Electric field fracturing coal seam anti-reflection gas extraction method

Abstract

The invention discloses an electric field fracturing coal seam anti-reflection gas extraction method, and belongs to the field of coal seam gas extraction. S2, generating a three-dimensional geologic model; s3, performing electric field fracturing simulation until an optimal fracturing path is obtained, and taking an electric field parameter corresponding to the optimal fracturing path as an electric field initial parameter; s4, based on the generated three-dimensional coal seam image, a distributed stress sensor, a strain sensor and a displacement sensor are arranged in the fracturing area so as to monitor stress concentration factors, strain and crack length and width changes in the coal seam; s5, applying an electric field initial parameter to the coal seam, and dynamically adjusting the electric field parameter to ensure that an optimal fracturing path is achieved; and S6, gas extraction. By the adoption of the electric field fracturing coal seam anti-reflection gas extraction method, the electric field parameters are dynamically adjusted based on real-time monitoring data, it is ensured that fracturing is conducted along the optimal path all the time, and unnecessary resource waste and potential safety hazards are avoided.

Description

Technical Field

[0001] The invention relates to the technical field of coal seam gas extraction, and in particular to a method for gas extraction by increasing the permeability of coal seams by electric field fracturing. Background Art

[0002] Gas, also known as coal-bed methane, refers to hydrocarbon gas stored in coal seams with methane as the main component, mainly adsorbed on the surface of coal matrix particles, and partially free in coal pores or dissolved in coal seam water. It is a mineral resource associated with coal and an unconventional natural gas. It is a clean, high-quality energy and chemical raw material that has emerged internationally in the past 10 to 20 years. When the gas-air concentration reaches 5%-16%, it will explode when exposed to open flames, resulting in explosion accidents in coal mines. In order to solve the above problem, gas is extracted before coal mining, thereby reducing the coal mine gas explosion rate to 70% to 85%.

[0003] There are many coal seam permeability enhancement technologies, such as high-pressure water fracturing, drilling, electric field fracturing, etc. Among them, the principle of electric fracturing of coal seams is mainly to use high-voltage electric pulse waves or plasma technology to generate physical effects such as stress waves, shock waves or water shock waves in the coal seams, thereby inducing the expansion of cracks inside the coal seams and the crushing of the coal body, so as to achieve the purpose of fracturing and permeability enhancement.

[0004] The existing electric field fracturing coal seam technology is as follows

[0005] CN202110158171.7 discloses a method for increasing coal seam permeability by integrating electric pulse directional cutting and hydraulic blasting. The method first constructs a positive electrode guide borehole and a negative electrode guide borehole at the terminal position of the motor drilling in the coal seam, and then injects a conductive ion solution into the positive electrode guide borehole and the negative electrode guide borehole, uses high-voltage electric pulse technology to impact the borehole, and performs hydraulic cutting. After the cutting is completed, the conductive ion solution in the electrode guide borehole is discharged by a drainage pump, and then the electrode guide borehole that has completed electric pulse hydraulic cutting is pulse blasted.

[0006] CN202310176030.7 discloses a device and method for controlling shock wave fracturing coal seams and combining gas injection to enhance gas extraction. The device includes a drilling fracturing component and a displacement extraction component; the drilling fracturing component includes a drilling rig, a hollow drill rod, a high-voltage electric pulse controller, a high-voltage electric pulse controllable shock wave generator, a multifunctional adapter and a multifunctional sealing plug; the displacement extraction component includes a CO2 gas permeability enhancement system, a CO2 gas injection pipe, a gas extraction pipe, a gas extraction system and a CO2 gas separation system. The method is: for low-permeability and difficult-to-desorb coal seams, first complete the drilling batch construction at the calibrated position of the gas extraction area in the coal seam, and the drilling is divided into conventional drilling and permeability enhancement drilling and alternately distributed; then, the coal seam is fractured and permeated by using a high-voltage electric pulse controllable shock wave, which effectively increases the degree of crack development in the coal seam, and then carbon dioxide is used to displace the gas in the coal seam, which effectively enhances the extraction effect of the difficult-to-desorb gas in the coal seam, and further reduces the risk of gas outburst and gas explosion.

