A method for calculating a lower limit of mobilizable fluid

Abstract

The application discloses a movable fluid lower limit calculation method, and steps are as follows: S1, taking a cylindrical rock sample of dense rock, and cutting the rock sample into two sections; S2, performing high-pressure mercury injection experiment on the first section of rock sample, and performing nuclear magnetic resonance water flooding experiment on the second section of rock sample; S3, calculating the maximum pore throat radius r max and fractal dimension D after high-pressure mercury injection; S4, calculating the irreducible water saturation S wi and residual oil saturation S or after water flooding experiment; and S5, calculating the movable fluid lower limit. The method has high experimental data measurement precision and simple operation, is based on fractal theory, combines nuclear magnetic resonance T2 spectrum and high-pressure mercury injection capillary pressure curve, and accurately calculates the movable fluid lower limit r c of a dense reservoir, thereby providing a new theoretical basis for effective evaluation of the dense reservoir.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil and gas exploration and development, and particularly relates to a method for calculating a lower limit of movable fluid. BACKGROUND

[0002] As a new hotspot in the field of unconventional oil and gas exploration and development, tight oil reservoirs gradually become one of important energy pillars for future development of China's oil and gas industry due to their wide distribution and great resource potential. Movable fluid saturation in the reservoir is an important parameter for understanding the oil-water flow characteristics of the tight reservoir, and is of great significance for effectively determining the recovery degree of the reservoir, evaluating the production capacity, and selecting the development blocks of the oilfield. The key to determining the movable fluid saturation lies in the calculation of the lower limit of the reservoir fluid. At present, domestic and foreign scholars have carried out research on the lower limit of movable fluid. Due to the complexity of the pore throat characteristics of the tight reservoir, it is difficult to determine the lower limit of the movable fluid. At present, the research methods for the fluid production of the tight reservoir mainly include various laboratory methods based on nuclear magnetic resonance technology.

[0003] The nuclear magnetic resonance technology is very sensitive to the complex pore structure and fluid medium therein, and the measured signal is not affected by the appearance shape of the rock. The nuclear magnetic resonance technology has the advantages of high efficiency and non-destructiveness. The pore structure and fluid flow and distribution of the tight reservoir can be accurately obtained by measuring the relaxation signal of hydrogen nuclei in the reservoir pore. The nuclear magnetic resonance technology mainly distinguishes the movable fluid and the bound fluid through the T2 cutoff value, that is, the fluid on the right side of the T2 cutoff value on the nuclear magnetic resonance relaxation spectrum curve is the movable fluid, and the fluid on the left side is the bound fluid.

[0004] However, there are some limitations in determining the lower limit by the T2 cutoff value. First, the current methods for determining the T2 cutoff value mainly include the experience method, the core experiment calibration method, and the original formation continuous calibration method. The experience method of 33ms is generally used, but it is not universally applicable to the tight reservoir. The result calculated by the core experiment calibration method is based on the analysis of the core experiment, and the experimental result is directly affected by the experimental pressure or centrifugal force, so the result precision is not enough. The cutoff value obtained by the original formation continuous calibration method is obtained by regional statistics, and the applicability is not high for other types of reservoirs and blocks. These conventional methods have some deficiencies in determining the T2 cutoff value. Second, the T2 cutoff value method for determining the movable fluid saturation is suitable for monitoring the case where the nuclear magnetic resonance response signal continuously decreases, and the application effect is good in the conventional reservoir. However, this method is not applicable to the case where the nuclear magnetic resonance response signal continuously increases.

[0005] In summary, the existing determination method of the lower limit of movable fluid is difficult to be applied to the tight reservoir due to certain limitations; in addition, as a subject of quantitatively studying irregular and disordered structures in nature, the fractal theory has a wide application in the characterization of pore structures. Therefore, the formula for accurately calculating the lower limit of movable fluid is established by means of the fractal theory, and the calculation process and method of the lower limit of movable fluid are provided according to the pore throat characteristics of the tight reservoir. SUMMARY

[0006] The present application aims at the technical defects that the existing technologies and methods cannot characterize the lower limit of movable fluid, and provides a movable fluid lower limit calculation method.

