Method for determining lower limit of movable fluid production in water-drive gas reservoir

Abstract

The application discloses a method for determining the lower limit of movable fluid in water drive gas reservoirs and a preparation method thereof, and belongs to the technical field of oilfield development. T 2The conversion relationship between the relaxation time and the pore throat radius is derived and established by considering the bound water film thickness, the bound water film thickness, the maximum pore throat radius and the minimum pore throat radius of the rock sample are calculated and obtained; the change characteristics of the nuclear magnetic resonance T 2The residual gas saturation of the rock sample after water drive gas is calculated and obtained by using the fractal theory, the theoretical expression of the lower limit of the movable fluid pore diameter, the lower limit of the movable fluid porosity and the lower limit of the movable fluid permeability considering the bound water film thickness is respectively derived and established; the lower limits of the movable fluid pore diameter, the porosity and the permeability in the water drive gas process are respectively calculated and evaluated by using the Newton iteration algorithm and the numerical integration algorithm in combination with the theoretical expression and the related parameters.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of oilfield development, and particularly relates to a method for determining the lower limit of movable fluid in a water-drive gas reservoir. BACKGROUND

[0002] At present, most of the gas reservoirs in China belong to water-drive gas reservoirs to varying degrees. In particular, among more than 500 gas reservoirs and gas-bearing structures discovered in the Sichuan Basin, water-drive gas reservoirs account for more than half, and the reserves scale accounts for more than 80% of the total natural gas reserves in the Sichuan Basin. Therefore, it is crucial to do a good job in the development of water-drive gas reservoirs for the development of the natural gas industry in China.

[0003] The mobility of reserves is a key factor affecting whether a water-drive gas reservoir can achieve profitable development, and accurate evaluation of the mobility of reserves is of great significance for improving the recovery efficiency of water-drive gas reservoirs. The evaluation of the mobility of reserves includes the evaluation of the lower limit of movable pore diameter and the lower limit of mobility (i.e., porosity and permeability). At present, the evaluation methods for the lower limit of movable pore diameter and the lower limit of mobility can be divided into static methods and dynamic methods. The static methods include mercury injection experiments, reservoir physical property parameters, and irreducible water saturation methods, etc. For example, a patent with publication number CN116337705A discloses a method for determining the lower limit of gas drive in conglomerate reservoirs. Based on the conversion relationship between nuclear magnetic resonance T2 relaxation time and pore throat radius, the lower limit of pore throat movement in the gas drive process of conglomerate reservoirs is determined by the change of T2 spectrum curve before and after gas drive, combined with the T2 cutoff value principle. A patent with publication number CN113945497A discloses a method for evaluating the mobility of oil and gas reservoir fluids. Based on the determination of the best conversion coefficient between nuclear magnetic resonance relaxation time and pore throat radius, the mobility of oil and gas reservoir fluids and the movement characteristics of fluids in different pore throat spaces are evaluated by the change of nuclear magnetic resonance cumulative curve before and after displacement. The dynamic methods mainly include well logging data method and productivity simulation experiment method. The well logging data method mainly determines the lower limit of physical properties under the existing economic and technical conditions by comprehensively comparing and analyzing well logging, oil testing, and reservoir physical property analysis, etc., combined with statistical principles and reservoir percolation mechanism. The productivity simulation experiment method determines the lower limit of reservoir physical properties by the change characteristics of recovery efficiency through indoor experiment simulation of gas reservoir development, and the lower limit of movable pore diameter can be obtained by nuclear magnetic resonance and CT scanning technology.

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

[0005] (1) The T2 cutoff value method of nuclear magnetic resonance is often used in the current static method, but a large number of indoor experiments have proved that the movable pore diameter lower limit determined by this method is not the true lower limit value, and the fluid below this value still shows good mobility, so the lower limit value determined by this method will be larger than the actual value.

[0006] (2) The theoretical calculation model of the lower limit of the movable aperture commonly used in the static method does not consider the influence of the thickness of the bound water film, which is inconsistent with the fluid occurrence characteristics of the actual gas reservoir, resulting in a calculated result that does not match the actual value of the gas reservoir.

