Method and system for determining gas-water two-phase relative permeability of low-permeability shale

Abstract

The invention relates to a low-permeability shale gas-water two-phase relative permeability determination method and system, T2 spectrum curves corresponding to different pretreatment modes are obtained based on nuclear magnetic resonance, the moisture occurrence state in sample pores is determined, the pore types of low-permeability shale are divided, and corresponding critical pore sizes are obtained; according to a Hagen-poiseuille equation and gas-water distribution characteristics of different pore types, determining the flow velocity of a gas phase and a water phase in a single shale pore, and determining the corresponding fluid flow in the single pore according to the flow velocity; the total flow of gas-water two-phase fluid in the low-permeability shale pore network is obtained based on the fractal theory, the critical pore diameter and the flow; based on the total flow velocity, the phase permeability of the gas phase and the water phase and the absolute permeability of the low-permeability shale are obtained through the Darcy law, and the relative permeability is determined; and according to the distribution characteristics of the gas-water two-phase fluid in the pores, determining the water saturation of the low-permeability shale, and determining the gas-water relative permeability of the low-permeability shale under different water saturation.

Description

Technical Field

[0001] The present invention relates to the technical field of energy engineering, and in particular to a method and system for determining relative permeability of low-permeability shale gas-water two-phases. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] In the process of shale gas extraction, due to the low permeability of the reservoir, a large amount of fracturing fluid is injected into the reservoir. This part of the fracturing fluid with water as the main component and the existing groundwater make the shale gas extraction process present a complex gas-water two-phase migration state, which greatly affects the shale gas extraction efficiency. In this case, the relative permeability of the gas-water two-phase is one of the important parameters to clarify the shale gas and water migration process. This parameter determines the transportation rate of shale gas and water in the reservoir, thereby affecting the production of shale gas wells.

[0004] Due to the low permeability of shale, it is unrealistic to directly use displacement experiments to obtain the relative permeability of gas and water. Therefore, the existing technology usually uses theoretical models to predict the relative permeability of shale gas and water. However, in the existing models, the distribution characteristics of water in pores and the fractal characteristics of shale pores are not fully considered, which makes the prediction results of the model different from the actual situation. Summary of the invention

[0005] In order to solve the technical problems existing in the above-mentioned background technology, the present invention provides a method and system for determining the relative permeability of low-permeability shale gas-water two-phase, which takes into account the distribution characteristics of water and reflects the fractal characteristics of shale pores, including the fractal characteristics of pore size distribution and the fractal characteristics of pore tortuosity. The distribution characteristics of water in shale are determined by a nuclear magnetic resonance test system, and low-permeability shale pores are classified accordingly. By calculating the flow rates of gas and water phases in different types of pores, the relative permeability of shale gas and water phases can be accurately obtained, making it closer to the actual shale gas production situation, and providing more accurate theoretical guidance for evaluating shale gas production and fracturing fluid return rate.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A first aspect of the present invention provides a method for determining the relative permeability of low-permeability shale gas-water two-phases, comprising the following steps:

[0008] Obtain low-permeability shale samples and pre-treat them, and obtain T2 spectrum curves corresponding to different pre-treatment methods based on nuclear magnetic resonance;

[0009] Through the T2 spectrum curve, the water storage state in the sample pores is determined, and the pore types of low-permeability shale are classified to obtain the corresponding critical pore size;

[0010] According to the Hagen-Poiseuille equation and the gas-water distribution characteristics of different pore types, the flow rates of gas and water phases in a single shale pore are determined, and based on the obtained flow rates, the corresponding fluid flow rate in a single pore is determined;

[0011] Based on fractal theory, critical pore size and flow rate of fluid in a single pore, the total flow rate of gas-water two-phase fluid in the pore network of low-permeability shale is obtained;

[0012] Based on the total flow rate of the fluid, Darcy's law is used to obtain the phase permeability of the gas phase and the water phase and the absolute permeability of the low-permeability shale, and further determine the relative permeability;

[0013] According to the distribution characteristics of gas-water two-phase fluid in the pores, the water saturation of low-permeability shale is determined, and finally the gas-water relative permeability of low-permeability shale at different water saturations is determined.