[0007] The new movable fluid lower limit calculation method provided by the present application comprises the following steps:

[0008] S1, taking a cylindrical rock sample of the tight rock with a length of about 8 cm, and performing cleaning and drying treatment.

[0009] S2, cutting the rock sample into two sections, the first section has a length of 2.5 cm, and the second section has a length of 4.5 cm, then drying the two sections again.

[0010] S3, performing high-pressure mercury injection experiment on the first section of the rock sample according to the national standard GB / T29171-2012 “Determination of Capillary Pressure Curve of Rock”, determining the capillary pressure curve of the rock, and recording the capillary pressure as P c .

[0011] S4, measuring the porosity φ He , permeability k, length L, diameter D, dry weight m0 and density ρ of the second section of the rock sample according to the national standard GB / T29172-2012 “Core Analysis Method”, and calculating the pore volume V p ,

[0012] S5, after the basic physical property measurement, vacuumizing the second section of the rock sample at a negative pressure of 0.1 MPa for 4 h, and pressurizing the manganese chloride solution with a concentration of 50000 ppm for 48 h or more at a pressure of 40 MPa, measuring the mass m1 of the rock sample after being completely saturated with water, and obtaining the T2 spectrum curve of the rock sample by using a nuclear magnetic resonance instrument, recording the area of the T2 spectrum curve as A1, and placing the saturated core into a core holder, calculating the effective pore volume V eff and effective porosity φ eff , when the relative error between the effective porosity of the rock sample and the gas measured porosity is less than 2%, that is, when the condition of is met, the saturation of the rock sample is considered to be completed, otherwise, the saturation is re-performed according to the above steps. The formula for calculating the effective pore volume V eff and effective porosity φ eff of the rock sample is as follows:

[0013]

[0014] wherein, - density of the manganese chloride solution.

[0015] S6, connect the displacement device, check the gas tightness of the instrument, put the saturated second section of the rock sample into the core holder, inject heavy water (D2O) into the core under the confining pressure of 5 MPa and a small displacement pressure difference, replace the saturated fluid in the core, gradually increase the displacement pressure difference during the replacement process (wherein the maximum displacement pressure is less than the confining pressure), and monitor the nuclear magnetic resonance signal until the nuclear magnetic resonance curve has no signal response, and stop the experiment.

[0016] S7, carry out oil-water displacement experiment on the rock sample replaced by heavy water with the original oil in the formation, continuously increase the displacement pressure difference under the confining pressure of 5 MPa, stop the experiment when the rock sample saturated with heavy water is completely displaced to the irreducible water saturation, unload the confining pressure and internal pressure, take out the core, and obtain the T2 spectrum curve of the rock sample by using the nuclear magnetic resonance instrument, record the T2 spectrum curve area as A2, then put the rock sample containing irreducible water into the experimental oil, and age for one week at the formation temperature, and measure the nuclear magnetic resonance T2 spectrum curve after aging.

[0017] S8, put the rock sample after aging and wiping off the surface oil into the core holder again, set the confining pressure and internal pressure to the experimental pressure, displace with heavy water to the residual oil saturation state, take out the rock sample and measure its T2 spectrum curve again, and record the T2 spectrum curve area as A3.

[0018] S9, data processing, specifically including the following steps:

[0019] S91, obtaining the pore throat distribution characteristics of the sample:

[0020] According to the capillary pressure curve of the sample obtained by the high-pressure mercury injection experiment, the pore throat distribution characteristics of the core can be obtained, and the maximum pore throat radius r max and the minimum pore throat radius r minn of the rock sample are determined, and the calculation formula is as follows:

[0021]

[0022] wherein: r is the capillary pressure corresponding to different capillary pressures, mm;

[0023] σ is the interfacial tension, N / m;

[0024] θ is the wetting contact angle, °;

[0025] pc is the capillary pressure;

[0026] For mercury, the interfacial tension and wetting angle are constant, σ is 480 mN / m, θ is 140°, and the formula can be expressed as:

[0027]

[0028] According to the obtained pore throat radius distribution characteristics, the maximum pore throat radius r max and the minimum pore throat radius r min .

