribution per unit mass of rock is obtained;

[0011] Step 6: Calculate the effective pore volume based on the lower limit of the movable water pore size and the pore volume-pore size distribution of the full-scale connected pores;

[0012] Step 7: Conduct low-field nuclear magnetic resonance experiments on the sample, perform signal intensity-pore volume conversion, calculate the total pore volume of large pores, and combine with low-temperature carbon dioxide experiments to obtain the total pore volume of small pores, and finally calculate the total pore volume;

[0013] Step 8: Calculate the effective pore volume ratio using the effective pore volume and the total pore volume.

[0014] Furthermore, in step 1, the calculation equation for the temperature of each sample under in-situ formation conditions is:

[0015] T=t+273.15=t0+βh / 100+273.15

[0016] Where T is the absolute temperature of the sample under in-situ formation conditions, K; t is the Celsius temperature of the sample under in-situ formation conditions, °C; t0 is the surface temperature, °C; β is the geothermal gradient, dimensionless; h is the burial depth, m.

[0017] Furthermore, in step 2, the correction equation for the surface tension coefficient of water in each sample under reservoir in situ conditions is:

[0018] σ=0.09537-2.24×10 -6 T-2.56×10 -7 T 2

[0019] Wherein, σ is the surface tension coefficient of water in the sample under the in-situ reservoir conditions, N / m; T is the temperature of the sample under the in-situ formation conditions, K.

[0020] Furthermore, in step three, by collecting relevant literature data, it was determined that the average contact angle between water and shale is θ = 37.5°.

[0021] Furthermore, in step 4, the calculation equation for reservoir fluid pressure is:

[0022] P R =ρgh*α

[0023] Where ρ is the density of water, which is 10 3 kg / m 3 ; g is the acceleration of gravity, which is 9.8N / kg; h is the burial depth, m; α is the reservoir pressure coefficient, which is dimensionless.

[0024] Furthermore, the calculation equation for the lower limit of movable water pore diameter under the reservoir in-situ temperature and pressure conditions is:

[0025]

[0026] Among them, P R Reservoir fluid pressure, Pa; Pc is capillary force, Pa; θ is the average contact angle between water and shale, rad; r c is the lower limit of movable water pore diameter under the reservoir in-situ temperature and pressure conditions, m.

[0027] Furthermore, step five includes the following sub-steps:

[0028] Low-temperature carbon dioxide adsorption experiments, low-temperature liquid nitrogen adsorption experiments, and high-pressure mercury injection experiments were conducted on the collected samples. The pore volume-pore size distribution data of all pores within a certain pore size range were obtained through the low-temperature carbon dioxide adsorption experiments and the low-temperature liquid nitrogen adsorption experiments; the pore size distribution of connected pores within a certain pore size range was obtained through the high-pressure mercury injection experiment.

[0029] The pore volume-pore size distribution data of all pores within a certain pore size range obtained from the low-temperature carbon dioxide adsorption experiment and the low-temperature liquid nitrogen adsorption experiment are multiplied by a correction coefficient to obtain the pore size distribution of connected pores within a certain pore size range;

[0030] According to the advantageous characterization ranges of various pore structure standard methods, the interception interval of the connected pore size distribution obtained in each experiment is selected to determine the connected pore size distribution within the full scale range in unit mass of shale.

[0031] Furthermore, the correction factor is determined by the crushed sample-plunger sample helium porosity experiment; the calculation equation of the correction factor is:

[0032]

[0033] Where, γ is the correction coefficient, %. colmnar and φ powder represent the porosity obtained by helium porosity test on the plug sample and crushed sample, respectively, %.

[0034] Furthermore, in step six, the calculation equation for the effective pore volume is:

[0035]

[0036] Among them, Vconnect(r i ) is the full-scale connected pore volume-pore size distribution function characterized by the three experiments, cm 3 / g;r c is the lower limit of movable water pore size, nm.

[0037] Furthermore, step seven includes:

[0038] Low-field NMR experiments were performed on the samples under saturated water and centrifuged conditions, and the conversion between NMR signals and pore volumes was performed using the water absorption volume and the difference in NMR signal intensity between saturated water and centrifuged. The total macropore volume was then calculated based on the NMR signal under saturated water conditions.

