Advanced energy storage fracturing process evaluation method

Abstract

The invention provides an advanced energy storage fracturing process evaluation method, and belongs to the technical field of petroleum engineering-oil-gas field development. The advanced energy storage fracturing process is evaluated by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on energy increasing effect evaluation and fracture pressure evaluation of pressure change and liquid storage amount, and based on crack form and complexity and roughness evaluation reconstructed after CT scanning. The evaluation indexes of the method are more comprehensive and objective, the experiment of fracturing after advanced energy storage under real conditions is simulated, and the evaluation result is more practical.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum engineering-oil and gas field development, and in particular to an advanced energy storage fracturing process evaluation method. Background Art

[0002] Unconventional oil and gas resources such as tight oil and gas have become an important direction for increasing reserves and production. However, due to the low porosity and low permeability characteristics of tight oil and gas, reservoir transformation is generally required to form fractures for production and development. For the middle and late stages of depleted oil fields or abnormally low-pressure formations, the formation energy deficit is serious, and advanced energy storage and post-fracturing are required. Advanced energy storage fracturing refers to a reservoir transformation process in which energy storage media is injected before fracturing to increase formation energy and then fracturing is performed. It has two purposes. On the one hand, the energy storage medium increases the pore pressure in the near-well zone and reduces the reservoir temperature, which can change the reservoir stress state, help reduce fracturing resistance and form a complex fracture network, and increase the oil leakage area; on the other hand, the energy stored in the reservoir after fracturing can be used as oil displacement power to increase production and efficiency. Therefore, it is of great significance to effectively characterize the energy enhancement effect and describe the fracture morphology, accurately quantify the fracture complexity, and establish an evaluation method for the effect of advanced energy storage fracturing technology to further guide and optimize the production increase operation of advanced energy storage fracturing.

[0003] A method of advanced energy storage fracturing is disclosed in Chinese patent application document CN110439521A. This method increases the pore pressure near the wellbore, expands the pore microcracks, reduces the effective stress of the rock, achieves the purpose of advanced energy storage and expansion, makes the later main fracturing reservoir fracture pressure lower, the fracture network density greater, and the fracturing fluid return easier, thereby greatly increasing the single well production.

[0004] Chinese patent application document CN111257540A discloses an experimental method and device for evaluating the effect of supercritical CO2 full-cycle fracturing energy storage and flowback. The invention uses the energy storage system and flowback system of the device to conduct multi-mode energy storage and flowback experiments on the core, and can accurately evaluate the energy enhancement effect and flowback efficiency of different modes of fracturing.

[0005] In the Chinese patent application document CN112459760B, a carbon dioxide energy storage composite fracturing experimental device was innovatively designed. This invention can evaluate the experimental method of the energy storage effect of core injection and "well soaking" process and the fluid return efficiency in the return process under different displacement modes of CO2 and slick water in different phases, and clarifies the energy storage effect and return efficiency under different CO2 displacement modes and displacement slug volumes.

[0006] A method for calculating the complexity of multi-cluster hydraulic fracturing cracks in a horizontal well section of a fractured reservoir is disclosed in Chinese patent application document CN110516407B. The invention is based on a two-dimensional displacement discontinuity method, introduces a natural fracture model and a criterion for interaction between hydraulic fractures and natural fractures, can accurately calculate the geometric morphology of hydraulic fracturing cracks in fractured reservoirs, and characterizes the complexity of the cracks formed after fracturing through fractural dimension.

[0007] A method for dynamic evaluation of hydraulic fracture complexity based on fractal theory is disclosed in Chinese patent application document CN111779477A. ​​The invention is based on event point data and fractal theory, fully considers the status and role of different levels of fractures formed by fracturing, and takes into account the density characteristics of the entire fracture system. It can objectively characterize the morphology of hydraulic fractures under different conditions and dynamically evaluate the complexity of hydraulic fractures on site.

[0008] The existing technology has at least the following deficiencies:

[0009] 1. The current evaluation method for advanced energy storage fracturing is mainly to simulate energy storage and flowback by displacement, and evaluate the energy storage and flowback effects based on pressure changes and flowback production. It cannot simulate the real advanced energy storage that changes the reservoir stress conditions and thus affects the fracturing process;

[0010] 2. Currently, there are few quantitative characterization indicators for the evaluation of fracture morphology, complexity and roughness. Therefore, there is an urgent need for a more comprehensive evaluation method for reservoir energy enhancement effect and fracture complexity in the actual advanced energy storage fracturing process. Summary of the invention

[0011] In order to solve the problems existing in the prior art, the present invention provides an evaluation method for advanced energy storage fracturing technology, which evaluates the advanced energy storage fracturing technology based on the evaluation of the energy enhancement effect of pressure change and the amount of stored liquid, the evaluation of the fracture pressure, and the evaluation of the fracture morphology, complexity and roughness reconstructed based on CT scanning by simulating the experimental method of fracturing after advanced energy storage under real conditions. The evaluation index of the present invention is more comprehensive and objective, and it simulates the experiment of fracturing after advanced energy storage under real conditions, and the evaluation result is more in line with reality.

[0012] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0013] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0014] Step 1: Prepare rock samples:

[0015] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0016] Step 2: Advanced energy storage fracturing experiment:

[0017] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0018] Step 3: Perform CT scanning on the sample after fracturing;

[0019] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0020] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation;

[0021] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0022] Preferably, step one comprises the following steps:

[0023] (1) Rock sample preparation: Take the target reservoir tight core from the same block, process the sample into a cuboid, and drill a wellbore at the center of the surface where the sample area is the smallest;

[0024] (2) Washing oil, salt and drying: Place the rock sample in a core basket, use carbon tetrachloride and ethanol to wash the oil and salt until the carbon tetrachloride and ethanol in the core basket are clarified, and then dry to constant weight;

[0025] (3) Saturated simulated oil: Place the sample in a vacuum and pressurized saturation instrument, evacuate the sample, inject simulated formation oil, pressurize the sample to the formation pressure, and age the sample for a preset number of days under the formation pressure. Stop the aging process after energy transfer is completed.

[0026] (4) Cementing and sealing: A bare hole section with a length of L is reserved in the middle of the core, and epoxy resin is used to bond the simulated wellbore for cementing. Then, a layer of organic potting glue is applied on the surface of the core to seal it.