[0007] It can be seen that the control parameters in the existing electric field cracking process are mostly set based on experience and cannot be dynamically adjusted according to the on-site environment, resulting in poor transmittance enhancement effect. Summary of the invention

[0008] The purpose of the present invention is to provide a method for gas extraction by electric field fracturing coal seam permeability enhancement to solve the above technical problems.

[0009] To achieve the above object, the present invention provides a method for gas extraction by electric field fracturing coal seam permeability enhancement, comprising the following steps:

[0010] S1. On-site investigation: Collect coal seam lithology data, physical property data, boundary and internal structure data, initial stress and historical fracturing experimental data;

[0011] S2, modeling: importing the coal seam lithology data, physical property data, boundary and internal structure data, initial stress and historical fracturing experimental data obtained in step S1 into a three-dimensional modeling software to generate a three-dimensional geological model;

[0012] S3, simulation experiment: based on the three-dimensional geological model generated in step S2, use numerical simulation software to perform electric field fracturing simulation until the optimal fracturing path is obtained, and take the electric field parameters corresponding to the optimal fracturing path as the initial electric field parameters;

[0013] S4, data monitoring: based on the three-dimensional coal seam image generated in step S2, distributed stress sensors, strain sensors and displacement sensors are arranged in the fracture area to monitor the stress concentration factor, strain and changes in the length and width of the fracture in the coal seam;

[0014] S5, applying the initial electric field parameters determined in step S3 to the coal seam, and dynamically adjusting the electric field parameters based on the crack width, stress change and temperature distribution collected in real time in step S4, so as to ensure that the optimal fracturing path described in step S3 is achieved;

[0015] S6. Use gas extraction equipment to extract gas at the entrance of the optimal fracturing path.

[0016] Preferably, in step S1, a core sample of the coal seam is taken to obtain lithology data of the coal seam;

[0017] Use geological radar to transmit electromagnetic waves to the coal seam, and then obtain the physical property data of the coal seam through the received reflected waves. The physical property data includes the dielectric constant, thickness, density and conductivity of the coal seam.

[0018] Using a seismic source to apply a seismic source to the coal seam, and then determining the boundary and internal structure data of the coal seam through the received seismic waves, and the internal structure data includes faults, joints, cracks and folds;

[0019] The initial stress field of the coal seam is obtained by borehole measurement.

[0020] Preferably, the three-dimensional modeling software described in step S2 is FLAC3D.

[0021] Preferably, the numerical simulation software described in step S3 is ANSYS or COMSOL;

[0022] And step S3 specifically includes the following steps:

[0023] S31, setting initial conditions: setting the initial electric field strength E0 and the initial frequency f0, and determining the boundary conditions of the simulation area;

[0024] S32, define the following multi-objective function with the comprehensive evaluation index F as the optimization target:

[0025] F=α1×L+α2×W+α3×D+α4×K t +α5×RII (1);

[0026] Where L is the crack length, in m; W is the crack width, in m; D is the crack density, in m; K is the crack width, in m. t represents stress concentration factor; RII represents rock integrity index; α1, α2, α3, α4 and α5 are respectively crack length L, crack width W, crack density D, stress concentration factor K t and the weight coefficient corresponding to the rock integrity index RII;

[0027] S33, numerical iteration simulation: change the electric field strength E or frequency f to perform numerical iteration simulation, and record the electric field strength E, frequency f, crack length L, crack width W, crack density D, stress concentration factor K of each simulation t and rock integrity index RII, until the change of the comprehensive evaluation index F is less than the set threshold, the iteration is stopped;

[0028] S34, determining the optimal fracture path: selecting the maximum comprehensive evaluation index F from the iterative results obtained in step S33 as the optimal fracture path;

[0029] S35. Determine the electric field strength E and frequency f corresponding to the maximum comprehensive evaluation index F as the initial electric field strength E0 and the initial frequency f0, respectively.

