Method for correcting multi-scale fracture physical property parameters

Abstract

The invention relates to a method for correcting physical property parameters of a multi-scale crack. The method comprises the following steps: measuring related parameters of the multi-scale crack, measuring opening degrees of cracks with different scales, measuring lengths of the cracks, analyzing a crack group system, and measuring sampling areas of samples with different scales. Preparing a rock sample, and calculating and correcting the crack opening; calculating fracture densities of different scales; the calculated fracture densities of different scales and the corrected fracture opening degrees of the corresponding scales are put into the cross plot, a multi-scale corrected fracture opening degree and fracture density relation chart is obtained, the relation between the multi-scale fracture densities and the corrected fracture opening degrees is established, and the multi-scale fracture development relation is established; multi-scale fracture density correction is carried out; and corrected multi-scale physical property parameters are calculated. Rock samples under different pressure conditions are selected, the fracture forming state is simulated, the multi-scale fracture development relation is established, the porosity and permeability of the corrected fractures of different scales are calculated, and more accurate data support can be provided for evaluation of contribution of the fractures to the reservoir.

Description

Technical Field:

[0001] The present invention relates to a technology for obtaining physical property parameters of reservoir fractures, and specifically to a method for correcting physical property parameters of multi-scale fractures. Background Art:

[0002] Reservoir fractures are effective reservoir spaces and good migration channels for oil and gas. Especially in tight reservoirs, fractures can increase the reservoir space for oil and gas, play an important role in the migration and accumulation of oil and gas, and provide an important guarantee for the exploration and development of tight reservoir oil and gas. The physical property parameters of fractures mainly refer to the porosity and permeability of fractures, and the aperture of fractures is an important parameter affecting the physical properties. Therefore, the accuracy of fracture aperture is very important for calculating the physical property parameters of reservoir fractures.

[0003] The traditionally measured fracture aperture is obtained by drilling underground cores to the ground for measurement. However, during the formation drilling process, due to the loss of a large amount of overlying formation pressure on the rock, the drilled rock on the surface experiences formation pressure release, resulting in the measured fracture aperture value on the surface being greater than the true aperture value of the fracture underground, making it impossible to accurately measure the underground fracture aperture. At the same time, due to the sampling deviation between fractures of different scales, there are problems such as some fractures with smaller scales not being recognized at their measurement scales and some fractures with larger scales exceeding their measurement scales and not being completely measured, which will make some of the statistically obtained data inaccurate, and further cause a certain error between the calculated fracture density and the true fracture density value. On this basis, the accuracy of the measured fracture porosity and permeability is relatively low, and it is not possible to evaluate the contribution of fractures to the reservoir well. Summary of the Invention:

[0004] The purpose of the present invention is to provide a method for correcting physical property parameters of multi-scale fractures, which is used to solve the problem of relatively low accuracy of fracture porosity and permeability of reservoir fractures measured by traditional methods in the prior art.

[0005] The technical solution adopted by the present invention to solve its technical problems is as follows: This method for correcting physical property parameters of multi-scale fractures includes the following steps;

[0006] (1) Measurement of multi-scale fracture related parameters. The multi-scale fractures are defined as nano-scale fractures, micro-scale fractures, core-scale fractures, and imaging logging-scale fractures according to the fracture development characteristics and fracture aperture distribution range in the study area; measurement of fracture aperture, fracture length, fracture set analysis, and sampling area measurement of different-scale samples for different-scale fractures;

[0007] (2) Fracture aperture correction: Prepare rock samples. The underground aperture of fractures is related to the rock mechanical parameters of the rock where the fractures are located, the normal stress on the fracture surface, and the pore fluid pressure. The rock mechanical parameters include Young's modulus and Poisson's ratio. Calculate the corrected fracture aperture according to the following formula to obtain the multi-scale corrected fracture aperture;

[0008]

[0009] In the formula: e—the corrected fracture aperture, μm; e0—the fracture aperture, μm; σ—the normal stress on the fracture surface, MPa; P—the pore fluid pressure, MPa; E—the elastic modulus of the rock, MPa; υ—the Poisson's ratio of the rock; C1 and C2—proportionality coefficients, C1=(2.5 - 4.5)×10 -2 , C2 = 1 - 1.4;

[0010] (3) Multi-scale fracture density calculation and correction: Calculate the fracture density at different scales; Input the calculated fracture density at different scales and the corrected fracture aperture at the corresponding scale into a cross-plot to obtain the relationship chart of multi-scale corrected fracture aperture and fracture density, and establish the relationship between multi-scale fracture density and corrected fracture aperture; Then correct and establish the relationship between multi-scale fracture density and corrected fracture aperture, fit the relationship between multi-scale fracture density and corrected fracture aperture, establish the multi-scale fracture development relationship, and perform multi-scale fracture density correction:

[0011] ρ = ae -b

[0012] In the formula: ρ is the fracture density, fractures / m; e is the corrected fracture aperture, μm; a and b are coefficients;

[0013] (4) Calculate the multi-scale physical property parameters after correction:

[0014] Calculate the corrected fracture porosity:

[0015]

[0016] In the formula: is the fracture porosity, %; e is the corrected fracture aperture, μm; e m is the average corrected fracture aperture, μm; e1 is the lower limit of the fracture aperture at different scales, μm; e2 is the upper limit of the fracture aperture at different scales, μm; ρ is the fracture density.

[0017] Calculate the corrected fracture permeability:

[0018]

[0019] In the formula: K f is the fracture permeability, mD; e is the corrected fracture aperture, μm; e me0 is the average aperture of the cracks for calibration, in μm; e1 is the lower limit of the crack aperture at different scales, in μm; e2 is the upper limit of the crack aperture at different scales, in μm; ρ is the crack density; S is the coefficient of the crack set system, which depends on the crack set system and geometry.