[0039] The total pore volume was obtained by adding the total pore volume of macropores obtained from the low-field NMR experiment and the total pore volume of small pores obtained from the low-temperature carbon dioxide adsorption experiment.

[0040] Furthermore, the calculation equation for the total pore volume of macropores is:

[0041]

[0042]

[0043] Where C is the conversion coefficient between NMR signal and pore volume, cm 3 / ms; A2(T2) is the NMR signal amplitude of the sample in the water-saturated state, dimensionless; A1(T2) is the base signal of the sample in the water-free state after centrifugation, dimensionless; T2 is the relaxation time of the low-field NMR experiment, ms; V water is the water absorption volume of the sample when it is saturated with water, cm 3 ; V(T2) is the pore volume corresponding to a certain T2 value, cm 3 ; m is the core mass, g; V NMR is the total macropore volume.

[0044] Furthermore, the calculation equation for the total pore volume is:

[0045]

[0046]

[0047] Among them, V tol is the total pore volume, is the total pore volume of small pores obtained from the low-temperature carbon dioxide adsorption experiment. is the pore volume-pore size distribution function obtained from the low-temperature carbon dioxide adsorption experiment, cm 3 .

[0048] Furthermore, in step eight, the calculation equation for the effective pore volume ratio is:

[0049] α=V eff / V tol

[0050] Where α is the effective pore volume ratio, V eff is the effective pore volume.

[0051] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0052] The present invention fully considers the influence of temperature and pressure conditions of real shale formations on the mobility of reservoir fluids. Compared with the existing related evaluation methods, the results obtained are more in line with the actual situation. The present invention solves the problems caused by the large difference between experimental conditions and actual formation conditions, high error in the lower limit of movable fluids, and low evaluation accuracy. In addition, the present invention proposes a joint characterization method through fluid injection experiments and low-field nuclear magnetic resonance experiments, which can achieve the simultaneous determination of the effective pore volume ratio of pore connectivity and pore fluid mobility, effectively improving the evaluation accuracy of the lower limit of pore diameter and effective pore volume ratio of movable fluids in shale reservoirs, reducing testing costs, and making up for the shortcomings of previous evaluation methods. As a clean energy source, the exploration and development of shale gas is also increasing, so this method not only has a wide range of applications, but also has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings in the embodiments will be briefly introduced below. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0054] Figure 1 3 is a flow chart of a method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio in an embodiment of the present invention.

[0055] Figure 2It is the lower limit of the movable fluid pore size of each small layer sample under the reservoir in situ temperature and pressure conditions calculated in the embodiment of the present invention.

[0056] Figure 3 is a pore volume-pore size distribution diagram of full-scale connected pores per unit mass of shale in an embodiment of the present invention;

[0057] Figure 4 3 is a waveform diagram of the nuclear magnetic resonance signals of the sample under the conditions of being saturated with water, not containing water, and containing some water in the embodiment of the present invention.

[0058] Figure 5 It is a statistical diagram of the effective pore volume ratio of each small layer sample in the embodiment of the present invention. DETAILED DESCRIPTION

[0059] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0060] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0061] Example

[0062] See also Figure 1 、 Figure 2 、 Figure 3 、 Figure 4 , Figure 5 This embodiment provides a method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio, comprising the following steps:

[0063] Step 1: Calculate the temperature of each sample under in-situ conditions based on the geothermal gradient and the burial depth of each sample:

[0064] T=t+273.15=t0+βh / 100+273.15

[0065] Where T is the absolute temperature of the sample under in-situ formation conditions, K; t is the Celsius temperature of the sample under in-situ formation conditions, °C; t0 is the surface temperature, °C; β is the geothermal gradient, dimensionless; h is the burial depth, m.

[0066] Step 2: Use the temperature of each sample under the in-situ formation conditions to calibrate the surface tension coefficient of water under the in-situ reservoir conditions of each sample:

[0067] σ=0.09537-2.24×10 -6 T-2.56×10 -7 T 2

[0068] Wherein, σ is the surface tension coefficient of water in the sample under the in-situ reservoir conditions, N / m; T is the temperature of the sample under the in-situ formation conditions, K.

