An evaluation method for quantitatively characterizing the fracturing degree of rock through fracture complexity

Abstract

The present invention discloses an evaluation method for quantitatively characterizing the fracturing degree of rocks through fracture complexity. By comparing the differences in induced polarization parameters before and after reservoir fracturing, the fracturing development complexity parameter C based on the induced polarization parameters can be obtained f , which is used to quantitatively characterize the fracturing degree of reservoir rocks. Then, combined with the average absolute gradient difference ΔD before and after fracturing, the correlation between C f and ΔD is obtained, and then the relationship fitting degree is obtained to verify that quantitatively characterizing the fracturing degree of rocks through fracture complexity is effective, providing technical support for establishing the quality evaluation of reservoir hydraulic fracturing based on the induced polarization method.

Description

Technical Field

[0001] The present invention relates to the technical fields of oil and gas exploration and development, unconventional and new energy technologies, and particularly relates to an evaluation method for quantitatively characterizing the fracturing degree of rocks through fracture complexity. Background Art

[0002] Hydraulic fracturing is the core technology for improving the physical properties of unconventional reservoirs and realizing the effective development of unconventional oil and gas resources. Dynamic monitoring of hydraulic fracturing can evaluate the fracture development degree and scale during the fracturing process, judge the flow direction of fracturing fluid, delineate the distribution range of proppants, and realize the evaluation of the fracturing reformed volume of the reservoir, which is crucial for reservoir productivity prediction and resource development. Compared with microseismic monitoring, electromagnetic monitoring has the advantages of low monitoring cost, high efficiency, and more sensitive electrical parameters than elastic parameters, providing a physical basis for monitoring the fracture initiation and subsequent propagation during the hydraulic fracturing process. Currently, the monitoring technologies for reservoir fracturing reform mainly focus on determining the fracturing range, lacking quantitative characterization technologies for the fracturing degree. Summary of the Invention

[0003] The present invention can quantitatively characterize the fracturing degree of rocks by providing an evaluation method for quantitatively characterizing the fracturing degree of rocks through fracture complexity.

[0004] The present invention provides an evaluation method for quantitatively characterizing the fracturing degree of rocks through fracture complexity, including:

[0005] For each sample of the reservoir under study, it is dried to obtain the mass of the sample at this time as m 1 , and it is combined with the formation water in the survey area to saturate the reservoir rocks to reach the formation salinity and saturation state, and the mass of the sample at this time is recorded as m 2 ;

[0006] For the rock sample that has reached the formation salinity and saturation state, complex resistivity measurement is carried out to simulate the reservoir temperature T and pressure P, a complex resistivity curve is obtained, and the pre-fracture true conductivity of the reservoir rocks before fracturing is obtained by combining the parameter estimation algorithm based on the MGEMTIP model and the pre-fracture polarization rate ;

[0007] The rock sample that has reached the formation salinity and saturation state is subjected to salt washing and drying operations, and the dried sample is subjected to image scanning processing and graying to obtain the pre-fracture average absolute gradient D of the rock measurement plane b ;

[0008] The rock sample that has reached the formation salinity and saturation state is subjected to rock fracturing, and the material loss during the fracturing process is preserved or supplemented to ensure that the mass of the sample at this time is m 1; After the sample is shaped, it is saturated with reservoir rock in combination with the formation water in the measurement area until the mass of the sample reaches m 2 ;

[0009] Carry out a simulation study on the saturated and fractured sample to measure the complex resistivity at reservoir temperature T and pressure P, obtain the complex resistivity curve, and obtain the true conductivity after fracturing of the reservoir rock by combining the parameter estimation algorithm based on the MGEMTIP model and the polarization rate after fracturing ;

[0010] Wash the salt and dry the saturated and fractured sample, perform image scanning and grayscale processing on the dried sample, and obtain the average absolute gradient after fracturing on the rock measurement plane D a ；

[0011] Through the formula Calculate the measured fracture complexity C f ;

[0012] Through the formula Calculate the difference in average absolute gradient before and after fracturing ;

[0013] Establish the correlation between the C f and the , and then obtain the relationship fitting degree, and obtain the evaluation result through the relationship fitting degree

[0014] Specifically, the operation of washing the salt and drying the rock sample in the state of formation salinity and saturation includes:

[0015] Wash the salt of the rock sample in the state of formation salinity and saturation, soak it in pure water for 24 hours, replace the pure water every 8 hours during this period, and then perform the drying operation

[0016] Specifically, the drying operation includes:

[0017] Dry the sample after the salt washing operation in a drying oven at 70 °C until the mass change of the sample is less than 0.01 g for 2 consecutive hours

[0018] Specifically, the average absolute gradient D b before fracturing of the rock measurement plane is obtained, including:

[0019] Through the formula Calculate the average absolute gradient D b before fracturing; where D bx and Dby The average absolute gradients in the pre - fracturing directions corresponding to the x - direction and y - direction respectively, and the corresponding expressions are:

[0020] ;

[0021] Wherein, is the set of pixel data points of the rock test surface image, is the gray - scale value of the corresponding pixel in the pre - fracturing image, G bx (i,j) is the pre - fracturing x - direction gradient of the corresponding coordinate point, G by (i,j) is the pre - fracturing y - direction gradient of the corresponding coordinate point.

[0022] Specifically, for the rock sample that has reached the formation salinity and saturation state, rock fracturing is carried out, and the material loss during the fracturing process is preserved or supplemented to meet the mass of the sample at this time as m 1 , including:

[0023] Carry out rock fracturing on the rock sample that has reached the formation salinity and saturation state by means of fracturing, splitting, and cutting; during the fracturing or splitting process, always wrap the rock sample in a colloid to ensure that the sample debris during the process is constrained inside the rock sample to meet the mass of the sample at this time as m 1 ; By comparing the mass difference of the sample before and after cutting, fill in the same - mass debris to supplement the material loss during the fracturing process to meet the mass of the sample at this time as m 1 .

[0024] Specifically, after the sample is shaped, it is saturated with reservoir rock in combination with the formation water in the survey area until the mass of the sample reaches m 2 After that, it further includes:

[0025] Pressurize the fracturing fluid of the sample with a mass of m 2 through a pressure chamber until the mass change of the sample is less than 0.01 g for 2 consecutive hours.

[0026] Specifically, obtaining the average absolute gradient after fracturing of the rock measurement plane D a , including:

[0027] Calculate the average absolute gradient after fracturing through the formula D a ; Wherein, and correspond to the average absolute gradients in the post - fracturing directions in the x - direction and y - direction respectively, and the corresponding expressions are:

[0028] ;

[0029] Among them, is the set of data point pixels of the rock test surface image, G a (i,j) is the gray value of the corresponding pixel in the post-fracture image, G ax (i,j) is the post-fracture x-direction gradient of the corresponding coordinate point, G ay (i,j) is the post-fracture y-direction gradient of the corresponding coordinate point.

[0030] Specifically, establishing the correlation between the said C f and the said , and then obtaining the relationship fitting degree, and obtaining the evaluation result through the said relationship fitting degree, including:

[0031] From the said C f and the said establish the correlation relationship formula C f * =a*exp(b ); where a and b are fitting coefficients, and C f * is to calculate the fracture complexity;

[0032] Through the formula calculate to obtain the relationship fitting degree R 2 ; where n is the total number of samples, is the measured fracture complexity of the i-th sample, is the calculated fracture complexity after the i-th sample is substituted into the said correlation relationship formula, is the average measured fracture complexity of n samples;

[0033] If the said relationship fitting degree R 2 is equal to or greater than the set threshold, it is evaluated as valid.

[0034] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0035] By comparing the differences in induced polarization parameters before and after reservoir fracturing, the fracturing development complexity parameter C f based on induced polarization parameters can be obtained, which is used to quantitatively characterize the fracturing degree of reservoir rocks. Then, combined with the average absolute gradient difference before and after fracturing, C f and The correlation therebetween is then obtained to get the relationship fitting degree, so as to verify that it is effective to quantitatively characterize the rock fracturing degree through the fracture complexity. Among them, the average absolute gradient difference is a quantitative characterization of the visually observed fracturing degree of the sample before and after fracturing, and the fracture complexity C f is an indirect quantitative characterization of the fracturing degree obtained based on the induced polarization method through the degree of development of the back-conductance of the rock after fracturing, and is very suitable for underground exploration. Underground exploration can directly obtain the induced polarization parameters, and then the underground fracturing effect can be evaluated. The correlation degree between the two can verify that the fracture complexity C f is an effective parameter for characterizing the rock fracturing degree, providing technical support for establishing the evaluation of the hydraulic fracturing quality of reservoirs based on the induced polarization method. Description of the Drawings

[0036] Figure 1 is a flowchart of the evaluation method for quantitatively characterizing the rock fracturing degree through the fracture complexity provided by the embodiment of the present invention;

[0037] Figure 2 is a schematic diagram of the measurement surface of the sample and the significant surface fracture development in the embodiment of the present invention;

[0038] Figure 3 is a fitting relationship diagram between the average absolute gradient difference and the fracture complexity based on the induced polarization parameters in the embodiment of the present invention. Detailed Embodiments

[0039] The embodiment of the present invention provides an evaluation method for quantitatively characterizing the rock fracturing degree through the fracture complexity, which can quantitatively characterize the rock fracturing degree.