[0014] Furthermore, low-permeability shale samples were obtained and pre-treated, and T2 spectrum curves corresponding to different pre-treatment methods were obtained based on nuclear magnetic resonance, specifically:

[0015] The target low-permeability shale is processed into samples of set size and shape. After drying, a set of T2 spectrum data is obtained by nuclear magnetic resonance, which is recorded as T 2_d ;

[0016] The dried sample was saturated under a set water pressure to obtain a shale sample in a water-saturated state. Another set of T2 spectrum data was obtained by nuclear magnetic resonance, denoted as T 2_s ;

[0017] The saturated low-permeability shale sample was centrifuged, and the third set of T2 spectrum data was obtained by nuclear magnetic resonance, which was recorded as T 2_c .

[0018] Furthermore, the T2 spectrum curve is used to determine the water storage state in the sample pores and classify the pore types of low-permeability shale, specifically:

[0019] All the water in the pores is bound water, corresponding to type I pores, i.e., T 2_s -T 2_d ≠0 and situation;

[0020] The water in the pores is composed of bound water and free water, corresponding to type II pores, that is, And T 2_c -T 2_d ≠0;

[0021] All the water in the pores is free water, corresponding to type III pores, i.e., T 2_s-T 2_d ≠0 and T 2_c -T 2_d =0.

[0022] Furthermore, the critical pore size refers to two specific pore size critical values, namely the critical pore size r between type I pores and type II pores. i , and the critical pore size r between type II and type III pores w .

[0023] Furthermore, the fluid flow rate in a single pore is determined based on the integral of the flow velocity over the cross-sectional area of ​​the pore.

[0024] Furthermore, fractal theory includes fractal theory of pore size distribution and fractal theory of pore tortuosity. The fractal theory of pore size distribution is used to describe the relationship between pore size and the corresponding number of pores, and the fractal theory of pore tortuosity is used to calculate the pore length after considering the tortuosity.

[0025] Furthermore, the absolute permeability of low-permeability shale is the phase permeability of the gas phase when all the pores are filled with gas, as shown in the following formula:

[0026]

[0027] In the formula, A p is the effective cross-sectional area of ​​low permeability shale matrix, is the pore pressure difference, μ g is the dynamic viscosity of the gas phase, L0 is the length of the pore without considering the tortuosity, r max and r min are the maximum and minimum pore diameters, respectively.

[0028] Furthermore, the relative permeability is determined by the ratio between the phase permeability of the gas phase and the water phase and the absolute permeability, respectively.

[0029] Furthermore, water saturation is determined by the gas-water distribution characteristics in the pores, as shown in the following formula:

[0030]

[0031] In the formula, h g is the radius of the gas in the pore, r is the pore radius, r max and r min are the maximum and minimum pore diameters, respectively.

[0032] A second aspect of the present invention provides a system for determining relative permeability of low-permeability shale gas-water two-phases, comprising:

[0033] Nuclear magnetic resonance equipment, used to obtain T2 spectrum curves of low-permeability shale samples corresponding to different pretreatment methods;

[0034] The processor is configured to: determine the water storage state in the pores of the sample through the T2 spectrum curve, and classify the pore types of the low-permeability shale to obtain the corresponding critical pore size;

[0035] The processor is further configured to: determine the flow rates of gas and water phases in a single shale pore according to the Hagen-Poiseuille equation and the gas-water distribution characteristics of different pore types, and determine the corresponding fluid flow rate in the single pore according to the obtained flow rates;

[0036] The processor is further configured to: obtain the total flow rate of gas-water two-phase fluid in the low-permeability shale pore network based on fractal theory, critical pore size, and flow rate of fluid in a single pore;

[0037] The processor is further configured to: obtain the phase permeabilities of the gas phase and the water phase and the absolute permeability of the low-permeability shale based on the total flow rate of the fluid using Darcy's law, and further determine the relative permeability;

[0038] The processor is further configured to determine the water saturation of the low permeability shale according to the distribution characteristics of the gas-water two-phase fluid in the pores, and finally determine the gas-water relative permeability of the low permeability shale at different water saturations.

[0039] Compared with the prior art, one or more of the above technical solutions have the following beneficial effects:

[0040] In the mathematical model for calculating the relative permeability of shale gas and water, the occurrence state of water is taken into account, and the fractal characteristics of shale pores are reflected. The distribution of water in low-permeability shale pores is digitized through the nuclear magnetic resonance system, and the shale pores are divided accordingly; by analyzing the distribution characteristics and seepage velocity of gas-water two-phase fluids in different types of pores, the relative permeability of gas phase and water phase at different water saturations of low-permeability shales is accurately obtained, providing more accurate theoretical guidance for evaluating shale gas production and fracturing fluid return rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0043] Figure 1 is a schematic diagram of a low-permeability shale T2 spectrum curve and corresponding Class I pore classification provided by one or more embodiments of the present invention;