[0020] Step (1) in the above solution is specifically as follows:

[0021] ① Measurement of multi-scale crack apertures: The crack apertures at the nano-scale are measured by a scanning electron microscope. When measuring the cracks, select 3 parts that can represent the overall aperture of the cracks, measure the crack aperture values, and take the average value as the crack aperture of this crack; the crack apertures at the micro-scale are measured by an optical microscope; the crack apertures at the core scale are measured by a ruler or a feeler gauge. For cracks with an aperture greater than 1 mm, use a ruler to measure, and for cracks with an aperture less than 1 mm, use a feeler gauge to measure; the crack apertures at the imaging logging scale are calculated based on the imaging logging data.

[0022] ② Measurement of multi-scale crack lengths: While measuring the crack apertures at different scales, measure the length of each crack for calculating the crack density.

[0023] ③ Analysis of crack set systems: Based on the development characteristics of cracks at different scales, comprehensively analyze the distribution of crack set systems.

[0024] ④ Measurement of the sampling areas of samples at different scales: The sampling areas at the nano-scale and the micro-scale samples are both the areas of the thin sections where the cracks are located; the core-scale samples are the surface areas of the cores; the imaging logging scale samples are the surface areas of the imaging logging boreholes.

[0025] Step (2) in the above solution is specifically as follows:

[0026] ① Preparation of rock samples: Select N rock samples with different lithologies, and select 2 parallel samples for each lithology sample point. Among them, parallel sample A has no cracks and is used for rock mechanics experiments; the other parallel sample B has cracks and is used for pressurization experiments under different confining pressures. According to the requirements of the experimental sample scale, prepare experimental samples, and the core sample scale is 25 mm × 50 mm.

[0027] ② Obtaining parameters related to calibrating crack apertures: The underground aperture of the cracks is related to the Young's modulus, Poisson's ratio of the rock where the cracks are located, the normal stress on the crack surface, and the pore fluid pressure:

[0028] a. Crack aperture e0: According to the measurement of multi-scale crack apertures in Step 1, obtain the crack apertures at different scales.

[0029] b. Normal stress σ on the crack surface: The specific calculation formula for the normal stress on the crack surface is as follows:

[0030]

[0031] Where: n ij is the direction cosine, and the stress tensor at a certain point on the fracture surface is defined as A:

[0032]

[0033] Where: σ1, σ2, and σ3 are the maximum, intermediate, and minimum principal stresses, respectively, in MPa; γ is the angle between the normal of the fracture surface and the minimum principal stress σ3, in °;

[0034] c. Pore fluid pressure P: The product of the hydrostatic pressure P’ and the fluid pressure coefficient is the pore fluid pressure P, where the hydrostatic pressure P′ = ρ 水 gH, g = 9.8, H is the depth of the rock sample, and the fluid pressure coefficient is obtained from the measured data of the oilfield company;

[0035] d. Rock elastic modulus E and rock Poisson's ratio υ: Using the parallel rock sample A, conduct rock mechanics experiments to obtain the elastic modulus and Poisson's ratio of different rock samples;

[0036] e. Proportionality coefficients C1 and C2: Using the parallel sample B, conduct pressurization experiments under different confining pressures to determine the specific values of the proportionality coefficients C1 and C2;

[0037] ③ Calculate the corrected fracture aperture: Use the following formula to perform corrected calculations on fracture apertures of different scales;

[0038]

[0039] Where: e—the corrected fracture aperture, in μm; e0—the fracture aperture, in μm; σ—the normal stress on the fracture surface, in MPa; P—the pore fluid pressure, in MPa; E—the rock elastic modulus, in MPa; υ—the rock Poisson's ratio; C1 and C2—the proportionality coefficients, C1 = (2.5 - 4.5)×10 -2 , C2 = 1 - 1.4.

[0040] Step (3) in the above scheme is specifically as follows:

[0041] ① Calculate the multi-scale fracture density:

[0042] Fracture parameter statistics: When measuring the multi-scale fracture apertures, statistically analyze the fracture lengths at different scales and the sampling areas of samples at different scales. Sort the fracture lengths from largest to smallest. Arrange the fracture numbers with fracture lengths ≥ the longest scale as No. 1 fracture, and the length of the No. 1 fracture is the longest fracture length L1; arrange the fracture numbers with fracture lengths ≥ the second longest as No. 2 fracture, and the length of the No. 2 fracture is the sum of the longest fracture length and the second longest fracture length L2... Then, successively perform fracture number arrangement and fracture length calculation;

[0043] Calculate the fracture densities at different scales based on the calculated lengths of the 1st, 2nd, 3rd... nth fractures and the corresponding sampling areas.

[0044] ② Multi-scale fracture density correction:

[0045] a. Establish the relationship between multi-scale fracture density and corrected fracture aperture: Input the fracture density calculated in step (3)①b and the corresponding multi-scale corrected fracture aperture calculated in step (2)③ into a crossplot to establish a relationship chart between multi-scale corrected fracture aperture and fracture density.

[0046] b. Multi-scale fracture density correction: According to the power-law distribution relationship between fracture density and fracture aperture, correct the relationship between multi-scale fracture density and corrected fracture aperture obtained in step (3)②a, fit the relationship between multi-scale fracture density and corrected fracture aperture, and establish a multi-scale fracture development relationship. The specific formula is:

[0047] ρ = ae -b

[0048] In the formula: ρ is the fracture density, fractures / m; e is the corrected fracture aperture, μm; a and b are coefficients.