[0007] (3) The dynamic method for determining the lower limit of the movable aperture needs to use a large amount of sample data and core experiment data, and the evaluation result is subjective. SUMMARY

[0008] The purpose of the present application is to overcome the shortcomings of the existing lower limit determination method, and to establish a theoretical model of the lower limit of the movable aperture and the physical property limit of the water drive gas reservoir from the occurrence characteristics of the gas reservoir fluid, and to establish a method for determining the lower limit of the movable fluid of the water drive gas reservoir, which is more accurate and objective, and provides guidance for the evaluation of the mobilizable reserves of the water drive gas reservoir.

[0009] The purpose of the present application is achieved by the following technical solutions:

[0010] A method for determining the lower limit of the movable fluid of a water drive gas reservoir, comprising the following steps:

[0011] Step S1, selecting a cylindrical core of a low-permeability reservoir, washing and drying the cylindrical core and measuring the dry weight m0, porosity and permeability k, then cutting the cylindrical core into two segments to divide it into a first segment of rock sample and a second segment of rock sample;

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

[0013] Step S3, performing vacuum pumping and pressurized saturation of formation water experiment on the second segment of rock sample, measuring the mass m1 of the second segment of rock sample after saturation is completed; at the same time, testing and obtaining the nuclear magnetic resonance T2 spectrum curve of the rock sample saturated with water;

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

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

[0016] Step S6, the pore throat distribution curve obtained by high pressure mercury injection is scaled and compared with the NMR T2 spectrum curve in step S3 in the same coordinate system, the conversion model among NMR T2 time, pore throat radius r and bound water film thickness h is used to fit the conversion coefficient C and the bound water film thickness h of the rock sample, and the maximum pore throat radius r of the rock sample is read from the converted pore throat distribution curve max and the minimum pore throat radius r min ;

[0017] Step S7, the fractal dimension D of the rock sample is fitted by using the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2, and the residual gas saturation S org ;

[0018] Step S8, the parameters obtained in steps S6 and S7 are used to calculate the lower limit of the movable fluid pore size r c ;

[0019] Step S9, the movable fluid pore is calculated, and the lower limit of the movable fluid porosity is calculated by using the relationship between the total pore volume and the movable fluid and the bound fluid

[0020] Step S10, the lower limit of the movable fluid permeability k m is calculated by using the numerical integration method in the water drive gas process.

[0021] Preferably, the length of the cylindrical rock sample in step S1 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 pressure saturation formation water experiment in step S4 is: first, the second section of the rock sample is vacuumed to 133 Pa, and then the second section of the rock sample is saturated with formation water solution under the pressure of 20 MPa for 48 hours, then the mass of the second section of the rock sample is measured, which is recorded as m1, and the effective pore volume V eff and the effective porosity of the second section of the rock sample are calculated. When the relative error between the two is less than 2%, the saturation of the rock sample is completed, otherwise, the saturation is re-saturated according to the above steps.

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

[0024] Step S51, after the rock sample is built to the bound water saturation, the confining pressure pump is used to increase the confining pressure to the overburden pressure, and the system temperature is increased to the formation temperature;

[0025] ​Step S52, after the system temperature is stable, open the constant pressure displacement pump, use the pressure balance valve to quickly balance the pressure at both ends to the formation pressure, and stabilize 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 between the two ends of the sample, the cumulative gas production at the outlet end, 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, stop the experiment after injecting 2 PV of water. At this time, the sample reaches the residual gas state, and the nuclear magnetic resonance T2 spectrum curve of the rock sample in the residual gas state is measured.

