Method for determining lower use limit of movable fluid of water-driven gas reservoir

Abstract

The invention discloses a method for determining the lower use limit of a movable fluid of a water-driven gas reservoir and a preparation method of the movable fluid, and belongs to the technical field of oilfield development, the conversion relation between the nuclear magnetic resonance T2 relaxation time and the pore throat radius when the thickness of a bound water film is considered is deduced and established from the actual occurrence characteristics and the pore throat form of the fluid of the gas reservoir, and the conversion relation between the relaxation time and the pore throat radius is obtained. The bound water film thickness, the maximum pore throat radius and the minimum pore throat radius of the rock sample are obtained through calculation; the residual gas saturation of a rock sample after water-driven gas is obtained through calculation according to the change characteristics of a nuclear magnetic resonance T2 map in the water-driven gas process, and theoretical expressions of a movable fluid aperture lower limit, a movable fluid porosity lower limit and a movable fluid permeability lower limit considering the bound water film thickness are respectively derived and established by utilizing a fractal theory; in combination with a theoretical expression and related parameters, a Newton iteration algorithm and a numerical integration algorithm are adopted to respectively calculate and evaluate the aperture, porosity and permeability lower limit of the movable fluid in the water-driven gas process.

Description

Technical Field

[0001] The invention belongs to the technical field of oilfield development, and in particular relates to a method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir. Background Art

[0002] At present, most of my country's gas reservoirs are water-driven gas reservoirs to varying degrees, especially in the Sichuan Basin. Among the more than 500 gas reservoirs and gas-bearing structures discovered, water-driven gas reservoirs account for more than half, and the reserves account for more than 80% of the total natural gas reserves in the Sichuan Basin. Therefore, the development of water-driven gas reservoirs is crucial to the development of my country's natural gas industry.

[0003] The mobility of reserves is a key factor affecting whether water-driven gas reservoirs can achieve profitable development. Accurate evaluation of reserve mobility is of great significance for improving the recovery of water-driven gas reservoirs. The evaluation of reserve mobility includes the evaluation of the lower limit of movable pore size and the lower limit of movable properties (i.e., porosity and permeability). At present, the evaluation methods for the lower limit of movable pore diameter and movable physical limit can be mainly divided into static method and dynamic method, among which the static method includes mercury injection experiment, reservoir physical property parameters and irreducible water saturation method, etc. For example, Chinese patent with publication number CN116337705A discloses an experimental method for determining the lower limit of gas drive in conglomerate reservoirs. This method determines the lower limit of pore throat activation in the conglomerate reservoir during gas drive by changing the T2 spectrum curve before and after gas drive and combining the T2 cutoff value principle on the basis of obtaining the conversion relationship between the nuclear magnetic resonance T2 relaxation time and the pore throat radius; Chinese patent with publication number CN113945497A discloses a method for evaluating the mobility of reservoir fluid in oil and gas reservoirs. This method fully combines CT scanning, nuclear magnetic resonance and error analysis, and evaluates the mobility of reservoir fluid in oil and gas reservoirs and the activation characteristics of fluids in different pore throat spaces by changing the nuclear magnetic resonance cumulative curve before and after displacement on the basis of determining the optimal conversion coefficient between the nuclear magnetic resonance relaxation time and the pore throat radius. The dynamic method mainly includes the well logging data method and the production capacity simulation experiment method. The well logging data method mainly determines the lower limit of physical properties under the existing economic and technical conditions based on comprehensive comparative analysis of various dynamic and static data such as well logging, oil testing, reservoir physical property analysis, etc., combined with statistical principles and reservoir seepage mechanism; the production capacity simulation experiment method simulates the gas reservoir development process with the help of indoor experiments, determines the lower limit of reservoir physical properties through the change characteristics of recovery rate, and at the same time, with the assistance of nuclear magnetic resonance and CT scanning technology, the lower limit of movable aperture can be obtained.

[0004] However, these current methods have the following shortcomings:

[0005] (1) The nuclear magnetic resonance T2 cutoff value method is currently commonly used in static methods. However, a large number of indoor experiments have confirmed that the lower limit of the movable aperture determined by this method is not the true lower limit. The fluid smaller than this value still exhibits good mobility, and the lower limit determined by this method will be larger than the actual value.