[0044] Figure 2 is a schematic diagram of a low-permeability shale T2 spectrum curve and corresponding Class II pore classification provided by one or more embodiments of the present invention;

[0045] Figure 3 is a schematic diagram of a low-permeability shale T2 spectrum curve and corresponding Class III pore classification provided by one or more embodiments of the present invention;

[0046] Figure 4 is a schematic diagram of the flow velocity distribution of gas-water two-phase fluid in Type I pores provided by one or more embodiments of the present invention;

[0047] Figure 5 is a schematic diagram of the flow velocity distribution of gas-water two-phase fluid in Type II pores provided by one or more embodiments of the present invention;

[0048] Figure 6 is a schematic diagram of the flow velocity distribution of gas-water two-phase fluid in Class III pores provided by one or more embodiments of the present invention;

[0049] In the figure: 1-type I pores, 2-type II pores, 3-type III pores, 4-gas phase, 5-bound water, 6-free water, 7-gas phase seepage velocity, 8-water phase seepage velocity. DETAILED DESCRIPTION

[0050] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0051] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0052] The Hagen-Poiseuille equation, also known as Poiseuille's law or Hagen-Poiseuille's law, is a basic law in fluid mechanics, which is used to describe the pressure loss generated when a fluid flows in a capillary (such as blood vessels, catheters, rock pores, etc.).

[0053] As introduced in the background technology, the existing technology usually uses theoretical model methods to predict the relative permeability of shale gas and water. However, in the existing models, the distribution characteristics of water in pores and the fractal characteristics of shale pores are not fully considered, which makes the prediction results of the model different from the actual situation.

[0054] Therefore, the following embodiment provides a method and system for determining the relative permeability of low-permeability shale gas-water two-phase, which takes into account the distribution characteristics of water and reflects the fractal characteristics of shale pores, including the fractal characteristics of pore size distribution and the fractal characteristics of pore tortuosity. The distribution characteristics of water in shale are determined by a nuclear magnetic resonance test system, and low-permeability shale pores are classified accordingly. By calculating the flow rates of gas and water phases in different types of pores, the relative permeability of shale gas and water phases can be accurately obtained, making it closer to the actual shale gas production situation, and providing more accurate theoretical guidance for evaluating shale gas production and fracturing fluid return rate.

[0055] Embodiment 1:

[0056] A method for determining the relative permeability of low-permeability shale gas-water two-phases comprises the following steps:

[0057] A. Conduct nuclear magnetic resonance testing on low permeability shale samples;

[0058] B. Draw the T2 spectrum curve of low permeability shale sample according to the nuclear magnetic resonance test results;

[0059] C. Determine the state of water in shale pores through the T2 spectrum curve, classify the pore types of low-permeability shale, and obtain the corresponding critical pore size;

[0060] D. Obtain the flow rates of gas and water phases in a single shale pore based on the Hagen-Poiseuille equation and gas-water distribution characteristics;

[0061] E. Calculate the corresponding fluid flow rate in a single pore based on the flow rates of the gas phase and the water phase in the single pore;

[0062] F. Based on fractal theory, critical pore size, and flow rate of fluid in a single pore, the total flow rate of gas-water two-phase fluid in the pore network of low-permeability shale is obtained;

[0063] G. Based on the total flow rate of the fluid, the phase permeability of the gas phase and the water phase and the absolute permeability of the low-permeability shale are obtained using Darcy's law, and the relative permeability is further calculated;

[0064] H. The water saturation of low permeability shale is determined based on the distribution characteristics of gas-water two-phase fluid in the pores, and finally the gas-water relative permeability of low permeability shale at different water saturations is determined.

[0065] The MRI test involves the following steps:

[0066] ① The target low-permeability shale was processed into a 50 mm × 25 mm cylindrical sample, dried at 60 ° C for 48 hours, and the dried sample was subjected to nuclear magnetic resonance testing to obtain a set of T2 spectrum data, recorded as T 2_d ;

[0067] ② The dry shale sample was saturated under a water pressure of 10 MPa for 12 hours, and then the shale sample in the water saturated state was tested by nuclear magnetic resonance to obtain another set of T2 spectrum data, recorded as T 2_s ;

[0068] ③ The saturated low-permeability shale sample was centrifuged at a centrifugal pressure of 300 psi (under this centrifugal pressure, the free water in the shale sample will be removed from the sample, while the bound water will be retained in the sample). The centrifuged shale sample was subjected to nuclear magnetic resonance testing to obtain the third set of T2 spectrum data, recorded as T 2_c The three groups of T2 spectrum data curves and the corresponding pore classifications are shown in Figure 1-Figure 3 shown.