[0029] S92, calculation of fractal dimension: the capillary pressure curve and the mercury saturation curve are plotted in the double logarithmic coordinates, and the slope k of the double logarithmic curve is calculated, and the specific calculation formula of the fractal dimension D is as follows:

[0030] D = 3-k

[0031] In the formula: D-fractal dimension;

[0032] k-slope of the curve;

[0033] S93, calculation of irreducible water saturation and residual oil saturation: the difference between the nuclear magnetic resonance T2 spectrum area A1 in the saturated water state and the nuclear magnetic resonance T2 spectrum curve area A2 in the heavy water state after saturation oil represents the distribution of irreducible water in the pore, and the nuclear magnetic resonance T2 spectrum curve area A3 in the residual oil state is the distribution of residual oil, and the formula for calculating the irreducible water saturation S wi and the residual oil saturation S or is as follows:

[0034] (1) Irreducible water saturation (%):

[0035]

[0036] In the formula, S wi -irreducible water saturation, %;

[0037] A1-nuclear magnetic resonance T2 spectrum area in the saturated water state, dimensionless;

[0038] A2-nuclear magnetic resonance T2 spectrum area in the heavy water state after saturation oil, dimensionless;

[0039] (2) Residual oil saturation (%):

[0040]

[0041] In the formula, S or -residual oil saturation, %;

[0042] A1-nuclear magnetic resonance T2 spectrum area in the saturated water state, dimensionless;

[0043] A3 - NMR T2 spectrum area in residual oil state, dimensionless;

[0044] S94, movable fluid lower limit calculation:

[0045] (1) Based on the fractal theory of porous media, a simplified capillary model is established, assuming that the storage space of the dense reservoir is composed of a series of capillaries with length L and radius r, and the bound water film thickness is uniformly distributed in the capillary. The number of pore throats with a pore throat radius greater than r in the capillary model can be expressed as:

[0046]

[0047] In the formula: N(>r) - pore throat number; P(r) - pore throat radius distribution density function; D - fractal dimension.

[0048] Assuming that the pore size distribution is continuous, the number of pore throats with a pore radius between r and r+dr is obtained by differentiating equation (1):

[0049] dN(>r) = -Dr -D-1 r max D dr (2)

[0050] (2) In the process of water flooding, the volume of residual oil V s that has not been driven in the pore space of the sample can be expressed as:

[0051]

[0052] In the formula, r c - movable fluid radius.

[0053] (3) The relationship between the bound water film thickness and the bound water saturation can be expressed as:

[0054]

[0055] In the formula, h - bound water film thickness.

[0056] It is found that there is a linear correlation between the capillary length and the capillary radius:

[0057] L = ar (5) In the formula, a - linear correlation coefficient.

[0058] (4) Substitute equations (2) and (4), (5) into equation (3), then the residual oil volume in the pore space can be expressed as:

[0059]

[0060]

[0061] ​(5) In formula (3), the critical pore throat radius r c is the maximum pore throat radius r max , and the thickness of the bound water film h is 0, at this time, the cumulative calculation volume is the total pore volume V p of the sample space:

[0062]

[0063] (6) Bringing formula (2) into formula (7), since there is a linear correlation between the capillary length and the capillary radius, the total pore volume V p of the sample space can be expressed as:

[0064]

[0065] (7) The residual oil saturation Sor is the ratio of the residual oil volume to the total pore volume, that is, the ratio of formula (6) to formula (8), and the residual oil saturation can be expressed as:

[0066]

[0067] Since the maximum pore throat radius is much larger than the minimum pore throat radius, the minimum pore throat radius in formula (9) can be ignored, and formula (9) can be expressed as:

[0068]

[0069] From formula (10), the calculation formula of the lower limit of movable fluid r c can be obtained:

[0070]

[0071] In the formula, r c - the lower limit of movable fluid, μm;

[0072] S or - residual oil saturation, %;

[0073] S wi - bound water saturation, %;

[0074] r max - the maximum pore throat radius, μm;

[0075] D- fractal dimension.