[0006] (2) The theoretical calculation models of the movable pore diameter lower limit commonly used in the static method do not consider the influence of the bound water film thickness, which is inconsistent with the fluid storage characteristics of the actual gas reservoir, resulting in the calculation results being inconsistent with the actual values ​​of the gas reservoir.

[0007] (3) The dynamic method for determining the lower limit of the movable aperture requires the use of a large amount of sample data and core test data, and its evaluation results are highly subjective. Summary of the invention

[0008] The purpose of the present invention is to address the shortcomings of the existing methods for determining the lower limit of production. Starting from the distribution characteristics of gas reservoir fluids, a theoretical model of the lower limit of movable pore size and physical property of water-driven gas reservoirs is derived and established, and a method for determining the lower limit of movable fluid production in water-driven gas reservoirs is established. The calculation results are more accurate and objective, providing guidance for the evaluation of reserve mobility of water-driven gas reservoirs.

[0009] The objective of the present invention is achieved through the following technical solutions:

[0010] A method for determining the lower limit of movable fluid production in a water-driven gas reservoir comprises the following steps:

[0011] Step S1: Select a cylindrical core of a low-permeability reservoir, wash the salt and dry the cylindrical core, and measure the dry weight m0 and porosity. After determining the permeability k, the cylindrical core is cut into two sections, namely the first section rock sample and the second section rock sample;

[0012] Step S2, performing a high-pressure mercury injection experiment on the first section of the rock sample, calculating and obtaining the pore throat radius of the rock sample according to the mercury injection curve of the rock sample, and drawing a corresponding frequency distribution diagram;

[0013] Step S3, performing a vacuum pressurization experiment on the second section of rock sample to saturate formation water, and measuring the mass m1 of the second section of rock sample after saturation is completed; and at the same time, obtaining a nuclear magnetic resonance T2 spectrum curve of the saturated underwater rock sample;

[0014] Step S4, placing the rock sample completely saturated with water into a core holder, using natural gas to displace the rock sample to irreducible water saturation by means of gradient pressurization, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under irreducible water;

[0015] Step S5, carrying out a water drive gas experiment under the formation temperature and pressure conditions until the cumulative gas production no longer increases or the water production rate tends to be stable, injecting 2PV of water and then stopping the experiment, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under the residual gas state;

[0016] Step S6: Plot the pore throat distribution curve obtained by high-pressure mercury injection and the nuclear magnetic resonance T2 spectrum curve in step S3 in the same coordinate system for scale comparison, and use the conversion model between the nuclear magnetic resonance T2 time, the pore throat radius r and the bound water film thickness h to obtain the conversion coefficient C and the bound water film thickness h corresponding to the rock sample, and read the maximum pore throat radius r corresponding to the rock sample from the converted pore throat distribution curve. max and minimum pore throat radius r min ;

[0017] Step S7: Use the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2 to obtain the fractal dimension D of the rock sample, and calculate the residual gas saturation S during the water drive process. org ;

[0018] Step S8: Using the parameters obtained in steps S6 and S7, calculate and obtain the lower limit r of the movable fluid pore size during the water-driven gas displacement process of the rock sample. c ;

[0019] Step S9: Calculate and obtain the movable fluid pores The lower limit of the porosity of the movable fluid is calculated by using the relationship between the total pore volume and the movable fluid and the bound fluid.

[0020] Step S10: Calculate the lower limit k of the permeability of the movable fluid in the water-displacing gas process by using a numerical integration method. m .

[0021] Preferably, in step S1, the length of the cylindrical rock sample is 4-6 cm and the diameter is 2.5 cm; the length of the first section of the rock sample is 1-2 cm, and the length of the second section of the rock sample is 3-4 cm.

[0022] Preferably, the specific process of the vacuum pressurized saturated formation water experiment in step S4 is: first, the second section of the rock sample is vacuumed to 133Pa, and after pressurizing the saturated formation water solution at a pressure of 20MPa for 48 hours, the mass of the second section of the rock sample is measured, recorded as m1, and the effective pore volume V of the second section of the rock sample is calculated. eff and effective porosity when and When the relative error between the two values ​​is less than 2%, the rock sample is saturated. Otherwise, re-saturate according to the above steps.

