Coal seam gas extraction method based on horizontal hydraulic slotting and gas injection displacement synergy

Abstract

The present application relates to the technical field of coal mine safety and coalbed gas development, and discloses a coalbed gas extraction method based on synergistic effect of horizontal hydraulic slotting and gas injection displacement, which comprises the following steps: alternately and intervally arranging slotting holes and displacement holes; performing multi-point fixed-point hydraulic slotting along the length direction in the slotting holes to form a string of horizontal slot grooves to build pressure relief compensation space; performing pulsating gas injection on the displacement holes to apply periodic pressure load; dividing the operation period into a gas injection stage, a stop-injection seepage absorption stage and an extraction stage; collecting gas extraction scalar quantity, gas injection pressure wave, acoustic emission signal and gas injection pump power consumption data; establishing a double-target optimization model and using an improved evolutionary algorithm to dynamically optimize the gas injection time and the extraction time; repeatedly performing gas injection, extraction, data collection and parameter dynamic optimization to form a closed-loop operation mode. The present application realizes the synergistic effect of pressure relief slotting and pulsating displacement, and improves the gas extraction efficiency and safety.

Description

Technical Field

[0001] This invention relates to the field of coal mine safety and coalbed methane development technology, specifically to a coalbed methane extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement. Background Technology

[0002] Currently, various technologies have been developed to improve the gas extraction efficiency of low-permeability coal seams. Hydraulic fracturing technology creates local pressure relief spaces by cutting grooves in the coal seam, but subsequent expansion of the fractures often relies on the coal's own stress or high-energy blasting. The former is slow and has a limited range, while the latter is costly and poses safety hazards. Gas injection displacement technology can replace gas, but in the original coal seam that has not been sufficiently permeable, the injection pressure is difficult to diffuse, easily producing a "gas lock" effect, resulting in a small displacement range and low efficiency. Existing synergistic technologies are mostly instantaneous "hydraulic fracturing + high-energy fracturing" modes, lacking a continuous, controllable, and low-cost synergistic approach. Furthermore, the determination of process parameters often relies on experience or single-factor experiments, making it unable to adapt to the dynamic changes in the coal seam stress-seepage field. Summary of the Invention

[0003] The purpose of this invention is to provide a coal seam gas extraction method based on the synergistic effect of horizontal hydraulic fracturing and gas injection displacement. By constructing a four-level synergistic system of macroscopic pressure relief channel, periodic pressure difference driving fracture expansion, gas replacement, and multi-objective dynamic optimization, a synergistic process is achieved to create pressure relief space by using horizontal hydraulic fracturing and dynamically optimize the pulsed gas injection parameters based on an improved evolutionary algorithm to achieve efficient coal seam gas extraction. This overcomes the shortcomings of existing technologies, such as limited pressure relief range, high fracturing cost, poor parameter matching, and low displacement efficiency.

[0004] To achieve the above objectives, the following technical solution is adopted:

[0005] This invention provides a coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement, comprising the following steps:

[0006] S1: Construct a set of horizontal boreholes in the coal seam, the boreholes including slotted holes and displacement holes arranged at alternating intervals;

[0007] S2: Perform multi-point fixed-point hydraulic cutting operations along the length of the cut hole to form multiple beaded horizontal slots distributed along the borehole axis to construct a pressure relief compensation space.

[0008] S3: Pulsed gas injection is performed on the displacement hole to apply a periodic pressure load to the coal body;

[0009] S4: A complete operation cycle is divided into a gas injection stage, a gas injection stoppage and seepage stage, and an extraction stage. In the gas injection stage, pulsating gas injection is performed through the displacement hole, and in the extraction stage, negative pressure extraction is performed through the slotted hole.

[0010] S5: Collects data on gas extraction volume, gas injection pressure wave, coal acoustic emission signal, and gas injection pump power consumption.

[0011] S6: An optimization model is pre-established with the dual objectives of increasing the pure gas extraction volume and reducing the unit extraction energy consumption. The gas injection duration and extraction duration are used as decision variables. During the operation, based on the collected data, an improved evolutionary algorithm is used to dynamically optimize the decision variables, and the execution parameters for the next cycle are determined according to the optimization results.

[0012] S7: Repeat steps S3 to S5, and repeat the dynamic optimization and parameter distribution process in step S6 according to the preset optimization cycle to form a closed-loop operation mode.

[0013] Furthermore, in step S1, the slotting holes and displacement holes are arranged alternately, and the hole spacing is determined to be 5-8 meters according to the coal seam permeability coefficient. The larger the permeability coefficient, the larger the hole spacing value is. In step S2, the position of the first slotting point is not less than 10 meters away from the hole opening to ensure that all slotting points are located in the working section deeper than the sealing section.

[0014] Furthermore, in step S2, the water jet pressure of the hydraulic slit is 50-80 MPa, the continuous cutting time of each slit point is 5-10 minutes, the width of the formed horizontal groove is 1-3 cm, the radial influence radius is 1.5-2.5 meters, and the distance between adjacent slit points is 3-5 meters.