[0030] Preferably, in step S32, the weight coefficient determination step is as follows:

[0031] Step 1: Initialize weight coefficients: α1, α2, α3, α4 and α5;

[0032] Step 2: Calculate the comprehensive evaluation index F under the initialization weight coefficient;

[0033] The third step is to calculate the partial derivative F′ of the objective function with respect to each weight coefficient:

[0034]

[0035] In the formula, α i is the weight coefficient, and i=1,2,3,4,5;

[0036] Step 4: Update the weight coefficient using the gradient descent formula:

[0037]

[0038] In the formula, η represents the learning rate;

[0039] Step 5: Repeat steps 1 to 4 until convergence or the maximum number of iterations is reached;

[0040] Step 6: Output α' after iteration i as the final weight coefficient.

[0041] Preferably, step S5 specifically includes the following steps:

[0042] S51, applying an electric field to the coal seam using an electric field applying device according to the initial parameters of the electric field, and monitoring the stress concentration factor, strain, and changes in the length and width of the cracks in the coal seam in real time;

[0043] S52, based on the real-time collected stress concentration factor, strain, and crack length and width changes, the electric field parameters are dynamically adjusted based on the adaptive control algorithm:

[0044]

[0045] In the formula, α (t+1) represents the electric field parameter at the t+1th iteration; α (t) represents the electric field parameter at the tth iteration; K p , K i and K d represent proportional gain, integral gain and differential gain respectively; e(t) represents the difference between the current fracture path and the optimal fracture path;

[0046] S53, evaluating the crack expansion situation, determining whether the crack grows along the optimal fracture path, if so, maintaining the current electric field parameters, otherwise returning to step S52.

[0047] Therefore, the present invention adopts the above-mentioned electric field fracturing coal seam permeability enhancement gas extraction method, which has the following beneficial effects:

[0048] 1. Multi-source data fusion: Through core samples, geological radar, seismic waves and borehole measurements, a variety of key data including coal seam lithology, physical properties, boundaries and internal structure, initial stress, etc. are obtained, providing a basis for subsequent processing;

[0049] 2. Introduce comprehensive evaluation indicators, consider multiple factors such as crack length, width, density, stress concentration factor and rock integrity, continuously optimize weight coefficients through gradient descent method, and finally determine the optimal fracturing path, making the results more realistic;

[0050] 3. Adopting adaptive control algorithm to fine-tune the electric field parameters makes the whole process more intelligent and flexible, and can better cope with the uncertainty under complex geological conditions;

[0051] 4. Through real-time monitoring of stress, strain and crack expansion in the fracture-causing area, problems can be discovered in a timely manner and corresponding adjustments can be made, thus improving the safety and success rate of the operation.

[0052] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 The present invention is a flow chart of a method for gas extraction by electric field fracturing coal seam permeability enhancement. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical scheme and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention are further described in detail in combination with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not used to limit the embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions.

[0055] It should be noted that the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or server that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units that are not explicitly listed or inherent to these processes, methods, products or devices.

[0056] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0057] like Figure 1 As shown, a method for gas extraction by electric field fracturing coal seam permeability enhancement comprises the following steps:

[0058] S1. On-site investigation: Collect coal seam lithology data, physical property data, boundary and internal structure data, initial stress and historical fracturing experimental data;

[0059] In step S1, a core sample of the coal seam is taken to obtain coal seam lithology data;

[0060] Use geological radar to transmit electromagnetic waves to the coal seam, and then obtain the physical property data of the coal seam through the received reflected waves. The physical property data includes the dielectric constant, thickness, density and conductivity of the coal seam.

[0061] Using a seismic source to apply a seismic source to the coal seam, and then determining the boundary and internal structure data of the coal seam through the received seismic waves, and the internal structure data includes faults, joints, cracks and folds;

[0062] The initial stress field of the coal seam is obtained by borehole measurement.

[0063] S2, modeling: importing the coal seam lithology data, physical property data, boundary and internal structure data, initial stress and historical fracturing experimental data obtained in step S1 into a three-dimensional modeling software to generate a three-dimensional geological model;

[0064] The three-dimensional modeling software described in step S2 is FLAC3D.