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

[0028]

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

[0030] Preferably, the residual gas saturation calculation formula in step S7 is as formula (2) and (3):

[0031]

[0032]

[0033] In the formula, S wi is the bound water saturation, a decimal; S org is the residual gas saturation, a decimal; m(r) 100％ is the nuclear magnetic resonance signal amplitude when the water is completely saturated; m(r) Swi is the nuclear magnetic resonance signal amplitude in the bound water state; m(r) Sorg is the nuclear magnetic resonance signal amplitude in 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, substitute the calculated fractal dimension D, bound water film thickness h, residual gas saturation S org , maximum pore throat radius r max , and minimum pore throat radius r min into formula (6) to establish a one-variable multiple equation about the lower limit r c of the movable fluid pore diameter.

[0036]

[0037] wherein: is the porosity of the rock sample, 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, the lower limit radius r c of the movable fluid is solved by using the Newton iteration format (7)~(9) to solve the one-dimensional multiple equation of the movable fluid, and the iteration is ended when the error of the current and the previous two times is less than 0.001, and the calculation result is the lower limit r c of the movable fluid pore diameter in the water drive gas process.

[0039]

[0040]

[0041]

[0042] wherein: n is the iteration number.

[0043] Preferably, in the step S81, the pore volume V p and the residual gas volume V org are calculated according to the formula (4)~(5):

[0044]

[0045]

[0046] wherein: is the porosity of the rock sample, 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 calculation formula of the movable fluid porosity in the step S9 is as formula (10)~(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 production in a water-flooding gas reservoir. Starting from the actual occurrence characteristics of the reservoir fluid and the pore throat morphology, the method first derives and establishes a conversion relationship between the nuclear magnetic resonance (NMR) T2 relaxation time and the pore throat radius when considering the thickness of the irreducible water film. The irreducible water film thickness, maximum pore throat radius, and minimum pore throat radius of the rock sample are calculated by comparing and scaling the NMR T2 spectrum of a fully saturated water sample with the high-pressure mercury injection pore size distribution. Then, the residual gas saturation of the rock sample after water flooding is calculated based on the changing characteristics of the NMR T2 spectrum during the water-flooding process. Furthermore, theoretical expressions for the lower limits of the movable fluid pore size, porosity, and permeability, considering the thickness of the irreducible water film, are derived using fractal theory. Finally, combining the theoretical expressions and relevant parameters, the Newton iteration algorithm and numerical integration algorithm are used to calculate and evaluate the lower limits of the movable fluid pore size, porosity, and permeability during the water-flooding process.

[0056] Second, the present invention provides a method for determining the lower limit of movable fluid production in water-flooded gas reservoirs. Based on the actual distribution characteristics of gas reservoir fluids, this method establishes theoretical expressions for the lower limits of movable fluid pore size and physical properties. This method overcomes the shortcomings of current methods for evaluating the lower limit of movable fluid production, making the evaluation results more objective and accurate. Furthermore, the present invention offers the advantages of a simple experimental method, fewer operational steps, and minimal risk of human error. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0059] Figure 3A T2 spectrum curve in a water drive gas process in the present application;

[0060] Figure 4 A fitting curve between a T2 relaxation time and a pore throat radius in the present application;

[0061] Figure 5 A schematic diagram for obtaining a fitting fractal dimension based on high-pressure mercury injection in the present application. DETAILED DESCRIPTION

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

[0063] Example 1

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

[0065] Step S1, selecting a cylindrical core of a low-permeability reservoir, washing and drying the cylindrical core and measuring the dry weight m0, porosity and permeability k of the cylindrical core, then cutting the cylindrical core into two segments to divide into a first segment of rock sample and a second segment of rock sample;

[0066] Step S2, performing a high-pressure mercury injection experiment on the first segment of 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 graph;

[0067] Step S3, performing a vacuum pressurized formation water saturation experiment on the second segment of rock sample, measuring the mass m1 of the second segment of rock sample after saturation is completed; and testing and obtaining a nuclear magnetic resonance T2 spectrum curve of the rock sample saturated with water;

[0068] Step S4, loading the rock sample fully saturated with water into a core holder, using a gradient pressurization method to displace the rock sample with natural gas to a bound water saturation, and measuring a nuclear magnetic resonance T2 spectrum curve of the rock sample under the bound water;

