Hard coal body weakening method based on static fracturing and hydraulic fracturing cooperation

Abstract

The invention relates to the technical field of safe and efficient mining of coal mines, and discloses a static fracturing and hydraulic fracturing synergy-based hard coal weakening method, which comprises the following steps of: determining a drill hole arrangement mode and a quantity ratio; drilling holes are constructed according to a preset interval meeting the physical penetration requirement, and track control is carried out; the constructed packaging type static expansion device is arranged in the static fracturing drill hole and sealed; water is injected into the hydraulic fracturing drill hole to generate a water guide crack, and communication characteristics are monitored; after confirmation of cut-through, an expansion device is activated through crack water flow, and generated radial expansion stress and a hydraulic fracturing stress field are superposed to form a synergistic weakening effect; and determining coal body parameters to evaluate the weakening degree. The static fracturing units are communicated and activated through the hydraulic fractures, the fractures are supported through static stress to prevent closing, meanwhile, composite stress is overlaid at the tip ends of the fractures to induce secondary fractures, the problems that single hydraulic fracturing fractures are prone to closing, and static fracturing deep crack initiation is difficult are solved, and efficient volume breaking of hard coal is achieved.

Description

Technical Field

[0001] This invention relates to the field of safe and efficient coal mining technology, specifically a method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing. Background Technology

[0002] Hard coal seams are widely distributed in my country, especially in areas subjected to tectonic compression or magmatic thermal metamorphism, where the coal and rock exhibit high strength and strong integrity. In fully mechanized mining operations, the presence of hard coal significantly increases the cutting resistance of the mining machine, leading to severe wear on the cutting teeth, drums, and transmission system, frequent equipment failures, and hindering mining efficiency. Furthermore, hard coal seams are often accompanied by hard roofs. Especially during the initial mining phase, because mining stress has not yet fully manifested and the roof is not prone to natural collapse, large areas of overhanging roofs can easily form. A sudden collapse of the overhang can induce intense mine pressure manifestations, posing a threat to the safety of personnel and equipment underground.

[0003] Currently, to reduce the difficulty of mining hard coal seams, engineering practices typically employ techniques such as deep-hole blasting, hydraulic fracturing, or static fracturing to pre-weaken the coal body. While deep-hole blasting boasts strong fracturing capabilities, the process involves shock waves, vibrations, and toxic gases, significantly disrupting the stability of the surrounding rock and posing high safety risks. Hydraulic fracturing utilizes high-pressure fluids to create fractures within the coal seam, offering relatively good safety. However, the resulting fracture morphology is heavily influenced by the geostress field, often resulting in a relatively simple fracture network. Furthermore, after water injection ceases, the fractures easily reclose due to the compaction caused by high geostress at depth, leading to a decline in the weakening effect over time. Static fracturing utilizes the expansion pressure generated by the hydration reaction of an expansive agent to crush coal and rock, offering advantages such as no vibration and no flyrock. However, the effective implementation of this technology relies on the free surfaces at the rock mass boundaries. Within deep, intact coal seams, the triaxial stress state and lack of free surfaces to provide expansion space make it difficult for the stress generated by the expansive agent to induce initial crack propagation, resulting in difficulty initiating fracturing and poor fracturing effects in deep-hole fracturing scenarios. Therefore, existing single weakening techniques are insufficient to simultaneously achieve effective crack initiation and long-term maintenance of cracks in coal seams under deep, high-stress environments. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing. This method solves the problems in existing technologies where hydraulic fracturing alone produces fractures with limited morphology, which are prone to closure under geostress, and static fracturing alone has a limited range and difficulty in initiating deep fracturing, resulting in insufficient coal body fragmentation and high resistance to subsequent mechanical cutting.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing includes the following steps: determining the layout pattern and quantity ratio of static fracturing boreholes and hydraulic fracturing boreholes based on the geological conditions of the target coal seam; constructing static fracturing boreholes and hydraulic fracturing boreholes in the hard coal body according to the determined layout pattern and quantity ratio, and performing trajectory control at predetermined intervals that meet the physical connectivity requirements; determining the size of a packaged static expansion device and constructing the device, filling the constructed packaged static expansion device into the constructed static fracturing borehole, and sealing it with a sealing device; injecting high-pressure water into the hydraulic fracturing borehole to generate water-conducting fractures, and monitoring the connectivity response characteristics during the water injection process; when the connectivity response characteristics confirm connectivity, activating the packaged static expansion device with the water flow in the water-conducting fractures, causing the packaged static expansion device to generate radial expansion stress, which superimposes with the hydraulic fracturing stress field to form a synergistic weakening effect, calculating the effective superimposed fracturing stress intensity to characterize the synergistic weakening effect; and measuring the coal body parameters after the synergistic weakening effect to assess the degree of weakening of the hard coal body.

[0007] Preferably, when determining the layout pattern, the spatial positions of static fracturing boreholes and hydraulic fracturing boreholes are set as either a bedding-parallel staggered layout pattern or a cut-hole vertical cross-sectional layout pattern. In the bedding-parallel staggered layout pattern, the borehole axis is parallel to the working face advance direction, and they exhibit a periodic staggered arrangement on a section perpendicular to the strike. In the cut-hole vertical cross-sectional layout pattern, the borehole axis is perpendicular to the working face advance direction, and the effective borehole depth is constrained based on the measured width of the cut-hole in the working face.

[0008] Preferably, the calculation steps for the predetermined spacing are as follows: obtain the engineering safety factor, the coal seam heterogeneous geological correction factor, the effective wetting radius of single-hole hydraulic fracturing, and the angle between the direction of the maximum horizontal principal stress and the normal of the line connecting the centers of the two holes, and define the above parameters as the spacing calculation parameter set; construct a fluid conduction constraint inequality that satisfies the fluid conduction requirements based on the spacing calculation parameter set; use the fluid conduction constraint inequality to calculate and limit the distance between the center axis of the static fracturing borehole and the center axis of the adjacent hydraulic fracturing borehole, and confirm the calculated distance as the predetermined spacing.