[0065] S3, simulation experiment: based on the three-dimensional geological model generated in step S2, use numerical simulation software to perform electric field fracturing simulation until the optimal fracturing path is obtained, and take the electric field parameters corresponding to the optimal fracturing path as the initial electric field parameters;

[0066] The numerical simulation software described in step S3 is ANSYS or COMSOL;

[0067] And step S3 specifically includes the following steps:

[0068] S31, setting initial conditions: setting the initial electric field strength E0 and the initial frequency f0, and determining the boundary conditions of the simulation area;

[0069] S32, define the following multi-objective function with the comprehensive evaluation index F as the optimization target:

[0070] F=α1×L+α2×W+α3×D+α4×K t +α5×RII (1);

[0071] Where L is the crack length, in m; W is the crack width, in m; D is the crack density, in m; K is the crack width, in m. t represents stress concentration factor; RII represents rock integrity index; α1, α2, α3, α4 and α5 are respectively crack length L, crack width W, crack density D, stress concentration factor K t and the weight coefficient corresponding to the rock integrity index RII;

[0072] In step S32, the weight coefficient determination steps are as follows:

[0073] Step 1: Initialize weight coefficients: α1, α2, α3, α4 and α5;

[0074] Step 2: Calculate the comprehensive evaluation index F under the initialization weight coefficient;

[0075] The third step is to calculate the partial derivative F of the objective function with respect to each weight coefficient:

[0076]

[0077] In the formula, α i is the weight coefficient, and i=1, 2, 3, 4, 5;

[0078] Step 4: Update the weight coefficient using the gradient descent formula:

[0079]

[0080] In the formula, η represents the learning rate;

[0081] Step 5: Repeat steps 1 to 4 until convergence or the maximum number of iterations is reached;

[0082] Step 6: Output α′ after iteration i as the final weight coefficient.

[0083] S33, numerical iteration simulation: change the electric field strength E or frequency f to perform numerical iteration simulation, and record the electric field strength E, frequency f, crack length L, crack width W, crack density D, stress concentration factor K of each simulation t and rock integrity index RII, until the change of the comprehensive evaluation index F is less than the set threshold, the iteration is stopped;

[0084] S34, determining the optimal fracture path: selecting the maximum comprehensive evaluation index F from the iterative results obtained in step S33 as the optimal fracture path;

[0085] S35. Determine the electric field strength E and frequency f corresponding to the maximum comprehensive evaluation index F as the initial electric field strength E0 and the initial frequency f0, respectively.

[0086] S4, data monitoring: based on the three-dimensional coal seam image generated in step S2, distributed stress sensors, strain sensors and displacement sensors are arranged in the fracture area to monitor the stress concentration factor, strain and changes in the length and width of the fracture in the coal seam;

[0087] S5, applying the initial electric field parameters determined in step S3 to the coal seam, and dynamically adjusting the electric field parameters based on the crack width, stress change and temperature distribution collected in real time in step S4, so as to ensure that the optimal fracturing path described in step S3 is achieved;

[0088] Step S5 specifically includes the following steps:

[0089] S51, applying an electric field to the coal seam using an electric field applying device according to the initial parameters of the electric field, and monitoring the stress concentration factor, strain, and changes in the length and width of the cracks in the coal seam in real time;

[0090] S52, based on the real-time collected stress concentration factor, strain, and crack length and width changes, the electric field parameters are dynamically adjusted based on the adaptive control algorithm:

[0091]

[0092] In the formula, α (t+1) represents the electric field parameter at the t+1th iteration; α (t) represents the electric field parameter at the tth iteration; K p , K i and K drepresent proportional gain, integral gain and differential gain respectively; e(t) represents the difference between the current fracture path and the optimal fracture path;

[0093] S53, evaluating the crack expansion situation, determining whether the crack grows along the optimal fracture path, if so, maintaining the current electric field parameters, otherwise returning to step S52.

[0094] S6. Use gas extraction equipment to extract gas at the entrance of the optimal fracturing path.

[0095] Example:

[0096] At a coal mine site, in order to verify the effectiveness of the electric field fracturing coal seam permeability enhancement gas extraction method of the present invention, the following steps were implemented:

[0097] 1. On-site exploration and data collection: By taking core samples from the coal seams, the lithology data of the coal seams, including the hardness and brittleness of the coal, were obtained.