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

[0070] Step S6, drawing 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 scaling and comparison, using a conversion model between the nuclear magnetic resonance T2 time, the pore throat radius r and the bound water film thickness h to fit and obtain the conversion coefficient C and the bound water film thickness h corresponding to the rock sample, and reading the maximum pore throat radius r max and the minimum pore throat radius r min;

[0071] Step S7, fitting the fractal dimension D of the rock sample using the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2, and calculating the residual gas saturation S in the water drive gas process org ;

[0072] Step S8, calculating the lower limit r of the movable fluid pore diameter in the water drive gas process of the rock sample using the parameters obtained in steps S6 and S7 c ;

[0073] Step S9, calculating the movable fluid pore and calculating the lower limit of the movable fluid porosity using the relationship between the total pore volume and the movable fluid and the bound fluid

[0074] Step S10, calculating the lower limit km of the movable fluid permeability in the water drive gas process using the numerical integration method.

[0075] Example 2

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

[0077] Step S1, selecting a cylindrical core with a length of 4-6 cm in a low permeability reservoir, and washing and drying the core, measuring the dry weight m0, porosity and permeability k of the core, cutting the core into two sections, the first section with a length of 1-2 cm, and the second section with a length of 3-4 cm;

[0078] Step S2, performing high-pressure mercury injection experiment on the first section of the core according to the national standard GB / T 29171-2012 "Determination of rock capillary pressure curve", calculating the pore throat radius of the rock sample according to the mercury injection curve of the rock sample, and drawing the corresponding frequency distribution graph (as Figure 2 shown);

[0079] Step S3, performing vacuum pumping and pressurized saturation of formation water experiment on the second section of the core, first pumping the second section of the core to 133 Pa, then pressurizing and saturating the formation water solution at 20 MPa for 48 hours, measuring the mass of the second section of the core, denoted as m1, calculating the effective pore volume V eff and effective porosity of the second section of the core, when the relative error between and is less than 2%, the saturation of the rock sample is completed, otherwise, re-saturate according to the above steps; at the same time, open the nuclear magnetic resonance instrument, set the parameters of the nuclear magnetic resonance instrument, and test the nuclear magnetic resonance T2 spectrum curve (as Figure 3 shown) of the saturated water second section of the core;

[0080] Step S4, the fully saturated water rock sample is loaded into the core holder, and the natural gas is used to displace the rock sample to the irreducible water saturation by using the gradient pressurization method, and the nuclear magnetic resonance T2 spectrum curve of the rock sample under the irreducible water is measured;

[0081] Step S5, after the rock sample is built to the irreducible water saturation, the confining pressure is increased to the overburden pressure by using the confining pressure pump, and the system temperature is increased to the formation temperature;

[0082] Step S6, after the system temperature is stable, the constant pressure displacement pump is opened, the pressure balance valve is used to quickly balance the pressure at the upstream and downstream ends to the formation pressure, and is stable for more than 2 h;

[0083] Step S7, the pressure balance valve is closed, the upstream pressure is increased to the experimental pressure, and then the water drive gas experiment is carried out, and the displacement time, the pressure difference between the two ends of the sample, the cumulative gas production at the outlet end, the water breakthrough time and the cumulative water production after the water breakthrough are recorded during the experiment, when the cumulative gas production no longer increases or the water production rate tends to be stable, 2 PV of water is injected, and then the experiment is stopped, at this time, the sample reaches the residual gas state, and the nuclear magnetic resonance T2 spectrum curve of the rock sample under the residual gas state is measured (as shown in Figure 3 );

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

[0085]

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

[0087] Step S9, the fractal dimension D of the rock sample is fitted and obtained by using the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2 (as shown in Figure 5 and Table 1), and the residual gas saturation S org in the water drive gas process is calculated (as shown in Table 1).