[0009] Preferably, when determining the quantity ratio, the measured value of the Protodyakonov hardness coefficient of the target coal seam, the benchmark hardness threshold without special reinforcement fracturing, and the hardness grade difference are obtained and defined as the density index calculation parameter set; the parameter set is substituted into the weakened unit density index calculation logic operation, and the quantity ratio of static fracturing boreholes and hydraulic fracturing boreholes per unit volume is determined based on the output weakened unit density index.

[0010] Preferably, the trajectory control steps include: using a directional drilling rig equipped with a measurement-while-drilling system to calculate the real-time actual distance between the static fracturing borehole and the hydraulic fracturing borehole; comparing the real-time actual distance with the predetermined allowable boundary value; and when the comparison result shows that the real-time actual distance is close to the boundary value, initiating the directional deviation correction program to prevent the rock bridge effect.

[0011] Preferably, when constructing the encapsulated static expansion device, it is assembled using an internal high-energy static crushing core and an external directional permeable constraint layer; the particle retention criterion is determined based on the effective particle size and safety screening coefficient of the high-energy static crushing core powder particles; the material of the directional permeable constraint layer is screened so that the average micropore size meets the particle retention criterion, allowing external moisture to penetrate and preventing internal powder from flowing out.

[0012] Preferably, when determining the size of the encapsulated static expansion device, the construction inner diameter of the static fracturing borehole is measured; the construction inner diameter is substituted into the radial filling matching coefficient calculation formula for calculation; and the outer diameter of the cylinder of the encapsulated static expansion device is determined based on the calculation result to achieve geometric matching between the device and the borehole.

[0013] Preferably, when calculating the effective superimposed fracturing stress intensity, the hydraulic coupling coefficient, residual pore water pressure, stress transfer efficiency factor, maximum radial expansion pressure, reagent reaction rate constant, and reaction time are obtained and defined as the potential energy influence factor set at the fracture tip; a calculation formula is constructed based on this factor set to calculate the effective superimposed fracturing stress intensity and quantitatively characterize the destructive potential energy at the fracture tip.

[0014] Preferably, when monitoring connectivity response characteristics, the data on the change of injection pressure over time is recorded; the data is analyzed to see if there is a characteristic in which the instantaneous pressure drop exceeds the set pressure threshold and is accompanied by a synchronous surge in injection flow, and this characteristic is defined as connectivity response characteristics; when this characteristic is identified, it is determined that the hydraulic fracture has physically penetrated the static fracturing borehole; the set pressure threshold is 3 MPa or 10% of the current system pressure.

[0015] Preferably, when assessing the degree of weakening, verification boreholes are drilled at the center and edge of the fracturing area to obtain the original coal body and the Protodyakonov hardness coefficient and the standard amount of drill cuttings per unit borehole length after synergistic weakening treatment, which are defined as the synergistic weakening effectiveness evaluation parameter set; the synergistic weakening effectiveness index is constructed and calculated using this parameter set, and the degree of weakening of the hard coal body is assessed based on this index.

[0016] This invention provides a method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing. It has the following beneficial effects: 1. This invention utilizes the water-conducting fractures formed by hydraulic fracturing as fluid transport channels to deliver the liquid water required for fracturing into the static fracturing borehole. This activates an encapsulated static expansion device, and the continuous radial stress generated by the static expansion acts on the fracture surface, effectively supporting and expanding the formed hydraulic fractures. This overcomes the defect of single hydraulic fracturing fractures easily closing under in-situ stress, and also solves the problem of difficulty in fracturing in deep coal seams due to the lack of free surfaces for static fracturing agents, achieving volumetric fracturing of hard coal seams. Especially during the initial mining phase of the working face, in special conditions where the roof has not yet collapsed and mining-induced stress is lacking, making coal and rock extremely difficult to fracture, this invention utilizes the superposition of static expansion stress and hydraulic stress to artificially construct a high-energy fracturing "stress field," effectively compensating for the lack of natural mining-induced stress. This active weakening mechanism allows hard coal bodies to form a network of interconnected fractures before mechanical cutting, significantly reducing the strength and integrity of the coal body. This greatly reduces the cutting resistance of the coal mining machine, solves the problem of slow advance speed caused by hard coal and rock, and significantly reduces the wear and failure rate of fully mechanized mining equipment such as cutting teeth, rollers, and scraper conveyors, greatly alleviating maintenance pressure.

[0017] 2. The present invention employs an encapsulated static expansion device with a directional permeable constraint layer, combined with specific micropore size screening criteria, to achieve bidirectional control of water infiltration and powder outflow. This structure ensures that the reagent inside the device can absorb water and react without being dispersed or diluted when high-pressure water flows through, thus ensuring the effective accumulation and transmission of expansion stress and improving the cracking stability under high-pressure water environment.

[0018] 3. This invention quantifies the predetermined spacing between static fracturing boreholes and hydraulic fracturing boreholes by constructing a spacing calculation model based on fluid conduction constraint inequalities, and determines the borehole quantity ratio according to coal seam hardness. This parametric design eliminates the blindness of empirical layout, ensures that hydraulic fractures can physically connect to static fracturing units, avoids fracturing blind zones caused by excessive spacing or engineering waste caused by excessive spacing, and ensures the adaptability of the synergistic weakening system under different geological conditions. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall workflow of a method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing, according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the planar arrangement of static fracturing boreholes and hydraulic fracturing boreholes in the present invention, which adopt a parallel and staggered arrangement along the bedding plane. Figure 3 This is a schematic diagram of the planar arrangement of the static fracturing borehole and the hydraulic fracturing borehole in the present invention, which adopts a vertical cross-sectional arrangement mode. Figure 4 This is a schematic diagram of the fluid-structure interaction mechanism and crack evolution in the synergistic cracking process of this invention; Figure 5 This is a pressure-time characteristic curve during the water injection process of the present invention.

[0020] Among them, 1. Static fracturing borehole; 2. Hydraulic fracturing borehole; 3. Coal pillar in roadway; 4. Auxiliary haulage roadway; 5. Main haulage roadway; 6. Coal seam in working face; 7. Return airway; 8. Goaf; 9. Water injection hole; 10. Fracturing charge; 11. Top cutting borehole wall; 12. Static expansion stress; 13. Extended microcracks; 14. Main fracturing fracture; 15. Water injection drill rod; 16. Fracturing borehole wall; 17. Fracturing water. Detailed Implementation

[0021] The technical solutions in 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.