[0027] Preferably, in step 1, the sample is made into a standard cuboid of 8 cm × 8 cm × 10 cm, and the inner diameter of the wellbore drilled in the center of the surface with the smallest sample area is 1.5 cm and the depth is 6.0 cm. The sample is prepared according to the size of the true triaxial fracturing equipment. The size of the wellbore inner diameter is related to the injection equipment, and the depth setting is to set the open hole section in the middle as much as possible.

[0028] Preferably, in step 1, the drying temperature is 100-105°C.

[0029] Preferably, in step 1, the drying temperature is 102°C.

[0030] Preferably, in step 1, vacuum is applied for 24 hours and aging is performed for 15 days.

[0031] Preferably, in step 1, the length L is 1-1.5 cm, and the simulated wellbore size has an outer diameter of 1.3 cm, an inner diameter of 0.6 cm, and a length of 5.8 cm. The wellbore size is selected according to the wellbore size, and there should be room for cementing between the wellbore outer diameter and the wellbore diameter.

[0032] Preferably, step 2 comprises the following steps:

[0033] (1) Prepare the experimental environment: install the rock sample and apply triaxial stress to the rock sample according to the triaxial stress of the formation; place the fracturing equipment in a constant temperature box and adjust the temperature of the constant temperature box to the formation temperature;

[0034] (2) Advanced energy storage fracturing: Using an ISCO pump, the energy storage medium is injected below the fracture pressure using different energy storage methods at the same displacement, and the energy is stored for T time to allow the pressure to diffuse. Then, the fracturing fluid is continuously injected until the rock sample fractures. During this process, the bottom hole pressure is recorded in real time using a pressure monitoring system.

[0035] (3) After the experiment is over, take out the experimental samples;

[0036] Among them, T is the preset energy storage time, which is determined according to the sample size and field experimental conditions. The smaller the sample size, the shorter the well blocking time; the better the core porosity, the shorter the well blocking time; conversely, the longer the well blocking time

[0037] Preferably, the fracturing equipment in step 2 (1) is a true triaxial fracturing equipment.

[0038] Preferably, the variables of different energy storage methods in step 2 (2) include different energy storage times and different energy storage rounds. One energy storage + well blocking equals one round, which is equivalent to the number of injections; the energy storage time is the injection time + well blocking time.

[0039] Preferably, the different energy storage methods in step two (2) are divided into two categories, one is a different energy storage method based on setting a target pressure to stop the pump, and the other is a different energy storage method based on setting a target liquid volume to stop the pump.

[0040] Preferably, in step 2 (2), the different energy storage methods based on setting the target pressure as the basis for stopping the pump include:

[0041] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0042] Method 2: store energy for one round, stop the pump when the target bottom hole pressure reaches P, record the stored fluid volume and storage time T;

[0043] Method 3: store energy for one round, stop the pump when the target bottom hole pressure reaches P, record the stored fluid volume and storage time nT;

[0044] Method 4: store energy for n rounds. When the bottom hole pressure increases by P / n after one round of storage, stop the pump, record the stored fluid volume and storage time T, and store energy for a total of nT time.

[0045] in,

[0046] n is a positive integer;

[0047] P is the target bottom hole pressure, which is lower than the closure stress;

[0048] V is the target liquid volume.

[0049] Preferably, in step 2 (2), the different energy storage methods based on setting the target liquid volume as the basis for stopping the pump include:

[0050] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0051] Method 2: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time T;

[0052] Method 3: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time nT;

[0053] Method 4: store energy for n rounds, store energy for T time per round, stop the pump when the liquid volume increases by V / n after one round of storage, record the stored liquid volume and storage time T, and store energy for a total of nT time.

[0054] Preferably, according to different energy storage modes 4 with the set target liquid volume as the basis for stopping the pump, a relationship curve between the pressure maintenance level and the total liquid volume stored after each energy storage well soaking can be drawn, so as to predict the required energy storage medium liquid volume according to the formation deficit degree for similar physical property reservoirs;

[0055] The fracture pressure monitoring is obtained from the results of the pressure monitoring system. When the sample is destroyed to the boundary, the bottom hole pressure drops to 0 MPa instantly.

[0056] The evaluation indexes of crack complexity reconstructed after CT scanning include crack three-dimensional morphology, fractal dimension, crack width distribution and standard deviation, crack area ratio, etc.

[0057] Preferably, in step six, the three-dimensional morphology of the crack is reconstructed, the fractal dimension, crack width distribution and standard deviation, and crack area ratio are calculated, and the crack morphology, complexity, and roughness are evaluated.

[0058] Preferably, the three-dimensional morphology extraction and image processing in the three-dimensional morphology reconstruction of the crack includes the following steps:

[0059] (1) Image segmentation: A series of two-dimensional slices obtained by CT scanning of the rock sample are combined and reconstructed into a three-dimensional rock image. The three-dimensional rock image is smoothed by digital filtering technology, and the fracture area is extracted by a preset gray value threshold;

[0060] (2) Crack digitization: Cracks are digitized by setting the unit length S of voxels (basic elements); the voxel size represents the smallest element that makes up the image;

[0061] (3) Crack surface extraction: The crack surface is extracted from the three-dimensional binary image using the homotopy thinning algorithm. After the crack surface is extracted, the local thickness filter is applied to the extracted crack surface to calculate the number of basic elements on the crack surface at the same corresponding position to obtain the distribution of crack width. The local thickness filter is aimed at the width. The same corresponding position refers to the two corresponding points on the cracking direction to calculate the width, which is related to the crack initiation direction.

[0062] (4) Standardization: The binary image of the crack surface is converted into a point cloud dataset by calculating the center point of each basic element on the crack surface. The normal vector of each crack point is calculated according to the position of the adjacent point cloud. After the normal vector is calculated, the crack deflection angle between the normal vector and the maximum principal stress direction is calculated.

[0063] Preferably, in the three-dimensional morphological reconstruction of the crack, the digital filtering technology in the image segmentation is Fourier wavelet filtering.

[0064] Preferably, in the step (4) of extracting the three-dimensional morphology of the crack and processing the image, the original point cloud is uniformly sampled.

[0065] Preferably, the calculation formula of fractal dimension (FD) is as follows:

[0066]

[0067] in,

[0068] h represents the size of the basic element; the basic element is a box or a cube;

[0069] N h Indicates the number of basic elements required to cover the crack.

[0070] Preferably, the fractal dimension is calculated using the box counting method, that is, by changing the size of the basic element h, the number of different basic elements N required to cover the crack is calculated. h , plot N in a double logarithmic plot h The relationship between FD and h is derived as the slope of the line.