[0049] Beneficial effects:

[0050] 1. The present invention selects rock samples under different pressure conditions, combines with rock mechanics experiments, restores the true pressure state of the rock in the underground environment, simulates the fracture formation state, determines the correction formula, and corrects the fracture-related parameters. On this basis, a multi-scale fracture development relationship is established, and the corrected fracture porosities and permeabilities at different scales are calculated, which can provide more accurate data support for the evaluation of the contribution of fractures to the reservoir.

[0051] 2. The present invention measures the fracture apertures at different scales, measures the fracture lengths, and measures the sampling areas of samples at different scales, prepares rock samples, obtains the parameters related to the corrected fracture aperture and calculates the corrected fracture aperture, calculates the fracture densities at different scales and corrects the fracture density, establishes a multi-scale fracture relationship, and corrects and calculates the physical property parameters of fractures at different scales. This method has been applied and verified in the actual calculation of fracture physical property parameters and the experiment of evaluating the contribution of fractures to the reservoir, proving that this method is feasible.

[0052] 3. The present invention can not only correct the fracture aperture according to rock mechanics experiments, but also establish a multi-scale fracture development relationship based on the corrected fracture aperture and fracture density, and can predict the fracture apertures and densities at different scales. More importantly, based on the corrected fracture aperture and density, combined with the fracture set system and geometric shape distribution characteristics, the calculated fracture porosity and permeability values are more accurate, which can provide more favorable data support for the evaluation of the contribution of fractures to the reservoir.

[0053] 4. The fracture parameters statistically analyzed in the present invention have multi-scale characteristics. The fracture parameters cover a variety of scales, quantitatively characterizing the distribution laws of fractures at different scales, and providing higher accuracy for the establishment of the development relationship of multi-scale fractures in the later stage.

[0054] 5. There are differences in the stress states of core samples with different lithologies. The present invention selects core samples under different stress states for rock mechanics experiments. There are more data such as fracture morphology and parameters obtained from the experiments, simulating as many different development characteristics of underground fractures as possible, and the experimental results have a smaller error.

[0055] 6. The present invention restores the actual confining pressure state of the core underground, simulates the real formation environment for fracture generation, and restores the real aperture when the core fracture is formed. According to the fracture aperture measured by the rock mechanics experiment, the error caused by the expansion of the fracture aperture due to pressure release when the core fracture is mined from underground to the surface can be effectively reduced.

[0056] 7. The fracture aperture and fracture density of different scales in the present invention follow the power-law distribution characteristics. The fracture data obtained by restoring the real fracture development environment through rock mechanics experiments are used to establish a more accurate multi-scale fracture development relationship, and the predicted fracture aperture values of different scales are more accurate. Description of the Drawings:

[0057] Figure 1 It is a display diagram of fractures at different observation scales in the case of the present invention;

[0058] Figure 2 It is a curve graph of the change in fracture aperture under different confining pressures in the rock mechanics experiment in the case of the present invention;

[0059] Figure 3 It is a relationship diagram between multi-scale fracture aperture and fracture density in the case of the present invention;

[0060] Figure 4 It is the relationship between multi-scale corrected fracture density and fracture aperture in the case of the present invention

[0061] Figure 5 It is a distribution diagram of different-scale corrected fracture porosity in the case of the present invention;

[0062] Figure 6 It is a distribution diagram of different-scale corrected fracture permeability in the case of the present invention;

[0063] Figure 7 It is a flow chart of the present invention. Detailed Embodiment:

[0064] The following further describes the present invention in conjunction with the attached drawings:

[0065] This method for correcting physical properties parameters of multi-scale fractures includes the following steps;

[0066] (1) Measurement of multi-scale fracture related parameters: including measurement of fracture aperture at different scales, fracture length measurement, fracture set analysis, sampling area measurement of samples at different scales, and definition of fractures at different scales

[0067] ① Measurement of multi-scale fracture aperture (e): Fracture aperture refers to the vertical distance between two fracture walls of the same fracture. Fracture aperture measurement is carried out at four scales, namely nano-scale, micro-scale, core-scale, and imaging logging scale. For nano-scale fracture aperture measurement, a scanning electron microscope is needed. When measuring the fracture, select 3 parts that can represent the overall aperture of the fracture, measure the fracture aperture values, and take their average as the measured aperture of this fracture. For micro-scale fracture measurement, an optical microscope is needed, and the measurement method is the same as that of nano-scale fracture aperture measurement. Core-scale fracture aperture measurement requires a ruler or a feeler gauge. For fractures with an aperture greater than 1 mm, a ruler can be used directly for measurement. For fractures with an aperture less than 1 mm, a feeler gauge can be used for measurement, and its accuracy can reach 20 μm. The fracture aperture at the imaging logging scale can be calculated according to the following formula:

[0068]

[0069] In the formula: e - fracture aperture at the imaging logging scale, mm; R xo — resistivity of the flushed zone, Ω·m; Rm - resistivity of the mud, Ω·m; C and b - constants related to the instrument, C = 0.004801, b = 0.863; A - abnormal current area caused by the fracture, calculated using

[0070] Note: All parameters can be obtained from imaging logging data.

[0071] ② Measurement of multi-scale fracture length (l): While measuring the fracture aperture at different scales, measure the length of each fracture for calculating fracture density.