[0069] Step 3: By collecting relevant literature data, the average contact angle between water and shale is determined to be θ = 37.5° (0.6545 rad).

[0070] Step 4: Use the Washburn equation to calculate the capillary force under the reservoir in-situ temperature and pressure conditions, make the capillary force equal to the reservoir fluid pressure, and calculate the lower limit of the movable water pore diameter under the reservoir in-situ temperature and pressure conditions.

[0071] The equation for calculating reservoir fluid pressure is:

[0072] P R =ρgh*α

[0073] Where ρ is the density of water, which is 10 3 kg / m 3 ; g is the acceleration of gravity, which is 9.8N / kg; h is the burial depth, m; α is the reservoir pressure coefficient, which is dimensionless.

[0074] The equation for calculating the lower limit of movable water pore diameter under the reservoir in-situ temperature and pressure conditions is:

[0075]

[0076] Among them, P R Reservoir fluid pressure, Pa; Pc is capillary force, Pa; θ is the average contact angle between water and shale, rad; r c is the lower limit of movable water pore diameter under the reservoir in-situ temperature and pressure conditions, m.

[0077] refer to Figure 2 The calculation results show that the lower limit of movable fluid in shale reservoir increases with decreasing burial depth. On the one hand, this is because the surface tension coefficient increases as the burial depth decreases; on the other hand, the reservoir fluid pressure decreases as the burial depth decreases (the burial depth of the Wufeng-7 sublayer gradually decreases).

[0078] The lower limit of the movable fluid pore size is less than 2 nm, indicating that under the conditions of high temperature and overpressure in deep shale gas reservoirs, the reservoir pore water does not hinder the migration of free gas, but mainly leads to the obstruction of the desorption-diffusion of adsorbed gas.

[0079] Step 5: Conduct low-temperature carbon dioxide adsorption experiments, low-temperature liquid nitrogen adsorption experiments, and high-pressure mercury injection experiments on the collected samples. By calibrating and combining the three experiments, the full-scale connected pore volume-pore size distribution of unit mass rock is obtained.

[0080] Taking a small layer of sample as an example, low-temperature carbon dioxide adsorption experiments, low-temperature liquid nitrogen adsorption experiments, and high-pressure mercury injection experiments were conducted on the sample. Since the low-temperature carbon dioxide adsorption and low-temperature liquid nitrogen adsorption experiments were conducted on powder samples, pore volume-pore size distribution data were obtained for all pores in the pore size range of 0-2nm and 2nm-100nm, respectively. The high-pressure mercury injection experiment was conducted on a bulk sample and obtained the pore size distribution of connected pores in the pore size range of >3nm.

[0081] The pore volume-pore size distribution data for all pores within a certain pore size range, obtained from low-temperature carbon dioxide adsorption experiments and low-temperature liquid nitrogen adsorption experiments, are multiplied by a correction factor to obtain the pore size distribution of connected pores within a certain pore size range. The correction factor is determined from the crushed-plug helium porosity experiment.

[0082] The calculation equation for the correction coefficient is:

[0083]

[0084] Where, γ is the correction coefficient, %. colmnar and φ powder represent the porosity obtained by helium porosity test on the plug sample and crushed sample, respectively, %.

[0085] Then, according to the advantageous characterization range of various pore structure standard methods, the intercept interval of the connected pore size distribution obtained in each experiment is selected to determine the connected pore size distribution in the full scale range of unit mass shale. Figure 3 As shown, the pore size range of 0-2nm uses carbon dioxide adsorption experimental data, the pore size range of 2nm-50nm uses low-temperature liquid nitrogen adsorption experimental data, and the pore size range of >50nm uses high-pressure mercury injection experimental data.

[0086] Step 6: Calculate the effective pore volume (per unit mass of rock) based on the lower limit of the movable water pore size and the pore volume-pore size distribution of the full-scale connected pores:

[0087]

[0088] Among them, Vconnect(r i) is the full-scale connected pore volume-pore size distribution function characterized by the three experiments, cm 3 / g;r c is the lower limit of movable water pore size, nm.