[0071] Preferably, the crack width W is calculated as follows:

[0072] W=s*N i

[0073] in,

[0074] N i It represents the number of basic elements in the crack width at the same corresponding position;

[0075] Preferably, after obtaining the crack widths at different positions, the maximum value, minimum value and average value of the crack widths can be obtained accordingly, which are used to characterize the crack properties;

[0076] Preferably, the standard deviation SD of crack width is a statistical item used to evaluate the degree of deviation of crack width from the mean value. A low SD value indicates that the crack distribution is close to the mean value, and a high SD value indicates that the crack width has a large variation range, which can be used to evaluate the roughness of the crack. The formula for calculating the standard deviation of crack width is as follows:

[0077]

[0078] in,

[0079] N represents the crack width statistics;

[0080] W i Indicates the width of each statistical point;

[0081] W represents the average width of each statistical point;

[0082] Preferably, the area ratio AR represents the actual crack area S f and the projected area S p ratio.

[0083] AR=S f / S p

[0084] in,

[0085] Projection area S p It is the cross-sectional area of ​​the rock sample minus the drilling area;

[0086] Preferably, the actual crack area S f Calculated by:

[0087] s f =N f *S0

[0088] in,

[0089] N f Represents the number of basic elements on the crack surface;

[0090] S0=s 2 ;

[0091] Preferably, the method further includes step seven, selecting an energy storage method according to the evaluation result.

[0092] Preferably, in step 2 (2), the ISCO pump performs constant displacement injection, and the injection volume is recorded in real time.

[0093] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0094] (1) The present invention adopts an energy storage method of injecting energy storage medium below the fracture pressure, which can make the experimental results of pre-storage and post-fracturing under simulated real conditions more realistic and in line with the actual situation.

[0095] (2) The present invention comprehensively evaluates the effect of the advanced energy storage fracturing process by comprehensively considering the energy enhancement effect, fracture pressure, fracture morphology and roughness complexity, based on a real simulation of the advanced energy storage fracturing experiment, and provides strong technical support for optimizing the advanced energy storage method including energy storage time and energy storage rounds to increase production and efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0096] Figure 1 This is a flow chart of the evaluation of the effect of the advanced energy storage fracturing process according to one embodiment of the present invention;

[0097] Figure 2 A flow chart of the evaluation of the effect of the advanced energy storage fracturing process according to another embodiment of the present invention;

[0098] Figure 3 A flowchart of three-dimensional morphology extraction and image processing of cracks according to an embodiment of the present invention;

[0099] Figure 4 A graph showing a relationship between a diffusion coefficient of energy storage pressure and time in energy storage mode 2 in which a target pressure is set as a basis for stopping a pump according to an embodiment of the present invention;

[0100] Figure 5 This is a pressure curve diagram of different energy storage modes based on setting a target liquid volume as a basis for stopping the pump in one embodiment of the present invention. DETAILED DESCRIPTION

[0101] The specific implementation modes of the present invention are described in detail below in conjunction with the accompanying drawings.

[0102] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0103] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0104] Step 1: Prepare rock samples:

[0105] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0106] Step 2: Advanced energy storage fracturing experiment:

[0107] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0108] Step 3: Perform CT scanning on the sample after fracturing;

[0109] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0110] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation;

[0111] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0112] According to a specific embodiment of the present invention, step one comprises the following steps:

[0113] (1) Rock sample preparation: Take the target reservoir tight core from the same block, process the sample into a cuboid, and drill a wellbore at the center of the surface where the sample area is the smallest;

[0114] (2) Washing oil, salt and drying: Place the rock sample in a core basket, use carbon tetrachloride and ethanol to wash the oil and salt until the carbon tetrachloride and ethanol in the core basket are clarified, and then dry to constant weight;

[0115] (3) Saturated simulated oil: Place the sample in a vacuum and pressurized saturation instrument, evacuate the sample, inject simulated formation oil, pressurize the sample to the formation pressure, and age the sample for a preset number of days under the formation pressure. Stop the aging process after energy transfer is completed.

[0116] (4) Cementing and sealing: A bare hole section with a length of L is reserved in the middle of the core, and epoxy resin is used to bond the simulated wellbore for cementing. Then, a layer of organic potting glue is applied on the surface of the core to seal it.

[0117] According to a specific embodiment of the present invention, in step 1, the sample is made into a standard cuboid of 8 cm × 8 cm × 10 cm, and the inner diameter of the wellbore drilled in the center of the surface with the smallest sample area is 1.5 cm and the depth is 6.0 cm. The sample is prepared according to the size of the true triaxial fracturing equipment; the size of the inner diameter of the wellbore is related to the injection equipment, and the depth setting is to set the open hole section in the middle as much as possible.

[0118] According to a specific embodiment of the present invention, in step 1, the drying temperature is 100-105°C.

[0119] According to a specific embodiment of the present invention, in step 1, the drying temperature is 102°C.

[0120] According to a specific embodiment of the present invention, in step 1, vacuum is applied for 24 hours and aging is performed for 15 days.

[0121] According to a specific embodiment of the present invention, in step 1, the length L is 1-1.5 cm, and the simulated wellbore size has an outer diameter of 1.3 cm, an inner diameter of 0.6 cm, and a length of 5.8 cm. The wellbore size is selected according to the wellbore size, and there should be room for cementing between the wellbore outer diameter and the wellbore diameter.

[0122] According to a specific embodiment of the present invention, step 2 comprises the following steps:

[0123] (1) Prepare the experimental environment: install the rock sample and apply triaxial stress to the rock sample according to the triaxial stress of the formation; place the fracturing equipment in a constant temperature box and adjust the temperature of the constant temperature box to the formation temperature;

[0124] (2) Advanced energy storage fracturing: Using an ISCO pump, the energy storage medium is injected below the fracture pressure using different energy storage methods at the same displacement, and the energy is stored for T time to allow the pressure to diffuse. Then, the fracturing fluid is continuously injected until the rock sample fractures. During this process, the bottom hole pressure is recorded in real time using a pressure monitoring system.

[0125] (3) After the experiment is over, take out the experimental samples;

[0126] Among them, T is the preset energy storage time, which is determined according to the sample size and field experimental conditions. The smaller the sample size, the shorter the well blocking time; the better the core porosity, the shorter the well blocking time, and vice versa.

[0127] According to a specific embodiment of the present invention, the fracturing equipment in step 2 (1) is a true triaxial fracturing equipment.

