Coal seam fatigue fracturing and permeability enhancement method by alternating liquid nitrogen cold shock and hot steam

Abstract

This application relates to the field of mining engineering technology, specifically disclosing a method for fatigue-induced fracturing and permeability enhancement in coal seams using alternating liquid nitrogen cold impact and hot steam. The system includes a liquid nitrogen injection system, a steam generation system with accompanying inert gas, a three-dimensional borehole network, and a monitoring and control system. The preparation method is as follows: first, a main branch borehole network is constructed and control equipment is installed; then, multi-stage gradient cyclic fracturing based on damage feedback is performed, sequentially alternating hot and cold treatments during crack initiation, propagation, and stabilization stages; during this process, process parameters and treatment orientation are dynamically adjusted based on acoustic emission and temperature field data; finally, pipeline purging and borehole sealing are performed. This method can be used for fracturing and permeability enhancement operations in low-permeability coal seams, and it has the advantages of strong controllability of the fracturing process, uniform development of the fracture network, and good connectivity; furthermore, the method has the advantages of dynamically adjustable process parameters and adaptability to the needs of different development stages of coal seam cracks.

Description

Technical Field

[0001] This application relates to the field of mining engineering technology, and more specifically, to a method for enhancing the permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold impact and hot steam. Background Technology

[0002] In the development of coalbed methane (CBM) in low-permeability coal seams, the permeability of the coal seam directly affects the extraction efficiency. To improve the permeability of low-permeability coal seams, fracturing and permeability enhancement technologies are often used. Liquid nitrogen cold impact and alternating hot steam fracturing and permeability enhancement technology, as a novel physical fracturing method, has gained widespread attention in the field of low-permeability coal seam enhancement due to its characteristics of no chemical pollution and a wide fracturing range. The core logic of this technology is to induce cold contraction cracks in the coal body through liquid nitrogen cold impact, and then use hot steam excitation to further expand the cracks, thereby constructing a connected fracture network, expanding the coal seam seepage channels, and achieving the goal of improving coalbed methane extraction efficiency.

[0003] In practical applications, existing technologies that combine liquid nitrogen cold shock with alternating hot steam fracturing and permeability enhancement often employ a continuous cyclic fracturing mode with fixed parameters. This lacks adaptability to different stages of crack development. Furthermore, uneven distribution of hot steam during injection is prone to occur, and it is difficult to determine the timing of stage switching through precise feedback mechanisms. This results in crack development often exhibiting localized concentrations or insufficient penetration, making it difficult to guarantee the uniformity and effectiveness of fracturing and permeability enhancement, thereby affecting the overall efficiency of coalbed methane extraction. Summary of the Invention

[0004] To address the problem that existing methods of simple alternating liquid nitrogen and hot steam fracturing are difficult to dynamically control energy input based on the coal seam fracture state, resulting in insufficient uniformity and effectiveness of permeability enhancement, this application provides a coal seam fatigue fracturing and permeability enhancement method using alternating liquid nitrogen cold impact and hot steam.

[0005] A method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam includes the following steps: S1. Construction and System Layout of Borehole Network: Construct a central main borehole and multiple branch boreholes arranged around it in the target coal seam to form a connected borehole network; install independently operable electrically controlled dynamic sealing valves at the entrance of each branch borehole; lay out liquid nitrogen injection pipelines for injecting atomized liquid nitrogen into the borehole network, and steam injection pipelines for injecting hot steam mixed with inert gas. S2. Multi-stage gradient cyclic fracturing: The fracturing process includes at least a crack initiation stage and a crack propagation and penetration stage, each stage including one or more alternating cycles of liquid nitrogen cold shock and hot steam excitation; wherein the liquid nitrogen shock pressure and hot steam temperature used in the crack propagation and penetration stage are both lower than the corresponding values ​​used in the crack initiation stage. S3. Damage Feedback and Dynamic Control: During the fracturing process, the acoustic emission signals and temperature field data of the coal seam are monitored in real time. Based on the cumulative energy of the acoustic emission signals, the cumulative number of acoustic emission events, and the non-uniformity index of the temperature field, the damage degree characterizing the degree of coal body fracturing is calculated. When the damage degree reaches a preset threshold, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered. S4. Operation Completion: After the fracturing operation is completed, stop the injection of medium and carry out pipeline purging and borehole sealing.

[0006] By adopting the above technical solutions, in the borehole network construction and system layout stage, the interconnected network formed by the central main borehole and the surrounding branch boreholes provides a channel foundation for the uniform diffusion of fracturing media; the electrically controlled dynamic sealing valve can selectively open and close to directionally guide the fracturing action; atomized liquid nitrogen can increase the contact area with the coal body, enhancing the uniformity and penetration depth of cold impact; inert gas injection can optimize the flow and distribution of hot steam in the fracture, reduce the phenomenon of local accumulation or condensation of hot steam, and avoid the closure or incomplete development of fractures due to uneven distribution of thermal stress. The multi-stage gradient cyclic fracturing stage, based on the differentiated parameters designed according to the characteristics of different stages of coal crack development, adapts the fracturing process to the dynamic needs of coal crack development through gradient energy regulation. Among them, the high parameters in the crack initiation stage break through the initial strength of the coal body through the intense thermal stress generated by the alternation of strong cold and strong heat, while the low parameters in the crack propagation and penetration stage guide the orderly propagation and penetration of the induced crack through gentle and continuous energy input, avoiding excessive damage to the coal matrix. In the damage feedback and dynamic control phase, real-time monitoring of acoustic emission signals and temperature field data is used. The cumulative energy of the acoustic emission signal, the cumulative number of acoustic emission events, and the temperature field inhomogeneity index are used as core indicators to calculate the damage degree, characterizing the extent of coal seam fracturing. A preset threshold for damage degree triggers a phase switch, achieving precise control of the fracturing process. In the final stage of the operation, dry nitrogen purging removes residual media from the pipeline, preventing interference with subsequent coalbed methane extraction. High-strength polyurethane sealing agent ensures the stability and sealing of the borehole, providing a safe seepage environment for the fracturing coal seam. These technologies work synergistically to optimize the distribution of fracturing media, regulate energy input rhythm, and precisely guide crack development, ensuring a complete and well-connected fracture network after fracturing.