[0076] Compared with the existing technology, the benefits of the present invention are: the existing method for determining the lower limit of movable fluid production is mainly based on the nuclear magnetic resonance T2 cutoff value method, which has a certain deviation from the actual reservoir movable fluid production situation. This method combines the characteristics of tight cores and proposes a theoretical calculation method based on fractal theory, which effectively overcomes the shortcomings of the current characterization technology of the lower limit of movable fluid production and provides new ideas and methods for effectively determining the lower limit of tight reservoir fluid production. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 , NMR T2 spectrum distribution diagram of water flooding experiment under different displacement pressure differences

[0078] Figure 2 , Schematic diagram of fractal dimension calculation

[0079] Figure 3 , Schematic diagram of capillary model DETAILED DESCRIPTION

[0080] The specific embodiments of the present invention are described in more detail below with reference to the accompanying drawings and preferred examples. The preferred examples described herein are only used for illustration and explanation and are not intended to limit the present invention.

[0081] A method for calculating the lower limit of movable fluid utilization comprises the following steps:

[0082] S1. Take a cylindrical rock sample of dense rock with a length of 8 cm and a diameter of 2.5 cm, and clean, deoil, and dry it.

[0083] S2. Cut the cleaned and dried rock sample into two sections, with the length of the first section being 2.5 cm and the length of the second section being 4.5 cm, and then dry the two sections of rock sample again.

[0084] S3. According to the national standard GB / T29171-2012 "Determination of rock capillary pressure curve", the capillary pressure test is carried out on the first section of rock sample to determine the rock capillary pressure curve. The capillary pressure is recorded as P c .

[0085] S4. Measure the length L, diameter D, and porosity φ of the second section of rock sample according to the national standard GB / T29172-2012 "Core Analysis Method" He , permeability k, dry weight m0 and density ρ, and calculate the pore volume V p , the calculation formula is

[0086] S5. After the basic physical property measurements are completed, the second section of rock sample is vacuumed at -0.1 MPa for 4 hours, and then pressurized with a saturated MnCl2 solution at 40 MPa for more than 48 hours, where the concentration of the MnCl2 solution is 50,000 ppm. The mass m1 of the rock sample after it is completely saturated with water is measured, and the T2 spectrum curve of the rock sample is obtained using a nuclear magnetic resonance instrument. The area of ​​the T2 spectrum curve is recorded as A1, and the effective pore volume V is recorded as eff and effective porosity φ eff When the relative error between the effective porosity of the rock sample and the gas porosity is less than 2%, the When the rock sample is saturated, it can be considered that the rock sample is fully saturated. Otherwise, follow the above steps to re-saturate. Calculate the effective pore volume V of the rock sample eff and effective porosity φ eff The formula is as follows:

[0087]

[0088] S6. Place the saturated second section of rock sample into the core holder, connect the displacement device, check the air tightness of the instrument, and at a confining pressure of 20 MPa, first use a small displacement pressure difference to inject heavy water (D2O) into the core to displace the saturated fluid in the core. During the displacement process, gradually increase the pressure difference and monitor the nuclear magnetic resonance signal until the experiment is stopped when there is no signal response from the nuclear magnetic resonance curve.

[0089] S7. Connect an oil-water separation metering tube to the outlet of the core holder, conduct an oil-water displacement experiment with formation crude oil under a certain pressure differential on the rock sample replaced with heavy water. Stop the experiment when the rock sample completely saturated with heavy water is displaced to the irreducible water saturation. After unloading the confining pressure and internal pressure, remove the core and obtain the T2 spectrum curve of the rock sample using a nuclear magnetic resonance instrument. Note the area of ​​the T2 spectrum curve as A2. Then, place the rock sample containing irreducible water in the experimental oil and age it at the formation temperature for one week. Measure the nuclear magnetic resonance T2 spectrum curve after aging.

[0090] S8. After wiping off the surface oil of the second aged rock sample, place it in the core holder again to conduct water flooding experiment. Figure 1 This is the distribution of nuclear magnetic resonance T2 spectra during water displacement at different displacement pressures. Under different experimental pressures, heavy water is used to displace the rock sample to the residual oil saturation state. The rock sample is taken out and its T2 spectrum curve is measured again. The area of ​​the T2 spectrum curve is recorded as A3.