[0022] See attached document Figure 1 This invention provides a method for weakening hard coal seams based on the synergy of static fracturing and hydraulic fracturing. This method relies on a synergistic weakening system constructed within the hard coal seam, primarily comprising a static fracturing unit and a hydraulic fracturing unit. The static fracturing unit generates continuous radial expansion stress, while the hydraulic fracturing unit generates initial macroscopic fractures and provides the liquid water medium required for the reaction. The static fracturing unit includes a static fracturing borehole 1 arranged within the coal seam, an encapsulated static expansion device inserted into the borehole, and a sealing device; the hydraulic fracturing unit includes a hydraulic fracturing borehole 2 arranged within the coal seam and a high-pressure water injection system connected to it.

[0023] In the physical architecture of the synergistic weakening system, static fracturing borehole 1 and hydraulic fracturing borehole 2 are arranged in an alternating pattern in three-dimensional space. There is no direct pipeline connection between the static fracturing unit and the hydraulic fracturing unit; instead, fluid and stress are coupled and interact through the fracture network in the coal and rock medium. The physical mechanism of their synergistic effect is as follows: high-pressure water in hydraulic fracturing borehole 2 generates water-conducting fractures that extend along the direction of maximum principal stress. When these fractures extend to the location of static fracturing borehole 1, the pressurized water within the fractures penetrates the permeable surface of the encapsulated static expansion device, triggering a hydration reaction of the calcium oxide-based material inside the device. The resulting volume expansion is converted into high-intensity radial compressive stress. This stress field is superimposed on the fracture surface generated by hydraulic fracturing, forcing the fractures to further open and generate secondary micro-fractures, thereby achieving volumetric fracturing of the coal body under deep confinement.

[0024] Combination Figure 1As shown, the overall workflow of this specific implementation method includes the following steps: Step S1: Overall Layout Design First, based on the geological exploration data of the target working face, an overall layout design is conducted. This design specifically includes: borehole combination method, borehole spacing and length, water injection pressure, duration, flow rate, and single-hole expansion agent mass. In the borehole combination method design, the relative positional relationship between static fracturing borehole 1 and hydraulic fracturing borehole 2 is determined, such as parallel or perpendicular arrangement. In the borehole spacing design, the effective wetting radius of a single-hole hydraulic fracturing borehole needs to be calculated, and the straight-line distance between static fracturing borehole 1 and the nearest neighboring hydraulic fracturing borehole 2 is set within the range that effectively affects the effective wetting radius of a single-hole hydraulic fracturing borehole to ensure that the fracture can achieve physical penetration. The single-hole expansion agent mass is determined based on the product of the borehole volume and the expansion agent density, and a margin for expansion space must be reserved.

[0025] Step S2: After completing the drilling and cleaning operations, the encapsulated static expansion device is sequentially inserted into the static fracturing borehole 1. The specific operational requirements for this step are: fix the expansion device inside the borehole and seal it using a sealing device. The encapsulated static expansion device uses a permeable material to encapsulate the dried static fracturing agent. This structure allows external moisture to penetrate but prevents internal powder from flowing out. After the device is inserted to the predetermined depth, a sealing device is installed at the opening of the static fracturing borehole 1. The sealing device has a sealing performance that can withstand pressures exceeding 20 MPa and is used to seal the borehole opening, preventing subsequent expansion pressure from leaking axially along the opening and ensuring that the expansion stress mainly acts radially on the borehole wall. During this stage, it is strictly forbidden to inject water into the static fracturing borehole 1; the device must be kept dry and ready for activation. Simultaneously, the hydraulic fracturing borehole 2 is sealed and connected to a high-pressure water injection system.

[0026] Step S3: Inject water into the fracturing borehole. Start the high-pressure water injection system to inject high-pressure water into the hydraulic fracturing borehole 2. As the pressure inside the borehole rises to the coal seam initiation pressure, the coal seam fractures and generates extended fractures. The physical process involved in this step is as follows: After the main fracturing fracture 14 penetrates the static fracturing borehole, the expansion device undergoes a fracturing reaction. The radial stress of the expansion acts on the fracturing borehole and the main fracturing fracture 14, generating a complex fracture network. Specifically, when the fracture penetrates the static fracturing borehole 1, the high-pressure water flow passes through the coating layer of the encapsulated static expansion device and contacts the internal reagents, triggering an exothermic and crystallization expansion reaction. Due to the constraint of the sealing device, the volume expansion is converted into a huge compressive force on the borehole wall. This compressive force acts perpendicularly on the main fracture surface formed by hydraulic fracturing, preventing fracture closure and inducing branch fractures at the tip of the main fracture.

[0027] Step S4: The coal body is fully fractured. Under the continuous superposition of hydraulic impact and static expansion stress 12, the original intact structure inside the hard coal body is destroyed. The static expansion stress 12 continues to increase over several hours to tens of hours, causing the crack network to develop, expand, and intertwine, ultimately leading to the full fracture of the coal body. The fractured coal body has significantly reduced strength and is filled with micro-cracks, providing pre-fracture conditions for subsequent cutting operations by the coal mining machine, reducing cutting resistance and equipment wear, and achieving synergistic weakening of the hard coal body during the initial mining phase.

[0028] See attached document Figure 2 and attached Figure 3 In this embodiment, the construction of the collaborative weakening system begins with the topology design and construction of the borehole array (corresponding to step S1). This process relies on the stress evolution characteristics during the working face mining stage and the geometric parameters of the coal seam, configuring the static fracturing units and hydraulic fracturing units within the system in either a parallel, staggered arrangement along the bedding plane or a vertical, transverse arrangement. These two modes are orthogonal in spatial geometry, but both follow the construction principle of "spatial complementarity between active fracturing sources (hydraulic) and passive fracturing sources (static)," aiming to construct an optimally connected fracture network within the hard coal body using minimal engineering work.