[0128] According to a specific embodiment of the present invention, the variables of different energy storage methods in step 2 (2) include different energy storage times and different energy storage rounds. One energy storage + well blocking equals one round, which is equivalent to the number of injections; the energy storage time is the injection time + well blocking time.

[0129] According to a specific embodiment of the present invention, the different energy storage methods in step two (2) are divided into two categories, one is a different energy storage method based on setting a target pressure to stop the pump, and the other is a different energy storage method based on setting a target liquid volume to stop the pump.

[0130] According to a specific embodiment of the present invention, the different energy storage methods based on setting the target pressure as the basis for stopping the pump in step 2 (2) include:

[0131] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0132] Method 2: store energy for one round, stop the pump when the target bottom hole pressure reaches P, record the stored fluid volume and storage time T;

[0133] Method 3: store energy for one round, stop the pump when the target bottom hole pressure reaches P, record the stored fluid volume and storage time nT;

[0134] Method 4: store energy for n rounds. When the bottom hole pressure increases by P / n after one round of storage, stop the pump, record the stored fluid volume and storage time T, and store energy for a total of nT time.

[0135] in,

[0136] n is a positive integer;

[0137] P is the target bottom hole pressure, which is lower than the closure stress;

[0138] V is the target liquid volume.

[0139] According to a specific embodiment of the present invention, the different energy storage methods based on setting the target liquid volume as the basis for stopping the pump in step 2 (2) include:

[0140] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0141] Method 2: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time T;

[0142] Method 3: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time nT;

[0143] Method 4: store energy for n rounds, store energy for T time per round, stop the pump when the liquid volume increases by V / n after one round of storage, record the stored liquid volume and storage time T, and store energy for a total of nT time.

[0144] According to a specific embodiment of the present invention, according to different energy storage modes 4 with the set target liquid volume as the basis for stopping the pump, a relationship curve between the pressure maintenance level and the total liquid volume stored after each energy storage well soaking can be drawn, so as to predict the required energy storage medium liquid volume according to the formation deficit degree for similar physical property reservoirs;

[0145] The fracture pressure monitoring is obtained from the results of the pressure monitoring system. When the sample is destroyed to the boundary, the bottom hole pressure drops to 0 MPa instantly.

[0146] The evaluation indexes of crack complexity reconstructed after CT scanning include crack three-dimensional morphology, fractal dimension, crack width distribution and standard deviation, crack area ratio, etc.

[0147] According to a specific embodiment of the present invention, in step six, the three-dimensional morphology of the crack is reconstructed, the fractal dimension, crack width distribution and standard deviation, and crack area ratio are calculated, and the crack morphology, complexity and roughness are evaluated.

[0148] According to a specific embodiment of the present invention, the three-dimensional morphology extraction and image processing in the three-dimensional morphology reconstruction of the crack includes the following steps:

[0149] (1) Image segmentation: A series of two-dimensional slices obtained by CT scanning of the rock sample are combined and reconstructed into a three-dimensional rock image. The three-dimensional rock image is smoothed by digital filtering technology, and the fracture area is extracted by a preset gray value threshold;

[0150] (2) Crack digitization: Cracks are digitized by setting the unit length S of voxels (basic elements); the voxel size represents the smallest element that makes up the image;

[0151] (3) Crack surface extraction: The crack surface is extracted from the three-dimensional binary image using the homotopy thinning algorithm. After the crack surface is extracted, a local thickness filter is applied to the extracted crack surface to calculate the number of basic elements on the crack surface at the same corresponding position to obtain the distribution of crack width.

[0152] (4) Standardization: The binary image of the crack surface is converted into a point cloud dataset by calculating the center point of each basic element on the crack surface. The normal vector of each crack point is calculated according to the position of the adjacent point cloud. After the normal vector is calculated, the crack deflection angle between the normal vector and the maximum principal stress direction is calculated.

[0153] According to a specific embodiment of the present invention, in the three-dimensional morphological reconstruction of the crack, the digital filtering technology in the image segmentation is Fourier wavelet filtering.

[0154] According to a specific embodiment of the present invention, in the step (4) of extracting the three-dimensional morphology of the crack and processing the image, the original point cloud is uniformly sampled.

[0155] According to a specific embodiment of the present invention, the calculation formula of the fractal dimension FD is as follows:

[0156]

[0157] in,

[0158] h represents the size of the basic element; the basic element is a box or a cube;

[0159] N h Indicates the number of basic elements required to cover the crack.

[0160] According to a specific embodiment of the present invention, the fractal dimension is calculated by the box counting method, that is, by changing the size of the basic element h, the number of different basic elements N required to cover the crack is calculated. h , plot N in a double logarithmic plot h The relationship between FD and h is derived as the slope of the line.

[0161] According to a specific embodiment of the present invention, the crack width W is calculated as follows:

[0162] W=s*N i

[0163] in,

[0164] N i It represents the number of basic elements in the crack width at the same corresponding position;

[0165] According to a specific embodiment of the present invention, after the crack widths at different positions are obtained, the maximum value, minimum value and average value of the crack width can be obtained accordingly, which are used to characterize the crack properties;

[0166] Preferably, the standard deviation SD of crack width is a statistical item used to evaluate the degree of deviation of crack width from the mean value. A low SD value indicates that the crack distribution is close to the mean value, and a high SD value indicates that the crack width has a large variation range, which can be used to evaluate the roughness of the crack. The formula for calculating the standard deviation of crack width is as follows:

[0167]

[0168] in,

[0169] N represents the crack width statistics;

[0170] W i Indicates the width of each statistical point;

[0171] Indicates the average value of the width of each statistical point;

[0172] According to a specific embodiment of the present invention, the area ratio AR represents the actual crack area S f and the projected area S p ratio.

[0173] AR=S f / S p

[0174] in,

[0175] Projection area S p It is the cross-sectional area of ​​the rock sample minus the drilling area;

[0176] According to a specific embodiment of the present invention, the actual crack area S f Calculated by:

[0177] s f =N f *S0

[0178] in,

[0179] N f Represents the number of basic elements on the crack surface;

[0180] S0=s 2 ;

[0181] According to a specific implementation scheme of the present invention, the method further includes step seven, selecting an energy storage method according to the evaluation result.

[0182] According to a specific embodiment of the present invention, in step two (2), the ISCO pump performs constant displacement injection, and the injection volume is recorded in real time.