[0040] To better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings of the specification and specific embodiments.

[0041] As Figure 1 shown, the evaluation method for quantitatively characterizing the rock fracturing degree through the fracture complexity provided by the embodiment of the present invention includes:

[0042] Step S110: For each sample of the reservoir under study, dry it to obtain the mass of the sample at this time as m 1 , combine with the formation water in the survey area to saturate the reservoir rock to reach the formation salinity and saturation state, and record the mass of the sample at this time as m 2 ;

[0043] Step S120: Conduct complex resistivity measurement on the rock sample that has reached the formation salinity and saturation state to simulate the reservoir temperature T and pressure P, obtain the complex resistivity curve, and combine with the parameter estimation algorithm based on the MGEMTIP model to obtain the pre-fracture true conductivity of the reservoir rock before fracturing and pre - fracturing polarizability ;

[0044] Step S130: Wash the salt and dry the rock samples that have reached the formation salinity and saturation state, perform image scanning and grayscale processing on the dried samples, and obtain the pre - fracturing average absolute gradient D of the rock measurement plane b ;

[0045] Specifically, washing the salt and drying the rock samples that have reached the formation salinity and saturation state includes:

[0046] Wash the salt of the rock samples that have reached the formation salinity and saturation state, soak them in pure water for 24 hours, replace the pure water every 8 hours during this period, and then perform the drying operation

[0047] Among them, the drying operation includes:

[0048] Dry the samples after the salt - washing operation in an oven at 70 °C until the mass change of the samples is less than 0.01 g for 2 consecutive hours

[0049] Obtain the pre - fracturing average absolute gradient D of the rock measurement plane b , including:

[0050] Calculate the pre - fracturing average absolute gradient D through the formula b ; where D bx and D by correspond to the pre - fracturing direction average absolute gradients in the x - direction and y - direction respectively, and the corresponding expressions are:

[0051] ;

[0052] Among them, is the set of data - point pixels of the rock test - surface image, is the grayscale value of the corresponding pixel of the pre - fracturing image, G bx (i, j) is the pre - fracturing x - direction gradient of the corresponding coordinate point, G by (i, j) is the pre - fracturing y - direction gradient of the corresponding coordinate point

[0053] Step S140: Fracture the rock samples that have reached the formation salinity and saturation state, save or supplement the material loss during the fracturing process, so that the mass of the samples at this time is m 1 ; After shaping the samples, saturate the reservoir rocks with the formation water in the survey area until the mass of the samples reaches m 2 ;

[0054] ​Specifically, rock fracturing is performed on rock samples that have reached the formation salinity and saturation state, and the material loss during the fracturing process is preserved or supplemented to ensure that the mass of the sample at this time is m 1 , including:

[0055] Rock fracturing is performed on rock samples that have reached the formation salinity and saturation state by means of fracturing, splitting, and cutting; during the fracturing or splitting process, the rock sample is always wrapped in a colloid to ensure that the sample debris during the process is confined within the rock sample to ensure that the mass of the sample at this time is m 1 ; by comparing the mass difference of the sample before and after cutting, the same mass of debris is filled to supplement the material loss during the fracturing process to ensure that the mass of the sample at this time is m 1 .

[0056] After the sample is shaped, it is saturated with reservoir rock in combination with the formation water in the survey area until the mass of the sample reaches m 2 After that, it also includes:

[0057] The fracturing fluid of the sample with a mass reaching m is pressurized through a pressure chamber until the mass change of the sample is less than 0.01 g for 2 consecutive hours, and the pressure value during the pressurization process is the pressure difference during the injection fracturing process 2 .