[0015] Furthermore, in step S3, the injection pressure of the pulsed gas injection is determined based on the original gas pressure of the coal seam: when the original gas pressure of the coal seam is between 0.8 MPa and 1.2 MPa, the injection pressure is a value between 0.8 MPa and a first upper limit value, where the first upper limit value is the smaller of 1.5 MPa and 1.2 times the original gas pressure; when the original gas pressure of the coal seam is below 0.8 MPa, the injection pressure is 0.8 MPa; and the injection pressure does not exceed 80% of the coal seam fracturing pressure, which is obtained through on-site hydraulic fracturing tests or empirical formulas.

[0016] The frequency and duty cycle of the pulsed gas injection are selected based on the permeability coefficient of the coal seam: for low-permeability coal seams with a permeability coefficient below 0.05 m² / (MPa²•d), a frequency of 0.5-1 Hz and a duty cycle of 60%-70% are used; for coal seams with a permeability coefficient between 0.05-0.1 m² / (MPa²•d), a frequency of 2-3 Hz and a duty cycle of 50% are used.

[0017] After completing the fixed-point hydraulic slit cutting operation, the slit hole is converted into an extraction hole and connected to the extraction system. The displacement hole remains connected to the pulsed gas injection device throughout the process. Furthermore, the opening sections of the slit hole and the displacement hole are sealed with high-pressure grouting to a depth of not less than 8 meters.

[0018] Furthermore, in step S4, the initial settings for the duration of the gas injection phase, the duration of the gas injection cessation and absorption phase, and the duration of the extraction phase are 30 minutes, 15 minutes, and 45 minutes, respectively.

[0019] The duration of the infiltration and absorption phase is also set according to the coal seam permeability coefficient: 20 minutes when the permeability coefficient is below 0.05 m² / (MPa²·d), 15 minutes when it is between 0.05 and 0.1 m² / (MPa²·d), and 10 minutes when it is above 0.1 m² / (MPa²·d).

[0020] Furthermore, in step S6, the optimization model takes maximizing the pure gas extraction volume and minimizing the unit extraction energy consumption as optimization objectives. Maximizing the pure gas extraction volume is converted into its reciprocal as the first objective function, and the unit extraction energy consumption is used as the second objective function. The decision variables include the gas injection duration and the extraction duration. The pure gas extraction volume is calculated by multiplying the mixed gas flow rate and the gas concentration on the extraction main pipeline, and the unit extraction energy consumption is calculated by the ratio of the gas injection pump power consumption to the pure gas extraction volume.

[0021] Furthermore, in step S6, the improved evolutionary algorithm employs an improved non-dominated sorting evolutionary algorithm, specifically including:

[0022] Fitness assessment: Calculate the first and second objective function values ​​for each individual based on the optimization model;

[0023] Chaotic initialization: The initial population is generated using Tent mapping, and the decision variables are mapped to their value ranges;

[0024] Non-dominated sorting and crowding distance calculation: The population is stratified using fast non-dominated sorting, and the crowding distance of each individual is calculated.

[0025] Adaptive mutation: The mutation probability is adaptively varied with the degree of population convergence, and the mutation operation is performed using a polynomial mutation operator;

[0026] Decision-making mechanism: Select the elbow point from the final Pareto front as a compromise solution, calculate the curvature of each point on the front, and select the decision variable corresponding to the point with the largest curvature as the execution parameter for the next cycle.

[0027] Furthermore, in the adaptive mutation, the initial mutation probability is 0.2, and the mutation probability decreases adaptively with the exponential decay of the ratio of the standard deviation of the Pareto front crowding distance in the current generation to the standard deviation of the initial generation; the distribution exponent of the polynomial mutation operator is 15; the population size is 20 to 30 individuals, and the number of iterations is 50 to 100 generations.

[0028] The mutation probability adapts adaptively to the degree of population convergence according to the following formula:

[0029]

[0030] in, Let be the adaptive mutation probability of generation t. The initial mutation probability is set to 0.2, and λ is an adjustment coefficient with a value ranging from 0.5 to 2.0. Let be the standard deviation of the Pareto front congestion distance in generation t. The standard deviation of the initial Pareto front crowding distance is given.

[0031] Furthermore, in the decision-making mechanism, the method for selecting the elbow point as a compromise solution from the final Pareto front surface includes:

[0032] The curvature at each point on the leading edge is calculated using the following formula:

[0033]

[0034] in, : Pareto frontier Curvature at a point; , , They represent the first and second digits on the Pareto front. The point, the first The point and the first The first objective function value at each point; , These represent the first and second digits on the Pareto front. The point, the first The second objective function value at each point; : Index of points on the Pareto front surface; ε' is a regularization term used to prevent division by zero;

[0035] The curvature at the boundary point is defined as 0;

[0036] The decision variable corresponding to the point with the greatest curvature is selected as the execution parameter for the next cycle.

[0037] Furthermore, in step S6, the preset optimization cycle is to trigger a parameter optimization cycle every 4 to 6 hours; the value range of the gas injection duration and extraction duration is preset with lower and upper limits based on the coal seam thickness, original gas pressure and permeability coefficient.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) Permeability enhancement effect: Under the conditions of coal seam with original gas pressure ≤1.2MPa and permeability ≤0.1mD, field tests show that compared with the traditional single-hole drilling and extraction method without permeability enhancement, the method of the present invention increases the effective extraction radius from about 1.5m to 4-5m (an increase of about 1.5-2.5 times); compared with the conventional single hydraulic slotting method (without gas injection), the effective extraction radius is increased by about 50%-100%. The specific increase varies with the geological conditions of the coal seam.