[0088] During the experiment, the nuclear magnetic resonance signal of the water phase is mainly monitored, so the T2 spectrum curve obtained only represents the water 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 the water drive gas can be calculated by formula (2)-(3):

[0089]

[0090]

[0091] wherein S wi is the irreducible water saturation, decimal; S org is the residual gas saturation, decimal; m(r) 100％ is the NMR signal amplitude of the fully water-saturated rock sample; m(r) Swi is the NMR signal amplitude of the irreducible water state; m(r) Sorg is the NMR signal amplitude of 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, using the parameters obtained in steps S8 and S9, calculate the lower limit r c of the movable fluid pore size in the water drive gas process of the rock sample, as follows:

[0093] Step S101, substitute the calculated fractal dimension D, irreducible water film thickness h, residual gas saturation S org , maximum pore throat radius r max , and minimum pore throat radius r min into equation (6) to establish a one-dimensional multiple equation about the lower limit r c of the movable fluid pore size:

[0094]

[0095]

[0096]

[0097] wherein: is the porosity of the rock sample, 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 format (7)~(9) to solve the one-dimensional multiple equation of the lower limit radius r c of the movable fluid pore size, and end the iteration when the error of the current and previous two solutions is less than 0.001, at which time the calculation result is the lower limit r c of the movable fluid pore size in the water drive gas process (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 movable fluid porosity is calculated using the relationship between the total pore volume and the movable fluid and bound fluid. Formulas (10) and (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 during the water displacement process 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 flooding of rock samples

[0111]

[0112] Finally, it should be noted that the above embodiments are only for illustration, not for limitation of 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 mobile fluid production in a water-drive gas reservoir, characterized in that, The method comprises the following steps: Step S1, select a low-permeability reservoir core, wash salt and dry the core and measure the dry weight m0, porosity φ and permeability k Then, cut the core into two segments, a first segment and a second segment. Step S2, performing high-pressure mercury injection experiment on the first section of rock sample, calculating the pore throat radius of the rock sample according to the mercury injection curve of the rock sample, and drawing a corresponding frequency distribution graph; Step S3, vacuumizing and pressurizing the second section of rock sample to saturate the formation water, and measuring the mass m1 of the second section of rock sample after saturation; meanwhile, testing and obtaining the nuclear magnetic resonance of the saturated rock sample under water T 2 spectrum curve; Step S4, the fully saturated water rock sample is loaded into the core holder, and the gradient pressurization is used to displace the rock sample with natural gas to the irreducible water saturation, and the nuclear magnetic resonance of the rock sample under the irreducible water is measured T 2 spectrum curve; Step S5, carry out water flooding gas experiment under the conditions of formation temperature and pressure until the cumulative gas production no longer increases or the water production rate tends to be stable, then stop the experiment after injecting 2PV water, measure the nuclear magnetic resonance of the rock sample under the state of residual gas T 2 spectrum curve; Step S6, the pore throat distribution curve obtained by high pressure mercury injection is scaled and compared with the nuclear magnetic resonance T2 spectrum curve in step S3 in the same coordinate system, and the conversion model between the relaxation time and the pore throat radius of the rock sample is fitted to obtain the conversion coefficient between the relaxation time and the pore throat radius of the rock sample T 2time, pore throat radius r and the bound water film thickness h The conversion model is fitted to obtain the conversion coefficient between the relaxation time and the pore throat radius of the rock sample C and the bound water film thickness h , and the maximum pore throat radius corresponding to the rock sample is read from the converted pore throat distribution curve r max and the minimum pore throat radius r min ; Step S7, the fractal dimension of the rock sample is obtained by fitting the relationship curve between the mercury injection pressure and the mercury injection saturation obtained in step S2 D and the residual gas saturation during water drive gas process is calculated S org ; Step S8, using the parameters obtained in steps S6 and S7, calculate the lower limit of the movable fluid pore size in the water drive gas process of the rock sample r c ; Step S9, calculating the movable fluid porosity φ m and calculating the movable fluid porosity lower limit using the relationship between the total porosity volume and the movable fluid and irreducible fluid φ im ; Step S10, using numerical integration method to calculate the lower limit of the movable fluid permeability in the water drive gas process k m .