[0029] like Figure 2 As shown in the figure, the mine roadway layout environment for implementing this method is first illustrated, specifically including the coal seam 6 to be mined, the adjacent goaf 8, the return airway 7 located on one side of the coal seam, and the main haulage roadway 5, auxiliary haulage roadway 4, and roadway pillars 3 used for roadway isolation located on the other side of the coal seam. Under this spatial framework, when implementing the in-seam parallel staggered layout mode, the borehole axis is parallel to the working face advance direction and the strike of the return airway 7 and the main haulage roadway 5. In this topology, the static fracturing borehole 1, as a passive fracturing component, and the hydraulic fracturing borehole 2, as an active fracturing component, exhibit a periodic staggered arrangement on a cross-section perpendicular to the strike. The spacing parameters marked in the figure are shown below. , and This characterizes the lateral cutting distance between adjacent heterogeneous boreholes. (Regarding the Protodyakonov hardness coefficient...) For extra-hard coal seams with a hardness greater than 8, in order to overcome the extremely high tensile strength, the system adopts a high-density "single-water, single-static" alternating topology. This means that for each hydraulic fracturing borehole 2, static fracturing boreholes 1 are arranged adjacent to each other on both sides to ensure the continuity of fracture propagation. Regarding the Protodyakonov hardness coefficient... In medium-hard coal seams between 6 and 8 degrees, the combined adjustment mechanism of stress relief types is activated. At this time, a sparse combination array of one water-two static or two water-one static formations can be adopted. This parallel arrangement mode utilizes the trajectory extension advantage of directional long boreholes, enabling strip-shaped pre-weakening of deep coal bodies before the working face advances. This not only effectively blocks the transmission of high stress to the roadway side but also avoids conflicts in time and space between mining operations and borehole construction.

[0030] like Figure 3 As shown, under the same geological background including the working face coal seam 6, goaf 8, and the roadway systems on both sides, a vertical cross-sectional layout pattern is adopted for the initial mining cut area of ​​the working face or special geological structural zones requiring forced roof caving. In this scenario, the axes of static fracturing borehole 1 and hydraulic fracturing borehole 2 are perpendicular to the working face advancement direction and extend along the dip of the coal seam. The spacing parameters under this pattern are... The spacing between rows along the direction of advancement directly determines the distribution frequency of artificial fault zones.

[0031] To prevent drilling operations from damaging the surrounding rock stability of the two roadways and coal pillar 3, such as Figure 3 The "Drill Length Adjustment" parameter shown at the bottom needs to be based on the actual measured width of the cut on the working face. To impose constraints, the effective drilling depth is typically set. satisfy This model utilizes a cantilever beam structure where transverse fractures cut through the coal seam roof, artificially creating a "structural discontinuity" (or "pre-fabricated fracture surface") before the initial pressure, thereby blocking the continuous transmission of stress and solving the dynamic disaster problem caused by the large-area cantilever without collapse of a hard roof. When determining the basic parameters of the above topology, to eliminate the blindness of traditional empirical borehole layout, and especially to ensure that the macroscopic main fracture generated by hydraulic fracturing can physically penetrate the adjacent static fracturing borehole 110, this embodiment uses a critical spacing calculation model based on the fluid-structure interaction mechanism to verify the design parameters. Unlike the conventional equal-spacing arrangement, this system sets a design spacing between the central axis of the static fracturing borehole 110 and the central axis of the adjacent hydraulic fracturing borehole 210. The following fluid conduction constraint inequalities must be satisfied: ; in: This represents the design spacing between the central axis of static fracturing borehole 1 and the central axis of the adjacent hydraulic fracturing borehole 2. This distance is a geometric criterion for determining whether a fracture can physically connect. If the actual spacing exceeds this value, the fracture will not be able to effectively connect the two boreholes.

[0032] This represents the safety factor for the project. It is usually taken as a constant of 0.9 to compensate for calculation deviations caused by construction errors and uncertainties in geological exploration, ensuring that the breakthrough can still be achieved under the most unfavorable conditions.

[0033] This represents the geological correction factor for coal seam heterogeneity. The value range is... Because of the natural bedding and joints within coal seams, hydraulic fractures often do not propagate along ideal straight lines. This coefficient is used to correct for the tortuous fracture path effect caused by the anisotropy of the medium. When the coal seam bedding is well-developed and the orientation is disordered, it is necessary to select the lower limit value of the interval to increase the conductivity redundancy.

[0034] This represents the effective wetting radius of a single-hole hydraulic fracturing system. This parameter characterizes the limit of high-pressure water penetration in the coal and rock matrix under a predetermined injection pressure (typically 20 MPa to 30 MPa). It needs to be obtained through field injection tests, and the typical value ranges from 15 meters to 40 meters.

[0035] This represents the angle between the direction of the maximum horizontal principal stress and the normal to the line connecting the centers of the two holes. The range of values ​​is... The physical meaning is that hydraulic fractures always tend to propagate perpendicular to the direction of minimum principal stress (i.e., along the direction of maximum principal stress). When At that time, the crack propagation path coincides with the inter-hole connection line, resulting in the highest probability of continuity; as... As the crack increases, it deflects, shortening the effective conduction distance.

[0036] In addition, to achieve quantitative control of the weakening intensity, a weakening unit density index is introduced. This is used to determine the borehole mix ratio per unit volume. This index reflects the static expansion source density required to break coal bodies of a specific hardness; its calculation logic is as follows: ; in: This represents the density index of the weakening unit. Specifically, it represents the number of statically fracturing boreholes within the same stress-relief assembly unit. Number of hydraulic fracturing boreholes The ratio; This indicates the number of static fracturing boreholes configured within the same pressure relief assembly unit; This indicates the number of hydraulic fracturing boreholes configured within the same depressurization unit. (Dimensionless); represents the measured value of the Protodyakonov hardness coefficient of the target coal seam; This represents the baseline hardness threshold for which no special fracturing enhancement is required. In this embodiment, the value is 4, representing the lower limit of coal seam hardness requiring synergistic weakening. This represents the hardness grade difference. In this embodiment, the typical value is 2, meaning that for every two additional hardness units, the system needs to increase the dynamic source density configuration by one level. This represents the numerical stability constant. It takes the value of... This is used to prevent the denominator from being zero (e.g., ...). Calculation divergence caused by setting an anomaly; This represents the floor operator.