[0091] S9, data processing, specifically including the following steps:

[0092] S91. Obtaining the pore throat distribution characteristics of samples:

[0093] According to the capillary pressure curve of the sample obtained by high-pressure mercury injection experiment, the pore throat distribution characteristics of the core can be obtained and the maximum pore throat radius r of the rock sample can be determined. max and minimum pore throat radius rmin ,, the calculation formula is as follows:

[0094]

[0095] Where: r-capillary pressure corresponding to different capillary pressures, mm;

[0096] σ-interfacial tension, N / m;

[0097] θ-wetting contact angle, °;

[0098] pc--capillary pressure;

[0099] For mercury, the interfacial tension and wetting angle are fixed values, σ is 480mN / m, θ is 140°, and the positive values ​​can be expressed as:

[0100]

[0101] According to the obtained pore throat radius distribution characteristics, the maximum pore throat radius r can be obtained. max and minimum pore throat radius r min .

[0102] S92. Calculation of fractal dimension:

[0103] like Figure 2 The figure below is a schematic diagram of calculating the fractal dimension based on the high-pressure mercury injection curve. The capillary pressure curve and the mercury injection saturation curve are plotted with logarithmic values ​​to obtain the curve slope k. The specific calculation formula for the fractal dimension D is as follows:

[0104] D=3-k

[0105] Where: D-fractal dimension;

[0106] k-curve slope;

[0107] S93. Calculation of bound water saturation and residual oil saturation: The area between the NMR T2 spectrum curve area A1 in the water-saturated state and the NMR T2 spectrum curve area A2 in the oil-saturated bound heavy water state represents the distribution of bound water in the pores. The area A3 of the NMR T2 spectrum curve in the residual oil state represents the distribution of residual oil. The bound water saturation S is calculated using the T2 spectrum area ratio method. wi and residual oil saturation S or The formula is as follows:

[0108] (3) Immune water saturation (%):

[0109]

[0110] Where S wi -Immobilized water saturation, %;

[0111] A1 - Nondimensional T2 spectrum area under saturated water state;

[0112] A2 - Nondimensional T2 spectrum area under bound heavy water state after oil saturation;

[0113] (4) Residual oil saturation (%):

[0114]

[0115] where S or - Residual oil saturation, %;

[0116] A1 - Nondimensional T2 spectrum area under saturated water state;

[0117] A3 - Nondimensional T2 spectrum area under residual oil state;

[0118] S94, Lower limit of movable fluid production:

[0119] (1) Based on the fractal theory of porous media, a simplified capillary model is established. It is assumed that the storage space of the tight reservoir is composed of a series of capillaries with length L and radius r, and the bound water film thickness is uniformly distributed in the capillary (L Figure 3 ), then the number of pore throats with a pore throat radius greater than r in the capillary model can be expressed as:

[0120]

[0121] where N(>r) - Number of pore throats; P(r) - Distribution density function of pore throat radius; D - Fractal dimension.

[0122] Assuming that the pore size distribution is continuous, the number of pore throats with a pore radius between r and r+dr is obtained by differentiating equation (1):

[0123] dN(>r) = -Dr -D-1 r max D dr (2)

[0124] (2) During water flooding, the volume of residual oil V s that is not driven in the pore space of the sample can be expressed as:

[0125]

[0126] where r c - Radius of movable fluid.

[0127] (3) The relationship between the bound water film thickness and the bound water saturation can be expressed as:

[0128]

[0129] In the formula, h is the thickness of the bound water film.

[0130] Studies have shown that there is a linear correlation between capillary length and capillary radius:

[0131] L = ar (5) In the formula, a is the linear correlation coefficient.