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

3. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 1, characterized in that: The specific process of the vacuumizing and pressurizing saturation experiment in step S4 is as follows: the second rock sample is first vacuumized to 133 Pa, and then pressurized and saturated with the formation water solution under a pressure of 20 MPa for 48 hours; then the mass of the second rock sample is measured and recorded as m1, and the effective pore volume of the second rock sample is calculated V eff and the effective porosity φ eff When the relative error between the mass m1 and the mass m2 is less than 2%, the saturation of the rock sample is completed, otherwise, the saturation is re-performed according to the above steps. φ eff and the effective porosity φ ​ 4. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 1, characterized in that: The specific process of the step S5 is as follows: Step S51, after the rock sample is built to the irreducible water saturation, the confining pressure pump is used to increase the confining pressure to the overburden pressure, and the system temperature is increased to the formation temperature; Step S52, after the system temperature is stable, the constant pressure displacement pump is opened, the pressure balance valve is used to quickly balance the pressure at the two ends of the upstream and downstream to the formation pressure, and is stable for more than 2 h; 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 between the two ends of the sample, the cumulative gas production at the outlet end, 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 2 PV of water and stop the experiment. At this time, the sample reaches the residual gas state, and the nuclear magnetic resonance spectrum of the rock sample in the residual gas state is measured. T 2 spectrum curve.

5. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 1, characterized in that: The step S6 is to obtain the nuclear magnetic resonance by fitting the formula (1) T 2The conversion relationship between the time, the pore throat radius r and the thickness of the bound water film h ​ Formula (1); wherein: T 2 is the transverse relaxation time, ms; C is the conversion factor between the relaxation time and the pore throat radius, pm / ms; r is the pore throat radius, pm; h is the irreducible water film thickness, pm.

6. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 1, characterized in that: The residual gas saturation calculation formula in the step S7 is as follows: Formula (2); Formula (3); In the formula: S wi Swb = fractional bound water saturation; S org R is the residual gas saturation, fraction; m(r) 100% R is the residual gas saturation, fraction; m(r) Swi R is the residual gas saturation, fraction; m(r) Sorg R is the residual gas saturation, fraction; r max R is the residual gas saturation, fraction; r min R is the residual gas saturation, fraction; 7. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 1, characterized in that, The specific process in the step S8 is as follows: Step S81, calculating the fractal dimension acquired D , thickness of the bound water film h , residual gas saturation S org , maximum pore throat radius r max and minimum pore throat radius r min Substitute equation (6) to establish a one-variable multiple power equation about the lower limit of the movable fluid pore diameter r c ; Formula (6); wherein: φ is the porosity of the rock sample, 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, solve the lower limit radius of movable fluid using Newton iteration format (7)~(9) for the one-dimensional multiple equation of movable fluid, and end the iteration when the error of the current and previous two times is less than 0.001, and the calculation result is the lower limit of movable fluid pore diameter in the water drive gas process r c r c ;​ Formula (7); Formula (8); Formula (9); In the formula, n is the iteration number.

8. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 7, characterized in that: In the step S81, the pore volume V p and the residual gas volume V org The calculation formulae are as formulae (4)~(5): Formula (4); Formula (5); wherein: φ is the porosity of the rock sample, 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 production in a water-drive gas reservoir according to claim 1, characterized in that: The lower limit calculation formula of the movable fluid porosity in the step S9 is as follows: Formula (10); Formula (11); wherein: τ is the tortuosity of the rock sample, dimensionless; φ m is the movable fluid porosity, decimal; φ im is the lower limit of movable fluid porosity, in decimal; V pc is the corresponding pore volume of movable fluid, in cm 3 ; V is the outer volume of the rock sample, in cm 3 .

10. The method for determining the lower limit of movable fluid production in a water-drive gas reservoir according to claim 1, characterized in that: The calculation formula in the step S10 is as follows: Formula (12); wherein: τ is the tortuosity of the rock sample, dimensionless; k m is the lower limit of the movable fluid permeability, mD.

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