[0007] Preferably, in step S2, during the crack initiation stage, the liquid nitrogen impact pressure is 4-5 MPa, the hot steam temperature is 280-300℃, and the volumetric injection ratio of inert gas to steam is 1.0:4.5-5.5; the crack initiation process also includes a crack stabilization stage performed after the crack propagation and penetration stage, wherein the liquid nitrogen impact pressure and hot steam temperature used in the crack stabilization stage are both lower than the corresponding values ​​used in the crack propagation and penetration stage.

[0008] By adopting the above technical solutions, the parameter settings in the crack initiation stage can create a strong alternation of cold and heat. Through the effects of intense cold contraction and instantaneous thermal expansion, a large thermal stress gradient is formed within the coal body, breaking through the initial structural strength of the coal and providing energy for crack initiation. Inert gas injection assists the rapid diffusion of hot steam within the borehole network, avoiding localized overheating or condensation caused by localized hot steam accumulation. Simultaneously, it ensures the temperature and thermal intensity of the hot steam, providing a stable thermal environment for crack initiation. The synergistic effect of the cold and hot gasification medium and the injection gas improves the uniformity of crack initiation. The parameter settings in the crack stabilization stage are based on the structural characteristics of the coal body after crack propagation and penetration. By reducing the intensity of the crack-inducing parameters, a mild alternation of cold and heat is formed, optimizing and stabilizing the existing fracture network. This avoids excessive impact from high energy input on penetrated cracks, prevents secondary fracturing of the coal matrix leading to fracture blockage, and guides the further development and improvement of micro-cracks, enhancing the connectivity and stability of the fracture network. By dividing the process into complete gradient stages and matching parameters, the fracturing process is adapted to the evolution law of coal body cracks throughout. Through gradient regulation of energy input and auxiliary optimization of accompanying gas, a foundation is provided for the formation of stable seepage channels.

[0009] Preferably, during the crack propagation and penetration stage, the liquid nitrogen impact pressure is 2-3 MPa, the hot steam temperature is 220-250℃, and the volumetric injection ratio of inert gas to steam is 1.0:2.8-3.2. During the crack stabilization stage, the liquid nitrogen impact pressure is 1-2 MPa, the hot steam temperature is 150-180℃, and the volumetric injection ratio of inert gas to steam is 1.0:0.9-1.1.

[0010] By adopting the above technical solution, the parameters of the crack propagation and penetration stage are adapted to the mechanical state of the coal body after the initial crack formation. Through gentle and continuous alternating hot and cold energy input, the driving force required for the directional propagation and penetration of the initial crack is provided, guiding the dispersed initial cracks to connect and form a continuous fracture network. The adjustment of the inert gas injection ratio in this stage allows the inert gas-assisted hot steam to diffuse precisely along the initial crack channel, ensuring uniform thermal coverage of the crack propagation path, while avoiding excessive dilution of the hot steam by the inert gas, which would lead to insufficient thermal intensity. The orderly penetration of the crack is assisted by the regulation of the medium distribution. In the crack stabilization stage, the parameters further reduce the cracking intensity. Through gentle alternating hot and cold action, the continuous fracture network is optimized and stabilized, driving the further extension and improvement of incompletely developed micro-cracks in the fracture network, enhancing the overall connectivity and stability of the fracture network. The adjustment of the inert gas injection ratio in this stage enhances the support for the fracture walls, inhibits the closure of cracks due to coal stress release, and reduces the amount of hot steam used to avoid excessive thermal action that would adversely affect the coal's seepage characteristics. The parameter gradient design and dynamic adjustment of the inert gas injection ratio at each stage form a full-cycle gradient fracturing system, realizing the dual dynamic adaptation of fracturing energy input and inert gas injection, and promoting the orderly formation and stable development of the fracture network.

[0011] Preferably, in step S2, the liquid nitrogen cold shock is performed in a pulsed microcirculation manner, and each microcirculation includes an injection period and an intermittent period; the microcirculation cycle of the crack initiation stage is shorter than the microcirculation cycle of the crack stabilization stage.