[0040] (2) Safety: The upper limit of the gas injection pressure is 1.5MPa, which is far lower than the rupture pressure of conventional hydraulic fracturing, thus avoiding the risk of abnormal gas outburst.

[0041] (3) Synchronous increase in concentration and flow rate: The periodic pressure waves generated by the pulsed gas injection not only drive the expansion of fractures, but also promote the premature desorption of adsorbed gas. The desorbed gas is carried by the injected gas to the extraction hole, thus achieving a simultaneous increase in extraction purity and concentration. In a comparative test at a mine (average value over 30 days), after adopting this method, the extracted gas concentration increased by about 15%-20% compared to the fixed-parameter pulsed gas injection method, and the mixed flow rate increased by about 120%.

[0042] (4) Adaptability: The improved evolutionary algorithm can adapt to the slow time-varying coal seam permeability without frequent human intervention.

[0043] It should be understood that the description in the Summary of the Invention is not intended to limit the key or essential features of the embodiments of the present invention, nor is it intended to restrict the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0044] The above and other features, advantages, and aspects of the various embodiments of the present invention will become more apparent from the accompanying drawings and the following detailed description. The drawings are provided for a better understanding of the invention and are not intended to limit the scope of the invention. In the drawings, the same or similar reference numerals denote the same or similar elements, wherein:

[0045] Figure 1 This is a process flow diagram of a coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement according to an embodiment of the present invention.

[0046] Figure 2This is a flowchart of the improved evolutionary algorithm used in this invention;

[0047] Figure 3 This is a trend diagram of the fracture penetration zone obtained through numerical simulation in this invention. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0050] The core of this invention lies in the coupling effect between the non-uniform stress field created by the "beaded" slits and the pulsating gas pressure. A single-point slit creates a localized stress concentration zone in the coal seam, with the surrounding coal seam in a state of unloading-loading transition. During propagation, the periodic pressure waves generated by pulsating gas injection preferentially enter the stress concentration zone along the high-permeability channels created by the slits, forming a stronger wavefront superposition in this region, which is beneficial for promoting fracture propagation. This synergistic effect of "stress guidance-wavefront focusing" makes the fracture propagation efficiency superior to the simple superposition of slits and pulsating gas injection (the specific effect varies depending on the coal seam properties, verified by numerical simulation and similar physical experiments). Specifically, it includes the following innovations:

[0051] (1) Dual-mode fracturing caused by "static load decompression + periodic differential pressure drive":

[0052] Unlike traditional high-energy instantaneous fracturing modes, this invention uses "horizontal hydraulic cutting" to macroscopically and statically depressurize and create fractures. Subsequently, low-pressure, pulsating gas injection is used to subject the coal fracture network to long-term, dynamic, and periodic pressure impacts, causing the microfractures to gradually expand and connect.

[0053] (2) Coupling of pulsating displacement and differential pressure drive:

[0054] Traditional gas injection is a steady-state injection, mainly utilizing the gas partial pressure displacement effect. This invention employs pulsed gas injection, converting pressure waves into mechanical waves, and applying a "gas wedge" cyclic loading to the coal body that has been depressurized but not completely fractured after fracturing, causing fatigue damage propagation of micro-fractures. This organically couples the two mechanisms of "displacement flow enhancement" and "pressure difference-driven fracturing".

[0055] (3) "Beaded" horizontal compensation space:

[0056] Conventional hydraulic fracturing produces a single slot. This invention proposes forming multiple beaded horizontal slots along the horizontal borehole direction. Combined with the stress distribution of the coal seam, this can induce a three-dimensional stress-relief fracture network over a larger area, providing a bottom-layer seepage channel for subsequent pulsed gas injection.

[0057] (4) Innovation in process flow – “Injection-Permeation-Extraction” cyclical intermittent operation mode:

[0058] This invention establishes a cyclical operation process of "pulsating gas injection - cessation of injection and seepage - intermittent extraction - dynamic optimization", which avoids the drawbacks of "gas lock" and premature formation of dominant channels.

[0059] (5) Innovation in parameter decision-making—multi-objective dynamic optimization based on improved evolutionary algorithms:

[0060] This invention is the first to introduce an improved multi-objective evolutionary algorithm into the field of coal seam gas injection displacement parameter control. Through chaotic initialization, adaptive mutation operators, and elbow rule decision-making, real-time optimization of gas injection / extraction duration is achieved, adapting to the time-varying characteristics of coal seam parameters.

[0061] Example 1:

[0062] Figure 1 This is a process flow diagram of a coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement according to an embodiment of the present invention. Figure 1 This demonstrates a complete closed-loop workflow, from borehole layout, horizontal hydraulic slotting, borehole sealing and orifice switching, intermittent extraction, data acquisition, to dynamic parameter optimization based on an improved evolutionary algorithm and subsequent feedback execution. Specifically, the method includes the following steps:

[0063] S1: Construct a set of horizontal boreholes in the coal seam, the boreholes including slotted holes and displacement holes arranged at alternating intervals;

[0064] Step S1 is used to execute the drilling site and borehole layout.

