A coal-rock interface determination method based on hydraulic fracturing and microseismic monitoring technology

Abstract

The present application belongs to the technical field of intelligent mining of coal mines, and specifically relates to a coal-rock interface determination method based on hydraulic fracturing and microseismic monitoring technology. The method comprises the following steps: S100, basic parameter testing and statistics are performed on the coal rock mass and interface at the top and bottom of the mining face; S200, the construction parameters of underground hydraulic fracturing are calculated according to the basic parameters of the coal rock mass and interface, and the hydraulic fractures can expand along the interface when reaching the coal-rock interface under the construction parameters; S300, the probes of the microseismic monitoring equipment are arranged in the mining area face, and the equipment is debugged; S400, hydraulic fracturing is performed according to the calculated construction parameters; S500, the micro-break vibration signals of the crack expansion feedback are detected in real time during the coal seam hydraulic fracturing process, and the position trajectory of the coal-rock interface of the mining face is determined after the signal analysis and processing. The present application overcomes the problems of the prior art, such as great influence of the coal mining operation environment, high requirement for equipment, low interface recognition accuracy, etc.

Description

Technical Field

[0001] This invention belongs to the field of intelligent coal mining technology, specifically a method for determining the coal-rock interface based on hydraulic fracturing and microseismic monitoring technology. Background Technology

[0002] Intelligent coal mining has been a research hotspot in recent years. To achieve intelligent coal mining in underground longwall faces, it is first necessary to accurately determine the coal cutting trajectory of the coal mining machine or accurately identify the interface between the coal seam and its roof and floor during the longwall mining process. This ensures the safety and cutting efficiency of the coal mining machine's cutting teeth and guarantees that the workload of subsequent coal preparation and the recovery rate of the working face are not increased.

[0003] Currently, mainstream coal-rock interface recognition methods rely on feature information such as images, sounds, pressure signals, current signals, and torque signals, combined with autonomous learning methods, to identify the coal-rock interface and further determine the coal cutting trajectory of the mining machine based on the recognition results. A number of invention patents, including those with publication numbers CN113435294A, CN110424964A, and CN112990169A, all utilize the aforementioned methods and principles for coal-rock interface recognition. However, the complex and variable underground environment of coal mines, the strong heterogeneity of coal and rock strata, and the harsh environment of the longwall mining face easily lead to difficulties in image recognition, low accuracy in sound transmission, and low signal recognition, resulting in significant challenges and inaccuracies in coal-rock interface recognition.

[0004] To address the aforementioned issues, this method proposes a coal-rock interface determination method based on hydraulic fracturing and microseismic monitoring technology. It primarily involves adjusting construction parameters to induce directional initiation and propagation of hydraulic fractures along the interface between the coal seam and its roof and floor. Pre-positioned monitoring probes capture micro-vibration signals during fracture propagation, and analysis of the signal data determines the location of the coal-rock interface in the longwall face, providing the mining machine with a precise cutting trajectory. Summary of the Invention

[0005] The purpose of this invention is to determine the coal-rock interface by integrating downhole hydraulic fracturing technology and microseismic monitoring technology, overcoming the problems of existing methods being greatly affected by the coal mining environment, having high equipment requirements, and having low interface identification accuracy, and providing a coal-rock interface determination method based on hydraulic fracturing and microseismic monitoring technology.

[0006] This invention adopts the following technical solution: a method for determining the coal-rock interface based on hydraulic fracturing and microseismic monitoring technology, comprising the following steps: S100: conducting basic parameter testing and statistics on the coal-rock mass and interface at the longwall face and its roof and floor; S200: calculating the construction parameters for underground hydraulic fracturing based on the statistically obtained basic parameters of the coal-rock mass and interface, under which the hydraulic fracture can extend along the interface when it reaches the coal-rock interface; S300: arranging the probes of the microseismic monitoring equipment in the longwall face and debugging the equipment; S400: performing hydraulic fracturing operations according to the calculated construction parameters; S500: detecting the micro-fracture vibration signal fed back by the fracture extension in real time during the coal seam hydraulic fracturing process, and determining the position trajectory of the coal-rock interface at the longwall face after analyzing and processing the signal.