[0012] By adopting the above technical solution, the injection phase of the pulsed microcirculation achieves rapid and directional injection of liquid nitrogen, enabling the liquid nitrogen to quickly contact the coal surface and penetrate into the micropores, forming an instantaneous strong cold impact effect that induces local cold contraction stress. The intermittent phase provides sufficient stress response and energy transfer time for the coal, avoiding excessive accumulation of cold energy or brittle fracture of the coal caused by a sudden drop in local temperature due to continuous injection, while reserving space for subsequent heat exchange induced by hot steam. The differences in microcirculation cycles and gradient parameter systems at different fracturing stages form a synergistic adaptation. The short cycle in the crack initiation stage can achieve high-frequency cold and hot alternation, quickly breaking the mechanical balance of the initial structure of the coal. With the corresponding stage parameters, high-frequency energy input rapidly accumulates thermal stress, promoting the initiation of the initial crack. The long cycle in the stable stage adapts to the requirements of low-intensity fracturing parameters. By extending the injection and intermittent phases, the cold impact of liquid nitrogen and the excitation of hot steam are more sufficient, avoiding secondary damage to the already penetrated cracks caused by high-frequency impacts, while providing sufficient time for the diffusion and support of inert gas in the cracks. The pulsed microcirculation design, along with the gradient parameters at each stage and the dynamic injection ratio, forms a triple synergy to achieve balanced transfer of hot and cold energy and precise distribution of the medium, ensuring the orderly evolution of cracks from initiation to expansion and stability.

[0013] Preferably, in step S3, the dynamic control further includes: based on the spatial distribution of the acoustic emission signal monitored in real time, controlling the opening of the electrically controlled dynamic sealing valves corresponding to the branch holes at different positions to guide the crack to expand in a specific direction.

[0014] By employing the aforementioned technical solution, the spatial distribution of acoustic emission signals can accurately characterize the spatial differences in coal crack development. Areas with dense signals correspond to areas of concentrated crack development, while areas with sparse or missing signals correspond to areas where fracturing action is insufficient and crack development is inadequate. Based on this monitoring result, by controlling the electrically controlled dynamic plugging valves corresponding to branch holes at different locations, active intervention in the flow path of the fracturing medium can be achieved: for areas with dense signals and sufficient crack development, the plugging valves of the corresponding branch holes can be closed or adjusted to reduce the amount of medium injected into that area; for areas with sparse signals and insufficient fracturing action, the plugging valves of the corresponding branch holes can be opened and kept unobstructed, leading to preferential injection of the fracturing medium through those branch holes, thus concentrating the fracturing energy on that area. This control mechanism, in synergy with multi-stage gradient fracturing and pulsed microcirculation injection, can directionally deliver the fracturing medium to a preset area during the crack initiation stage, thereby ensuring that the initial crack initiates along a predetermined orientation; during the crack propagation and penetration stage, it can guide the crack to propagate orderly along the branch hole extension direction; and during the crack stabilization stage, it can specifically enhance the fracturing action in weak areas, improving the overall uniformity of the fracture network.

[0015] Preferably, in step S3, the damage degree is calculated by weighted summation of the normalized cumulative acoustic emission energy, the temperature field inhomogeneity index, and the cumulative number of acoustic emission events. The weighting coefficient of the cumulative acoustic emission energy is 0.4-0.6, the weighting coefficient of the temperature field inhomogeneity index is 0.2-0.3, the weighting coefficient of the cumulative number of acoustic emission events is 0.2-0.3, and the sum of the three weighting coefficients is 1.

[0016] By adopting the above technical solution, the normalization processing of damage degree calculation can eliminate the differences in data dimensions across different monitoring dimensions, ensuring the comparability between indicators and the rationality of calculation results. The differentiated setting of weighting coefficients aligns with the core value of each indicator in characterizing the degree of coal fracture. The cumulative acoustic emission energy directly reflects the energy release intensity during crack initiation and propagation in the coal body, and is given the highest weight as a core indicator characterizing the degree of fracture. The temperature field inhomogeneity index reflects the uniformity of heat exchange between the fracturing medium and the coal body, indirectly related to the uniformity of crack development. The cumulative number of acoustic emission events characterizes the frequency and density of crack development per unit time; these two, as supplements to the core indicators, are given corresponding weights. The design that the sum of the weights of the three is 1 ensures the completeness and effectiveness of the calculation results. The synergistic weighting of multi-dimensional indicators can comprehensively and accurately quantify the coal body fracture state, providing an objective and reliable basis for judging the switching of fracturing stages. The directional control of fracturing orientation and the calculation of damage degree work together to form a feedback control closed loop of overall control and local optimization. The damage degree calculation judges the macroscopic state of coal body fracturing from the overall level and guides the switching of fracturing stages. The control of fracturing orientation solves the problem of disorder in local fracturing orientation. The synergy of the two enables the control to take into account both the orderly advancement of the overall fracturing process and the optimization of local fracturing effect.

[0017] Preferably, in step S3, when the calculated damage degree reaches the first threshold of 0.15-0.25, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered; when the damage degree reaches the second threshold of 0.55-0.65, the switch from the crack propagation and penetration stage to the crack stabilization stage is triggered.

[0018] By adopting the above technical solution, the damage threshold range is precisely matched with the characteristic states of different development stages of coal cracks, forming a quantitative standard for stage switching. The first threshold corresponds to the state where the initial cracks in the coal body have fully initiated but have not yet become excessively fragmented. At this point, a switch to the crack propagation and penetration stage is triggered, allowing the fracturing process to promptly enter a parameter range that meets the requirements for crack extension. The second threshold corresponds to the state where the cracks have fully expanded and initially penetrated to form a continuous fracture network. At this point, a switch to the crack stabilization stage is triggered, using lower-intensity fracturing parameters to optimize and stabilize the already formed fracture network. The threshold triggering mechanism and the damage weighted calculation method work together to form a complete feedback control logic, guiding the orderly evolution of cracks from initiation to expansion and stabilization through precise stage switching.