[0065] A drilling site is set up within the coal seam excavation face or mining roadway, and a set of horizontal boreholes parallel to the dip or strike of the coal seam are constructed. The boreholes are divided into slotting holes and displacement holes, which are arranged alternately. The spacing between the holes is determined based on the coal seam's permeability coefficient, generally 5-8 meters; the higher the permeability coefficient, the larger the spacing. Slotting holes are used for hydraulic slotting and, after slotting, are converted into extraction holes; displacement holes (not slotted) are used entirely as gas injection holes.

[0066] S2: Perform multi-point fixed-point hydraulic cutting operations along the length of the cut hole to form multiple beaded horizontal slots distributed along the borehole axis to construct a pressure relief compensation space.

[0067] Step S2 is used to perform a horizontal hydraulic cut to create a “beaded” pressure relief compensation space.

[0068] A high-pressure hydraulic slitting device is inserted into the slitting hole. The front end of the device is equipped with a rotating nozzle (2-3 mm in diameter, jet angle 60°-90°), which is connected to the ground pump station via a high-pressure hose; the middle of the device is equipped with a hydraulic jaw or packer, which can be anchored at a predetermined position inside the hole.

[0069] The water jet pressure of the hydraulic cutting is controlled at 50-80MPa, and the cutting is continued for 5-10 minutes at each cutting point to form a flat horizontal groove with a width of 1-3cm and a radial influence radius of 1.5-2.5 meters.

[0070] Along the entire length of the slotted hole, the above-mentioned fixed-point slotting operation is performed every 3-5 meters to form multiple bead-like horizontal compensation spaces. Near each slotted point, the effective bearing cross section of the coal body decreases, and stress concentration zones and local pressure relief zones are generated around the slot. This causes the existing micro-fractures in this area to open and extend under the action of in-situ stress, forming a fracture extension zone with the slotted point as the core.

[0071] Spatial relationship between the slit point and the sealing section: The first slit point should be located at least 10 meters away from the borehole opening (8 meters for sealing depth plus a 2-meter safety margin) to ensure that all slit points are located within the working section deeper than the sealing section, thus avoiding blockage by cement slurry.

[0072] S3: Pulsed gas injection is performed on the displacement hole to apply a periodic pressure load to the coal body;

[0073] Step S3 is used to perform pulsating gas injection displacement to achieve periodic pressure difference-driven fracture expansion and gas replacement.

[0074] After completing the fixed-point hydraulic slit cutting operation in step S2, the slit hole is converted into an extraction hole and connected to the extraction system, while the displacement hole remains connected to the pulsed gas injection device throughout the process.

[0075] Sealing and Connection: High-pressure grouting is performed to seal the orifice sections of slotted holes (converted to extraction holes) and displacement holes (gas injection holes), with a sealing depth of not less than 8 meters. Gas injection / extraction flange connections are installed at the orifice openings, dividing the borehole into a sealing section (orifice section) and a working section (deeper than the sealing section). The working section is maintained with open hole or screen completion. After slotting operations are completed, the gas injection valve is closed and the extraction valve is opened to connect to the extraction system for slotted holes; for displacement holes, the extraction valve is closed, maintaining connection to the pulsed gas injection device.

[0076] Pulsating gas injection device: It adopts a pneumatic pulse generator (composed of an electromagnetic reversing valve and a gas storage tank), and uses a PLC to control the switching frequency and duty cycle of the electromagnetic valve to generate a low-pressure pulsating gas flow that approximates a square wave. This gas flow enters the working section of the displacement hole through the gas injection pipeline, applying a periodic pressure load to the coal body.

[0077] Process parameter range:

[0078] (1) This method is applicable to the original gas pressure of coal seams. The coal seam. The injection pressure of the pulsating gas injection is determined based on the original gas pressure of the coal seam: when At that time, the injection pressure was taken as 0.8 MPa to... Values ​​within the interval; when At that time, the gas injection pressure is 0.8 MPa. The gas injection pressure shall not exceed 80% of the coal seam fracturing pressure.

[0079] (2) The frequency and duty cycle of the pulsating gas injection are selected according to the permeability coefficient of the coal seam: For low-permeability coal seams with a permeability coefficient <0.05m² / (MPa²·d), low frequency (0.5~1Hz) and high duty cycle (60%~70%) are recommended; for coal seams with a permeability coefficient between 0.05-0.1m² / (MPa²•d), medium frequency (2~3Hz) and 50% duty cycle are recommended.

[0080] S4: A complete operation cycle is divided into a gas injection stage, a gas injection stoppage and seepage stage, and an extraction stage. In the gas injection stage, pulsating gas injection is performed through the displacement hole, and in the extraction stage, negative pressure extraction is performed through the slotted hole.

[0081] Step S4 is used to implement the division of roles between intermittent extraction and the injection / extraction port. A complete operation cycle is divided into three stages: the injection stage (duration...). (From displacement pulsating gas injection) and the cessation of infusion absorption phase (duration) (Stop gas injection, allowing gas to diffuse in the fracture), extraction phase (duration) Extraction is carried out under negative pressure through the slotted holes and some displacement holes.