[0058] Step S150: Conduct a simulation study on the complex resistivity measurement of the saturated and fractured sample for the reservoir temperature T and pressure P, obtain the complex resistivity curve, and obtain the post-fracture true conductivity and post-fracture polarization rate of the reservoir rock in combination with the parameter estimation algorithm based on the MGEMTIP model and the post-fracture polarization rate ;

[0059] Step S160: Wash the salt and dry the saturated and fractured sample, perform image scanning and grayscale processing on the dried sample, and obtain the post-fracture average absolute gradient D of the rock measurement plane a ;

[0060] Specifically, washing the salt and drying the saturated and fractured sample includes:

[0061] Wash the salt of the saturated and fractured sample, soak it in pure water for 24 hours, replace the pure water every 8 hours during this period, and then perform the drying operation

[0062] Among them, the drying operation includes:

[0063] The sample after the salt washing operation is dried in an oven at 70 °C until the mass change of the sample is less than 0.01 g for 2 consecutive hours

[0064] Obtain the post-fracture average absolute gradient D of the rock measurement planea , including:

[0065] The average absolute gradient D after pressing is calculated through the formula ; where a ; among them, and correspond to the average absolute gradient in the pressing direction in the x and y directions respectively, and the corresponding expressions are:

[0066] ;

[0067] Among them, is the set of pixel data points of the rock test surface image, G a (i, j) is the gray value of the corresponding pixel of the fractured image, G ax (i, j) is the gradient in the x direction after pressing at the corresponding coordinate point, G ay (i, j) is the gradient in the y direction after pressing at the corresponding coordinate point.

[0068] Step S170: Calculate the measured fracture complexity C through the formula ; f ;

[0069] Step S180: Calculate the average absolute gradient difference before and after fracturing through the formula ; ;

[0070] Step S190: Establish the correlation between C f and , and then obtain the relationship fitting degree, and obtain the evaluation result through the relationship fitting degree.

[0071] Specifically explain this step. Establish the correlation between C f and , and then obtain the relationship fitting degree, and obtain the evaluation result through the relationship fitting degree, including:

[0072] Establish the correlation relationship C f and from C f * =a*exp(b ); where a and b are fitting coefficients, C f * is the calculated fracture complexity;

[0073] Calculate the relationship fitting degree R through the formula ; where n is the total number of samples, 2 ; among them, is the measured fracture complexity of the i-th sample, is the calculated fracture complexity after substituting the i-th sample into the correlation relationship formula, is the average measured fracture complexity of n samples;

[0074] If the relationship fitting degree R 2 is equal to or greater than the set threshold, it is evaluated as valid. The larger the fitting degree R 2 , the better the fracture complexity C f characterizes the effectiveness of this measurement area.

[0075] The method provided by the embodiment of the present invention was verified for the reservoir shale in a certain measurement area. Experiments before and after fracturing were carried out on 5 samples, and the test results are shown in Table 1.

[0076] Table 1 Correspondence table between average absolute gradient difference of samples and measured electrical fracture complexity

[0077] ;

[0078] The columnar samples before and after fracturing are shown in the appendix Figure 2 , and the samples are fractured by various methods such as wire cutting, splitting, and fracturing. The fracture development is sorted from simple to complex in sequence. The appendix Figure 3 is the correlation analysis between the average absolute gradient difference and the fracture complexity based on induced polarization parameters. The results show a high fitting degree, indicating that the induced polarization fracture complexity defined by this method can effectively characterize the complexity of the development of fractured fractures in rocks and can be further applied to actual measurement areas, providing technical support for establishing a reservoir hydraulic fracturing quality assessment based on the induced polarization method.

[0079] The embodiment of the present invention extracts the development complexity parameter based on complex conductivity by comparing the differences in induced polarization parameters of reservoir rocks before and after fracturing, which is used to quantitatively characterize the fracturing degree of reservoir rocks, realizes the assessment of fracturing quality in fracturing monitoring, and provides technical support for establishing a reservoir hydraulic fracturing quality assessment based on the induced polarization method.