[0072] ③ Analysis of fracture set (S): Based on the fracture development characteristics at different scales, comprehensively analyze the distribution of fracture sets;

[0073] ④ Measurement of sampling area (A) of samples at different scales: Sampling area refers to the thin section area where the fracture is located (for nano-scale and micro-scale samples), core surface area, or imaging logging borehole surface area;

[0074] ⑤ Definition of multi-scale fractures: According to the fracture development characteristics and fracture aperture distribution range in the study area, define different ranges for fractures at different scales.

[0075] (2) Fracture aperture correction: including rock sample preparation, acquisition of parameters related to corrected fracture aperture, and calculation of corrected fracture aperture ​

[0076] ① Rock sample preparation: Select N rock samples with different lithologies (for each lithology sample point, 2 parallel samples are selected. Among them, parallel sample ① has no cracks and is used for rock mechanics experiments; parallel sample ② has cracks and is used for pressurization experiments under different confining pressures). According to the requirements of the experimental sample size, prepare experimental samples with a core sample size of 25mm×50mm;

[0077] ② Obtaining parameters related to crack aperture correction: A large number of experiments and previous studies have shown that the underground aperture of a crack is related to the rock mechanics parameters (Young's modulus, Poisson's ratio) of the rock where it is located, the normal stress on the crack surface, and the pore fluid pressure. Therefore, it can be corrected by the following formula:

[0078]

[0079] In the formula: e—corrected crack aperture, μm; e0—crack aperture, μm; σ—normal stress on the crack surface, MPa; P—pore fluid pressure, MPa; E—rock elastic modulus, MPa; υ—rock Poisson's ratio; C1 and C2—proportionality coefficients, C1=(2.5 - 4.5)×10 -2 , C2 = 1 - 1.4.

[0080] Among them:

[0081] a. Crack aperture (e0): According to the measurement of multi-scale crack apertures in step 1(1), obtain crack apertures e0 of different scales;

[0082] b. Normal stress on the crack surface (σ): The specific calculation formula for the normal stress on the crack surface is as follows:

[0083]

[0084] In the formula: n ij is the direction cosine, and define the stress tensor at a certain point on the crack surface as A:

[0085]

[0086] In the formula: σ1, σ2, and σ3 are the maximum, intermediate, and minimum principal stresses respectively, MPa; γ is the angle between the normal of the crack surface and the minimum principal stress σ3, °;

[0087] c. Pore fluid pressure (P): The product of the hydrostatic pressure (P’) and the fluid pressure coefficient is the pore fluid pressure (P), where the hydrostatic pressure P′ = ρ 水 gH, g = 9.8, H is the sample depth, and the fluid pressure coefficient is obtained from the measured data of the oilfield company;

[0088] d. Elastic modulus (E) and Poisson's ratio (υ) of the rock: For the parallel rock samples ① with different lithologies prepared in step (2) ①, conduct rock mechanics experiments to obtain the elastic modulus (E) and Poisson's ratio (υ) of different rock samples;

[0089] e. Proportionality coefficients (C1 and C2): For the parallel rock samples ② with different lithologies prepared in step (2) ①, conduct pressurization experiments under different confining pressures to determine the specific values of the proportionality coefficients C1 and C2;

[0090] ③ Calibration of crack aperture calculation: Based on the multi-scale crack apertures measured in step 1 (1), the correlation coefficients obtained in step 2

[0091] (2), and the calibration crack aperture formula, the crack apertures of different scales can be calibrated and calculated.

[0092] (3) Calculation and calibration of multi-scale crack density: Include calculation of multi-scale crack density and crack density calibration

[0093] ① Calculation of multi-scale crack density: Include statistical processing of crack parameters and calculation of multi-scale crack density

[0094] a. Statistical analysis of crack parameters: When measuring the multi-scale crack apertures (e) in step (1) ①, statistically analyze other crack-related parameters at different scales, including crack length (l) and sampling area (A). Then sort the crack lengths from largest to smallest. Arrange the crack numbers with crack lengths ≥ the longest scale as No. 1 crack, and the length of No. 1 crack is the longest crack length (L1); arrange the crack numbers with crack lengths ≥ the second-longest as No. 2 crack, and the length of No. 2 crack is the sum of the longest crack length and the second-longest crack length (L2)..., and then sequentially perform crack number arrangement and crack length calculation.

[0095] Statistical processing of crack parameters: Based on the multi-scale calibrated crack apertures (e) calculated in step (2) ③, the crack lengths (l) measured in step (1) ①, and the sampling area (A) statistics, process the crack parameters. First, sort the crack lengths from largest to smallest. Arrange the crack numbers with crack lengths ≥ the longest scale as No. 1 crack, and the length of No. 1 crack is the longest crack length (L1); arrange the crack numbers with crack lengths ≥ the second-longest as No. 2 crack, and the length of No. 2 crack is the sum of the longest crack length and the second-longest crack length (L2)..., and then sequentially perform crack number arrangement, crack aperture (average crack aperture within the sampling area), crack length, and sampling area calculation (Table 1);

[0096] Table 1:

[0097]

[0098] b. Multi-scale fracture density calculation: According to the lengths (L) of the No. 1, No. 2, No. 3... No. n fractures calculated in step (3) ①a, calculate the fracture densities (ρ) at different scales;

[0099] ρ = L / A

[0100] In the formula: ρ is the fracture density, mm / mm 2 ; L is the fracture length, mm; A is the sampling area, mm 2 .