[0082] The initial setting value is minute, minute, Minutes, subsequently adjusted dynamically by the optimization algorithm. and The proportion.

[0083] Duration of the infusion and absorption phase Optimization variables can be incorporated, for example, by setting different time intervals based on the coal seam permeability coefficient: 20 minutes when the permeability coefficient is below 0.05 m² / (MPa²·d), 15 minutes when it is between 0.05 and 0.1 m² / (MPa²·d), and 10 minutes when it is above 0.1 m² / (MPa²·d), to simplify implementation.

[0084] S5: Collects data on gas extraction volume, gas injection pressure wave, coal acoustic emission signal, and gas injection pump power consumption.

[0085] Step S5 is used to achieve data collection and feedback.

[0086] Install the following sensors near the drilling site:

[0087] (1) Pipeline gas concentration sensor and flow meter (installed on the main extraction pipeline) to calculate the pure gas extraction volume. .

[0088] (2) Pressure sensor for gas injection pipeline (sampling frequency ≥ 10 Hz) is used to calculate pressure wave attenuation rate.

[0089] (3) Acoustic emission monitoring probes (placed on the surface of the coal seam between boreholes, frequency range 100Hz~10kHz, probe spacing ≤10m, sensitivity threshold ≥40dB) are used to obtain the cumulative ring count. The original signal is bandpass filtered (100Hz~10kHz) and background noise is removed by a threshold to obtain the effective cumulative ring count. .

[0090] (4) Power meter of air injection pump, record real-time power consumption. .

[0091] S6: An optimization model is pre-established with the dual objectives of increasing the pure gas extraction volume and reducing the unit extraction energy consumption. The gas injection duration and extraction duration are used as decision variables. During the operation, based on the collected data, an improved evolutionary algorithm is used to dynamically optimize the decision variables, and the execution parameters for the next cycle are determined according to the optimization results.

[0092] Step S6 is used to achieve dynamic optimization of injection and extraction parameters based on the improved evolutionary algorithm.

[0093] S6.1, Bi-objective optimization model:

[0094] The dual-objective optimization model aims to maximize the pure gas extraction volume and minimize the energy consumption per unit extraction volume. Maximizing the pure gas extraction volume is transformed into its reciprocal as the first objective function, and energy consumption per unit extraction volume is used as the second objective function. The decision variables include the injection duration and the extraction duration. The pure gas extraction volume is calculated by multiplying the mixed gas flow rate and gas concentration on the main extraction pipeline. The energy consumption per unit extraction volume is calculated by the ratio of the power consumption of the injection pump to the pure gas extraction volume. Specifically:

[0095] The system triggers a parameter optimization cycle every 4-6 hours. A dual-objective optimization model is established, with the gas injection duration as the decision variable. With sampling duration The optimization objective is to maximize the extracted net volume while minimizing the energy consumption per unit of extraction, which can be transformed into the following two minimization objectives:

[0096]

[0097]

[0098]

[0099] in, : The objective function vector of a multi-objective optimization problem; The first objective function is the time cost coefficient, in units of... , used to measure the relative time required for gas extraction per unit volume; The second objective function is the energy consumption per unit of extracted pure energy, in units of... x: Decision variable vector; Inflation time; Sampling duration; The pure amount of gas extracted is a function of the decision variable. ; : The flow rate of the mixed gas on the main extraction pipeline; : Gas concentration on the main extraction pipeline; The power consumption of the air injection pump is a function of the decision variable. Obtained by integration from the power meter.

[0100] The two objectives constitute a typical Pareto trade-off: pursuing high output often comes with increased energy consumption, while minimizing energy consumption per unit may limit output.

[0101] S6.2 Improved Evolutionary Algorithm:

[0102] An improved non-dominated sorting evolutionary algorithm is used to solve the problem, requiring low computational resources (2-dimensional decision space, single optimization time does not exceed 5 minutes on a mining industrial control computer with a main frequency of 1GHz or higher). Figure 2 The diagram shows a flowchart of the improved evolutionary algorithm used in this invention, illustrating the entire process from fitness evaluation, chaotic initialization, non-dominated sorting and crowding distance calculation, adaptive mutation, to preserving dominant individuals and finally, the elbow rule decision outputting optimal parameters. The improved evolutionary algorithm specifically includes:

[0103] 1) Fitness assessment: Based on the optimization model, calculate the first objective function value for each individual. and the second objective function value .

[0104] 2) Chaotic Initialization: An initial population is generated using Tent mapping, and decision variables are mapped to their value ranges. The mapping formula is as follows:

[0105]

[0106] in, : The value of the chaotic variable at step k in the Tent mapping; :Tent mapping The chaotic variable values ​​of the step; The lower limit of the decision variable (injection duration or extraction duration); The upper limit of the decision variable (injection duration or extraction duration); : The initial value of the i-th individual and the j-th dimension decision variable; : The lower bound of the j-th dimension decision variable; : The upper bound of the j-th dimension decision variable; : The chaotic value corresponding to the i-th individual and the j-th dimension decision variable.

[0107] The values ​​for gas injection duration and extraction duration are predetermined based on engineering experience, with lower and upper limits set in advance.

[0108] Then map the decision variables to Interval: .