[0007] In step S100, the basic parameters of the longwall face and its roof and floor coal and rock mass include layer thickness, elastic modulus, Poisson's ratio, tensile and compressive strength, fracture toughness, and the magnitude and direction of in-situ stress. The in-situ stress includes vertical in-situ stress, maximum horizontal principal stress, and minimum horizontal principal stress. The basic parameters of the interface are the shear strength of the coal-roof interface and the coal-floor interface. When the direct roof or direct floor of the coal seam is too thin to test other basic parameters besides the thickness, the shear strength of the interface is calculated as 1.2 to 1.5 times the basic parameters of the coal seam. The shear strength of the interface is calculated as 0.9 to 1.1 times that of the coal seam.

[0008] In step S200, the construction parameters for downhole hydraulic fracturing include the fracturing initiation location, perforation direction and number, injection rate, and the location and spacing of the fracturing holes.

[0009] The initiation point is set in the middle of the longwall face; the perforation direction is perpendicular to the longwall face and dips upward or downward. When the difference in shear strength between the coal-roof interface and the coal-floor interface is within ±10%, and the difference in tensile strength between the roof and floor is within ±20%, bidirectional symmetrical perforation is used; under other conditions, unidirectional perforation is used. For bidirectional perforation, one perforation operation is required at each initiation point; for unidirectional perforation, two perforation operations are required at each initiation point. First, the upper perforation operation is carried out, and the lower perforation operation is carried out at an interval of 20cm. The height of both bidirectional and unidirectional perforations is 10cm.

[0010] The fracturing hole is located at the midpoint of the coal seam thickness within the cut, along the strike of the coal seam. The fracturing fluid can be water-based, and the appropriate injection rate range is calculated using the following inequality:

[0011]

[0012]

[0013]

[0014]

[0015] In the formula, P 0 represents the water pressure inside the well, in MPa; P 1 represents the critical water pressure (MPa) required for the hydraulic fracture to propagate across the layer when it reaches the interface; P 2 represents the critical water pressure required for the hydraulic fracture to propagate along the interface when it reaches the interface, in MPa; q The fracturing fluid discharge rate is m. 3 / s; ρ is the density of the fluid, kg / m³ 3 ; h The crack height is in meters (m). γ The viscosity of the fracturing fluid is expressed in mPa·s. E Let GPa be the elastic modulus of the reservoir. K CC The type I fracture toughness of the coal seam, MPa·m 1 / 2 ; l Let m be the half-length of the crack. K RC The fracture toughness of the top or bottom plate, in MPa·m 1 / 2 ; σ V The vertical ground stress is expressed in MPa. σ h The minimum horizontal ground stress is given in MPa; π / 2- α The dip angle of the coal seam is , in °.

[0016] In step S300, the microseismic monitoring equipment includes a microseismometer and a probe. The probe of the microseismic monitoring equipment is an accelerometer. The probe is deployed in two locations: a fixed probe deployment area and a mobile probe deployment area. The fixed probe deployment area is located in the working face cut and the conveyor belt roadway in the mining area, arranged in two rows, with the upper row 5-10 cm from the roof and the lower row 5-10 cm from the floor. Each row has 5 sets of probes, and each set has 1-3 probes. The mobile probe deployment area is located in the return air roadway and auxiliary transport roadway on both sides 100 m before and after each fracture initiation point, arranged in two rows, with the upper and lower rows 5-10 cm from the roof and floor, respectively. Each row has 10 sets of probes, and each set has 3 probes.

[0017] Step S400 adopts a double-end sealing and retreating fracturing method. Each retreating distance is less than or equal to the length of the working face cut. The fracturing process is as follows: after perforation operation by high-pressure water cutting, the sealing device and hydraulic pressure gauge are installed and adjusted. After pushing the sealing device to the predetermined fracturing initiation position, the sealing device is pressurized to 10MPa. Then, the water pump is turned on to start fracturing until water seeps or gushes out in the roadway roof and floor, coal face or borehole. Following the above steps, the fracturing operation is completed by retreating along the fracturing hole from the working face stop line to the cut.