[0183] Example 1

[0184] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0185] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0186] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0187] Step 1: Prepare rock samples:

[0188] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0189] Step 2: Advanced energy storage fracturing experiment:

[0190] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0191] Step 3: Perform CT scanning on the sample after fracturing;

[0192] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0193] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and evaluate the fracture pressure;

[0194] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0195] Example 2

[0196] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0197] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0198] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0199] Step 1: Prepare rock samples:

[0200] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0201] Step 2: Advanced energy storage fracturing experiment:

[0202] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0203] Step 3: Perform CT scanning on the sample after fracturing;

[0204] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0205] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and evaluate the fracture pressure;

[0206] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0207] Wherein, step one includes the following steps:

[0208] (1) Rock sample preparation: Take the target reservoir tight core from the same block, process the sample into a cuboid, and drill a wellbore at the center of the surface where the sample area is the smallest;

[0209] (2) Washing oil, salt and drying: Place the rock sample in a core basket, use carbon tetrachloride and ethanol to wash the oil and salt until the carbon tetrachloride and ethanol in the core basket are clarified, and then dry to constant weight;

[0210] (3) Saturated simulated oil: Place the sample in a vacuum and pressurized saturation instrument, evacuate the sample, inject simulated formation oil, pressurize the sample to the formation pressure, and age the sample for a preset number of days under the formation pressure. Stop the aging process after energy transfer is completed.

[0211] (4) Cementing and sealing: A bare hole section with a length of L is reserved in the middle of the core, and epoxy resin is used to bond the simulated wellbore for cementing. Then, a layer of organic potting glue is applied on the surface of the core to seal it.

[0212] Example 3

[0213] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0214] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0215] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0216] Step 1: Prepare rock samples:

[0217] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0218] Step 2: Advanced energy storage fracturing experiment:

[0219] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0220] Step 3: Perform CT scanning on the sample after fracturing;

[0221] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0222] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation;

[0223] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0224] Wherein, step one includes the following steps:

[0225] (1) Rock sample preparation: Take the target reservoir tight core from the same block, process the sample into a cuboid, and drill a wellbore at the center of the surface where the sample area is the smallest;

[0226] (2) Washing oil, salt and drying: Place the rock sample in a core basket, use carbon tetrachloride and ethanol to wash the oil and salt until the carbon tetrachloride and ethanol in the core basket are clarified, and then dry to constant weight;

[0227] (3) Saturated simulated oil: Place the sample in a vacuum and pressurized saturation instrument, evacuate the sample, inject simulated formation oil, pressurize the sample to the formation pressure, and age the sample for a preset number of days under the formation pressure. Stop the aging process after energy transfer is completed.

[0228] (4) Cementing and sealing: A bare hole section with a length of L is reserved in the middle of the core, and epoxy resin is used to bond the simulated wellbore for cementing. Then, a layer of organic potting glue is applied on the surface of the core to seal it.

[0229] In step 1, the sample is made into a standard cuboid of 8cm×8cm×10cm, and the inner diameter of the wellbore drilled in the center of the surface with the smallest sample area is 1.5cm and the depth is 6.0cm. The sample is prepared according to the size of the true triaxial fracturing equipment; the size of the inner diameter of the wellbore is related to the injection equipment, and the depth setting is to set the open hole section in the middle as much as possible.

[0230] Wherein, in step 1, the drying temperature is 100-105°C.

[0231] Among them, in step 1, vacuum is applied for 24 hours and aging is carried out for 15 days.

[0232] Among them, in step 1, the length L is 1-1.5 cm, and the simulated wellbore size is 1.3 cm in outer diameter, 0.6 cm in inner diameter, and 5.8 cm in length. The wellbore size is selected according to the wellbore size, and there should be room for cementing between the wellbore outer diameter and the wellbore diameter.

[0233] Example 4

[0234] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0235] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0236] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0237] Step 1: Prepare rock samples:

[0238] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0239] Step 2: Advanced energy storage fracturing experiment:

[0240] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0241] Step 3: Perform CT scanning on the sample after fracturing;

[0242] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0243] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation;

[0244] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0245] Wherein, step 2 includes the following steps:

[0246] (1) Prepare the experimental environment: install the rock sample and apply triaxial stress to the rock sample according to the triaxial stress of the formation; place the fracturing equipment in a thermostatic box and adjust the temperature of the thermostatic box to the formation temperature;

[0247] (2) Advanced energy storage fracturing: Using an ISCO pump, the energy storage medium is injected below the fracture pressure using different energy storage methods at the same displacement, and the energy is stored for T time to allow the pressure to diffuse. Then, the fracturing fluid is continuously injected until the rock sample fractures. During this process, the bottom hole pressure is recorded in real time using a pressure monitoring system.

[0248] (3) After the experiment is over, take out the experimental samples;

[0249] Among them, T is the preset energy storage time, which is determined according to the sample size and field experimental conditions. The smaller the sample size, the shorter the well blocking time; the better the core porosity, the shorter the well blocking time, and vice versa.

[0250] Example 5

[0251] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0252] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0253] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0254] Step 1: Prepare rock samples:

[0255] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0256] Step 2: Advanced energy storage fracturing experiment:

[0257] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0258] Step 3: Perform CT scanning on the sample after fracturing;

[0259] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0260] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation;

[0261] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0262] Wherein, step 2 includes the following steps:

[0263] (1) Prepare the experimental environment: install the rock sample and apply triaxial stress to the rock sample according to the triaxial stress of the formation; place the fracturing equipment in a thermostatic box and adjust the temperature of the thermostatic box to the formation temperature;

[0264] (2) Advanced energy storage fracturing: Using an ISCO pump, the energy storage medium is injected below the fracture pressure using different energy storage methods at the same displacement, and the energy is stored for T time to allow the pressure to diffuse. Then, the fracturing fluid is continuously injected until the rock sample fractures. During this process, the bottom hole pressure is recorded in real time using a pressure monitoring system.

[0265] (3) After the experiment is over, take out the experimental samples;

[0266] Among them, T is the preset energy storage time, which is determined according to the sample size and field experimental conditions. The smaller the sample size, the shorter the well blocking time; the better the core porosity, the shorter the well blocking time, and vice versa.

[0267] Wherein, the fracturing equipment in step 2 (1) is a true triaxial fracturing equipment.

[0268] Among them, the variables of different energy storage methods in step 2 (2) include different energy storage times and different energy storage rounds. One energy storage + well blocking equals one round, which is equivalent to the number of injections; the energy storage time is the injection time + well blocking time

[0269] Among them, the different energy storage methods in step 2 (2) are divided into two categories, one is a different energy storage method based on setting a target pressure to stop the pump, and the other is a different energy storage method based on setting a target liquid volume to stop the pump.