[0019] Preferably, step S3 further includes dynamically adjusting the process parameters based on the real-time monitored acoustic emission signal amplitude and temperature data, specifically including: When the acoustic emission signal amplitude is monitored to be below 100µV for more than 5 minutes, increase the liquid nitrogen or steam injection pressure at the current stage by 0.5-1.0MPa and reduce the liquid nitrogen atomization particle size by 20-50µm. When the temperature deviation in a specific area is detected to be more than 50°C, reduce the steam injection flow rate at the corresponding location by 2-3 m³ / h and increase the inert gas injection ratio by 0.5-1.0 times.

[0020] By adopting the above technical solutions, the dynamic adjustment of process parameters supplements the damage feedback control system, forming a dual-layer control logic with the stage switching control based on damage threshold. When the acoustic emission signal amplitude remains at a low level, it indicates that the crack development process inside the coal body is slow and the interaction strength between the fracturing medium and the coal body is insufficient. Increasing the current stage of liquid nitrogen or steam injection pressure can enhance the penetration ability and impact strength of the fracturing medium, and strengthen the transmission effect of thermal stress inside the coal body; reducing the liquid nitrogen atomization particle size can increase the contact area between liquid nitrogen and the coal body, and improve the uniformity and efficiency of cold impact. The two measures work together to enhance the fracturing energy input intensity and activate the crack development process in the coal body. When the temperature deviation in a specific area is continuously too large, it reflects the imbalance of the distribution of hot steam in the coal seam, which easily leads to uneven distribution of thermal stress, resulting in disordered crack development or local crack closure. Reducing the steam injection flow rate at corresponding locations can decrease the excessive accumulation of hot steam in local areas; increasing the proportion of inert gas injection can optimize the flow path of hot steam within the fracture network by utilizing the diffusion characteristics of inert gas, promoting uniform distribution of hot steam, balancing the coal seam temperature field, and ensuring the orderly propagation of cracks along the preset direction. This dynamic adjustment mechanism enables real-time adaptive control of process parameters within a stage, working in synergy with the directional control of fracturing orientation to improve the stability and controllability of the fracturing process.

[0021] Preferably, in step S1, the diameter of the main borehole is 150-200mm, the diameter of the branch hole is 100-150mm, the branch hole extends along the direction of the maximum principal stress of the coal seam at an angle of 30-45°, and the length is 20-50m; in step S4, the pipeline purging uses dry nitrogen gas at a pressure of 1-2MPa and continues to purge for 30-40min.

[0022] By adopting the above technical solutions, the main borehole diameter design can meet the high-flow-rate transportation requirements of atomized liquid nitrogen and hot steam mixed with inert gas, providing a channel foundation for the rapid arrival of fracturing media in the target coal seam area. The branch hole diameter design takes into account both media diffusion efficiency and coal structure protection, avoiding damage to the integrity of the coal matrix due to excessive hole diameter. The design of the branch holes extending along the direction of the maximum principal stress of the coal seam utilizes the mechanical property that the crack resistance of the coal seam is relatively low in the direction of the maximum principal stress, making it easier for cracks to initiate and propagate along the extension direction of the branch holes during fracturing. The coordinated setting of the dip angle and length parameters can achieve uniform coverage of the target coal seam area by the branch holes, providing a structural prerequisite for the subsequent guidance of crack directional propagation by the electrically controlled dynamic sealing valve. The dry nitrogen purging of the pipeline can effectively remove residual liquid nitrogen and hot steam in the pipeline, avoiding pressure fluctuations caused by the condensation or vaporization of residual media from affecting subsequent borehole sealing operations; at the same time, the dry nitrogen can remove moisture in the pipeline, preventing moisture from entering the coal seam and blocking micro-fractures, ensuring the unobstructed flow channels of the coal seam after fracturing. The drilling parameters, in conjunction with subsequent multi-stage gradient fracturing and directional sealing valve control techniques, provide a channel guarantee for the uniform diffusion of the fracturing medium and the directional development of cracks; the purging parameters, in conjunction with the borehole sealing process, create a favorable seepage environment for coalbed methane extraction after fracturing.

[0023] Preferably, in step S4, the borehole sealing is performed using a high-strength polyurethane sealant. The initial curing time of the sealant is 15-30 minutes, and after complete curing, the compressive strength is ≥25MPa, and the temperature resistance range is -50℃ to 200℃.

[0024] By adopting the above technical solution, the initial curing time design of the high-strength polyurethane sealant balances the convenience of construction and operation efficiency, allowing sufficient time to complete the sealant injection and smoothing work, avoiding voids or defects in the sealing layer due to excessively rapid curing, and preventing delays in subsequent procedures due to excessively long curing time. The fully cured compressive strength can withstand the formation pressure underground in the coal seam, preventing deformation and damage to the borehole due to pressure, and maintaining the integrity of the borehole sealing structure; the temperature resistance range can adapt to the temperature fluctuation environment underground in the coal seam, and withstand the drastic temperature changes caused by liquid nitrogen cold shock and hot steam excitation during fracturing, avoiding cracking, peeling, and other failures of the sealant due to sudden temperature rises and falls. The borehole sealing process works synergistically with the preceding dry nitrogen purging process. Dry nitrogen purging removes residual liquid nitrogen, hot vapor, and impurities from the pipeline, preventing the residual medium from reacting with the sealing agent and affecting the curing effect. The sealing agent seals the borehole after purging, preventing external moisture and impurities from entering the coal seam and blocking the seepage channels. Together, they form a comprehensive system to ensure the successful completion of the fracturing operation, creating a complete closed loop with the core technologies used earlier, ensuring that the fracture network formed after fracturing can continue to play a seepage role.