[0023] Preferably, the specific process of step S5 is:

[0024] Step S51: After the irreducible water saturation of the rock sample is established, the confining pressure is increased to the overburden pressure by using a confining pressure pump, and the system temperature is raised to the formation temperature;

[0025] Step S52: After the system temperature is stable, turn on the constant pressure displacement pump, use the pressure balance valve to quickly balance the pressure at both ends of the upstream and downstream to the formation pressure, and stabilize it for more than 2 hours;

[0026] Step S53, close the pressure balance valve, increase the upstream pressure to the experimental pressure and then carry out the water drive gas experiment. During the experiment, record the displacement time, the pressure difference at both ends of the sample, the cumulative gas production at the outlet, the water breakthrough time and the cumulative water production after water breakthrough. When the cumulative gas production no longer increases or the water production rate tends to be stable, inject 2PV of water and stop the experiment. At this time, the sample reaches the residual gas state, and measure the nuclear magnetic resonance T2 spectrum curve of the rock sample in the residual gas state.

[0027] Preferably, in step S6, the conversion relationship between the nuclear magnetic resonance T2 time, the pore throat radius r and the bound water film thickness h is obtained by fitting formula (1);

[0028]

[0029] Where: T2 is the transverse relaxation time, ms; C is the conversion coefficient between relaxation time and pore throat radius, μm / ms; r is the pore throat radius, μm; h is the bound water film thickness, μm.

[0030] Preferably, the residual gas saturation calculation formula in step S7 is as follows:

[0031]

[0032]

[0033] Where: S wi is bound water saturation, decimal; S org is the residual gas saturation, decimal; m(r) 100％ is the NMR signal amplitude when fully saturated with water; m(r) Swi is the NMR signal amplitude in the bound water state; m(r) Sorg is the amplitude of the NMR signal under the residual gas state; r max is the maximum pore throat radius of the rock sample, μm; r min is the minimum pore throat radius of the rock sample, μm.

[0034] Preferably, the specific process in step S8 is:

[0035] Step S81: calculate the fractal dimension D, bound water film thickness h, and residual gas saturation S org , maximum pore throat radius r max and minimum pore throat radius r min Substituting into equation (6) we can establish the lower limit r of the movable fluid aperture: c A polynomial equation of one variable;

[0036]

[0037] Where: is the porosity of the rock sample, a decimal; d is the diameter of the rock sample, cm; L is the length of the rock sample, cm; τ is the tortuosity of the rock sample, dimensionless; V p is the pore volume, cm 3 ; V org is the residual gas volume, cm 3 ;

[0038] Step S82: Use Newton iteration scheme (7) to (9) to calculate the lower limit radius r of the movable fluid. c The one-variable multi-time equation is solved. When the error between the first and last solutions is less than 0.001, the iteration is terminated. At this time, the calculation result is the lower limit of the movable fluid pore size r in the water-driven gas process. c ;

[0039]

[0040]

[0041]

[0042] Where: n is the number of iterations.

[0043] Preferably, in step S81, the pore volume V p and residual gas volume V org The calculation formula is as follows:

[0044]

[0045]

[0046] Where: is the porosity of the rock sample, a decimal; d is the diameter of the rock sample, cm; L is the length of the rock sample, cm; τ is the tortuosity of the rock sample, dimensionless; V p is the pore volume, cm 3 ; V org is the residual gas volume, cm 3 .

[0047] Preferably, the lower limit of the porosity of the movable fluid in step S9 is calculated as shown in formulas (10) to (11):

[0048]

[0049] φ im =φ-φ m Formula (11);

[0050] Where: is the movable fluid porosity, a decimal; is the lower limit of the porosity of the movable fluid, a decimal; Vpc is the pore volume corresponding to the movable fluid, cm 3 ; V is the surface volume of the rock sample, cm 3 .

[0051] Preferably, the calculation formula in step S10 is as follows:

[0052]

[0053] Where: k m is the lower limit of movable fluid permeability, mD.

[0054] The beneficial effects of this technical solution are as follows:

[0055] 1. The present invention provides a method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir. Starting from the actual occurrence characteristics and pore throat morphology of the gas reservoir fluid, the method first derives and establishes the conversion relationship between the nuclear magnetic resonance T2 relaxation time and the pore throat radius when the irreducible water film thickness is considered, and the irreducible water film thickness, the maximum pore throat radius and the minimum pore throat radius of the rock sample are obtained by comparing and calculating the nuclear magnetic resonance T2 spectrum curve of the fully saturated water and the high-pressure mercury injection pore size distribution; then, the residual gas saturation of the rock sample after water-driven gas is calculated through the change characteristics of the nuclear magnetic resonance T2 spectrum during the water-driven gas process, and the theoretical expressions of the movable fluid pore size lower limit, the movable fluid porosity lower limit and the movable fluid permeability lower limit considering the irreducible water film thickness are derived and established by using the fractal theory; finally, combining the theoretical expressions and related parameters, the Newton iteration algorithm and the numerical integration algorithm are used to calculate and evaluate the movable fluid pore size, porosity and permeability lower limits during the water-driven gas process.