[0270] Example 6

[0271] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0272] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0273] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0274] Step 1: Prepare rock samples:

[0275] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0276] Step 2: Advanced energy storage fracturing experiment:

[0277] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0278] Step 3: Perform CT scanning on the sample after fracturing;

[0279] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0280] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and evaluate the fracture pressure;

[0281] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0282] Wherein, step 2 includes the following steps:

[0283] (1) Prepare the experimental environment: install the rock sample and apply triaxial stress to the rock sample according to the triaxial stress of the formation; place the fracturing equipment in a constant temperature box and adjust the temperature of the constant temperature box to the formation temperature;

[0284] (2) Advanced energy storage fracturing: Using an ISCO pump, the energy storage medium is injected below the fracture pressure using different energy storage methods at the same displacement, and the energy is stored for T time to allow the pressure to diffuse. Then, the fracturing fluid is continuously injected until the rock sample fractures. During this process, the bottom hole pressure is recorded in real time using a pressure monitoring system.

[0285] (3) After the experiment is over, take out the experimental samples;

[0286] Among them, T is the preset energy storage time, which is determined according to the sample size and field experimental conditions. The smaller the sample size, the shorter the well blocking time; the better the core porosity, the shorter the well blocking time, and vice versa.

[0287] Wherein, the fracturing equipment in step 2 (1) is a true triaxial fracturing equipment.

[0288] Among them, the variables of different energy storage methods in step 2 (2) include different energy storage times and different energy storage rounds. One energy storage + well blocking equals one round, which is equivalent to the number of injections; the energy storage time is the injection time + well blocking time

[0289] Among them, the different energy storage methods in step 2 (2) are divided into two categories, one is a different energy storage method based on setting a target pressure to stop the pump, and the other is a different energy storage method based on setting a target liquid volume to stop the pump.

[0290] Among them, in step 2 (2), different energy storage methods based on setting the target pressure as the basis for stopping the pump include:

[0291] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0292] Method 2: store energy for one round, stop the pump when the target bottom hole pressure P is reached, record the stored fluid volume and storage time T;

[0293] Method 3: store energy for one round, stop the pump when the target bottom hole pressure P is reached, record the stored fluid volume and storage time nT;

[0294] Method 4: store energy for n rounds. When the bottom hole pressure increases by P / n after one round of storage, stop the pump, record the stored fluid volume and storage time T, and store energy for a total of nT time.

[0295] in,

[0296] n is a positive integer;

[0297] P is the target bottom hole pressure, which is lower than the closure stress;

[0298] V is the target liquid volume.

[0299] Among them, in step 2 (2), different energy storage methods based on setting the target liquid volume as the basis for stopping the pump include:

[0300] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0301] Method 2: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time T;

[0302] Method 3: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time nT;

[0303] Method 4: store energy for n rounds, store energy for T time per round, stop the pump when the liquid volume increases by V / n after one round of storage, record the stored liquid volume and storage time T, and store energy for a total of nT time.

[0304] Example 7

[0305] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0306] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0307] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0308] Step 1: Prepare rock samples:

[0309] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0310] Step 2: Advanced energy storage fracturing experiment:

[0311] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0312] Step 3: Perform CT scanning on the sample after fracturing;

[0313] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0314] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation;

[0315] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0316] Among them, in step six, the three-dimensional morphology of the crack is reconstructed, the fractal dimension, crack width distribution and standard deviation, and crack area ratio are calculated, and the crack morphology, complexity and roughness are evaluated.

[0317] Example 8

[0318] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0319] The present invention provides an advanced energy storage fracturing process evaluation method, which evaluates the advanced energy storage fracturing process by simulating an experimental method of fracturing after advanced energy storage under real conditions, based on the evaluation of energy enhancement effect of pressure change and stored liquid volume, the evaluation of fracture pressure, and the evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning;

[0320] The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps:

[0321] Step 1: Prepare rock samples:

[0322] Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing;

[0323] Step 2: Advanced energy storage fracturing experiment:

[0324] Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time;

[0325] Step 3: Perform CT scanning on the sample after fracturing;

[0326] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0327] Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and evaluate the fracture pressure;

[0328] Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

[0329] Among them, the energy enhancement coefficient α in step 4 E The definition is as follows:

[0330]

[0331] in,

[0332] P is the target bottom hole pressure, which is lower than the closure stress;

[0333] V is the target liquid volume.

[0334] Among them, the pressure diffusion coefficient α in step 4 is P The definition is as follows:

[0335]

[0336] in,

[0337] P(t) is the bottom hole pressure at any time t during the soaking process;

[0338] Δt is the time change.

[0339] Example 9

[0340] According to a specific embodiment of the present invention, the advanced energy storage fracturing process evaluation method of the present invention is described in detail below.

[0341] This example uses dense sandstone cores to evaluate the effects of advanced energy storage fracturing processes with different energy storage methods. The specific methods are as follows:

[0342] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0343] Method 2: Store energy once until the target bottom hole pressure reaches 7.5 MPa, stop pumping, record the amount of stored fluid, and store energy for 2 hours;

[0344] The advanced energy storage fracturing process effect method of different energy storage modes in this embodiment includes the following steps:

[0345] Step 1: Prepare rock samples

[0346] (1) Rock sample preparation: Take the target reservoir tight core of the same block and process the sample into a standard cuboid of 8cm×8cm×10cm. Drill a wellbore with an inner diameter of 1.5cm and a depth of 6.0cm in the center of the 8cm×8cm surface of the sample;

[0347] (2) Washing oil and salt and drying: Use carbon tetrachloride and ethanol to wash the oil and salt until the organic solvent in the core basket is clarified, and then dry at 102°C to constant weight;

[0348] (3) Saturated simulated oil: Place the sample in a vacuum pressurized saturation instrument, evacuate for 24 h, inject simulated formation oil, pressurize to formation pressure, and age at formation pressure for 15 days;

[0349] (4) Cementing and sealing: A 1.5 cm open hole section was reserved in the middle of the core, and epoxy resin was used to bond the simulated wellbore (outer diameter 1.3 cm, inner diameter 0.6 cm, length 5.8 cm) for cementing, and a layer of organic potting glue was applied on the surface of the core for sealing;

[0350] Step 2: Advanced energy storage fracturing experiment

[0351] (1) Prepare the experimental environment: install the sample and apply triaxial stress to the rock sample according to 9 / 15 / 21 MPa; place the equipment in a constant temperature box and adjust the formation temperature to 75°C;

[0352] (2) Advanced energy storage fracturing process: ISCO pump was used with a constant displacement of 30 ml / min. After different energy storage methods, fracturing fluid was continuously injected until the rock sample broke. During this process, a pressure monitoring system was used to record the bottom hole pressure in real time.