[0025] In summary, this application has the following beneficial effects: 1. This application adopts a multi-stage gradient cyclic fracturing process that includes a crack initiation stage and a crack propagation and penetration stage. In the crack propagation and penetration stage, the liquid nitrogen impact pressure and hot steam temperature are set to values ​​lower than those in the crack initiation stage. At the same time, the damage degree calculated based on acoustic emission and temperature field data is introduced as the triggering basis for stage switching. This allows the fracturing process to adapt to changes in the coal body fracture state, achieving precise guidance of the crack propagation process and gradient optimization of energy input. This helps to form a more uniform and more interconnected fracture network, improving the overall efficiency and effect of the permeability enhancement operation.

[0026] 2. This application preferably installs an electrically controlled dynamic sealing valve at the inlet of the branch borehole. This design can control the opening and closing of the valve in real time according to the spatial distribution of acoustic emission signals, thereby actively guiding the direction of crack propagation, allowing the crack network to extend in an orderly manner along a preset path, and improving the controllability of the crack initiation range.

[0027] 3. The method of this application, by designing liquid nitrogen cold impact as a pulsed microcirculation mode, with each microcirculation including an injection period and an intermittent period, and the microcirculation cycle in the crack initiation stage being shorter than that in the stable stage, makes the contact between liquid nitrogen and coal seam more uniform, the heat exchange efficiency higher, and avoids excessive local cold concentration, thus achieving the effect of enhancing the uniformity of cold shrinkage stress.

[0028] 4. Since this application adopts a dynamic adjustment scheme for process parameters based on acoustic emission signal amplitude and temperature data, when the acoustic emission signal amplitude is detected to be too low or the temperature deviation is too large, parameters such as injection pressure and atomization particle size can be adjusted in real time, so that the process parameters can continuously adapt to the coal body fracture state, thereby improving the stability of the fracturing process.

[0029] 5. In this application, a weighted summation method is preferred to calculate the damage degree. By reasonably allocating the weights of the cumulative acoustic emission energy, the temperature field inhomogeneity index, and the cumulative number of acoustic emission events, the damage degree can accurately characterize the degree of coal body fracture. Since the stage switching can be accurately triggered based on the damage degree threshold, under-fracture or over-fracture can be avoided, thereby improving the accuracy of the fracturing process. Attached Figure Description

[0030] Figure 1 This is a flowchart of a method for enhancing the permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold impact and hot steam, as provided in this application. Detailed Implementation

[0031] The present application will be further described in detail below with reference to embodiments and comparative examples. Unless otherwise specified, the experimental methods used below are conventional methods. Unless otherwise specified, the materials, reagents, methods and instruments used are all conventional materials, reagents, methods and instruments in the art, which can be obtained by those skilled in the art through commercial channels or prepared according to literature methods.

[0032] Technical concept: In related technologies, the method of alternating liquid nitrogen cold shock and hot steam fracturing is usually based on fixed parameters and number of cycles. During implementation, the intensity and rhythm of energy input are not correlated with the actual initiation and propagation process of cracks within the coal body. Due to the heterogeneity of the coal structure and the staged nature of crack development, this fixed mode of action is difficult to dynamically adapt to the differentiated demands for thermal and contraction stresses at different stages. This easily leads to uneven energy distribution, with some areas experiencing excessive fracturing while others are insufficiently fractured, thus limiting the uniformity and overall effectiveness of the final permeability enhancement range.

[0033] This technical solution addresses the aforementioned problems by proposing a multi-stage gradient cycle with different process parameters for the fracturing process, and introducing damage degree calculated based on real-time monitoring data as the core criterion for process control. Specifically, a multi-stage model is constructed to represent crack initiation, propagation, and stabilization, with decreasing liquid nitrogen pressure and steam temperature parameters set for each stage. Simultaneously, acoustic emission and temperature field data are comprehensively utilized to calculate coal body damage degree online, thereby intelligently triggering the switching between stages and fine-tuning local parameters. This design allows the energy input strategy to automatically and dynamically adjust according to changes in the coal body fracturing state, thus optimizing the orderly propagation and network connection of cracks. Example 1: This example provides a method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam. The specific steps are as follows: S1. Construction and System Layout of Borehole Network: Construct a central main borehole and multiple branch boreholes arranged around it in the target coal seam to form a connected borehole network; install independently operable electrically controlled dynamic sealing valves at the entrance of each branch borehole; lay out liquid nitrogen injection pipelines for injecting atomized liquid nitrogen into the borehole network, and steam injection pipelines for injecting hot steam mixed with inert gas.

[0034] The main borehole has a diameter of 175 mm, the branch borehole has a diameter of 125 mm, the branch borehole extends along the direction of the maximum principal stress of the coal seam at an angle of 37.5°, and has a length of 35 m; the inert gas is dry nitrogen, and the water used for steam generation is deionized water.