[0018] The specific process of step S500 is as follows: the peak value, average value and effective value data of each microseismic instrument channel are transmitted to the data analysis and processing software in real time. After the data processing is completed, a three-dimensional scatter plot of the fracture location is output. Furthermore, the coal-rock interface trajectory of the mining face can be obtained by performing surface fitting on the scatter plot.

[0019] Compared with existing technologies, this invention accurately sets the construction parameters for downhole fracturing, causing the hydraulic fractures, after initiation, to extend along the interface as they reach the coal seam and roof / floor plates. It combines this with microseismic monitoring technology to record the rupture signals during the hydraulic fracture propagation process in real time, and finally uses the microseismic monitoring signals to deduce the position and changes of the coal-rock interface. This invention overcomes the problems of existing methods, such as being greatly affected by the coal mining environment, having high equipment requirements, and low interface identification accuracy. Attached Figure Description

[0020] Figure 1 This is a flowchart of the method for determining the coal-rock interface of the present invention;

[0021] Figure 2 This is a schematic diagram of the location of the fracturing borehole and the fracturing initiation point in the method for determining the coal-rock interface of the present invention;

[0022] Figure 3 yes Figure 2 Cross-sectional view;

[0023] Figure 4 This is a schematic diagram showing the position and height of the perforation hole in the bidirectional perforation of the present invention;

[0024] Figure 5 This is a cross-sectional view of the perforation during bidirectional perforation according to the present invention;

[0025] Figure 6 This is a schematic diagram showing the position and height of the perforation hole in the unidirectional perforation of the present invention;

[0026] Figure 7 This is a cross-sectional view of the unidirectional perforation of the present invention;

[0027] Figure 8 This is a schematic diagram of the microseismic monitoring probe deployment area and probe deployment point locations in the coal-rock interface determination method described in this invention.

[0028] Figure 9 This is a schematic diagram showing the locations of the probe deployment points of the present invention;

[0029] In the diagram, 1-conveyor roadway in the mining area, 2-fracture initiation point, 3-fracturing borehole, 4-return airway, 5-auxiliary haulage roadway, 6-cut hole, 7-cut hole coal wall, 8-coal-roof interface, 9-coal-floor interface, 10-bidirectional perforation location, 11-upper perforation location, 12-lower perforation location, 13-fixed probe deployment area, 14-moving and densified probe deployment area, 15-moving and densified probe deployment area, 16-probe deployment point, L is the cut hole length in the working face; h is the coal seam thickness; L1 is the distance between fracture initiation points, and h1 is the perforation height. Detailed Implementation

[0030] This invention provides a method for determining the coal-rock interface based on hydraulic fracturing and microseismic monitoring technology. The principle of this method is as follows: by accurately setting the downhole fracturing construction parameters, the hydraulic fractures, after initiation, extend along the interface when they reach the coal seam and roof / floor plate in their height direction. Microseismic monitoring technology is used to record the rupture signals during the hydraulic fracture propagation process in real time. Finally, the location and changes of the coal-rock interface are deduced based on the microseismic monitoring signals.

[0031] The above-mentioned objectives of this invention are achieved through the following technical solutions.

[0032] like Figure 1 As shown, a method for determining the coal-rock interface based on hydraulic fracturing and microseismic monitoring technology is presented, and the method for determining the coal-rock interface is carried out according to the following steps.

[0033] S100: Conduct basic parameter testing and statistics on the coal and rock mass and interfaces at the longwall face and its roof and floor.

[0034] The basic parameters of the longwall face and its roof and floor coal and rock mass should include layer thickness, elastic modulus, Poisson's ratio, tensile and compressive strength, fracture toughness, and magnitude and direction of in-situ stress. In-situ stress should include vertical in-situ stress, maximum horizontal principal stress, and minimum horizontal principal stress. The basic parameters of the interfaces are mainly the shear strength of the coal-roof interface and the coal-floor interface. When the immediate roof or floor of the coal seam is too thin to test other basic parameters besides thickness, they can be statistically calculated at 1.2 to 1.5 times the basic parameters of the coal seam. The shear strength of the interfaces can be statistically calculated at 0.9 to 1.1 times the shear strength of the coal seam.