[0037] ) represents the extreme value constraint function. This ensures that even at lower hardness, the number of statically fracturing boreholes is not less than the number of hydraulically fracturing boreholes, in order to maintain the basic synergistic mechanism.

[0038] After parameter verification based on the above model, the directional drilling and trajectory control stage begins. Given the high sensitivity of this method to borehole spacing, if the borehole trajectory deviates, causing the actual spacing to exceed [a certain value], [further action may be required]. When the critical conduction range is reached, a "rock bridge effect" will occur, causing the static fracturing unit to fail due to the inability to obtain reactive water.

[0039] Therefore, as a preferred implementation method, the construction process must employ a ZDY series fully hydraulic directional drilling rig equipped with a drilling measurement-while-drilling system. Specific control parameters include: the three-dimensional coordinate deviation of the borehole position must not exceed ±50 mm; during drilling, trajectory measurements are performed every 30 to 50 meters to monitor the borehole's inclination and azimuth in real time. A trajectory parallelism deviation threshold of 1.5% is set; once the monitoring system detects that the designed distance (referring here to the distance calculated from actual measurements) between the center axis of the static fracturing borehole 1 and the center axis of the adjacent hydraulic fracturing borehole 2 approaches a certain value... If the boundary value is reached, the directional drilling and correction procedure must be initiated immediately. For long boreholes with a depth exceeding 100 meters, additional measuring points must be added in the bottom area of ​​the borehole to ensure that the entire borehole section is within the effective coverage envelope of the hydraulic fractures.

[0040] In this embodiment, with the completion of the drilling construction process, the construction of the synergistic weakening system enters the assembly and placement stage of the encapsulated static fracturing unit (corresponding to step S2). The core task of this stage is to construct a stable fracturing source that can both sensitively respond to hydraulic signals and resist high-pressure fluid erosion. Given that the static fracturing borehole 1 inside the hard coal body is prone to retaining cooling water or seeping into the formation water during construction, if the agent is directly loaded, it will inevitably lead to premature hydration and failure of the static fracturing agent. Therefore, as a prerequisite for the loading operation, strict pretreatment of removing and drying water inside the borehole must be performed. The operators use a compressed air pipeline with a wind pressure of not less than 0.6 MPa to extend into the bottom of the hole and repeatedly blow the entire section of the static fracturing borehole 1 until there is no obvious water mist in the airflow discharged from the borehole opening. For boreholes with continuous trace water seepage, this embodiment also adopts a process of adding desiccant or using water-absorbing sponges for deep drying to ensure that the relative humidity of the borehole environment is reduced to below the critical reaction threshold.

[0041] After the drilling environment meets the drying requirements, the encapsulated static expansion device is assembled and filled. Addressing the drawback of traditional bulk static fracturing agents being easily dispersed and diluted by high-pressure water flow at the moment of hydraulic fracture penetration, thus losing their expansion capacity, this embodiment employs a special water-permeable and powder-retaining encapsulation structure. The encapsulated static expansion device consists of an internal high-energy static fracturing core and an external directional water-permeable constraint layer. The high-energy static fracturing core is made of modified calcium oxide-based powder and doped with 3% to 5% by mass of a water-reducing agent to regulate the hydration reaction rate. The external directional water-permeable constraint layer is made of high-strength industrial non-woven fabric or a microporous polymer membrane, whose physical properties must simultaneously meet the requirements of high permeability to liquid water and high retention of agent particles.

[0042] To quantify the screening performance of the confinement layer and ensure that water molecules can freely enter the device under pressure while preventing internal drug particles from flowing out under high-pressure turbulence, this embodiment sets the average micropore size of the directional permeable confinement layer material based on the principle of porous media filtration. The following particle retention criteria must be met: ; in: This represents the average micropore size of the directional permeable confinement layer material. This parameter directly determines the fluid penetration resistance and particle interception efficiency.

[0043] This represents the effective particle size of the high-energy statically broken core powder, specifically the grid value corresponding to a cumulative particle distribution of 1%. This parameter characterizes the lower limit of the smallest particle distribution in the agent and is a key control indicator for preventing "fine powder leakage."

[0044] The safety screening coefficient ranges from [0.6, 08]. This coefficient is designed to prevent the risk of particle escape caused by the increase in effective pore size due to elastic deformation of the fabric pores under the instantaneous impact of high-pressure water flow.

[0045] Furthermore, to ensure the encapsulated static expansion device can be successfully inserted into deep holes and, after absorbing water and expanding, tightly adhere to the hole wall to maximize stress transfer efficiency, the device's dimensions must be designed according to the principles of rheology and tribology. The outer diameter of the cylinder of the encapsulated static expansion device... Construction inner diameter of static fracturing borehole A specific geometric matching relationship must be maintained between them, which is determined by radial fill matching coefficients. The characterization and calculation formula are as follows: ; In the formula: This represents the radial fill matching coefficient. The preferred range for this coefficient is [0.75, 0.85]. When... When the pressure is below 0.75, the gap between the device and the borehole wall is too large, and a large amount of stress in the initial stage of expansion will be consumed by the voids, resulting in a significant decrease in the effective fracturing pressure; when When the value is higher than 0.85, it not only increases the frictional resistance during long-distance feeding, but may also cause the device to jam due to deformation of the hole wall; This indicates the outer diameter of the cylinder of the encapsulated static expansion device; Indicates the construction inner diameter of the static fracturing borehole (which must meet the following requirements). .

[0046] During the loading process, multiple encapsulated static expansion devices are connected end to end and sequentially fed into the predetermined depth of the static fracturing borehole 1 using PVC delivery pipes or flexible push rods. The axial coverage of the device within the borehole should spatially overlap with the predetermined fracturing section of the adjacent hydraulic fracturing borehole 2 (belonging to a hydraulic fracturing unit) to ensure effective activation when the fracture penetrates.