[0035] S2. Multi-stage gradient cyclic fracturing: The fracturing process includes a crack initiation stage, a crack propagation and penetration stage, and a crack stabilization stage. Each stage includes one or more alternating cycles of liquid nitrogen cold shock and hot steam excitation. The liquid nitrogen shock pressure and hot steam temperature used in the crack propagation and penetration stage are lower than the corresponding values ​​used in the crack initiation stage. The liquid nitrogen shock pressure and hot steam temperature used in the crack stabilization stage are also lower than the corresponding values ​​used in the crack propagation and penetration stage.

[0036] During the crack initiation stage, the liquid nitrogen impact pressure was 4.5 MPa, the hot steam temperature was 290℃, and the volumetric injection ratio of inert gas to steam was 1.0:5.0. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode. Each micro-circulation included an injection period and an interval period. The micro-circulation cycle was 4.5 s, the injection period was 3.0 s, and the interval period was 1.5 s. In this stage, the alternating cycle of liquid nitrogen cold impact and hot steam excitation was performed once, with a cycle interval of 7.5 min.

[0037] During the crack propagation and penetration stage, the liquid nitrogen impact pressure was 2.5 MPa, the hot steam temperature was 235℃, and the volumetric injection ratio of inert gas to steam was 1.0:3.0. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode, with a micro-circulation cycle of 5.5 s, an injection period of 3.35 s, and an interval of 2.0 s. In this stage, the alternating cycles of liquid nitrogen cold impact and hot steam excitation were performed 4 times, with a cycle interval of 4 min.

[0038] During the crack stabilization stage, the liquid nitrogen impact pressure was 1.5 MPa, the hot steam temperature was 165℃, and the volumetric injection ratio of inert gas to steam was 1.0:1.0. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode, with a micro-circulation cycle of 6.5 s, an injection period of 3.75 s, and an interval of 2.25 s. In this stage, the alternating cycle of liquid nitrogen cold impact and hot steam excitation was performed once, with a cycle interval of 2.5 min.

[0039] S3. Damage Feedback and Dynamic Control: During the fracturing process, the acoustic emission signals and temperature field data of the coal seam are monitored in real time. Based on the cumulative energy of the acoustic emission signals, the cumulative number of acoustic emission events, and the non-uniformity index of the temperature field, the damage degree, which characterizes the degree of coal body fracturing, is calculated. When the damage degree reaches a preset threshold, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered.

[0040] The dynamic control also includes: based on the spatial distribution of acoustic emission signals monitored in real time, controlling the opening of the electrically controlled dynamic sealing valves corresponding to the branch holes at different positions to guide the crack to expand in a specific direction.

[0041] The damage degree is calculated by weighted summation of the normalized cumulative acoustic emission energy, the temperature field inhomogeneity index, and the cumulative number of acoustic emission events. The specific formula is as follows: ; In the formula, D represents the degree of damage; α is the weighting coefficient of the cumulative acoustic emission energy, with a value of 0.5; β is the weighting coefficient of the temperature field inhomogeneity index, with a value of 0.25; γ is the weighting coefficient of the cumulative number of acoustic emission events, with a value of 0.25; α + β + γ = 1; E n T represents the normalized cumulative acoustic emission energy. n N is the normalized temperature field inhomogeneity index;n This represents the cumulative number of acoustic emission events after normalization.

[0042] Specifically, when the calculated damage level reaches the first threshold of 0.20, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered; when the damage level reaches the second threshold of 0.60, the switch from the crack propagation and penetration stage to the crack stabilization stage is triggered.

[0043] This includes dynamically adjusting process parameters based on real-time monitoring of acoustic emission signal amplitude and temperature data. Specifically, when the acoustic emission signal amplitude is continuously below 100μV for more than 5 minutes, the liquid nitrogen or steam injection pressure at the current stage is increased by 0.75MPa, and the liquid nitrogen atomization particle size is reduced by 35μm; when the temperature deviation in a specific area is continuously exceeded 50℃, the steam injection flow rate at the corresponding location is reduced by 2.5m³ / h, and the inert gas accompaniment ratio is increased by 0.75 times.

[0044] S4. Operation Completion: After the fracturing operation is completed, stop the injection of medium and carry out pipeline purging and borehole sealing.

[0045] The pipeline purging process uses dry nitrogen gas at a pressure of 1.5 MPa for 35 minutes. The borehole sealing is performed using a high-strength polyurethane sealant with an initial curing time of 22.5 minutes. After complete curing, the compressive strength is ≥25 MPa and the temperature range is -50℃ to 200℃.

[0046] Example 2: This example provides a method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam. The specific steps are as follows: S1. Construction and System Layout of Borehole Network: Construct a central main borehole and multiple branch boreholes arranged around it in the target coal seam to form a connected borehole network; install independently operable electrically controlled dynamic sealing valves at the entrance of each branch borehole; lay out liquid nitrogen injection pipelines for injecting atomized liquid nitrogen into the borehole network, and steam injection pipelines for injecting hot steam mixed with inert gas.

[0047] The main borehole has a diameter of 150 mm, the branch borehole has a diameter of 100 mm, and the branch borehole extends at a 30° angle along the direction of the maximum principal stress of the coal seam with a length of 20 m. Dry nitrogen is used as the inert gas, and deionized water is used for steam generation.