[0035] In the specific process, the coal blocks, roof and floor rock blocks, and coal-rock mixtures containing interfaces that have fallen from the longwall face are processed into standard specimens. Then, a high-precision servo press is used to test the elastic modulus, Poisson's ratio, tensile and compressive strength, and fracture toughness. After drilling a borehole perpendicular to the coal face, a rigid borehole deformable gauge is installed in the measuring hole. The ground stress is obtained by measuring the pressure change on the deformable gauge before and after the stress is released.

[0036] S200: The construction parameters for downhole hydraulic fracturing are calculated based on the statistical basic parameters of the coal-rock mass and interface. Under these construction parameters, the hydraulic fractures can extend along the interface when they reach the coal-rock interface.

[0037] Downhole hydraulic fracturing parameters should include fracturing initiation location, perforation direction and number, injection rate, and the location and spacing of fracturing holes. Among these, to ensure symmetry during hydraulic fracture propagation as much as possible, such as... Figure 2 , 3 As shown, the crack initiation point is located in the middle of the working face. The perforation direction is perpendicular to the working face and dips upwards or downwards; when the difference in shear strength between the coal-roof interface and the coal-floor interface is within ±10%, and the difference in tensile strength between the roof and floor is within ±20%, bidirectional symmetrical perforation is used (e.g., Figure 6 , 7 As shown), under other conditions, staggered single-direction perforations are used (such as...). Figure 4 , 5 (As shown); for bidirectional perforation, one perforation operation is required at each fracturing point; for unidirectional perforation, two perforation operations are required at each fracturing point, first the upper perforation, then the lower perforation approximately 20cm apart. The height of both bidirectional and unidirectional perforations is approximately 10cm. Fracturing holes can be drilled along the coal seam strike at the midpoint of the cut, at about 1 / 2 the thickness of the coal seam. The fracturing fluid type can be water-based. The reasonable range of injection rate is calculated based on the following inequality:

[0038]

[0039]

[0040]

[0041]

[0042] In the formula, P 0 represents the water pressure inside the well, in MPa; P 1 represents the critical water pressure (MPa) required for the hydraulic fracture to propagate across the layer when it reaches the interface; P 2 represents the critical water pressure required for the hydraulic fracture to propagate along the interface when it reaches the interface, in MPa; q The fracturing fluid discharge rate is m. 3 / s; ρ is the density of the fluid, kg / m³ 3 ; h The crack height is in meters (m). γ The viscosity of the fracturing fluid is expressed in mPa·s. E Let GPa be the elastic modulus of the reservoir. K CC The type I fracture toughness of the coal seam, MPa·m 1 / 2 ; l Let m be the half-length of the crack.K RC The fracture toughness of the top or bottom plate, in MPa·m 1 / 2 ; σ V The vertical ground stress is expressed in MPa. σ h The minimum horizontal ground stress is given in MPa; π / 2- α The dip angle of the coal seam is , in °.

[0043] Without the aforementioned parameters, hydraulic fractures cannot propagate along the coal-rock interface, making it difficult to achieve the desired effect. There are three propagation modes when hydraulic fractures reach the coal-rock interface: passivation, propagation along the interface, and cross-layer propagation. Construction parameters and geological conditions are the main influencing factors on the fracture propagation mode. Based on the coupling theory of fluid flow and solid elastic deformation, a discrimination model for the propagation path of hydraulic fractures reaching the coal-rock interface is established. By substituting geological parameters into the model, the critical values ​​of the construction parameters corresponding to the three propagation modes can be calculated. Only when the actual construction parameters are set within the aforementioned range will the hydraulic fractures propagate along the interface.

[0044] S300: Deploy probes for underground microseismic monitoring equipment within the working face of the mining area and perform equipment debugging.