[0047] After the grout cartridge is filled, the sealing device is installed immediately. This device is a key component for maintaining a high-pressure environment inside the borehole and preventing stress leakage along the axial direction. In this embodiment, a two-component polyurethane grouting sealer or a mechanical capsule sealer capable of withstanding at least 25 MPa pressure is selected. The sealing depth (i.e., the length of the sealing material inside the borehole) is not less than 8 meters and must avoid the loose zone of the surrounding rock in the roadway (usually within 3-5 meters outside the roadway outline) to ensure that the sealing section is in stable rock strata. After the grout has cured or mechanically tightened, an airtightness test is performed on the sealing device. Low-pressure gas (approximately 0.5 MPa) is injected into the borehole through a pre-embedded pipeline, and the pressure gauge reading is observed to decrease over 10 minutes. If the pressure drop exceeds 5% of the initial value, the sealing is considered to have failed, and secondary grouting or resealing is required. The adequate sealing ensures that when the hydraulic fracture is subsequently established, the high-pressure water entering the static fracturing borehole 1 will be forcibly retained inside the borehole and forced deep into the explosive cartridge. Simultaneously, the expansion stress generated by the reaction of the explosive will be forced to damage the coal seam radially, rather than ejecting from the borehole opening. At this time, the static fracturing unit is in a "ready-to-go" state, containing no liquid water except for natural air inside the borehole, awaiting the fluid activation signal from the hydraulic fracturing unit.

[0048] See attached document Figure 4 In this embodiment, the dynamic implementation process of hydraulic static synergistic fracturing (corresponding to steps S3 and S4) demonstrates the spatiotemporal relay mechanism of fluid dynamics and quasi-static solid mechanics. This process is not physically two isolated steps, but a continuous evolution of energy transformation achieved through fracture penetration.

[0049] After the high-pressure water injection system is activated, high-pressure fluid is injected into the hydraulic fracturing borehole 2 through the injection drill rod 15 via the injection hole 9. As the fluid pressure inside the borehole rapidly increases and exceeds the tensile strength threshold of the hard coal body, the fracturing borehole wall 16 fractures, forming a macroscopic main fracturing fracture 14. Under the guidance of the maximum principal stress, this fracture propagates directionally along a predetermined path. In this stage, the fracturing water 17 acts not only as a fracture-creating medium but also as an energy transfer carrier, driving the fracture tip to approach the adjacent static fracturing borehole 1 at a dynamic fracturing velocity. Thanks to the precise design of the borehole spacing in the preceding steps, the main fracturing fracture 14 can accurately penetrate the coal and rock medium physically, ultimately connecting with the top-cutting borehole wall 11 (i.e., the borehole wall of the static fracturing borehole 1).

[0050] When the leading edge of the main fracturing fracture 14 exposes the static fracturing borehole 1, high-pressure water flow from the fracture immediately rushes into the borehole and comes into contact with the pre-installed encapsulated static expansion device. At this time, the directional permeable constraint layer on the surface of the device allows water to rapidly penetrate into the core material based on the principle of porous media permeation, triggering a violent hydration reaction of the calcium oxide-based material. This process requires no manual intervention or additional detonation device; it is passively activated entirely by the fluid "signal" brought about by the fracture penetration, thereby ensuring the automatic synchronization of the fracturing timing and the hydraulic fracturing process. Figure 4 The connection section clearly illustrates this fluid-structure interaction node: the fracturing water 17 acts as the triggering medium for switching from "hydraulic fracturing" to "static expansion." For example... Figure 4 As shown, the high-energy static fracturing core inside the encapsulated static expansion device is the fracturing charge 10 shown in the figure.

[0051] As the hydration reaction proceeds inside the encapsulated static expansion device, the reagent undergoes irreversible volume expansion (corresponding to...). Figure 4 The agent expands in region I within the confined space, and transforms into high-intensity static expansion stress 12 (corresponding to...). Figure 4 (The thick arrow radiates outwards from the center). This stress field is not an isolated effect, but rather superimposed with the pore water pressure field left over from hydraulic fracturing, forming a composite fracturing effect. To quantify the rock-breaking capacity under this synergistic effect, this embodiment constructs a stress superposition model based on spatiotemporal evolution, employing the effective superposition of fracturing stress intensity. The destructive potential energy at the crack tip is characterized by the following formula: ; in: This represents the effective superimposed stress intensity that causes cracking. This value reflects the stress intensity at a given moment. The total driving stress acting on the fracture surface of the coal seam. When When the tensile strength of the hard coal exceeds that of the coal seam, the crack will continue to propagate.

[0052] This represents the hydraulic coupling coefficient, with a value range of [0.4, 0.6]. This coefficient is used to correct for the effective transmission efficiency of hydraulic pressure at the fracture tip caused by the surface roughness of the fracture and the viscosity of the fluid. This represents the residual pore water pressure remaining in the fracture network. During the static expansion phase after water injection stops, although this pressure field decays over time due to permeation, it still serves as the basic background stress, providing initial support for keeping the fractures open. This represents the stress transfer efficiency factor, with a value range of [0.8, 0.9]. This factor takes into account the contact thermal resistance and stress loss that may be caused by micro-deformation of the hole wall or incomplete filling of the encapsulated static expansion device; This represents the maximum radial expansion pressure. This parameter depends on the chemical formulation of the reagent and the rigid constraints of the borehole, and in this embodiment it is typically 30 MPa to 50 MPa. This represents the reaction rate constant of the reagent. This constant reflects the rate of increase in expansion stress and is usually positively correlated with ambient temperature and the content of water-reducing agent in the reagent. This indicates the reaction time calculated from the moment the hydraulic fracture is breached.

[0053] exist Driven by continuous efforts, such as Figure 4 As shown on the right, the original single fracturing main fracture 14 could not close, and a large number of secondary extended micro-cracks 13 were induced around the top-cut borehole wall 11 (corresponding to...). Figure 4 (The thin blue lines in the image). These micro-fractures develop radially from the static fracturing borehole 1 and intertwine with the main hydraulic fractures, completely destroying the original layered structure of the hard coal body into a fragmented structure. This mechanism of "first creating hydraulic fractures and then static expansion" overcomes the shortcomings of single hydraulic fracturing fractures being singular and easily closed, and also solves the problems of small range and difficulty in initiating single static fracturing, achieving all-round volumetric fracturing of the hard coal body.