[0109] 3) Non-dominated sorting and crowding distance calculation: The population is stratified using fast non-dominated sorting, and the crowding distance for each individual is calculated. (Crowding distance) The calculation formula is as follows (the boundary individuals are taken as infinity):

[0110]

[0111] in, : The crowding distance of the i-th individual; : Index of the objective function (value is 1 or 2); : after sorting A vector of decision variables for each individual; : after sorting A vector of decision variables for each individual; : No. The function values ​​of each individual on the m-th objective; : No. The function values ​​of each individual on the m-th objective; The maximum value of the m-th objective function in the population; : The minimum value of the m-th objective function in the population.

[0112] 4) Adaptive mutation operator: The mutation probability is adaptively changed with the degree of population convergence, and the mutation operation is performed using a polynomial mutation operator.

[0113] The mutation probability adapts adaptively to the degree of population convergence according to the following formula:

[0114]

[0115] in, : The adaptive mutation probability of generation t; Initial mutation probability, with a value of 0.2; Adjustment coefficient, recommended value 1.0, range 0.5~2.0; For the first Standard deviation of the Pareto front congestion distance; Standard deviation of the initial Pareto front crowding distance.

[0116] The mutation probability adaptively decreases as the ratio of the standard deviation of the Pareto front crowding distance in the current generation to the standard deviation in the initial generation decreases exponentially.

[0117] The mutation operation uses polynomial mutation:

[0118]

[0119]

[0120] In the formula, : No. Individual, the first The new values ​​of the decision variables after mutation; : No. Individual, the first The values ​​of the decision variables before mutation; : Step scaling factor for polynomial variation; : The upper limit of the decision variable in the 3rd dimension; : The lower bound of the decision variable in the 3rd dimension; : No. Uniformly distributed random numbers in the interval [0,1] of dimension; : Distribution index of polynomial variation, recommended value 15, range 10-20.

[0121] 5) Population size and number of iterations: The population size is 20-30 individuals, and convergence can be achieved after 50-100 iterations.

[0122] 6) Decision-making mechanism: Select the "elbow point" as a compromise solution from the final Pareto front surface, and calculate the curvature of each point on the front surface. (Add regularization term) Protection against zero):

[0123]

[0124] in, : The curvature of the i-th point on the Pareto front surface; : Regularization term, with values ​​of , used to prevent division by zero; : Pareto frontier The first objective function value at each point; The first objective function value at the i-th point on the Pareto front. : Pareto frontier The first objective function value at each point; : Pareto frontier The second objective function value at each point; : Pareto frontier The second objective function value at each point; : Index of points on the Pareto front surface, boundary points or The curvature is defined as 0; The total number of points on the Pareto front; : Optimal decision variable vector; Optimal gas injection duration; Optimal sampling duration.

[0125] choose The decision variable corresponding to the largest point This parameter serves as the execution parameter for the next cycle. The system automatically sends the parameter to the PLC for execution, thus initiating the subsequent cycle.

[0126] S7: Repeat steps S3 to S5, and repeat the dynamic optimization and parameter distribution process in step S6 according to the preset optimization cycle to form a closed-loop operation mode.

[0127] Step S7 is used to implement the cyclic operation.

[0128] Repeat steps S3 to S5, and repeat the dynamic optimization and parameter distribution process in step S6 according to the preset optimization cycle to form a closed-loop operation mode of "pulsating gas injection - data acquisition - dynamic optimization - intermittent extraction".

[0129] To verify the synergistic effect mechanism of this invention, the displacement of coal and rock mass in the numerical simulation model of horizontal hydraulic cut was calculated using PFC2D software. The displacement trend of coal and rock mass in the horizontal hydraulic cut was obtained, as follows: Figure 3 As shown. From Figure 3 It can be seen that the displacement of the coal and rock mass around the beaded grooves is regularly distributed, and an obvious stress release zone is formed near the cutting point, allowing the fractures to be effectively connected.

[0130] Example 2:

[0131] 1. Low to medium gas pressure, relatively small inclination angle:

[0132] A certain mine The coal seam is buried at a depth of 426 m, with an initial gas pressure of 1.1 MPa and a permeability coefficient of [missing information]. The inclination angle is 7°. Drilling parameters: 5 horizontal holes (3 slotted holes, 2 displacement holes), hole depth 80 m, hole diameter 113 mm, slotted hole and displacement hole spacing 6 m. Slotting pressure 70 MPa, slot spacing 4 m, first slotting point 12 m from the hole opening. Sealing length 10 m. Initial pulsation frequency 2 Hz, duty cycle... The injection pressure was 1.2 MPa. An improved algorithm was run with a population size of 30 and 100 generations. After optimization minute, minute, Minutes (fixed according to permeability coefficient). Average gas concentration after 30 days of continuous sampling. The effective extraction radius reached 4.2 m. Compared with the control borehole (single-hole extraction) under the same geological conditions without permeability enhancement, the effective extraction radius increased by approximately 1.8 times.