[0132] (4) Substitute formula (2) and formula (4), (5) into formula (3), then the remaining oil volume in the pore space can be expressed as:

[0133]

[0134] (5) In formula (3), the critical pore throat radius r c is the maximum pore throat radius r max , and the bound water film thickness h is 0, at this time the cumulative calculation volume is the total pore volume V p of the sample space:

[0135]

[0136] (6) Substitute formula (2) into formula (7), because there is a linear correlation between capillary length and capillary radius, the total pore volume V p of the sample space can be expressed as:

[0137]

[0138] (7) The residual oil saturation Sor is the ratio of the cumulative oil displacement volume and the total pore volume, that is, the ratio of formula (6) and formula (8), the residual oil saturation can be expressed as:

[0139]

[0140] Because the maximum pore throat radius is much larger than the minimum pore throat radius, the minimum pore throat radius in formula (9) can be ignored, then formula (9) can be expressed as:

[0141]

[0142] From formula (10), the calculation formula of the lower limit of movable fluid r c can be obtained:

[0143]

[0144] In the formula, r c is the lower limit of movable fluid, μm;

[0145] S or is the residual oil saturation, %

[0146] S wi - irreducible water saturation, %;

[0147] r max - maximum pore throat radius, pm;

[0148] D - fractal dimension.

[0149] In summary, the application is based on the fractal theory to establish a formula for accurately calculating the lower limit of movable fluid, and proposes a movable fluid calculation process and method for the pore throat characteristics of tight reservoirs. The method is simple to operate, and the experimental results obtained have high measurement accuracy, fully utilizes the pore throat structure characteristics and related percolation theory of tight reservoirs, reflects the real situation of tight reservoirs, and can more accurately evaluate the movable fluid production of tight oil reservoirs.

[0150] The above description does not limit the application in any form, and any person skilled in the art can make certain changes, additions or modifications to the equivalent embodiments disclosed above without departing from the concept and technical solution of the application. It should be understood that any modification, equivalent change and replacement made within the technical scope of the application without departing from the technical solution of the application shall fall within the protection scope of the technical solution of the application.