[0045] Microseismic monitoring equipment includes a microseismometer and a probe. The probe is an accelerometer with a sensitivity of 1 V / g and a dynamic range of 104 dB. For example... Figure 8 , 9 As shown, the probe deployment locations are divided into a fixed probe deployment area 13 and a mobile probe deployment area 14. The fixed probe deployment area 13 is located within the working face cut 6 and the mining area conveyor belt haulage roadway 1, arranged in two rows, with the upper row 5-10cm from the roof and the lower row 5-10cm from the floor. Each row has 5 sets of probes, with 1-3 probes in each set. The mobile probe deployment area 14 is located within the return air roadway 4 and auxiliary transport roadway 5 on both sides 100m before and after each fracture initiation point 2, arranged in two rows, with the upper and lower rows 5-10cm from the roof and floor respectively. Each row has 10 sets of probes, with 3 probes in each set.

[0046] S400: Perform downhole hydraulic fracturing operations according to the calculated construction parameters.

[0047] Hydraulic fracturing employs a double-end sealing and retreating fracturing method. Each retreating distance is less than or equal to the length of the working face cut. The fracturing process is as follows: after perforation using high-pressure water cutting, a sealing device and a hydraulic pressure gauge are installed and adjusted. The sealing device is pushed to the predetermined fracturing initiation position and pressurized to 10 MPa. Then, water and electricity are supplied to the pump to begin fracturing until water seeps or gushes out from the roadway roof, floor, coal face, or borehole. Following these steps, the fracturing operation is completed sequentially along the fracturing borehole from the working face stop line back to the cut.

[0048] S500: Real-time detection of micro-fracture vibration signals fed back by fracture propagation during coal seam hydraulic fracturing, and determination of the location trajectory of the coal-rock interface in the longwall face after signal analysis and processing.

[0049] Microseismic monitoring equipment mainly includes microseismometers and monitoring probes. The peak, average, and RMS data of each microseismometer channel are transmitted in real-time to the data analysis and processing software built into the microseismic monitoring equipment. The specific processing steps are as follows: The acquired microseismic waves are denoised and fitted using wavelet thresholding or Fourier transform denoising methods. Based on the processing results, the time window size and location are determined. Combining the velocity models of microseismic waves in different media, a velocity-time difference-distance model is established based on the least squares algorithm. The location coordinates of the rupture point can be obtained through matrix operations. After data processing, a three-dimensional scatter plot of the rupture location is output. Further surface fitting of the scatter plot using MATLAB or Origin software yields the coal-rock interface trajectory of the longwall face. The three-dimensional scatter plots of the rupture location are then aggregated and statistically analyzed to form a point cloud. The xyz data of each point in the point cloud is rasterized, and then the mesh or surf functions in MATLAB software are used to plot the surface of the point cloud.