[0056] Second, the present invention provides a method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir. Starting from the actual occurrence characteristics of gas reservoir fluids, a theoretical expression for the lower limit of movable fluid pore size and physical properties is established, which makes up for the shortcomings of the current movable fluid lower limit evaluation method and makes the evaluation result more objective and accurate. At the same time, the present invention also has the beneficial effects of simple experimental method, fewer operation links, and less human factors. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a flow chart of the present invention;

[0058] Figure 2 It is a pore throat radius distribution diagram obtained based on the high-pressure mercury injection curve in the present invention;

[0059] Figure 3It is the nuclear magnetic resonance T2 spectrum curve in the water displacement process of the present invention;

[0060] Figure 4 It is the fitting curve between the nuclear magnetic resonance T2 relaxation time and the pore throat radius in the present invention;

[0061] Figure 5 This is a schematic diagram of obtaining the fitted fractal dimension based on high-pressure mercury injection in the present invention. DETAILED DESCRIPTION

[0062] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0063] Example 1

[0064] This embodiment provides a method for determining the lower limit of movable fluid production in a water drive gas reservoir, comprising the following steps:

[0065] Step S1: Select a cylindrical core of a low-permeability reservoir, wash the salt and dry the cylindrical core, and measure the dry weight m0 and porosity. After determining the permeability k, the cylindrical core is cut into two sections, namely the first section rock sample and the second section rock sample;

[0066] Step S2, performing a high-pressure mercury injection experiment on the first section of the rock sample, calculating and obtaining the pore throat radius of the rock sample according to the mercury injection curve of the rock sample, and drawing a corresponding frequency distribution diagram;

[0067] Step S3, performing a vacuum pressurization experiment on the second section of rock sample to saturate formation water, and measuring the mass m1 of the second section of rock sample after saturation is completed; and at the same time, obtaining a nuclear magnetic resonance T2 spectrum curve of the saturated underwater rock sample;

[0068] Step S4, placing the rock sample completely saturated with water into a core holder, using natural gas to displace the rock sample to irreducible water saturation by means of gradient pressurization, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under irreducible water;

[0069] Step S5, carrying out a water drive gas experiment under the formation temperature and pressure conditions until the cumulative gas production no longer increases or the water production rate tends to be stable, injecting 2PV of water and then stopping the experiment, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under the residual gas state;

[0070] Step S6: Plot the pore throat distribution curve obtained by high-pressure mercury injection and the nuclear magnetic resonance T2 spectrum curve in step S3 in the same coordinate system for scale comparison, and use the conversion model between the nuclear magnetic resonance T2 time, the pore throat radius r and the bound water film thickness h to obtain the conversion coefficient C and the bound water film thickness h corresponding to the rock sample, and read the maximum pore throat radius r corresponding to the rock sample from the converted pore throat distribution curve. max and minimum pore throat radius r min;

[0071] Step S7: Use the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2 to obtain the fractal dimension D of the rock sample, and calculate the residual gas saturation S during the water drive process. org ;

[0072] Step S8: Using the parameters obtained in steps S6 and S7, calculate and obtain the lower limit r of the movable fluid pore size during the water-driven gas displacement process of the rock sample. c ;

[0073] Step S9: Calculate and obtain the movable fluid pores The lower limit of the porosity of the movable fluid is calculated by using the relationship between the total pore volume and the movable fluid and the bound fluid.

[0074] Step S10, using a numerical integration method to calculate and obtain the lower limit km of the movable fluid permeability during the water-displacing gas process.