Claims

1. A method for calculating the lower limit of movable fluid utilization, characterized in that: The steps include: S1. Take a cylindrical rock sample of dense rock with a length greater than 8 cm and wash and dry it; S2. Cut the rock sample into two sections, the first section is 2.5 cm, and the second section is 4.5 cm. Dry the two sections again; S3, performing a high-pressure mercury injection experiment on the first section of rock sample, and measuring the capillary pressure curve by the high-pressure mercury injection method; S4. Measure the porosity φ of the second section of rock sample He , permeability k, length L, diameter D, dry weight m0 and density ρ, and calculate the pore volume V p After measuring the basic physical properties, the core is vacuumed and saturated with formation water solution. After measuring the mass m1 and NMR T2 curve of the rock sample after it is completely saturated with water, heavy water (D2O) is used to displace the saturated formation water in the core. During the displacement process, NMR curves are continuously obtained until there is no signal in the NMR curve. S5. After heavy water displacement, the core is placed in a core holder and connected to a displacement device. Under a confining pressure of 5 MPa, the heavy water-saturated core is displaced with formation crude oil until the core reaches a bound water state. Displacement is then stopped, and the core is removed and the nuclear magnetic resonance T2 spectrum curve in the bound water state is measured. The core is then placed in experimental oil for aging for one week, and the nuclear magnetic resonance T2 spectrum curve of the aged core is measured. S6. Place the aged core back into the holder, displace it with heavy water under the experimental pressure until it reaches a residual oil state, and measure the nuclear magnetic resonance T2 spectrum curve in the residual oil state; S7. Data processing includes the following steps: S71. Obtain the pore throat distribution characteristics of the sample: According to the obtained capillary pressure curve, obtain the pore throat distribution characteristics of the core and determine the maximum pore throat radius r max and minimum pore throat radius r min , the formula is as follows: Where: r-pore throat radius corresponding to different capillary pressures, mm; σ-interfacial tension, N / m; θ-wetting contact angle, (°); p c -capillary pressure; For mercury, the interfacial tension and wetting angle are fixed values, σ is 480mN / m, θ is 140°, and the positive values ​​can be expressed as: S72. Calculation of fractal dimension: Take the logarithmic values ​​of the capillary pressure curve and the mercury saturation curve and plot them. Calculate the slope k of the curve. The calculation formula for the fractal dimension D is as follows: D=3-k S73. Calculation of bound water saturation and residual oil saturation. The difference between the area A1 of the NMR T2 spectrum in the water-saturated state and the area A2 of the NMR T2 spectrum curve in the oil-saturated bound heavy water state represents the distribution of bound water in the pores. The area A3 of the NMR T2 spectrum curve in the residual oil state represents the distribution of residual oil. The bound water saturation S is calculated using the T2 spectrum area ratio method. wi and residual oil saturation S or The formula is as follows: Where: S wi -Immobilized water saturation, %; S or --Residual oil saturation, %; A1-NMR T2 spectrum area in saturated water state, dimensionless; A2-NMR T2 spectrum area in the bound heavy water state after saturation of oil, dimensionless; A3-NMR T2 spectrum area in the residual oil state, dimensionless; S74. Determine the lower limit r of movable fluid based on the fractal principle of porous media c , as follows: (1) Based on the fractal theory of porous media, a simplified capillary model is established. Assuming that the storage space of a tight reservoir consists of a series of capillaries with a length of L and a radius of r, and the thickness of the bound water film is uniformly distributed in the capillaries, the number of pore throats with a pore throat radius greater than r in the capillary model can be expressed as: Where: N (>r) - number of pore throats; P (r) - distribution density function of pore throat radius; D - fractal dimension; Assuming that the pore size distribution is continuous, the number of pore throats with pore radius between r and r+dr can be obtained by differentiating equation (1): dN(>r)=-Dr -D-1 r max D dr (2) (2) During the water displacement process, the volume of residual oil in the pore space of the sample that is not driven is V s It can be expressed as: Where r c - movable fluid radius; (3) The relationship between bound water film thickness and bound water saturation can be expressed as: Where, h is the thickness of bound water film; There is a linear correlation between capillary length and capillary radius: L=ar (5) Where, a-linear correlation coefficient; (4) Substituting formula (2), formula (4) and formula (5) into formula (3), the volume of remaining oil in the pore space can be expressed as: (5) In formula (3), the critical pore throat radius r c is the maximum pore throat radius r max , and when the bound water film thickness h is 0, the cumulative calculated volume at this time is the total pore volume of the sample space V p : (6) Substituting formula (2) into formula (7), the total pore volume V of the sample space is p It can be expressed as: (7) Residual oil saturation S or is the ratio of the remaining oil volume to the total pore volume, that is, the ratio of equation (6) to equation (8). The residual oil saturation can be expressed as: Since the maximum pore throat radius is much larger than the minimum pore throat radius, the minimum pore throat radius in formula (9) can be ignored, and formula (9) can be expressed as: From formula (10), we can get the lower limit r of movable fluid: c The calculation formula is: Where: r c -Lower limit of movable fluid, μm; S or - Residual oil saturation, %; S wi -Immobilized water saturation, %; r max - Maximum pore throat radius, μm; D - fractal dimension.

2. A method for calculating the lower limit of movable fluid utilization according to claim 1, characterized in that: In step S1, the pore volume V p The calculation formula is:

3. A method for calculating the lower limit of movable fluid utilization according to claim 1, characterized in that: The step S4 specifically includes: first, evacuating the second section of rock sample at -0.1 MPa for 4 hours, pressurizing a saturated MnCl2 solution at 40 MPa for more than 48 hours, measuring the mass of the rock sample, recorded as m1, and calculating the effective pore volume V of the rock sample. eff and effective porosity φ eff , when satisfied If the rock sample is saturated, it is completed. Otherwise, re-saturate it according to the above steps.

4. A method for calculating the lower limit of movable fluid utilization according to claim 3, characterized in that: Calculate the effective pore volume V of the rock sample eff and effective porosity φ eff The formula is as follows: Where, --Density of MnCl2 solution.

5. A method for calculating the lower limit of movable fluid utilization according to claim 4, characterized in that: The concentration of the MnCl2 solution is 50000 ppm.

6. A method for calculating the lower limit of movable fluid utilization according to claim 1, characterized in that: In step S1, the cylindrical core has a length of 8 to 10 cm and a diameter of 2 to 3 cm.

7. A method for calculating the lower limit of movable fluid utilization according to claim 1, characterized in that: The capillary pressure test was carried out on the first section of rock sample according to the national standard GB / T29171-2012 “Determination of capillary pressure curve of rock”.

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