[0080] Details not described in the embodiments of the present invention are all well-known techniques in the technical field. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for evaluating the degree of rock fracturing by quantitatively characterizing the degree of rock fracturing by means of fracture complexity, characterized in that: include: For each sample of the studied reservoir, the mass of the sample is obtained by drying, which is m1. The reservoir rock is saturated with formation water in the measured area to reach the mineralization and saturation state of the formation. The mass of the sample is recorded as m2. The complex resistivity measurement of the reservoir temperature T and pressure P is simulated for the rock samples that have reached the state of formation mineralization and saturation, and the complex resistivity curve is obtained. The true conductivity before fracturing of the reservoir rock is obtained by combining the parameter estimation algorithm based on the MGEMTIP model. and pre-pressure polarizability ; The rock samples that have reached the state of formation mineralization and saturation are washed with salt and dried, and the dried samples are image scanned and grayed to obtain the average absolute gradient D before pressure on the rock measurement plane. b ; Perform rock fracturing on the rock samples that have reached the formation mineralization and saturation state, preserve or supplement the material loss during the fracturing process, and satisfy the mass of the sample at this time is m1; after the sample is shaped, the reservoir rock is saturated with the formation water in the measurement area until the mass of the sample reaches m2; The complex resistivity measurement of reservoir temperature T and pressure P was simulated for saturated fractured samples to obtain complex resistivity curves. The post-fracture true conductivity of the reservoir rock was obtained by combining the parameter estimation algorithm based on the MGEMTIP model. and post-compression polarization ; The saturated fracturing sample is washed with salt and dried, and the dried sample is image scanned and grayed to obtain the average absolute gradient of the rock measurement plane after fracturing. D a ； By formula The measured crack complexity C is calculated f ; By formula The average absolute gradient difference before and after fracturing was calculated ; Establish the C f and stated The correlation between them is then used to obtain the relationship fit, and the evaluation result is obtained through the relationship fit; The establishment of the C f and stated The correlation between them is then used to obtain the relationship fit, and the evaluation results are obtained through the relationship fit, including: By the C f and stated Establish the correlation relationship C f * =a*exp(b ); where a and b are fitting coefficients, C f * To calculate the crack complexity; By formula Calculate the relationship fit R 2 ; Where n is the total number of samples, is the measured crack complexity of the ith sample, is the calculated crack complexity after the i-th sample is brought into the correlation equation, is the average measured crack complexity of n samples; If the relationship fit R 2 If it is equal to or greater than the set threshold, it is evaluated as valid.

2. The evaluation method for quantitatively characterizing the degree of rock fracturing by fracture complexity according to claim 1, characterized in that: The salt washing and drying operation of the rock sample that has reached the formation mineralization and saturation state includes: The rock samples that have reached the formation mineralization and saturation state are subjected to salt washing operation, soaked in pure water for 24 hours, during which the pure water is replaced every 8 hours, and then dried.

3. The evaluation method for quantitatively characterizing the degree of rock fracturing by fracture complexity according to claim 2, characterized in that: The drying operation comprises: The sample after the salt washing operation was dried in a drying oven at 70°C until the mass change of the sample was less than 0.01g for 2 consecutive hours.

4. The evaluation method for quantitatively characterizing the degree of rock fracturing by fracture complexity according to claim 1, characterized in that: The average absolute gradient D before the rock measurement plane is obtained b ,include: By formula The average absolute gradient D before pressure is calculated b ;in, D bx and D by The average absolute gradients in the x-direction and y-direction before compression are respectively expressed as: ； in, is the pixel set of data points of the rock test surface image, is the gray value of the corresponding pixel in the image before fracturing, G bx (i,j) is the x-direction gradient before compression at the corresponding coordinate point, G by (i,j) is the y-direction gradient before compression of the corresponding coordinate point.

5. The evaluation method for quantitatively characterizing the degree of rock fracturing by fracture complexity according to claim 1, characterized in that: The rock fracturing is performed on the rock sample that has reached the formation mineralization and saturation state, and the material loss during the fracturing process is preserved or supplemented, so that the mass of the sample at this time is m1, including: The rock sample that has reached the state of formation mineralization and saturation is subjected to rock fracturing by means of fracturing, splitting and cutting; during the fracturing or splitting process, the rock sample is always wrapped in a colloid to ensure that the sample debris in the process is confined inside the rock sample to meet the mass of the sample at this time being m1; by comparing the mass difference of the sample before and after cutting, the same mass of debris is filled to make up for the material loss in the fracturing process to meet the mass of the sample at this time being m1.

6. The evaluation method for quantitatively characterizing the degree of rock fracturing by fracture complexity according to claim 1, characterized in that: After the sample is shaped, the reservoir rock is saturated with formation water in the measuring area until the mass of the sample reaches m2, and the method further includes: The fracturing fluid of a sample with a mass of m2 is pressurized through a pressure chamber until the mass change of the sample is less than 0.01g for 2 consecutive hours.

7. The evaluation method for quantitatively characterizing the degree of rock fracturing by fracture complexity according to claim 1, characterized in that: The obtained rock measurement plane has an average absolute gradient after compression. D a , include: By formula The average absolute gradient after compression is calculated D a ;in, and The average absolute gradient in the post-compression direction corresponding to the x-direction and y-direction respectively, the corresponding expressions are: ； in, is the pixel set of data points of the rock test surface image, G a (i,j) is the gray value of the corresponding pixel in the image after fracturing, G ax (i,j) is the x-direction gradient after compression at the corresponding coordinate point, G ay (i,j) is the y-direction gradient after compression at the corresponding coordinate point.

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