[0069] In this embodiment, the T2 spectrum curve includes two curves, namely, T 2_s -T 2_d and T 2_c -T 2_d The corresponding data curve.

[0070] In this embodiment, the water state includes three situations. The first situation is that all the water in the pores is bound water (T 2_s -T 2_d ≠0 and The second case is that the water in the pores is composed of bound water and free water ( And T 2_c -T 2_d ≠0), and the third case is that all the water in the pores is free water (t 2_s -T 2_d ≠0 and T 2_c -T 2_d =0).

[0071] In this embodiment, according to the water occurrence state, the pore types of low permeability shale also include three types, corresponding to the three conditions of water occurrence state. Type I pore 1 refers to pores where only bound water 5 exists, Type II pore 2 refers to pores where both bound water 5 and free water 6 exist, and Type III pore 3 refers to pores where only free water 6 exists. In the three types of pores, the gas phase seepage velocity 7 and the water phase seepage velocity 8 show different flow rates, such as Figure 4-Figure 6 shown.

[0072] In this embodiment, the critical pore size refers to two specific pore size critical values, namely the critical pore size between type I pores and type II pores (r i ) and the critical pore size between type II and type III pores (r w ).

[0073] In this embodiment, the flow rates of the gas phase and the water phase in a single shale pore include three cases. The first case is the flow rate of the gas phase in a single type I pore, which is calculated by formula (1) according to the Hagen-Poiseuille equation and boundary conditions:

[0074]

[0075] In the formula, v ga is the flow rate of the gas phase in a single type I pore, is the pore pressure difference, L is the pore length, μ g is the dynamic viscosity of the gas phase, h g is the radius of gas in the pores, and r is the pore radius.

[0076] The second case is the flow rate of gas and water phases in a single type II pore, which is calculated by formula (2) based on the Hagen-Poiseuille equation and boundary conditions:

[0077]

[0078] Where v gb and v wb are the flow rates of gas phase and water phase in type II pores, μ w is the dynamic viscosity of the water phase, h m is the difference between the pore radius and the radius of the gas in the pore.

[0079] The third case is the flow rate of gas and water phases in a single type III pore, which is calculated by formula (3) according to the Hagen-Poiseuille equation and boundary conditions:

[0080]

[0081] In this embodiment, the fluid flow rate in a single pore is calculated by integrating the flow velocity over the pore cross-sectional area, as shown in equations (4) to (6).

[0082]

[0083] In the formula, q ga ,q gb ,q gc are the gas flow rates in a single type I pore, type II pore, and type III pore, respectively, and q wb ,q wc are the flow rates of water phase in single type II pores and type III pores respectively, A is the cross-sectional area of ​​the pore, dA is the area element on the cross section, and r0 is the pore radius.

[0084] In this embodiment, the fractal theory includes two parts, namely the fractal theory of pore size distribution and the fractal theory of pore tortuosity.

[0085] In this embodiment, the fractal theory of pore size distribution is used to describe the relationship between the pore size and the corresponding pore number, which is represented by formula (7):

[0086]

[0087] Where N is the number of pores, D N is the fractal dimension of pore size distribution, r max is the maximum pore diameter, d is the differential sign, and r is the pore radius.

[0088] In this embodiment, the fractal theory of pore tortuosity is used to calculate the pore length after considering the tortuosity, which is represented by formula (8):

[0089]

[0090] Where L(r) and L0 are the lengths of the pore when tortuosity is considered and not considered, respectively, and D T is the fractal dimension of pore tortuosity.

[0091] In this embodiment, the total flow rate of the gas-water two-phase fluid in the pore network is calculated by equation (9) and equation (10):

[0092] Q g =Q ga +Q gb +Q gc (9)

[0093] Q w =Q wb +Q wc (10)

[0094] In the formula, Q g and Q w are the total flow rates of gas and water phases in the pore network, Q ga , Q gb , Q gc The total flow rate of gas phase in all type I pores, all type II pores, and all type III pores, Q wb and Q wc are the total flow rates of water phase in all type II pores and all type III pores, respectively. According to fractal theory, Q ga , Q gb , Q gc , Q wb and Q wc Calculated by the following formula:

[0095]

[0096] in:

[0097]

[0098] D P =D T -D N -2 (15)

[0099] In this embodiment, the phase permeability of the gas phase and the water phase is obtained by the fluid flow rate and Darcy's law, and is calculated by equations (16) and (17):

[0100]

[0101] In the formula, A p It is the effective cross-sectional area of ​​low permeability shale matrix.