[0048] S2. Multi-stage gradient cyclic fracturing: The fracturing process includes a crack initiation stage, a crack propagation and penetration stage, and a crack stabilization stage. Each stage includes one or more alternating cycles of liquid nitrogen cold shock and hot steam excitation. The liquid nitrogen shock pressure and hot steam temperature used in the crack propagation and penetration stage are lower than the corresponding values ​​used in the crack initiation stage. The liquid nitrogen shock pressure and hot steam temperature used in the crack stabilization stage are also lower than the corresponding values ​​used in the crack propagation and penetration stage.

[0049] During the crack initiation stage, the liquid nitrogen impact pressure was 4 MPa, the hot steam temperature was 280℃, and the volumetric injection ratio of inert gas to steam was 1.0:4.5. The liquid nitrogen cold impact was performed in a pulsed micro-circulation mode, with each micro-circulation including an injection period and an interval period. The micro-circulation cycle was 3s, with the injection period being 1.8s and the interval period being 0.9s. In this stage, the alternating cycle of liquid nitrogen cold impact and hot steam excitation was performed once, with a cycle interval of 5min.

[0050] During the crack propagation and penetration stage, the liquid nitrogen impact pressure was 2 MPa, the hot steam temperature was 220℃, and the volumetric injection ratio of inert gas to steam was 1.0:2.8. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode, with a micro-circulation cycle of 4s, an injection period of 2.2s, and an interval of 1.2s. In this stage, the liquid nitrogen cold impact and hot steam excitation were alternated three times, with a cycle interval of 3min.

[0051] During the crack stabilization stage, the liquid nitrogen impact pressure was 1 MPa, the hot steam temperature was 150℃, and the volumetric injection ratio of inert gas to steam was 1.0:0.9. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode, with a micro-circulation cycle of 5s, an injection period of 2.5s, and an interval of 1.5s. In this stage, the liquid nitrogen cold impact and hot steam excitation were alternated once, with a cycle interval of 2min.

[0052] S3. Damage Feedback and Dynamic Control: During the fracturing process, the acoustic emission signals and temperature field data of the coal seam are monitored in real time. Based on the cumulative energy of the acoustic emission signals, the cumulative number of acoustic emission events, and the non-uniformity index of the temperature field, the damage degree, which characterizes the degree of coal body fracturing, is calculated. When the damage degree reaches a preset threshold, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered.

[0053] The dynamic control also includes: based on the spatial distribution of acoustic emission signals monitored in real time, controlling the opening of the electrically controlled dynamic sealing valves corresponding to the branch holes at different positions to guide the crack to expand in a specific direction.

[0054] The damage degree is calculated by weighted summation of the normalized cumulative acoustic emission energy, the temperature field inhomogeneity index, and the cumulative number of acoustic emission events. The specific formula is as follows: ; In the formula, D represents the degree of damage; α is the weighting coefficient of the cumulative acoustic emission energy, with a value of 0.4; β is the weighting coefficient of the temperature field inhomogeneity index, with a value of 0.2; γ is the weighting coefficient of the cumulative number of acoustic emission events, with a value of 0.2; α + β + γ = 1; E n T represents the normalized cumulative acoustic emission energy. n N is the normalized temperature field inhomogeneity index; n This represents the cumulative number of acoustic emission events after normalization.

[0055] Specifically, when the calculated damage level reaches the first threshold of 0.15, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered; when the damage level reaches the second threshold of 0.55, the switch from the crack propagation and penetration stage to the crack stabilization stage is triggered.

[0056] This includes dynamically adjusting process parameters based on real-time monitoring of acoustic emission signal amplitude and temperature data. Specifically, when the acoustic emission signal amplitude is continuously below 100μV for more than 5 minutes, the liquid nitrogen or steam injection pressure at the current stage is increased by 0.5MPa, and the liquid nitrogen atomization particle size is reduced by 20μm; when the temperature deviation in a specific area is continuously exceeded 50℃, the steam injection flow rate at the corresponding location is reduced by 2m³ / h, and the inert gas accompaniment ratio is increased by 0.5 times.

[0057] S4. Operation Completion: After the fracturing operation is completed, stop the injection of medium and carry out pipeline purging and borehole sealing.

[0058] The pipeline purging process uses dry nitrogen gas at a pressure of 1 MPa for 30 minutes. The borehole sealing process uses a high-strength polyurethane sealant with an initial curing time of 15 minutes. After complete curing, the compressive strength is ≥25 MPa and the temperature range is -50℃ to 200℃.

[0059] Example 3: This example provides a method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam. The specific steps are as follows: S1. Construction and System Layout of Borehole Network: Construct a central main borehole and multiple branch boreholes arranged around it in the target coal seam to form a connected borehole network; install independently operable electrically controlled dynamic sealing valves at the entrance of each branch borehole; lay out liquid nitrogen injection pipelines for injecting atomized liquid nitrogen into the borehole network, and steam injection pipelines for injecting hot steam mixed with inert gas.

[0060] The main borehole has a diameter of 200mm, the branch borehole has a diameter of 150mm, and the branch borehole extends at a 45° angle along the direction of the maximum principal stress of the coal seam, with a length of 50m. Dry nitrogen is used as the inert gas, and deionized water is used for steam generation.