[0133] 2. Lower gas pressure, larger inclination angle:

[0134] No. 3 coal seam in a certain mine, dip angle Thickness 2.2 m, original gas pressure 0.9 MPa, air permeability coefficient The slit spacing was adjusted to 3.5 m, the injection pressure to 1.0 MPa, the pulsation frequency to 1.5 Hz, and the duty cycle to... The algorithm optimization result is as follows: minute, 35 minutes Minutes. The extraction radius reached 5.1 m, with a peak gas concentration of 78%. Compared to a single well in the same coal seam without permeability enhancement, the effective extraction radius increased by approximately 2.4 times.

[0135] 3. High gas pressure, close to the applicable upper limit:

[0136] The No. 5 coal seam in a certain mine has an original gas pressure of 1.15 MPa and a permeability coefficient of... (Low permeability). Based on the applicable conditions, the injection pressure is taken as 1.38 MPa. The pulsation frequency is set to a low frequency of 0.8 Hz with a duty cycle of 65%. After algorithm optimization... minute, minute, Minutes (based on low permeability rating). Average gas concentration after 30 days of extraction was 55%, with an effective extraction radius of 3.8m. This represents an improvement of approximately 1.5 times compared to a single well without permeability enhancement. This example demonstrates that the method remains effective at the boundaries of its applicable range, but the effect is slightly lower than in medium-pressure coal seams.

[0137] Parameter adjustment principles: When the coal seam thickness is small or the dip angle is large, appropriately reduce the slot spacing (3-5m); when the original gas pressure is low, take the lower limit of the gas injection pressure; the lower the permeability coefficient, the lower the pulsation frequency (0.5-1Hz), the higher the duty cycle (60%-70%), and the higher the infiltration time (20 minutes). The algorithm will automatically adjust. and In response to the aforementioned changes.

[0138] Feasibility Study: The equipment required for this invention consists of existing or customizable equipment found in coal mines: a high-pressure hydraulic slitting device (an industrial prototype already exists), a pneumatic pulse generator (explosion-proof solenoid valve + air tank), an acoustic emission monitoring system (intrinsically safe for mining), and a PLC control cabinet. The sealing process uses conventional cement slurry or chemical slurry, and the orifice flange interface is a standard mining connector. The algorithm program can be embedded in a mining data acquisition station or ground server, communicating with the PLC via the Modbus protocol. The entire method has been verified through numerical simulation and similar physical experiments, and is ready for field implementation.

[0139] In summary, this invention presents a coal seam gas extraction method based on the synergistic effect of horizontal hydraulic fracturing and gas injection displacement. It constructs a four-level synergistic system: horizontal hydraulic fracturing creates a beaded pressure relief space; low-pressure pulsating gas injection achieves periodic pressure difference-driven fracture propagation; gas replacement enhances flow; and multi-objective dynamic parameter optimization based on an improved evolutionary algorithm. Unlike the instantaneous action mode of traditional high-energy fracturing, this invention employs a gradual fracturing mechanism coupled with static load pressure relief and dynamic pressure waves, organically integrating displacement enhancement and pressure difference-driven fracturing. Furthermore, it introduces for the first time an improved non-dominated sorting evolutionary algorithm into the field of coal seam gas injection parameter control, achieving adaptive optimization of injection / extraction duration. The improved evolutionary algorithm can adapt to the slow time-varying nature of coal seam permeability, requiring no frequent manual intervention and possessing good field adaptability.

[0140] The above description is only a preferred embodiment of the present invention and is used only to explain the present invention. It is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0141] It should also be noted that, in the embodiments of this application, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0142] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined in the embodiments of this application may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown in this application, but is to be accorded the widest scope consistent with the principles and novel features disclosed in the embodiments of this application.

Claims

1. A method for coal seam gas extraction based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement, characterized in that, Includes the following steps: S1: Construct a set of horizontal boreholes in the coal seam, the boreholes including slotted holes and displacement holes arranged at alternating intervals; S2: Perform multi-point fixed-point hydraulic cutting operations along the length of the cut hole to form multiple beaded horizontal slots distributed along the borehole axis to construct a pressure relief compensation space. S3: Pulsed gas injection is performed on the displacement hole to apply a periodic pressure load to the coal body; S4: A complete operation cycle is divided into a gas injection stage, a gas injection stoppage and seepage stage, and an extraction stage. In the gas injection stage, pulsating gas injection is performed through the displacement hole, and in the extraction stage, negative pressure extraction is performed through the slotted hole. S5: Collects data on gas extraction volume, gas injection pressure wave, coal acoustic emission signal, and gas injection pump power consumption. S6: An optimization model is pre-established with the dual objectives of increasing the pure gas extraction volume and reducing the unit extraction energy consumption. The gas injection duration and extraction duration are used as decision variables. During the operation, based on the collected data, an improved evolutionary algorithm is used to dynamically optimize the decision variables, and the execution parameters for the next cycle are determined according to the optimization results. The optimization model aims to maximize the pure gas extraction volume and minimize the energy consumption per unit extraction volume. Maximizing the pure gas extraction volume is converted into its reciprocal as the first objective function, and the energy consumption per unit extraction volume is used as the second objective function. The improved evolutionary algorithm employs an improved non-dominated sorting evolutionary algorithm, specifically including: Fitness assessment: Calculate the first and second objective function values ​​for each individual based on the optimization model; Chaotic initialization: The initial population is generated using Tent mapping, and the decision variables are mapped to their value ranges; Non-dominated sorting and crowding distance calculation: The population is stratified using fast non-dominated sorting, and the crowding distance of each individual is calculated. Adaptive mutation: The mutation probability is adaptively varied with the degree of population convergence, and a polynomial mutation operator is used to perform the mutation operation. Decision-making mechanism: Select the elbow point as a compromise solution from the final Pareto front surface, calculate the curvature of each point on the front surface, and select the decision variable corresponding to the point with the largest curvature as the execution parameter for the next cycle; S7: Repeat steps S3 to S5, and repeat the dynamic optimization and parameter distribution process in step S6 according to the preset optimization cycle to form a closed-loop operation mode.

2. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In step S1, the slotting holes and displacement holes are arranged alternately, and the hole spacing is determined to be 5-8 meters according to the coal seam permeability coefficient. The larger the permeability coefficient, the larger the hole spacing value is. In step S2, the position of the first slotting point is not less than 10 meters away from the hole opening to ensure that all slotting points are located in the working section deeper than the sealing section.

3. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In step S2, the water jet pressure of the hydraulic slit is 50-80 MPa, the continuous cutting time of each slit point is 5-10 minutes, the width of the formed horizontal groove is 1-3 cm, the radial influence radius is 1.5-2.5 meters, and the distance between adjacent slit points is 3-5 meters.

4. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In step S3, the injection pressure of the pulsed gas injection is determined based on the original gas pressure of the coal seam: when the original gas pressure of the coal seam is between 0.8 MPa and 1.2 MPa, the injection pressure is a value between 0.8 MPa and a first upper limit value, where the first upper limit value is the smaller of 1.5 MPa and 1.2 times the original gas pressure; when the original gas pressure of the coal seam is below 0.8 MPa, the injection pressure is 0.8 MPa; and the injection pressure does not exceed 80% of the coal seam fracturing pressure, which is obtained through on-site hydraulic fracturing tests or empirical formulas. The frequency and duty cycle of the pulsed gas injection are selected based on the permeability coefficient of the coal seam: for low-permeability coal seams with a permeability coefficient below 0.05 m² / (MPa²•d), a frequency of 0.5-1 Hz and a duty cycle of 60%-70% are used; for coal seams with a permeability coefficient between 0.05-0.1 m² / (MPa²•d), a frequency of 2-3 Hz and a duty cycle of 50% are used. After completing the fixed-point hydraulic slit cutting operation, the slit hole is converted into an extraction hole and connected to the extraction system. The displacement hole remains connected to the pulsed gas injection device throughout the process. Furthermore, the opening sections of the slit hole and the displacement hole are sealed with high-pressure grouting to a depth of not less than 8 meters.

5. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In step S4, the initial settings for the duration of the gas injection phase, the duration of the gas injection cessation and seepage phase, and the duration of the extraction phase are 30 minutes, 15 minutes, and 45 minutes, respectively. The duration of the infiltration and absorption phase is also set according to the coal seam permeability coefficient: 20 minutes when the permeability coefficient is below 0.05 m² / (MPa²·d), 15 minutes when it is between 0.05 and 0.1 m² / (MPa²·d), and 10 minutes when it is above 0.1 m² / (MPa²·d).

6. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In step S6, the gas extraction volume is calculated by multiplying the mixed gas flow rate and gas concentration on the extraction main pipeline, and the energy consumption per unit extraction volume is calculated by the ratio of the gas injection pump power consumption to the gas extraction volume.

7. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In the adaptive mutation, the initial mutation probability is 0.2, and the mutation probability decreases adaptively with the exponential decay of the ratio of the standard deviation of the Pareto front crowding distance in the current generation to the standard deviation of the initial generation; the distribution exponent of the polynomial mutation operator is 15; the population size is 20 to 30 individuals, and the number of iterations is 50 to 100 generations. The mutation probability adapts adaptively to the degree of population convergence according to the following formula: in, Let be the adaptive mutation probability of generation t. The initial mutation probability is set to 0.2, and λ is an adjustment coefficient with a value ranging from 0.5 to 2.

0. Let be the standard deviation of the Pareto front congestion distance in generation t. The standard deviation of the initial Pareto front crowding distance is given.

8. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 7, characterized in that, The decision-making mechanism includes methods for selecting the elbow point as a compromise solution from the final Pareto front surface, including: The curvature at each point on the leading edge is calculated using the following formula: in, : Pareto frontier Curvature at a point; , , They represent the first and second digits on the Pareto front. The point, the first The point and the first The first objective function value at each point; , They represent the first and second digits on the Pareto front. The point, the first The second objective function value at each point; : Index of points on the Pareto front surface; ε' is a regularization term used to prevent division by zero; The curvature at the boundary point is defined as 0; The decision variable corresponding to the point with the greatest curvature is selected as the execution parameter for the next cycle.

9. The coal seam gas extraction method based on the synergistic effect of horizontal hydraulic slotting and gas injection displacement as described in claim 1, characterized in that, In step S6, the preset optimization cycle is to trigger a parameter optimization cycle every 4 to 6 hours; the value range of the gas injection duration and extraction duration is preset with lower and upper limits based on the coal seam thickness, original gas pressure and permeability coefficient.

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