[0353] The different energy storage methods include:

[0354] Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed;

[0355] Method 2: Store energy once until the target bottom hole pressure reaches 7.5 MPa, stop pumping, record the amount of stored fluid, and store energy for 2 hours;

[0356] (3) After the experiment is over, take out the experimental samples;

[0357] Step 3: Perform CT scanning on the sample after fracturing;

[0358] Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect;

[0359] The energy enhancement effect evaluation indicators based on pressure changes include:

[0360] (1) Energy enhancement coefficient (α E ): The ratio of the bottom hole pressure (P) to the total energy storage fluid volume (v) after the last soaking of the advance energy storage is completed, that is, the formation pressure increased by a unit of energy storage fluid, which represents the energy-increasing capacity of the energy storage fluid. The higher the amplitude of the formation pressure increased by a unit of fluid, the stronger the energy-increasing capacity of the energy storage fluid is, which can be expressed by the following method:

[0361]

[0362] Method 1: No energy storage, no energy increase coefficient;

[0363] Method 2: Advanced energy storage. After 2 hours of energy storage, the equilibrium pressure is 4.5MPa, the energy storage liquid volume is 195ml, and the energy increase coefficient is 0.0231MPa / ml;

[0364] (2) Pressure diffusion coefficient (α P ): The slope of the curve of the bottom hole pressure changing with time during the well soaking process can represent the difficulty of energy enhancement. The faster the bottom hole pressure decreases during the well soaking process, the easier the energy enhancement is, and vice versa. It can be expressed by the following method:

[0365]

[0366] Where, P(t) is the curve of bottom hole pressure changing with time during well soaking, and t is any time during well soaking;

[0367] Method 1: No energy storage, no pressure diffusion coefficient;

[0368] Method 2: Advance energy storage The relationship between the pressure diffusion coefficient and time during the 2h energy storage period is as follows Figure 3 , the pressure diffusion coefficient has a rule of first large and then small, which means that energy enhancement is easy at first and then difficult;

[0369] like Figure 3 The graph showing the relationship between the storage pressure diffusion coefficient and time in mode 2 is shown.

[0370] Step 5: Obtain the fracture pressure according to the bottom hole pressure monitoring curve to evaluate the fracture pressure. The lower the fracture pressure, the easier the fracturing is.

[0371] Pressure curves of different energy storage methods are as follows Figure 4 , the bursting pressure of method 2 is 13.51MPa, which is lower than the bursting pressure of method 1, 13.88MPa;

[0372] Step 6: Reconstruct the three-dimensional morphology of the cracks and calculate the fractal dimension, crack width distribution and standard deviation, and crack area ratio to evaluate the crack morphology, complexity, and roughness.

[0373] The three-dimensional morphology extraction and image processing steps of the crack include:

[0374] (1) Image segmentation: A series of two-dimensional slices are obtained by scanning the rock sample with CT. The two-dimensional slices are combined and reconstructed into a three-dimensional rock image. The image is smoothed and denoised by digital filtering technology (Fourier wavelet filtering), while retaining the sharp crack edges. Since the crack has a gray value different from that of the rock matrix, the crack area can be extracted by specifying a gray value threshold;

[0375] (2) Crack digitization: Since the image is represented by voxels, the cracks are digitized by setting the unit length of the voxel (the side length of the basic element cube) S. The voxel size represents the smallest element (basic element) that makes up the image and will be scaled to the physical size. By calculating the number of basic elements, the crack width and so on can be further obtained.

[0376] (3) Crack surface extraction: The homotopy thinning algorithm is used to extract the crack surface from the three-dimensional binary image. After the crack surface is extracted, a local thickness filter is applied and mapped onto the surface. The number of basic elements on the crack surface at the same corresponding position is calculated to obtain the distribution of crack width.

[0377] (4) Standardization: The binary image of the crack surface is converted into a point cloud dataset by calculating the center point of each basic element on the crack surface. The open source point cloud processing library is used to calculate the normal vector of each crack point according to the positions of adjacent points. After the normal vectors are calculated, the crack deflection angle between these normal vectors and the direction of the maximum principal stress can be calculated.

[0378] In order to save computational effort in the three-dimensional morphology extraction and image processing step (4), the original point cloud is uniformly sampled and some points are eliminated. The simplified point cloud can reflect the distribution trend of the original crack attributes.

[0379] The calculation formula of fractal dimension (FD) is as follows:

[0380]

[0381] Where h represents the size of the basic element (box or cube), N h Indicates the number of primitives (boxes or cubes) needed to cover the crack.

[0382] The fractal dimension is calculated using the box counting method, that is, by changing the size of the basic element h, the number of different basic elements N required to cover the crack is calculated. h , plot N in a double logarithmic ploth The relationship between FD and h can be derived as the slope of the line.

[0383] The calculation formula of crack width (W) is as follows:

[0384] W=s*N i

[0385] Where N i It represents the number of basic elements in the crack width at the same corresponding position;

[0386] After obtaining the crack widths at different locations, the maximum, minimum and average values ​​of the crack widths can also be obtained to characterize the crack properties;

[0387] The standard deviation (SD) of crack width is a statistical term used to evaluate the degree of deviation of crack width from the mean. A low SD value indicates that the crack distribution is close to the mean, and a high SD value indicates that the crack width varies widely, which can be used to evaluate the roughness of the crack. The formula for calculating the standard deviation of crack width is as follows:

[0388]

[0389] Where N is the crack width statistic, W is i Indicates the width of each statistical point, represents the average width of each statistical point; area ratio (AR) represents the actual crack area (S f ) and the projected area (S p ) ratio.

[0390] AR=S f / S p

[0391] The projected area (S p ) is the cross-sectional area of ​​the rock sample minus the borehole area;

[0392] The actual crack area (S f ) can be calculated by the following method:

[0393] s f =N f *S0

[0394] Where N f Represents the number of basic elements on the crack surface, S0 = s 2 ;

[0395] The fracture characterization parameters of advanced energy storage fracturing under the two energy storage methods are shown in Table 1. The energy storage fracturing process effect of method 2 is better than that of method 1.