[0101] ② Multi-scale fracture density correction: It includes establishing the relationship between multi-scale fracture density and corrected fracture aperture, and multi-scale fracture density correction

[0102] a. Establishing the relationship between multi-scale fracture density and corrected fracture aperture: Input the fracture density calculated in step (3) ①b and the corresponding multi-scale corrected fracture aperture calculated in step (3) ①a into the cross plot to establish a relationship chart of multi-scale corrected fracture aperture and fracture density;

[0103] b. Multi-scale fracture density correction: At a single scale, due to sampling deviation, some fractures with smaller scales cannot be well identified, and some fractures with larger scales exceed the sampling range. Therefore, according to the power-law distribution relationship between fracture density and fracture aperture, correct the relationship between multi-scale fracture density and corrected fracture aperture established in step (3) ②a, fit the relationship between multi-scale fracture density and corrected fracture aperture, and establish the multi-scale fracture development relationship. The specific formula is:

[0104] αe

[0105] In the formula: ρ is the fracture density, fractures / m; e is the corrected fracture aperture, μm; a and b are coefficients.

[0106] (4) Calculation of multi-scale physical properties parameters after correction: It includes the definition of fractures at different scales, and the calculation of corrected multi-scale fracture porosity and permeability

[0107] According to the multi-scale corrected fracture aperture calculated in step (2) ③, the corresponding fracture density calculated in step (3) ①a, and the relationship chart of multi-scale fracture density and corrected fracture aperture corrected in step (3) ②, the multi-scale physical properties parameters can be calculated.

[0108] ① Calculation of corrected multi-scale fracture porosity: According to the corrected fracture aperture calculated in step (2) ③ and the relationship between corrected fracture aperture and fracture density fitted in step (3) ②b, the corrected fracture physical property parameters for fractures at different scales can be calculated. Among them, the formula for calculating the corrected fracture porosity is:

[0109]

[0110] In the formula: is the fracture porosity, %; e is the corrected fracture aperture, μm; e m is the average corrected fracture aperture, μm; e1 is the lower limit of fracture aperture at different scales, μm; e2 is the upper limit of fracture aperture at different scales, μm; ρ is the fracture density.

[0111] ② Calculation of multi-scale fracture permeability after correction: Based on the analysis of fracture groups in step (1) ③, the corrected fracture aperture calculated in step (2) ③, and the relationship between the corrected fracture aperture and fracture density fitted in step (3) ②b, the corrected fracture physical property parameters for fractures of different scales can be calculated. Among them, the formula for calculating the corrected fracture permeability is as follows:

[0112]

[0113] In the formula: K f is the fracture permeability, mD; e is the corrected fracture aperture, μm; e m is the average corrected fracture aperture, μm; e1 is the lower limit of fracture aperture at different scales, μm; e2 is the upper limit of fracture aperture at different scales, μm; ρ is the fracture density; S is the coefficient of the fracture group system, which depends on the fracture group system and geometry (Table 2).

[0114] Table 2:

[0115]

[0116] Example:

[0117] The case is "Calculation of physical property parameters of tectonic fractures in deep tight sandstone reservoirs of the Shahejie Formation in the Nanpu Sag, Bohai Bay Basin". The imaging logging, cores, and microscopic thin sections involved in the case are all from the Shahejie Formation strata. The fractures in the data are well-developed, and the tectonic fractures can be observed and described in detail. Based on the measurement of fracture-related parameters such as fracture aperture, length, and sampling area at different scales, combined with means such as rock sample preparation, acquisition of corrected fracture aperture-related parameters, calculation of corrected fracture aperture, calculation of fracture density at different scales, and fracture density correction, a multi-scale fracture development relationship is established, and the porosity and permeability of fractures at different scales are corrected and calculated. All the geological data involved in the case are from the Shahejie Formation. The formation conditions of fractures at different scales are the same, and the fracture parameter laws are consistent, which can effectively correct the physical property parameters of fractures at different scales.

[0118] 1. Measurement of multi-scale fracture-related parameters: including measurement of fracture aperture at different scales, fracture length measurement, fracture group system analysis, measurement of sampling area of samples at different scales, and definition of fractures at different scales

[0119] (1) Multi-scale fracture aperture (e) measurement: Fracture aperture refers to the vertical distance between two fracture walls of the same fracture. Fracture aperture measurement is carried out at four scales, namely nano-scale, micro-scale, core-scale, and imaging logging scale. For nano-scale fracture aperture measurement, a scanning electron microscope is required. When measuring the fracture, select 3 parts that can represent the overall aperture of the fracture, measure the fracture aperture values, and take their average as the measured aperture of this fracture. For micro-scale fracture measurement, an optical microscope is needed, and the measurement method is the same as that of nano-scale fracture aperture measurement. Core-scale fracture aperture measurement requires a ruler or a feeler gauge. For fractures with an aperture greater than 1 mm, a ruler can be used directly for measurement. For fractures with an aperture less than 1 mm, a feeler gauge can be used for measurement, and its accuracy can reach 20 μm. The imaging logging scale fracture aperture can be calculated according to the following formula:

[0120]

[0121] In the formula: e—the fracture aperture at the imaging logging scale, mm; R xo —the resistivity of the flushed zone, Ω·m; Rm—the resistivity of the mud, Ω·m; C and b—constants related to the instrument, C = 0.004801, b = 0.863; A—the abnormal current area caused by the fracture, which is calculated by . All parameters can be obtained from imaging logging data.

[0122] (2) Multi-scale fracture length (l) measurement: While measuring the aperture of fractures at different scales, measure the length of each fracture for calculating the fracture density.