[0098] The geological radar was used to transmit electromagnetic waves to the coal seam and receive the reflected waves to obtain the physical property data of the coal seam, including the dielectric constant, thickness (average 5m), density (average 1.4g / cm 3 ) and conductivity.

[0099] By using a seismic source to apply a seismic source to the coal seam and receiving seismic waves, the boundary and internal structure data of the coal seam, including the distribution of faults, joints, cracks and folds, are determined.

[0100] Through borehole measurement, the initial stress field data of the coal seam was obtained, providing a basis for subsequent simulation experiments.

[0101] 2. Construction of three-dimensional geological model: All data obtained from the on-site survey were imported into the FLAC3D three-dimensional modeling software to generate a high-precision three-dimensional geological model, which provided an accurate physical basis for electric field fracturing simulation.

[0102] 3. Electric field fracturing simulation and optimization: In the ANSYS numerical simulation software, electric field fracturing simulation was carried out based on the generated three-dimensional geological model. At the same time, the initial electric field strength and frequency were set, and the boundary conditions of the simulation area were determined.

[0103] Through multi-objective function optimization, the optimal fracturing path and the corresponding initial electric field parameters were determined by comprehensively considering the crack length, crack width, crack density, stress concentration factor (reduced to 0.5 on average) and rock integrity index.

[0104] 4. Data monitoring and electric field parameter adjustment: Distributed stress sensors, strain sensors and displacement sensors are arranged in the fracture area to monitor the stress concentration factor, strain, and changes in crack length and width in the coal seam in real time.

[0105] 5. According to the optimized initial parameters of the electric field, use the electric field application equipment to apply the electric field to the coal seam, and dynamically adjust the electric field parameters according to the real-time monitoring data to ensure that the cracks grow along the optimal fracture path.

[0106] 6. Gas extraction: Gas extraction equipment was arranged at the entrance of the optimal fracturing path, and gas extraction operations were carried out.

[0107] The average length of the cracks obtained by the above method reached 25m, which effectively increased the permeability of the coal seam. The average width of the cracks was 0.025m, which provided a good channel for gas extraction. And the crack density of the coal seam after fracturing increased to an average of 5 / m, further improving the permeability of the coal seam.

[0108] Stress concentration factor: Through electric field fracturing, the stress concentration factor of the coal seam is reduced to 0.5 on average, effectively alleviating the stress concentration phenomenon of the coal seam.

[0109] Gas extraction efficiency: Compared with coal seams that have not been subjected to electric field fracturing, the gas extraction efficiency has increased by more than 55%, significantly improving the safety production level of coal mines.

[0110] In summary, the implementation of the electric field induced fracturing coal seam permeability enhancement and gas extraction method not only effectively increases the permeability and permeability of the coal seam, but also significantly improves the gas extraction efficiency, providing a strong guarantee for the safe production of coal mines.

[0111] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for gas extraction by electric field fracturing coal seam permeability enhancement, characterized in that: The following steps are involved: S1. On-site investigation: Collect coal seam lithology data, physical property data, boundary and internal structure data, initial stress and historical fracturing experimental data; S2, modeling: importing the coal seam lithology data, physical property data, boundary and internal structure data, initial stress and historical fracturing experimental data obtained in step S1 into a three-dimensional modeling software to generate a three-dimensional geological model; S3, simulation experiment: based on the three-dimensional geological model generated in step S2, use numerical simulation software to perform electric field fracturing simulation until the optimal fracturing path is obtained, and take the electric field parameters corresponding to the optimal fracturing path as the initial electric field parameters; S4, data monitoring: based on the three-dimensional coal seam image generated in step S2, distributed stress sensors, strain sensors and displacement sensors are arranged in the fracture area to monitor the stress concentration factor, strain and changes in the length and width of the cracks in the coal seam; S5, applying the initial electric field parameters determined in step S3 to the coal seam, and dynamically adjusting the electric field parameters based on the crack width, stress change and temperature distribution collected in real time in step S4, so as to ensure that the optimal fracturing path described in step S3 is achieved; S6. Use gas extraction equipment to extract gas at the entrance of the optimal fracturing path.