[0075] Example 2

[0076] like Figure 1 As shown, a method for determining the lower limit of movable fluid production in a water-driven gas reservoir comprises the following steps:

[0077] Step S1: Select a cylindrical core of a low-permeability reservoir with a length of 4-6 cm, wash the salt and dry it, and measure the dry weight m0 and porosity of the core. After determining the permeability k, the core was cut into two sections, the first section was 1-2 cm long and the second section was 3-4 cm long;

[0078] Step S2: According to the national standard GB / T 29171-2012 "Determination of rock capillary pressure curve", a high-pressure mercury injection experiment is performed on the first section of the core, and the pore throat radius of the rock sample is calculated according to the mercury injection curve of the rock sample, and the corresponding frequency distribution diagram is drawn (such as Figure 2 shown);

[0079] Step S3: Perform a vacuum pressurized saturated formation water experiment on the second core. First, vacuum the second core to 133Pa. After pressurizing the saturated formation water solution at 20MPa for 48 hours, measure the mass of the second core, record it as m1, and calculate the effective pore volume V of the second core. eff and effective porosity when and When the relative error between the two is less than 2%, the rock sample is saturated. Otherwise, re-saturate according to the above steps. At the same time, turn on the NMR instrument, set the parameters of the NMR instrument, and test the NMR T2 spectrum curve of the second section of the core saturated with water (such as Figure 3 shown);

[0080] Step S4, placing the rock sample completely saturated with water into a core holder, using natural gas to displace the rock sample to irreducible water saturation by means of gradient pressurization, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under irreducible water;

[0081] Step S5: After the irreducible water saturation of the rock sample is established, the confining pressure is increased to the overburden pressure by using a confining pressure pump, and the system temperature is raised to the formation temperature;

[0082] Step S6: After the system temperature is stable, turn on the constant pressure displacement pump, use the pressure balance valve to quickly balance the pressure at both ends of the upstream and downstream to the formation pressure, and stabilize it for more than 2 hours;

[0083] Step S7, close the pressure balance valve, increase the upstream pressure to the experimental pressure, and then carry out the water drive gas experiment. During the experiment, record the displacement time, the pressure difference between the two ends of the sample, the cumulative gas production at the outlet, the water breakthrough time, and the cumulative water production after water breakthrough. When the cumulative gas production no longer increases or the water production rate tends to be stable, inject 2PV of water and stop the experiment. At this time, the sample reaches the residual gas state, and measure the nuclear magnetic resonance T2 spectrum curve of the rock sample in the residual gas state (such as Figure 3 shown);

[0084] Step S8: Plot the pore throat distribution curve obtained by high pressure mercury injection and the nuclear magnetic resonance T2 spectrum curve under fully saturated water in the same coordinate system for scale comparison (e.g. Figure 4 The conversion relationship between the NMR T2 time and the pore throat radius is fitted using formula (1), so as to calculate the corresponding conversion coefficient, the bound water film thickness and the maximum pore throat radius r corresponding to the rock sample. max and minimum pore throat radius r min (As shown in Table 1):

[0085]

[0086] Where: T2 is the transverse relaxation time, ms; C is the conversion coefficient between relaxation time and pore throat radius, μm / ms; r is the pore throat radius, μm; h is the bound water film thickness, μm.

[0087] Step S9, using the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2 to fit the fractal dimension D of the rock sample (e.g. Figure 5 and Table 1), and calculate the residual gas saturation S during water drive gas org (As shown in Table 1).

[0088] During the experiment, the nuclear magnetic resonance signal of the water phase was mainly monitored, so the T2 spectrum curve obtained only represents the water phase signal; the change of the T2 spectrum curve reflects the change of the water phase in the experimental core, so the residual gas saturation after water displacement can be calculated by formulas (2) to (3):

[0089]

[0090]

[0091] Where: S wi is bound water saturation, decimal; S org is the residual gas saturation, decimal; m(r) 100％ is the NMR signal amplitude when fully saturated with water; m(r) Swi is the NMR signal amplitude in the bound water state; m(r) Sorg is the amplitude of the NMR signal under the residual gas state; r max is the maximum pore throat radius of the rock sample, μm; r min is the minimum pore throat radius of the rock sample, μm.