[0102] In this embodiment, the absolute permeability of low-permeability shale is the relative permeability of the gas phase when all pores are filled with gas, which is calculated by formula (18):

[0103]

[0104] In this embodiment, the relative permeability is obtained by the ratio between the phase permeability and the absolute permeability, as shown in equations (19) and (20):

[0105]

[0106] In the formula, k rg and k rw are the relative permeabilities of gas phase and water phase, respectively.

[0107] In this embodiment, the water saturation is determined by the gas-water distribution characteristics in the pores and is calculated by formula (21):

[0108]

[0109] The above process takes into account the occurrence state of water in the mathematical model for calculating the relative permeability of shale gas and water, and reflects the fractal characteristics of shale pores. The distribution of water in low-permeability shale pores is digitized through the nuclear magnetic resonance system, and the shale pores are divided accordingly; by analyzing the distribution characteristics and seepage velocity of gas-water two-phase fluids in different types of pores, the relative permeability of gas and water phases at different water saturations of low-permeability shales is accurately obtained, providing more accurate theoretical guidance for evaluating shale gas production and fracturing fluid return rate.

[0110] Embodiment 2:

[0111] A system for determining relative permeability of low-permeability shale gas-water two-phases, comprising:

[0112] Nuclear magnetic resonance equipment, used to obtain T2 spectrum curves of low-permeability shale samples corresponding to different pretreatment methods;

[0113] The processor is configured to: determine the water storage state in the pores of the sample through the T2 spectrum curve, and classify the pore types of the low-permeability shale to obtain the corresponding critical pore size;

[0114] The processor is further configured to: determine the flow rates of gas and water phases in a single shale pore according to the Hagen-Poiseuille equation and the gas-water distribution characteristics of different pore types, and determine the corresponding fluid flow rate in the single pore according to the obtained flow rates;

[0115] The processor is further configured to: obtain the total flow rate of gas-water two-phase fluid in the low-permeability shale pore network based on fractal theory, critical pore size, and flow rate of fluid in a single pore;

[0116] The processor is further configured to: obtain the phase permeabilities of the gas phase and the water phase and the absolute permeability of the low-permeability shale based on the total flow rate of the fluid using Darcy's law, and further determine the relative permeability;

[0117] The processor is further configured to determine the water saturation of the low permeability shale according to the distribution characteristics of the gas-water two-phase fluid in the pores, and finally determine the gas-water relative permeability of the low permeability shale at different water saturations.

[0118] In the mathematical model for calculating the relative permeability of shale gas and water, the occurrence state of water is taken into account, and the fractal characteristics of shale pores are reflected. The distribution of water in low-permeability shale pores is digitized through the nuclear magnetic resonance system, and the shale pores are divided accordingly; by analyzing the distribution characteristics and seepage velocity of gas-water two-phase fluids in different types of pores, the relative permeability of gas phase and water phase at different water saturations of low-permeability shales is accurately obtained, providing more accurate theoretical guidance for evaluating shale gas production and fracturing fluid return rate.

[0119] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for determining the relative permeability of low-permeability shale gas-water two-phase, characterized in that: The following steps are involved: Obtain low-permeability shale samples and pre-treat them, and obtain T2 spectrum curves corresponding to different pre-treatment methods based on nuclear magnetic resonance; Through the T2 spectrum curve, the water storage state in the sample pores is determined, and the pore types of low-permeability shale are classified to obtain the corresponding critical pore size; According to the Hagen-Poiseuille equation and the gas-water distribution characteristics of different pore types, the flow rates of gas and water phases in a single shale pore are determined, and based on the obtained flow rates, the corresponding fluid flow rate in a single pore is determined; Based on fractal theory, critical pore size and flow rate of fluid in a single pore, the total flow rate of gas-water two-phase fluid in the pore network of low-permeability shale is obtained; Based on the total flow rate of the fluid, Darcy's law is used to obtain the phase permeability of the gas phase and the water phase and the absolute permeability of the low-permeability shale, and further determine the relative permeability; According to the distribution characteristics of gas-water two-phase fluid in the pores, the water saturation of low-permeability shale is determined, and finally the gas-water relative permeability of low-permeability shale at different water saturations is determined.