2. The method for gas extraction by electric field fracturing coal seam permeability enhancement according to claim 1, characterized in that: In step S1, a core sample of a coal seam is taken to obtain lithology data of the coal seam; Use geological radar to transmit electromagnetic waves to the coal seam, and then obtain the physical property data of the coal seam through the received reflected waves. The physical property data includes the dielectric constant, thickness, density and conductivity of the coal seam. Using a seismic source to apply a seismic source to the coal seam, and then determining the boundary and internal structure data of the coal seam through the received seismic waves, and the internal structure data includes faults, joints, cracks and folds; The initial stress field of the coal seam is obtained by borehole measurement.

3. The method for gas extraction by electric field fracturing coal seam permeability enhancement according to claim 2, characterized in that: The three-dimensional modeling software described in step S2 is FLAC3D.

4. The method for gas extraction by electric field fracturing coal seam permeability enhancement according to claim 3, characterized in that: The numerical simulation software described in step S3 is ANSYS or COMSOL; And step S3 specifically includes the following steps: S31, setting initial conditions: setting the initial electric field strength E0 and the initial frequency f0, and determining the boundary conditions of the simulation area; S32, define the following multi-objective function with the comprehensive evaluation index F as the optimization target: F=α1×L+α2×W+α3×D+α4×K t +α5×RII (1); Where L is the crack length, in m; W is the crack width, in m; D is the crack density, in m; K is the crack width, in m. t represents stress concentration factor; RII represents rock integrity index; α1, α2, α3, α4 and α5 are respectively crack length L, crack width W, crack density D, stress concentration factor K t and the weight coefficient corresponding to the rock integrity index RII; S33, numerical iteration simulation: change the electric field strength E or frequency f to perform numerical iteration simulation, and record the electric field strength E, frequency f, crack length L, crack width W, crack density D, stress concentration factor K of each simulation t and rock integrity index RII, until the change of the comprehensive evaluation index F is less than the set threshold, the iteration is stopped; S34, determining the optimal fracture path: selecting the maximum comprehensive evaluation index F from the iterative results obtained in step S33 as the optimal fracture path; S35. Determine the electric field strength E and frequency f corresponding to the maximum comprehensive evaluation index F as the initial electric field strength E0 and the initial frequency f0, respectively.

5. The method for gas extraction by electric field fracturing coal seam permeability enhancement according to claim 4, characterized in that: In step S32, the weight coefficient determination steps are as follows: Step 1: Initialize weight coefficients: α1, α2, α3, α4 and α5; Step 2: Calculate the comprehensive evaluation index F under the initialization weight coefficient; The third step is to calculate the partial derivative F' of the objective function with respect to each weight coefficient: In the formula, α i is the weight coefficient, and i=1,2,3,4,5; Step 4: Update the weight coefficient using the gradient descent formula: In the formula, η represents the learning rate; Step 5: Repeat steps 1 to 4 until convergence or the maximum number of iterations is reached; Step 6: Output α' after iteration i as the final weight coefficient.

6. The method for gas extraction by electric field fracturing coal seam permeability enhancement according to claim 5, characterized in that: Step S5 specifically includes the following steps: S51, applying an electric field to the coal seam using an electric field application device according to initial parameters of the electric field, and monitoring in real time the stress concentration factor, strain, and changes in the length and width of cracks in the coal seam; S52, based on the real-time collected stress concentration factor, strain, and crack length and width changes, the electric field parameters are dynamically adjusted based on the adaptive control algorithm: In the formula, α (t+1) represents the electric field parameter at the t+1th iteration; α (t) represents the electric field parameter at the tth iteration; K p , K i and K d represent proportional gain, integral gain and differential gain respectively; e(t) represents the difference between the current fracture path and the optimal fracture path; S53, evaluating the crack expansion situation, determining whether the crack grows along the optimal fracture path, if so, maintaining the current electric field parameters, otherwise returning to step S52.

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