[0396] Table 1 Crack characterization parameters for different energy storage methods

[0397] Crack characterization parameters Method 1 Method 2 Fractal dimension (FD) 1.83579 1.87467 Crack width standard deviation (SD) 0.0906 0.171 Crack area ratio (AR%) 1.49 1.52

[0398] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for evaluating an advanced energy storage fracturing process, characterized in that: Through the experimental method of pre-energy storage and fracturing simulating real conditions, the pre-energy storage fracturing process was evaluated based on the evaluation of energy enhancement effect based on pressure change and stored fluid volume, evaluation of fracture pressure, and evaluation of fracture morphology, complexity and roughness reconstructed based on CT scanning; The experimental method for simulating real conditions of advanced energy storage and post-fracturing comprises the following steps: Step 1: Prepare rock samples: Select the target reservoir's tight core as a sample, wash the oil, salt and dry it, create the reservoir's oil-water distribution, and perform cementing and sealing; Step 2: Advanced energy storage fracturing experiment: Create an experimental environment, use ISCO pumps, and use different energy storage methods with the same displacement to inject energy storage media below the fracture pressure, and then keep the well for a preset time to allow the pressure to diffuse, and then continue to inject fracturing fluid until the rock sample breaks. During this process, the bottom hole pressure and energy storage fluid volume are recorded in real time; Step 3: Perform CT scanning on the sample after fracturing; Step 4: Calculate the energy enhancement coefficient and pressure diffusion coefficient to evaluate the energy enhancement effect; Step 5: Obtain the fracture pressure according to the monitored bottom hole pressure curve and conduct fracture pressure evaluation; Step 6: Reconstruct the three-dimensional morphology of the crack and evaluate the crack morphology, complexity and roughness based on the crack parameters.

2. The method for evaluating the advanced energy storage fracturing process according to claim 1, characterized in that: Step 1 includes the following steps: (1) Rock sample preparation: Take the target reservoir tight core from the same block, process the sample into a cuboid, and drill a wellbore at the center of the surface where the sample area is the smallest; (2) Washing oil, salt and drying: Place the rock sample in a core basket, use carbon tetrachloride and ethanol to wash the oil and salt until the carbon tetrachloride and ethanol in the core basket are clarified, and then dry to constant weight; (3) Saturated simulated oil: Place the sample in a vacuum and pressurized saturation instrument, evacuate the sample, inject simulated formation oil, pressurize the sample to the formation pressure, and age the sample for a preset number of days under the formation pressure. Stop the aging process after energy transfer is completed. (4) Cementing and sealing: A bare hole section with a length of L is reserved in the middle of the core, and epoxy resin is used to bond the simulated wellbore for cementing. Then, a layer of organic potting glue is applied on the surface of the core to seal it.

3. The method for evaluating the advanced energy storage fracturing process according to claim 1, characterized in that: Step 2 includes the following steps: (1) Prepare the experimental environment: install the rock sample and apply triaxial stress to the rock sample according to the triaxial stress of the formation; place the fracturing equipment in a constant temperature box and adjust the temperature of the constant temperature box to the formation temperature; (2) Advanced energy storage fracturing: Using an ISCO pump, the energy storage medium is injected below the fracture pressure using different energy storage methods at the same displacement, and the energy is stored for T time to allow the pressure to diffuse. Then, the fracturing fluid is continuously injected until the rock sample fractures. During this process, the bottom hole pressure is recorded in real time using a pressure monitoring system. (3) After the experiment is over, take out the experimental samples; Among them, T is the preset energy storage time, which is determined according to the sample size and on-site experimental conditions.

4. The method for evaluating the advanced energy storage fracturing process according to claim 3, characterized in that: The fracturing equipment in step 2 (1) is a true triaxial fracturing equipment.

5. The method for evaluating the advanced energy storage fracturing process according to claim 3, characterized in that: The variables of different energy storage methods in step 2 (2) include different energy storage times and different energy storage rounds.

6. The method for evaluating the advanced energy storage fracturing process according to claim 5, characterized in that: The different energy storage methods in step 2 (2) are divided into two categories: one is a different energy storage method based on setting a target pressure as a basis for stopping the pump, and the other is a different energy storage method based on setting a target liquid volume as a basis for stopping the pump.

7. The method for evaluating the advanced energy storage fracturing process according to claim 6, characterized in that: In step 2 (2), different energy storage methods based on setting the target pressure as the basis for stopping the pump include: Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed; Method 2: store energy for one round, stop the pump when the target bottom hole pressure P is reached, record the stored fluid volume and storage time T; Method 3: store energy for one round, stop the pump when the target bottom hole pressure P is reached, record the stored fluid volume and storage time nT; Method 4: store energy for n rounds. When the bottom hole pressure increases by P / n after one round of storage, stop the pump, record the stored fluid volume and storage time T, and store energy for a total of nT time. in, n is a positive integer; P is the target bottom hole pressure, which is lower than the closure stress.

8. The method for evaluating the advanced energy storage fracturing process according to claim 6, characterized in that: In step 2 (2), different energy storage methods based on setting the target liquid volume as the basis for stopping the pump include: Method 1: No energy storage, just inject fracturing fluid continuously until the sample is destroyed; Method 2: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time T; Method 3: store energy for one round, stop the pump when the target liquid volume V is reached, record the stored liquid volume and storage time nT; Method 4: store energy for n rounds, store energy for T time per round, stop the pump when the liquid volume increases by V / n after one round of storage, record the stored liquid volume and storage time T, and store energy for a total of nT time.

9. The method for evaluating the advanced energy storage fracturing process according to claim 1, characterized in that: In step six, the three-dimensional morphology of the cracks is reconstructed, and the fractal dimension, crack width distribution and standard deviation, and crack area ratio are calculated to evaluate the crack morphology, complexity, and roughness.

10. The method for evaluating advanced energy storage fracturing technology according to claim 1, characterized in that: The method also includes step seven: selecting an energy storage method according to the evaluation results.

11. The method for evaluating the advanced energy storage fracturing process according to claim 1, characterized in that: Energy enhancement coefficient α in step 4 E The definition is as follows: in, P is the target bottom hole pressure, which is lower than the closure stress; V is the target liquid volume.

12. The method for evaluating the advanced energy storage fracturing process according to claim 1, characterized in that: The pressure diffusion coefficient α in step 4 P The definition is as follows: in, P(t) is the bottom hole pressure at any time t during the soaking process; Δt is the time change.

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