[0089] Step 7: Conduct low-field NMR experiments on the sample under both water-saturated and centrifuged conditions. Convert the NMR signal to pore volume using the water absorption volume and the difference in the saturated-water-centrifuged NMR signal intensity. The total pore volume is then calculated based on the water-saturated NMR signal. Note that the NMR experiment can characterize a limited range of pore sizes, as water may not necessarily enter small pores. Therefore, the total pore volume per unit mass of rock is calculated by adding the total pore volume of large pores obtained from the low-field NMR experiment and the total pore volume of small pores (pore size less than 2 nm) obtained from the low-temperature CO2 adsorption experiment. Figure 4 The NMR signals of the sample saturated with water, without water, and with some water are shown. The method for calculating the total pore volume of macropores using NMR signals is:

[0090]

[0091]

[0092] Furthermore, the total pore volume of all-scale pores is calculated as:

[0093] V tol =V NMR +V CO2

[0094]

[0095] Where C is the conversion coefficient between NMR signal and pore volume, cm 3 / ms; A2(T2) is the NMR signal amplitude of the sample in the water-saturated state, dimensionless; A1(T2) is the base signal of the sample in the water-free state after centrifugation, dimensionless; T2 is the relaxation time of the low-field NMR experiment, ms; V water is the water absorption volume of the sample when it is saturated with water, cm 3 ; V(T2) is the pore volume corresponding to a certain T2 value, cm 3 ; m is the core mass, g; V tol is the total pore volume, is the total pore volume of small pores obtained from the low-temperature carbon dioxide adsorption experiment. is the pore volume-pore size distribution function obtained from the low-temperature carbon dioxide adsorption experiment, cm 3 .

[0096] Step 8: Calculate the effective pore volume ratio using the effective pore volume and total pore volume. Figure 5As shown, the effective pore volume of layer 1 is 11.13%. The calculation equation is:

[0097] α=V eff / V tol

[0098] Where α is the effective pore volume ratio, V eff is the effective pore volume.

[0099] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for determining the lower limit of movable fluid pore size and the effective pore volume ratio, characterized in that: The steps include: Step 1: Calculate the temperature of each sample under in-situ conditions in the formation based on the geothermal gradient and the burial depth of each sample; Step 2: using the temperature of each sample under the in-situ formation conditions to correct the surface tension coefficient of water in each sample under the in-situ reservoir conditions; Step 3: Determine the average contact angle between water and shale; Step 4: Calculate the capillary force under the reservoir in-situ temperature and pressure conditions, make the capillary force equal to the reservoir fluid pressure, and calculate the lower limit of the movable water pore diameter under the reservoir in-situ temperature and pressure conditions; Step 5: Conduct low-temperature carbon dioxide adsorption experiments, low-temperature liquid nitrogen adsorption experiments, and high-pressure mercury injection experiments on the collected samples. By calibrating and combining the three experiments, the full-scale connected pore volume-pore size distribution per unit mass of rock is obtained; Step 6: Calculate the effective pore volume based on the lower limit of the movable water pore size and the pore volume-pore size distribution of the full-scale connected pores; Step 7: Conduct low-field nuclear magnetic resonance experiments on the sample, perform signal intensity-pore volume conversion, calculate the total pore volume of large pores, and combine with low-temperature carbon dioxide experiments to obtain the total pore volume of small pores, and finally calculate the total pore volume; Step 8: Calculate the effective pore volume ratio using the effective pore volume and the total pore volume.

2. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 1, characterized in that: In step 1, the calculation equation for the temperature of each sample under in-situ formation conditions is: T=t+273.15=t0+βh / 100+273.15 Where T is the absolute temperature of the sample under in-situ formation conditions, K; t is the Celsius temperature of the sample under in-situ formation conditions, °C; t0 is the surface temperature, °C; β is the geothermal gradient, dimensionless; h is the burial depth, m.

3. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 2, characterized in that: In step 2, the correction equation for the surface tension coefficient of water in each sample under reservoir in situ conditions is: σ=0.09537-2.24×10 -6 T-2.56×10 -7 T 2 Wherein, σ is the surface tension coefficient of water in the sample under the in-situ reservoir conditions, N / m; T is the temperature of the sample under the in-situ formation conditions, K.

4. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 3, characterized in that: In step three, by collecting relevant literature data, the average contact angle between water and shale was determined to be θ = 37.5°.

5. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 4, characterized in that: In step 4, the calculation equation for reservoir fluid pressure is: P R =ρgh*a Where ρ is the density of water, which is 10 3 kg / m 3 ; g is the acceleration of gravity, which is 9.8N / kg; h is the burial depth, m; α is the reservoir pressure coefficient, which is dimensionless.

6. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 5, characterized in that: The calculation equation for the lower limit of movable water pore diameter under the reservoir in-situ temperature and pressure conditions is: Among them, P R Reservoir fluid pressure, Pa; Pc is capillary force, Pa; θ is the average contact angle between water and shale, rad; r c is the lower limit of movable water pore diameter under the reservoir in-situ temperature and pressure conditions, m.

7. The method for determining the lower limit of movable fluid pore size and the effective pore volume ratio according to claim 6, characterized in that: Step 5 includes the following sub-steps: Low-temperature carbon dioxide adsorption experiments, low-temperature liquid nitrogen adsorption experiments, and high-pressure mercury injection experiments were conducted on the collected samples. The pore volume-pore size distribution data of all pores within a certain pore size range were obtained through the low-temperature carbon dioxide adsorption experiments and the low-temperature liquid nitrogen adsorption experiments; the pore size distribution of connected pores within a certain pore size range was obtained through the high-pressure mercury injection experiment. The pore volume-pore size distribution data of all pores within a certain pore size range obtained from the low-temperature carbon dioxide adsorption experiment and the low-temperature liquid nitrogen adsorption experiment are multiplied by a correction coefficient to obtain the pore size distribution of connected pores within a certain pore size range; According to the advantageous characterization ranges of various pore structure standard methods, the interception interval of the connected pore size distribution obtained in each experiment is selected to determine the connected pore size distribution within the full scale range in unit mass of shale.

8. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 7, characterized in that: The correction factor is determined by the crushed sample-plunger sample helium porosity experiment; the calculation equation of the correction factor is: Where, γ is the correction coefficient, %. colmnar and φ powder represent the porosity obtained by helium porosity test on the plug sample and crushed sample, respectively, %.

9. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 8, characterized in that: In step 6, the calculation equation for the effective pore volume is: Among them, Vconnect(r i ) is the full-scale connected pore volume-pore size distribution function characterized by the three experiments, cm 3 / g;r c is the lower limit of movable water pore size, nm.

10. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 9, characterized in that: Step seven includes: Low-field NMR experiments were conducted on the samples under saturated water and centrifuged conditions, respectively. The water absorption volume and the difference in saturated water-centrifuged NMR signal intensity were used to convert the NMR signal to pore volume. The total macropore volume was then calculated based on the NMR signal under saturated water conditions. The total pore volume was obtained by adding the total pore volume of macropores obtained from the low-field NMR experiment and the total pore volume of small pores obtained from the low-temperature carbon dioxide adsorption experiment.

11. The method for determining the lower limit of the pore size and the effective pore volume ratio of the movable fluid according to claim 10, characterized in that: The calculation equation for the total macropore volume is: Where C is the conversion coefficient between NMR signal and pore volume, cm 3 / ms; A2(T2) is the NMR signal amplitude of the sample in the water-saturated state, dimensionless; A1(T2) is the base signal of the sample in the water-free state after centrifugation, dimensionless; T2 is the relaxation time of the low-field NMR experiment, ms; V water is the water absorption volume of the sample when it is saturated with water, cm 3 ; V(T2) is the pore volume corresponding to a certain T2 value, cm 3 ; m is the core mass, g; V NMR is the total macropore volume.

12. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 11, characterized in that: The calculation equation for the total pore volume is: Among them, V tol is the total pore volume, is the total pore volume of small pores obtained from the low-temperature carbon dioxide adsorption experiment, is the pore volume-pore size distribution function obtained from the low-temperature carbon dioxide adsorption experiment, cm 3 .

13. The method for determining the lower limit of the movable fluid pore size and the effective pore volume ratio according to claim 12, characterized in that: In step eight, the calculation equation for the effective pore volume ratio is: α=V eff / V tol Where α is the effective pore volume ratio, V eff is the effective pore volume.

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