[0054] In this embodiment, the monitoring and verification of the synergistic weakening effect constitutes a key link in the closed-loop control, with the core objective of quantitatively evaluating the actual operational effectiveness of the hydraulic static coupling fracturing mechanism within hard coal bodies. This process relies on the data acquisition module integrated into the high-pressure water injection system and subsequent physical exploration methods, achieving full-process coverage from real-time fracturing state diagnosis during step S3 (water injection) to comprehensive assessment of the weakening degree after fracturing is completed in step S4.

[0055] During the water injection operation in step S3, the high-frequency pressure sensor built into the high-pressure water injection system continuously records the change in water injection pressure over time at a millisecond-level sampling frequency, generating a water injection pressure-time characteristic curve. The fluctuation pattern of this curve directly maps the evolution behavior of the underground fracture network in the coal and rock medium. In a normal synergistic fracturing process, the pressure curve usually exhibits obvious staged evolution characteristics: initially, as fluid fills the hydraulic fracturing borehole 2 and micro-gaps, the pressure shows an approximately linear upward trend; when the pressure inside the borehole exceeds the fracturing strength of the hard coal body, the curve shows its first peak drop, which marks the physical formation of the main fracturing fracture 14; subsequently, as the fracture extends deeper into a steady state, the pressure usually remains at a relatively stable fluctuation platform. At this time, if the monitoring system captures an obvious "connection response characteristic" in the pressure curve, that is, an instantaneous pressure drop exceeding 3 MPa or exceeding 10% of the current system pressure, accompanied by a synchronous surge in water injection flow, it indicates that the hydraulic fracture has successfully connected the low flow resistance region, that is, physically connected the static fracturing borehole 1. This characteristic signal is defined as the "connectivity response threshold," which is the direct basis for determining whether the hydraulic fracturing unit and the static fracturing unit have achieved fluid interaction, confirming that the prerequisite for the passive activation of the subsequent encapsulated static expansion device has been met.

[0056] With the end of the static expansion reaction cycle (step S4), in order to accurately quantify the changes in the physical and mechanical properties of the hard coal body after being subjected to the dual effects of hydraulic impact and static expansion, this embodiment constructs verification boreholes at the center and edge of the fractured area, and uses a combination of drill cuttings method and strength testing method for verification. Compared with the original untreated coal body, the coal body after synergistic weakening treatment shows obvious differences in physical response: the drilling resistance during the construction of the verification borehole is significantly reduced, the fluctuation of drilling rig torque tends to be smoother, and the amount of drill cuttings discharged per unit depth is significantly increased; the particle size distribution of the discharged drill cuttings changes from the original coarse and blocky particles to powdery and fine particles, indicating that the integrity structure inside the coal body has been effectively cut and disintegrated by a high-density fracture network.

[0057] To establish a standardized evaluation system for weakening effects, this embodiment, based on the principles of elastoplastic mechanics and rock fracture mechanics, selects two dimensions—hardness attenuation and volume expansion—to construct a synergistic weakening effectiveness index. Calculation Model. This model comprehensively considers the attenuation of the coal body's resistance to deformation and the increase in the degree of macroscopic fracturing, and is used to determine whether the target area meets the pre-splitting requirements of a fully mechanized mining face. Its calculation formula is as follows: ; in: This represents the synergistic weakening effectiveness index. This index is a comprehensive evaluation indicator greater than 0; a higher value indicates a deeper degree of weakening of the coal body. In this embodiment, when... When the synergistic fracturing effect is judged to be "excellent," it meets the requirement of high-efficiency cutting; when When it is judged as "good" If the weakening is insufficient, supplementary crack-inducing measures should be considered; This represents the Protodyakonov hardness coefficient of the original coal seam. This value is measured through sampling boreholes drilled in the unfractured area and reflects the initial firmness of the coal seam. Given that this embodiment targets hard coal seams, it physically satisfies… This ensures the mathematical validity of the denominator; This represents the residual Protodyakonov hardness coefficient after synergistic weakening treatment. This value is obtained through verification drilling within the fracturing influence zone and directly reflects the reduction in coal strength. Data verification is required before calculation; otherwise, measurement errors may lead to inconsistencies. If the point is not effectively weakened, then it is determined that no effective weakening has occurred. ; This parameter represents the standard amount of drill cuttings per unit borehole length of the original coal seam. As a benchmark, it eliminates the influence of drilling rig model and drill bit diameter on the amount of cuttings removed, and physically satisfies... ; This represents the average amount of drill cuttings per unit borehole length after synergistic weakening treatment. An increase in this value reflects the volume expansion and fracturing effect within the coal seam caused by the development of microfractures and stress release.

[0058] Through the above monitoring and calculations, technicians can intuitively grasp the actual operational efficiency of the synergistic weakening system. Field test data shows that, under typical hard coal seam conditions, after applying this system, the water injection pressure curve clearly captures the connectivity signal conforming to the above definition, and the calculated synergistic weakening efficiency index... The values ​​generally remained between 1.6 and 2.2, verifying the significant advantages of hydraulic fracturing and static expansion mechanisms in reducing cutting resistance, increasing lump coal rate, and improving operational safety, thus achieving efficient pre-fracture control of deep and difficult-to-mine coal bodies.

[0059] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing, characterized in that, Includes the following steps: The layout pattern and quantity ratio of static fracturing boreholes and hydraulic fracturing boreholes are determined based on the geological conditions of the target coal seam. According to the layout pattern and the quantity ratio, the static fracturing boreholes and the hydraulic fracturing boreholes are constructed in the hard coal body according to the design spacing that meets the physical connection requirements, and trajectory control is performed. Determine the size of the encapsulated static expansion device and construct the encapsulated static expansion device. Fill the constructed encapsulated static expansion device into the static fracturing borehole after construction and seal it using a sealing device. High-pressure water is injected into the hydraulic fracturing borehole to generate water-conducting fractures, and the connectivity response characteristics during the water injection process are monitored. Once the connectivity response characteristics are confirmed to be connected, the water flow in the water-conducting fracture is used to activate the encapsulated static expansion device, causing the radial expansion stress generated by the encapsulated static expansion device to be superimposed with the hydraulic fracturing stress field to form a synergistic weakening effect. The effective superimposed fracturing stress intensity is calculated to characterize the synergistic weakening effect. The parameters of the coal body after the synergistic weakening effect are measured to assess the degree of weakening of the hard coal body.

2. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing as described in claim 1, characterized in that, The steps for determining the arrangement pattern of static fracturing boreholes and hydraulic fracturing boreholes include: The spatial positions of the static fracturing borehole and the hydraulic fracturing borehole are set as either a parallel staggered arrangement along the bedding plane or a vertical transverse arrangement of the cut-hole. The aforementioned parallel staggered arrangement pattern is set so that the borehole axis is parallel to the working face advance direction and presents a periodic staggered arrangement on the cross section perpendicular to the strike. The vertical cross-sectional arrangement of the cut holes is set so that the borehole axis is perpendicular to the working face advance direction, and the effective depth of the borehole is constrained based on the measured width of the cut holes on the working face.

3. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing as described in claim 1, characterized in that, The calculation steps for the design spacing include: The engineering safety factor, the coal seam heterogeneous geological correction factor, the effective wetting radius of single-hole hydraulic fracturing, and the angle between the direction of the maximum horizontal principal stress and the normal of the line connecting the centers of the two holes are obtained. The obtained parameters are defined as the spacing calculation parameter set. Based on the aforementioned spacing calculation parameter set, a fluid conduction constraint inequality that satisfies the fluid conduction requirements is constructed; The distance between the center axis of the static fracturing borehole and the center axis of the adjacent hydraulic fracturing borehole is calculated and defined using the fluid conduction constraint inequality, and the calculated distance is confirmed as the design spacing.

4. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing as described in claim 1, characterized in that, The step of determining the ratio of static fracturing boreholes to hydraulic fracturing boreholes includes: The measured values ​​of the Protodyakonov hardness coefficient of the target coal seam, the benchmark hardness threshold without special fracturing and the hardness grade difference are obtained, and the obtained parameters are defined as the density index calculation parameter set. The density index calculation parameter set is substituted into the weakened unit density index calculation logic for calculation; Based on the weakened unit density index output by the weakened unit density index calculation logic, the ratio of the number of static fracturing boreholes and hydraulic fracturing boreholes per unit volume is determined.

5. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing according to claim 1, characterized in that, The steps for trajectory control include: The directional drilling rig equipped with a measurement-while-drilling system was used for construction, and the real-time actual distance between the static fracturing borehole and the hydraulic fracturing borehole was calculated in real time. Compare the real-time actual spacing with the allowable boundary value of the design spacing; When the comparison results show that the real-time actual distance is close to the boundary value, the directional tilting correction program is started to prevent the rock bridge effect, thereby completing the trajectory control.

6. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing according to claim 1, characterized in that, The steps for constructing the packaged static expansion device include: It is assembled using an internal high-energy static fragmentation core and an external directional water-permeable constraint layer; The particle retention criteria are determined based on the effective particle size and safety screening coefficient of the high-energy statically broken core powder particles. The material of the directional permeable constraint layer is selected so that the average micropore size of the directional permeable constraint layer meets the particle retention criterion, allowing external moisture to penetrate and preventing internal powder from flowing out, thereby completing the construction of the encapsulated static expansion device.

7. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing according to claim 1, characterized in that, The step of determining the dimensions of the packaged static expansion device includes: Measure the construction inner diameter of the statically fractured borehole; Substitute the construction inner diameter into the radial filling matching coefficient calculation formula for calculation; The outer diameter of the cylinder of the encapsulated static expansion device is determined based on the calculation result of the radial filling matching coefficient calculation formula, so as to achieve geometric matching between the encapsulated static expansion device and the static fracturing borehole.

8. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing according to claim 1, characterized in that, The step of calculating the effective superimposed fracture-inducing stress intensity to characterize the synergistic weakening effect includes: The hydraulic coupling coefficient, residual pore water pressure, stress transfer efficiency factor, maximum radial expansion pressure, reagent reaction rate constant, and reaction time are obtained, and the obtained parameters are defined as the set of potential energy influence factors at the fracture tip. A formula for calculating the effective superimposed crack-inducing stress intensity is constructed based on the set of potential energy influencing factors at the crack tip. The effective superimposed crack-inducing stress intensity is calculated using the aforementioned formula, which quantitatively characterizes the destructive potential energy at the crack tip in the synergistic weakening effect.

9. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing according to claim 1, characterized in that, The steps for monitoring the connectivity response characteristics during the water injection process include: Record the data on the change of water injection pressure over time during the water injection process; Analyze the data on the change of water injection pressure over time, identify whether there is a feature in the data on the change of water injection pressure over time that the instantaneous pressure drop exceeds the set pressure threshold and is accompanied by a synchronous surge in water injection flow, and define the identified feature as a connectivity response feature. When the connectivity response feature is detected, it is determined that the hydraulic fracture has physically penetrated the static fracturing borehole. The set pressure threshold is 3 MPa or the set pressure threshold is 10% of the current system pressure.

10. The method for weakening hard coal bodies based on the synergy of static fracturing and hydraulic fracturing according to claim 1, characterized in that, The steps for assessing the degree of weakening of the hard coal body include: Verification boreholes were drilled at the center and edge of the fractured area to obtain the Protodyakonov hardness coefficient of the original coal body, the residual Protodyakonov hardness coefficient after synergistic weakening treatment, the standard amount of drill cuttings per unit borehole length of the original coal body, and the average amount of drill cuttings per unit borehole length after synergistic weakening treatment. The obtained parameters were defined as the synergistic weakening effectiveness evaluation parameter set. A calculation model for the synergistic weakening effectiveness index is constructed using the aforementioned synergistic weakening effectiveness evaluation parameter set; The synergistic weakening effectiveness index is calculated using the aforementioned synergistic weakening effectiveness index calculation model, and the degree of weakening of the hard coal body is evaluated based on the synergistic weakening effectiveness index.

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