[0092] Step S10: Calculate the lower limit r of the movable fluid pore size during the water-driven gas displacement process of the rock sample using the parameters obtained in steps S8 and S9. c , as follows:

[0093] Step S101: calculate the fractal dimension D, bound water film thickness h, and residual gas saturation S org , maximum pore throat radius r max and minimum pore throat radius r min Substituting into equation (6) we can establish the lower limit r of the movable fluid aperture: c The polynomial equation of one variable:

[0094]

[0095]

[0096]

[0097] Where: is the porosity of the rock sample, a decimal; d is the diameter of the rock sample, cm; L is the length of the rock sample, cm; τ is the tortuosity of the rock sample, dimensionless; V p is the pore volume, cm 3 ; V org is the residual gas volume, cm 3

[0098] Step S102: Use Newton iteration scheme (7) to (9) to calculate the lower limit radius r of the movable fluid. c The one-variable multi-time equation is solved. When the error between the first and last solutions is less than 0.001, the iteration is terminated. At this time, the calculation result is the lower limit of the movable fluid pore size r in the water-driven gas process. c (As shown in Table 1):

[0099]

[0100]

[0101]

[0102] Where: n is the number of iterations;

[0103] Step S11: Calculate and obtain the porosity of the movable fluid The lower limit of the porosity of the movable fluid is calculated by using the relationship between the total pore volume and the movable fluid and the bound fluid. Formulas (10) to (11) are as follows:

[0104]

[0105] φ im =φ-φ m Formula (11);

[0106] Where: is the movable fluid porosity, a decimal; V is the lower limit of the porosity of the movable fluid, a decimal; pc is the pore volume corresponding to the movable fluid, cm 3 ; V is the surface volume of the rock sample, cm 3 .

[0107] Step S12: Calculate the lower limit k of the permeability of the movable fluid in the water-displacing gas process by using a numerical integration method. m (as shown in Table 1), formula (12) is as follows:

[0108]

[0109] Where: k m is the lower limit of movable fluid permeability, mD.

[0110] Table 1 Summary of calculation results during water displacement of rock samples

[0111]

[0112] Finally, it should be noted that the above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir, characterized in that: The following steps are involved: Step S1: Select a cylindrical core of a low-permeability reservoir, wash the salt and dry the cylindrical core, and measure the dry weight m0 and porosity. After determining the permeability k, the cylindrical core is cut into two sections, namely the first section rock sample and the second section rock sample; Step S2, performing a high-pressure mercury injection experiment on the first section of the rock sample, calculating and obtaining the pore throat radius of the rock sample according to the mercury injection curve of the rock sample, and drawing a corresponding frequency distribution diagram; Step S3, performing a vacuum pressurization experiment on the second section of rock sample to saturate formation water, and measuring the mass m1 of the second section of rock sample after saturation is completed; and at the same time, obtaining a nuclear magnetic resonance T2 spectrum curve of the saturated underwater rock sample; Step S4, placing the rock sample completely saturated with water into a core holder, using natural gas to displace the rock sample to irreducible water saturation by means of gradient pressurization, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under irreducible water; Step S5, carrying out a water drive gas experiment under the formation temperature and pressure conditions until the cumulative gas production no longer increases or the water production rate tends to be stable, injecting 2PV of water and then stopping the experiment, and measuring the nuclear magnetic resonance T2 spectrum curve of the rock sample under the residual gas state; Step S6: Plot the pore throat distribution curve obtained by high-pressure mercury injection and the nuclear magnetic resonance T2 spectrum curve in step S3 in the same coordinate system for scale comparison, and use the conversion model between the nuclear magnetic resonance T2 time, the pore throat radius r and the bound water film thickness h to obtain the conversion coefficient C and the bound water film thickness h corresponding to the rock sample, and read the maximum pore throat radius r corresponding to the rock sample from the converted pore throat distribution curve. max and minimum pore throat radius r min ; Step S7: Use the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2 to obtain the fractal dimension D of the rock sample, and calculate the residual gas saturation S during the water drive process. org ; Step S8: Using the parameters obtained in steps S6 and S7, calculate and obtain the lower limit r of the movable fluid pore size during the water-driven gas displacement process of the rock sample. c ; Step S9: Calculate and obtain the movable fluid pores The lower limit of the porosity of the movable fluid is calculated by using the relationship between the total pore volume and the movable fluid and the bound fluid. Step S10: Calculate the lower limit k of the permeability of the movable fluid in the water-displacing gas process by using a numerical integration method. m .

2. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: In step S1, the length of the cylindrical rock sample is 4-6 cm and the diameter is 2.5 cm; the length of the first section of the rock sample is 1-2 cm, and the length of the second section of the rock sample is 3-4 cm.

3. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: The specific process of the vacuum pressurized saturated formation water experiment in step S4 is: first, the second section of the rock sample is vacuumed to 133Pa, and after pressurizing the saturated formation water solution at a pressure of 20MPa for 48 hours, the mass of the second section of the rock sample is measured, recorded as m1, and the effective pore volume V of the second section of the rock sample is calculated. eff and effective porosity when and When the relative error between the two values ​​is less than 2%, the rock sample is saturated. Otherwise, re-saturate according to the above steps.

4. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: The specific process of step S5 is as follows: Step S51: After the irreducible water saturation of the rock sample is established, the confining pressure is increased to the overburden pressure by using a confining pressure pump, and the system temperature is raised to the formation temperature; Step S52: After the system temperature is stable, turn on the constant pressure displacement pump, use the pressure balance valve to quickly balance the pressure at both ends of the upstream and downstream to the formation pressure, and stabilize it for more than 2 hours; Step S53, close the pressure balance valve, increase the upstream pressure to the experimental pressure and then carry out the water drive gas experiment. During the experiment, record the displacement time, the pressure difference at both ends of the sample, the cumulative gas production at the outlet, the water breakthrough time and the cumulative water production after water breakthrough. When the cumulative gas production no longer increases or the water production rate tends to be stable, inject 2PV of water and stop the experiment. At this time, the sample reaches the residual gas state, and measure the nuclear magnetic resonance T2 spectrum curve of the rock sample in the residual gas state.

5. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: In step S6, the conversion relationship between the nuclear magnetic resonance T2 time, the pore throat radius r and the bound water film thickness h is obtained by fitting formula (1); Where: T2 is the transverse relaxation time, ms; C is the conversion coefficient between relaxation time and pore throat radius, μm / ms; r is the pore throat radius, μm; h is the bound water film thickness, μm.

6. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: The residual gas saturation calculation formula in step S7 is as shown in formulas (2) to (3): Where: S wi is bound water saturation, decimal; S org is the residual gas saturation, decimal; m(r) 100％ is the NMR signal amplitude when fully saturated with water; m(r) Swi is the amplitude of the NMR signal in the bound water state; m(r) Sorg is the amplitude of the NMR signal under the residual gas state; r max is the maximum pore throat radius of the rock sample, μm; r min is the minimum pore throat radius of the rock sample, μm.

7. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: The specific process in step S8 is: Step S81: calculate the fractal dimension D, bound water film thickness h, and residual gas saturation S org , maximum pore throat radius r max and minimum pore throat radius r min Substituting into equation (6) we can establish the lower limit r of the movable fluid aperture: c A polynomial equation of one variable; Where: is the porosity of the rock sample, a decimal; d is the diameter of the rock sample, cm; L is the length of the rock sample, cm; τ is the tortuosity of the rock sample, dimensionless; V p is the pore volume, cm 3 ; V org is the residual gas volume, cm 3 ; Step S82: Use Newton iteration scheme (7) to (9) to calculate the lower limit radius r of the movable fluid. c The one-variable multi-time equation is solved. When the error between the first and last solutions is less than 0.001, the iteration is terminated. At this time, the calculation result is the lower limit of the movable fluid pore size r in the water-driven gas process. c ; Where: n is the number of iterations.

8. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 7, characterized in that: In step S81, the pore volume V p and residual gas volume V org The calculation formula is as follows: Where: is the porosity of the rock sample, a decimal; d is the diameter of the rock sample, cm; L is the length of the rock sample, cm; τ is the tortuosity of the rock sample, dimensionless; V p is the pore volume, cm 3 ; V org is the residual gas volume, cm 3 .

9. The method for determining the lower limit of movable fluid utilization in a water-driven gas reservoir according to claim 1, characterized in that: The calculation of the lower limit of the porosity of the movable fluid in step S9 is shown in equations (10) to (11): f im =φ-φ m expression(11); Where: is the movable fluid porosity, a decimal; V is the lower limit of the porosity of the movable fluid, a decimal; pc is the pore volume corresponding to the movable fluid, cm 3 ; V is the surface volume of the rock sample, cm 3 .

10. The method for determining the lower limit of movable fluid utilization in a water-drive gas reservoir according to claim 1, characterized in that: The calculation formula in step S10 is as follows: Where: k m is the lower limit of movable fluid permeability, mD.

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