[0123] (3) Fracture set (S) analysis: According to the fracture development characteristics at different scales, comprehensively analyze the distribution of fracture sets. There are two mutually perpendicular fracture sets developed in the study area;

[0124] (4) Measurement of the sampling area (A) of samples at different scales: The sampling area refers to the area of the thin section where the fracture is located (for nano-scale and micro-scale samples), the surface area of the core, or the surface area of the imaging logging wellbore;

[0125] (5) Definition of multi-scale fractures: According to the fracture development characteristics and the distribution range of fracture apertures in the study area, different ranges are defined for fractures at different scales. Among them, the nano-scale fracture aperture range is 100 nm - 1 μm, the micro-scale fracture aperture range is 1 μm - 40 μm, the core-scale fracture aperture range is 40 μm - 100 μm, and the imaging logging scale fracture aperture range is 100 μm - 200 μm.

[0126] Note: The distribution range of fracture apertures at different scales can be defined according to the fracture development characteristics of different study areas.

[0127] 2. Fracture Aperture Calibration: It includes rock sample preparation, obtaining parameters related to the calibration of fracture aperture, and calculating the calibration of fracture aperture.

[0128] (1) Rock sample preparation: Select 12 rock samples with different lithologies (for each lithology sample point, select 2 parallel samples. Among them, parallel sample ① has no fractures and is used for rock mechanics experiments; parallel sample ② has fractures and is used for pressurization experiments under different confining pressures). According to the requirements of the experimental sample scale, prepare experimental samples with a core sample scale of 25 mm × 50 mm.

[0129] (2) Obtaining parameters related to the calibration of fracture aperture: A large number of experiments and previous studies have shown that the underground aperture of fractures is related to the rock mechanics parameters (Young's modulus, Poisson's ratio) of the rock where the fractures are located, the normal stress on the fracture surface, and the pore fluid pressure. Therefore, the calibration can be carried out through the following formula:

[0130]

[0131] In the formula: e—calibrated fracture aperture, μm; e0—fracture aperture, μm; σ—normal stress on the fracture surface, MPa; P—pore fluid pressure, MPa; E—rock elastic modulus, MPa; υ—rock Poisson's ratio; C1 and C2—proportionality coefficients, C1 = (2.5 - 4.5) × 10 -2 , C2 = 1 - 1.4.

[0132] Among them:

[0133] ① Fracture aperture (e0): According to the measurement of multi-scale fracture aperture in step 1(1), obtain fracture apertures e0 of different scales.

[0134] ② Normal stress on the fracture surface (σ): The specific calculation formula for the normal stress on the fracture surface is as follows:

[0135]

[0136] In the formula: n ij is the direction cosine, and define the stress tensor at a certain point on the fracture surface as A:

[0137]

[0138] In the formula: σ1, σ2, and σ3 are the maximum, intermediate, and minimum principal stresses respectively, MPa; γ is the angle between the normal of the fracture surface and the minimum principal stress σ3, °.

[0139] ③ Pore fluid pressure (P): The product of the hydrostatic pressure (P’) and the fluid pressure coefficient is the pore fluid pressure (P), where the hydrostatic pressure P′ = ρ 水 gH, g = 9.8, H is the sample depth, and the fluid pressure coefficient is obtained according to the measured data of the oilfield company.

[0140] ④ Rock elastic modulus (E) and rock Poisson's ratio (υ): For the parallel rock samples ① of different lithologies prepared in step 2(1), conduct rock mechanics experiments to obtain the elastic modulus (E) and rock Poisson's ratio (υ) of different rock samples;

[0141] ⑤ Proportionality coefficients (C1 and C2): For the parallel rock samples ② of different lithologies prepared in step 2(1), conduct pressurization experiments under different confining pressures to determine the specific values of the proportionality coefficients C1 and C2 ( Figure 2 ). In the study area, the proportionality coefficient C1 = 3.28×10 -2 , C2 = 1.08;

[0142] (3) Calibrated fracture aperture calculation: According to the multi-scale fracture apertures measured in step 1(1), the obtained correlation coefficients in step 2(2), and the calibrated fracture aperture formula, the calibrated calculations of fracture apertures at different scales can be carried out.

[0143] 3. Multi-scale fracture density calculation and calibration: Including the calculation of fracture densities at different scales and the calibration of fracture density

[0144] (1) Multi-scale fracture density calculation: Including the statistical processing of fracture parameters and the calculation of multi-scale fracture density

[0145] ① Statistical processing of fracture parameters: On the basis of the statistics of the multi-scale calibrated fracture apertures (e) calculated in step 2(3), the fracture lengths (l) measured in step 1(1), and the sampling area (A), the fracture parameters are processed. First, sort the fracture lengths from largest to smallest. Arrange the fracture numbers with fracture lengths ≥ the longest scale as No. 1 fracture, and the length of the No. 1 fracture is the longest fracture length (L1); arrange the fracture numbers with fracture lengths ≥ the second longest as No. 2 fracture, and the length of the No. 2 fracture is the sum of the longest fracture length and the second longest fracture length (L2)..., and then sequentially carry out fracture number arrangement, fracture aperture (average fracture aperture within the sampling area), fracture length, and sampling area calculation (Table 1: Statistical and Calculation of Nano-scale Fracture Parameters);

[0146] Table 1:

[0147]

[0148] ② Multi-scale fracture density calculation: According to the lengths (L) of the No. 1, No. 2, No. 3... No. n fractures calculated in step 3(1)①, calculate the fracture densities (ρ) at different scales of the fractures (Table 1):

[0149] ρ = L / A

[0150] In the formula: ρ is the fracture density, mm / mm 2; L is the crack length, in mm; A is the sampling area, in mm 2 .