Claims

1. A coal-rock interface determination method based on hydraulic fracturing and microseismic monitoring technology, characterized in that: The method comprises the following steps, S100: basic parameter testing and statistics are performed on the coal rock mass and interface at the top and bottom of the coal seam of the mining face; In step S100, the basic parameters of the coal rock mass at the top and bottom of the mining face include layer thickness, elastic modulus, Poisson's ratio, tensile and compressive strength, fracture toughness, and the size and direction of the ground stress, wherein the ground stress includes the vertical ground stress, the maximum horizontal principal stress and the minimum horizontal principal stress; the basic parameters of the interface are the shear strength of the coal-roof interface (8) and the coal-floor interface (9); when the other basic parameters of the coal seam except the thickness at the direct roof or the direct floor are too thin to be tested, the statistics are performed according to 1.2-1.5 times of the basic parameters of the coal seam; and the shear strength of the interface is calculated according to 0.9-1.1 times of the coal seam; S200: construction parameters of the underground hydraulic fracturing are calculated according to the basic parameters of the coal rock mass and the interface; the hydraulic fracture can expand along the interface when reaching the coal rock interface under the construction parameters; In step S200, the construction parameters of the underground hydraulic fracturing include the cracking position, the perforation direction and number, the injection rate, and the position and spacing of the fracturing hole; The fracturing hole is constructed at the position of 1 / 2 of the coal seam thickness in the middle of the cut, the fracturing fluid is clear water fracturing fluid, and the reasonable range of the injection rate is calculated according to the following inequality: wherein, P 0 is the water pressure in the wellbore, MPa; P 1 is the critical water pressure of trans-layer propagation when the hydraulic fracture reaches the interface, MPa; P 2 is the critical water pressure of interface propagation when the hydraulic fracture reaches the interface, MPa; q is the fracturing fluid discharge, m 3 / s; ρ is the density of the fluid, kg / m 3 ; h is the fracture height, m; S300: the probes of the microseismic monitoring equipment are arranged in the mining face, and the equipment is debugged; is the viscosity of the fracturing fluid, mPa·s; E is the elastic modulus of the reservoir, GPa; K CC is the I-type fracture toughness of the coal seam, MPa·m 1 / 2 ; l is the half-length of the fracture, m; K RC is the fracture toughness of the roof or floor, MPa·m 1 / 2 ; In step S300, the microseismic monitoring equipment includes a microseismic instrument and probes, the probes of the microseismic monitoring equipment are acceleration sensors, the arrangement position of the probes is divided into a fixed probe arrangement area (13) and a mobile probe arrangement encryption area (14), the fixed probe arrangement area (13) is located in the cut (6) and the belt conveyor roadway (1) of the mining face, and is arranged in two rows, the upper row is 5-10 cm away from the roof, the lower row is 5-10 cm away from the floor, 5 groups of probes are arranged in each row, and 1-3 probes are arranged in each group; the mobile probe arrangement encryption area (14) is located in the air return gallery (4) and the auxiliary transportation gallery (5) on both sides of the cracking point (2) of each time, and is arranged in two rows, the upper and lower rows are 5-10 cm away from the roof and the floor respectively, 10 groups of probes are arranged in each row, and 3 probes are arranged in each group; V is the vertical in-situ stress, MPa; S400: the hydraulic fracturing operation is performed according to the calculated construction parameters; h is the horizontal minimum in-situ stress, Mpa; π / 2- α is the coal seam dip angle, °; S500: the micro-breakage vibration signals of the crack expansion feedback are detected in real time during the hydraulic fracturing of the coal seam, the signals are analyzed and processed to determine the position trajectory of the coal rock interface of the mining face. The cracking position (2) is arranged in the middle of the mining face; the perforation direction is perpendicular to the inclination of the mining face and upward or downward; when the difference between the shear strengths of the coal-roof interface (8) and the coal-floor interface (9) is within ±10%, and the difference between the tensile strengths of the roof and the floor is within ±20%, bidirectional symmetrical perforation is adopted; under other conditions, unidirectional perforation is adopted; one perforation operation is performed for each cracking point when bidirectional perforation is adopted, two perforation operations are performed for each cracking point when unidirectional perforation is adopted, the upper perforation operation is performed first, the lower perforation operation is performed at an interval of 20 cm, and the height of bidirectional perforation and unidirectional perforation is 10 cm. ​ ​ 2. The method for determining coal-rock interface based on hydraulic fracturing and microseismic monitoring technology according to claim 1, characterized in that: ​ 3. The method for determining coal-rock interface based on hydraulic fracturing and microseismic monitoring technology according to claim 1, characterized in that: The step S400 adopts a double-end plugging and retreating fracturing method, each time the retreating distance is less than or equal to the length of the working face cut eye, and the fracturing process is as follows: after perforating operation by using high-pressure water slotting, a hole packer and a water pressure instrument are installed and debugged, the hole packer is pushed to a predetermined cracking position, the hole packer is pressurized to 10 MPa, then the water pump is connected to water and power to start fracturing until water seeps out or bursts out from the roadway roof and floor, coal side or borehole, and the fracturing operation is completed by retreating from the working face stop mining line to the cut eye along the fracturing hole according to the above steps.

4. The method for determining coal-rock interface based on hydraulic fracturing and microseismic monitoring technology according to claim 1, characterized in that: The specific process of the step S500 is as follows: the peak value, average value and effective value data of each microseismic instrument channel are transmitted to a data analysis processing software in real time, a three-dimensional scatter plot of the fracture position is output after data processing is completed, and further, the scatter plot is surface-fitted to obtain the coal-rock interface trajectory of the mining working face.

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