2. A method for determining the relative permeability of low-permeability shale gas-water two-phase according to claim 1, characterized in that: The low-permeability shale samples were obtained and pre-treated, and the T2 spectrum curves corresponding to different pre-treatment methods were obtained based on nuclear magnetic resonance, specifically: The target low-permeability shale is processed into samples of set size and shape. After drying, a set of T2 spectrum data is obtained by nuclear magnetic resonance, which is recorded as T 2_d ; The dried sample was saturated under a set water pressure to obtain a shale sample in a water-saturated state. Another set of T2 spectrum data was obtained by nuclear magnetic resonance, denoted as T 2_s ; The saturated low-permeability shale sample was centrifuged, and the third group of T2 spectrum data was obtained by nuclear magnetic resonance, which was recorded as T 2_c .

3. A method for determining the relative permeability of low-permeability shale gas-water two-phase according to claim 2, characterized in that: The T2 spectrum curve is used to determine the water storage state in the sample pores and classify the pore types of low-permeability shale, specifically: All the water in the pores is bound water, corresponding to type I pores, i.e., T 2_s -T 2_d ≠0 and situation; The water in the pores is composed of bound water and free water, corresponding to type II pores, that is, And T 2_c -T 2_d ≠0; All the water in the pores is free water, corresponding to type III pores, i.e., T 2_s -T 2_d ≠0 and T 2_c -T 2_d =0.

4. A method for determining the relative permeability of low-permeability shale gas-water two-phase according to claim 1, characterized in that: Critical pore size refers to two specific pore size critical values, namely the critical pore size r between type I pores and type II pores. i , and the critical pore size r between type II and type III pores w .

5. The method for determining the relative permeability of low-permeability shale gas-water two-phase according to claim 1, characterized in that: The fluid flow rate in a single pore is determined by integrating the flow velocity over the cross-sectional area of ​​the pore.

6. A method for determining the relative permeability of low permeability shale gas and water phases according to claim 1, characterized in that: Fractal theory includes the fractal theory of pore size distribution and the fractal theory of pore tortuosity. The fractal theory of pore size distribution is used to describe the relationship between the pore size and the corresponding number of pores, and the fractal theory of pore tortuosity is used to calculate the pore length after considering the tortuosity.

7. A method for determining the relative permeability of low permeability shale gas and water phases according to claim 1, characterized in that: The absolute permeability of low-permeability shale is the phase permeability of the gas phase when all the pores are filled with gas, as shown in the following formula: In the formula, A p is the effective cross-sectional area of ​​low permeability shale matrix, is the pore pressure difference, μ g is the dynamic viscosity of the gas phase, L0 is the length of the pore without considering the tortuosity, r max and r min are the maximum and minimum pore diameters, respectively.

8. A method for determining the relative permeability of low-permeability shale gas-water two-phase according to claim 1, characterized in that: The relative permeability is determined by the ratio of the phase permeability of the gas phase and the water phase to the absolute permeability, respectively.

9. A method for determining the relative permeability of low permeability shale gas and water phases according to claim 1, characterized in that: The water saturation is determined by the gas-water distribution characteristics in the pores, as shown in the following formula: In the formula, h g is the radius of the gas in the pore, r is the pore radius, r max and r min are the maximum and minimum pore diameters, D N is the fractal dimension of pore size distribution, and d is the differential symbol.

10. A system for determining relative permeability of low permeability shale gas and water phases, characterized in that: include: Nuclear magnetic resonance equipment, used to obtain T2 spectrum curves of low-permeability shale samples corresponding to different pretreatment methods; The processor is configured to: determine the water storage state in the pores of the sample through the T2 spectrum curve, and classify the pore types of the low-permeability shale to obtain the corresponding critical pore size; The processor is further configured to: determine the flow rates of gas and water phases in a single shale pore according to the Hagen-Poiseuille equation and the gas-water distribution characteristics of different pore types, and determine the corresponding fluid flow rate in the single pore according to the obtained flow rates; The processor is further configured to: obtain the total flow rate of gas-water two-phase fluid in the low-permeability shale pore network based on fractal theory, critical pore size, and flow rate of fluid in a single pore; The processor is further configured to: obtain the phase permeabilities of the gas phase and the water phase and the absolute permeability of the low-permeability shale based on the total flow rate of the fluid using Darcy's law, and further determine the relative permeability; The processor is further configured to determine the water saturation of the low permeability shale according to the distribution characteristics of the gas-water two-phase fluid in the pores, and finally determine the gas-water relative permeability of the low permeability shale at different water saturations.

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