[0151] (2) Multi-scale crack density correction: including establishing the relationship between multi-scale crack density and corrected crack aperture, and multi-scale crack density correction

[0152] ① Establishing the relationship between multi-scale crack density and corrected crack aperture: Input the crack density calculated in step 3(1)② and the corresponding multi-scale corrected crack aperture calculated in step 3(1)① into a crossplot to establish a relationship chart of multi-scale corrected crack aperture and crack density ( Figure 4 );

[0153] ② Multi-scale crack density correction: At a single scale, due to sampling deviation, some cracks with smaller scales are not well identified, and some cracks with larger scales are beyond the sampling range. Therefore, according to the power-law distribution relationship between crack density and crack aperture, correct the relationship between multi-scale crack density and corrected crack aperture established in step 3(2)①, fit the relationship between multi-scale crack density and corrected crack aperture, and establish the multi-scale crack development relationship. The specific formula is:

[0154] ρ = 272.15e -0981

[0155] In the formula: ρ is the crack density, in number of cracks per meter; e is the corrected crack aperture, in μm.

[0156] 4. Calculation of multi-scale physical properties after correction: including the definition of cracks at different scales, and the calculation of multi-scale crack porosity and permeability after correction

[0157] Based on the multi-scale corrected crack aperture calculated in step 2(3), the corrected crack density calculated in step 3(1)②, and the relationship chart of multi-scale crack density and corrected crack aperture corrected in step 3(2), the multi-scale physical properties can be calculated.

[0158] (1) Calculation of multi-scale crack porosity after correction: Based on the corrected crack aperture calculated in step 2(3) and the relationship between the corrected crack aperture and crack density fitted in step 3(2)②, the corrected crack physical property parameters for cracks at different scales can be calculated ( Figure 5 ). Among them, calculate the corrected crack porosity (Table 2: Average porosity at different scales). The specific formula is:

[0159]

[0160] In the formula: is the crack porosity, in %; e is the corrected crack aperture, in μm; e mδ is the average crack opening, μm; e1 is the lower limit of the crack opening at different scales, μm; e2 is the upper limit of the crack opening at different scales, μm.

[0161] Table 2:

[0162]

[0163] (2) Calculation of the multi-scale crack permeability after correction: According to the analysis of the crack group system in step 1(3), the corrected crack opening calculated in step 2(3), and the relationship between the corrected crack opening and the crack density fitted in step 3(2)②, the corrected crack physical property parameters can be calculated for cracks at different scales ( Figure 6 ). Among them, the crack permeability after correction is calculated (Table 2: Average permeability at different scales), and the specific formula is:

[0164]

[0165] Where: K f is the crack permeability, mD; e is the corrected crack opening, μm; e m is the average corrected crack opening, μm; e1 is the lower limit of the crack opening at different scales, μm; e2 is the upper limit of the crack opening at different scales; S is the coefficient of the crack group system. There are two mutually perpendicular and orthogonal crack groups developed in the study area, and S = 1.71 (Table 4).

[0166] Table 4:

[0167]

[0168] The accuracy of the basic data (corrected crack opening and density) of the present invention is relatively high, and the crack porosity and permeability are calculated considering various factors such as the crack group system and geometric morphology, and the accuracy of the corrected crack physical property parameters is high.

Claims

1. A method for correcting physical property parameters of multi-scale fractures includes the following steps; characterized in that It includes the following steps: (1) Measurement of multi-scale fracture related parameters. The multi-scale fractures are defined as nano-scale fractures, micro-scale fractures, core-scale fractures, and imaging logging-scale fractures according to the fracture development characteristics and fracture aperture distribution range in the study area, with different ranges defined for fractures of different scales. Measure the aperture, length, fracture set analysis, and sampling area of samples of different scales for fractures of different scales; (2) Fracture aperture correction: Prepare rock samples. The underground aperture of fractures is related to the rock mechanical parameters of the rock where the fractures are located, the normal stress on the fracture surface, and the pore fluid pressure. The rock mechanical parameters include Young's modulus and Poisson's ratio. Calculate the corrected fracture aperture according to the following formula to obtain the multi-scale corrected fracture aperture; where: e—corrected crack aperture, μm; e0—crack aperture, μm; σ—normal stress on the crack surface, MPa; P—pore fluid pressure, MPa; E—rock elastic modulus, MPa; υ—rock Poisson's ratio; C1 and C2—proportionality coefficients, C1 = (2.5 - 4.5)×10 -2 , C2 = 1 - 1.4; (3) Calculation and correction of multi-scale fracture density: Calculate the fracture density of different scales; Input the calculated fracture density of different scales and the corrected fracture aperture of the corresponding scale into a cross-plot to obtain a relationship chart of multi-scale corrected fracture aperture and fracture density, and establish a relationship between multi-scale fracture density and corrected fracture aperture; Then correct and establish the relationship between multi-scale fracture density and corrected fracture aperture, fit the relationship between multi-scale fracture density and corrected fracture aperture, establish a multi-scale fracture development relationship, and perform multi-scale fracture density correction: ρ = ae -b In the formula: ρ is the fracture density, fractures per meter; e is the corrected fracture aperture, micrometers; a and b are coefficients; (4) Calculate the multi-scale physical properties parameters after correction: Calculate the fracture porosity after correction: In the formula: is the fracture porosity, %; e is the corrected fracture aperture, μm; e m is the average corrected fracture aperture, μm; e1 is the lower limit of fracture aperture at different scales, μm; e2 is the upper limit of fracture aperture at different scales, μm; ρ is the fracture density. Calculate the fracture permeability after correction: Where: K f is the fracture permeability, mD; e is the corrected fracture aperture, μm; e m is the average corrected fracture aperture, μm; e1 is the lower limit of the fracture aperture at different scales, μm; e2 is the upper limit of the fracture aperture at different scales, μm; ρ is the fracture density; S is the coefficient of the fracture set system, which depends on the fracture set system and geometry.

2. The calibration method for multi-scale fracture physical properties parameters according to claim 1 includes the following steps, characterized in that: The specific content of step (1) is as follows: ① Measurement of multi-scale fracture aperture: The aperture of nano-scale fractures is measured by a scanning electron microscope. When measuring the fractures, select 3 parts that can represent the overall aperture of the fractures, measure the fracture aperture values, and take the average value as the fracture aperture of this fracture; The aperture of micro-scale fractures is measured by an optical microscope; The aperture of core-scale fractures is measured by a ruler or a feeler gauge. For fractures with an aperture greater than 1 mm, use a ruler to measure, and for fractures with an aperture less than 1 mm, use a feeler gauge to measure; The aperture of imaging logging-scale fractures is calculated based on imaging logging data; ② Measurement of multi-scale fracture length: While measuring the apertures of fractures of different scales, measure the length of each fracture for calculating the fracture density. ③ Fracture set analysis: According to the fracture development characteristics of different scales, comprehensively analyze the distribution of fracture sets; ④ Measurement of sampling areas of samples of different scales: The sampling areas of nano-scale and micro-scale samples are both the areas of the thin sections where the fractures are located; The core-scale samples are the surface areas of the cores; The imaging logging-scale samples are the surface areas of the imaging logging boreholes.

3. The calibration method for multi-scale fracture physical properties parameters according to claim 2 includes the following steps; characterized in that: The specific content of step (2) is as follows: ① Prepare rock samples: Select N rock samples of different lithologies, and select 2 parallel samples for each lithology sample point. Among them, parallel sample A has no fractures and is used for rock mechanics experiments; The other parallel sample B has fractures and is used for pressurization experiments under different confining pressures. Prepare experimental samples according to the requirements of the experimental sample scale. The scale of the core samples is 25 mm × 50 mm; ② Obtain parameters related to the calibrated fracture aperture: The in-situ aperture of the fracture is related to the Young's modulus, Poisson's ratio of the rock where it is located, the normal stress on the fracture surface, and the pore fluid pressure: a. Fracture aperture e0: Obtain the fracture apertures at different scales according to the measurement of the multi-scale fracture aperture in Step 1; b. Normal stress σ on the fracture surface: The specific calculation formula for the normal stress on the fracture surface is as follows: where: n ij is the direction cosine, and the stress tensor at a certain point on the fracture surface is defined as A: In the formula: σ1, σ2, and σ3 are the maximum, intermediate, and minimum principal stresses respectively, in MPa; γ is the angle between the normal of the fracture surface and the minimum principal stress σ3, in °; c. Pore fluid pressure P: The pore fluid pressure P is the product of the hydrostatic pressure P’ and the fluid pressure coefficient, where the hydrostatic pressure P′ = ρ 水 gH, g = 9.8, H is the depth of the rock sample, and the fluid pressure coefficient is obtained from the measured data of the oilfield company; d. Rock elastic modulus E and rock Poisson's ratio υ: Use the parallel rock sample A to conduct rock mechanics experiments to obtain the elastic modulus and Poisson's ratio of different rock samples; e. Proportionality coefficients C1 and C2: Use the parallel sample B to conduct pressurization experiments under different confining pressures to determine the specific values of the proportionality coefficients C1 and C2; ③ Calculate the calibrated fracture aperture: Use the following formula to perform calibration calculations on the fracture apertures at different scales; where: e—corrected crack aperture, μm; e0—crack aperture, μm; σ—normal stress on the crack surface, MPa; P—pore fluid pressure, MPa; E—rock elastic modulus, MPa; υ—rock Poisson's ratio; C1 and C2—proportionality coefficients, C1 = (2.5 - 4.5) × 10 -2 , C2 = 1 - 1.

4.

4. The calibration method for multi-scale fracture physical properties parameters according to claim 3 comprises the following steps; characterized in that: The specific content of Step (3) is as follows: ① Calculate the multi-scale fracture density: Fracture parameter statistics: When measuring the multi-scale fracture aperture, statistically obtain the fracture lengths at different scales and the sampling areas of samples at different scales. Sort the fracture lengths from largest to smallest. Arrange the fracture numbers with fracture lengths ≥ the longest scale as the No. 1 fracture, and the length of the No. 1 fracture is the longest fracture length L1; Arrange the fracture numbers with fracture lengths ≥ the second longest as the No. 2 fracture, and the length of the No. 2 fracture is the sum of the longest fracture length and the second longest fracture length L2... Then, perform fracture number arrangement and fracture length calculation in sequence; Calculate the fracture densities at different scales according to the calculated lengths of the No. 1, No. 2, No. 3... No. n fractures and their corresponding sampling areas; ② Calibrate the multi-scale fracture density: a. Establish the relationship between the multi-scale fracture density and the calibrated fracture aperture: Input the fracture density calculated in Step (3)①b and the corresponding multi-scale calibrated fracture aperture calculated in Step (2)③ into the cross plot to establish a relationship chart of the multi-scale calibrated fracture aperture and the fracture density; b. Calibrate the multi-scale fracture density: According to the power-law distribution relationship between the fracture density and the fracture aperture, calibrate the relationship between the multi-scale fracture density and the calibrated fracture aperture obtained in Step (3)②a, fit the relationship between the multi-scale fracture density and the calibrated fracture aperture, and establish the multi-scale fracture development relationship. The specific formula is: ρ = ae -b In the formula: ρ is the fracture density, in fractures / m; e is the calibrated fracture aperture, in μm; a and b are coefficients.

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