[0061] S2. Multi-stage gradient cyclic fracturing: The fracturing process includes a crack initiation stage, a crack propagation and penetration stage, and a crack stabilization stage. Each stage includes one or more alternating cycles of liquid nitrogen cold shock and hot steam excitation. The liquid nitrogen shock pressure and hot steam temperature used in the crack propagation and penetration stage are lower than the corresponding values ​​used in the crack initiation stage. The liquid nitrogen shock pressure and hot steam temperature used in the crack stabilization stage are also lower than the corresponding values ​​used in the crack propagation and penetration stage.

[0062] During the crack initiation stage, the liquid nitrogen impact pressure was 5 MPa, the hot steam temperature was 300℃, and the volumetric injection ratio of inert gas to steam was 1.0:5.5. The liquid nitrogen cold impact was performed in a pulsed microcirculation mode, with each microcirculation including an injection period and an interval period. The microcirculation cycle was 6s, the injection period was 4.2s, and the interval period was 2.4s. In this stage, the alternating cycles of liquid nitrogen cold impact and hot steam excitation were performed twice, with an interval of 10min.

[0063] During the crack propagation and penetration stage, the liquid nitrogen impact pressure was 3 MPa, the hot steam temperature was 250℃, and the volumetric injection ratio of inert gas to steam was 1.0:3.2. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode, with a micro-circulation cycle of 7s, an injection period of 4.5s, and an interval of 2.8s. In this stage, the alternating cycles of liquid nitrogen cold impact and hot steam excitation were performed 5 times, with a cycle interval of 5min.

[0064] During the crack stabilization stage, the liquid nitrogen impact pressure was 2 MPa, the hot steam temperature was 180℃, and the volumetric injection ratio of inert gas to steam was 1.0:1.1. The liquid nitrogen cold impact was carried out in a pulsed micro-circulation mode, with a micro-circulation cycle of 8s, an injection period of 5.0s, and an interval of 3.0s. In this stage, the liquid nitrogen cold impact and hot steam excitation were alternated twice, with a cycle interval of 3min.

[0065] S3. Damage Feedback and Dynamic Control: During the fracturing process, the acoustic emission signals and temperature field data of the coal seam are monitored in real time. Based on the cumulative energy of the acoustic emission signals, the cumulative number of acoustic emission events, and the non-uniformity index of the temperature field, the damage degree, which characterizes the degree of coal body fracturing, is calculated. When the damage degree reaches a preset threshold, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered.

[0066] The dynamic control also includes: based on the spatial distribution of acoustic emission signals monitored in real time, controlling the opening of the electrically controlled dynamic sealing valves corresponding to the branch holes at different positions to guide the crack to expand in a specific direction.

[0067] The damage degree is calculated by weighted summation of the normalized cumulative acoustic emission energy, the temperature field inhomogeneity index, and the cumulative number of acoustic emission events. The specific formula is as follows: ; In the formula, D represents the degree of damage; α is the weighting coefficient of the cumulative acoustic emission energy, with a value of 0.6; β is the weighting coefficient of the temperature field inhomogeneity index, with a value of 0.3; γ is the weighting coefficient of the cumulative number of acoustic emission events, with a value of 0.3; α + β + γ = 1; E n T represents the normalized cumulative acoustic emission energy. n N is the normalized temperature field inhomogeneity index; n This represents the cumulative number of acoustic emission events after normalization.

[0068] Specifically, when the calculated damage level reaches the first threshold of 0.25, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered; when the damage level reaches the second threshold of 0.65, the switch from the crack propagation and penetration stage to the crack stabilization stage is triggered.

[0069] This includes dynamically adjusting process parameters based on real-time monitoring of acoustic emission signal amplitude and temperature data. Specifically, when the acoustic emission signal amplitude is continuously below 100μV for more than 5 minutes, the liquid nitrogen or steam injection pressure at the current stage is increased by 1.0MPa, and the liquid nitrogen atomization particle size is reduced by 50μm; when the temperature deviation in a specific area is continuously exceeded by 50℃, the steam injection flow rate at the corresponding location is reduced by 3m³ / h, and the inert gas accompaniment ratio is increased by 1.0 times.

[0070] S4. Operation Completion: After the fracturing operation is completed, stop the injection of medium and carry out pipeline purging and borehole sealing.

[0071] The pipeline purging process uses dry nitrogen gas at a pressure of 2 MPa for 40 minutes. The borehole sealing process uses a high-strength polyurethane sealant with an initial curing time of 30 minutes. After complete curing, the compressive strength is ≥25 MPa and the temperature range is -50℃ to 200℃.

[0072] Comparative Example 1: The only difference in Comparative Example 1 is that, in all stages of the steam excitation step in S2, no inert gas is injected; only pure saturated steam at the same temperature, pressure, and flow rate is injected into the borehole network. All other steps, equipment, and process parameters are completely consistent with Example 1.

[0073] Comparative Example 2: The only difference in Comparative Example 1 is that in step S2, a multi-stage gradient cycle is not performed. The entire fracturing process uses a fixed set of parameters for alternating cycles of liquid nitrogen cold shock and hot steam excitation. The fixed parameters are: liquid nitrogen shock pressure 3.0 MPa, hot steam temperature 245°C, and inert gas to steam volume ratio of 1.0:3.0. All other steps and parameters, such as damage-based feedback control, pulse injection, and directional control, are completely consistent with Example 1.

[0074] Comparative Example 3: The only difference in Comparative Example 1 is that in step S3, damage degree is not calculated, and stage switching is not triggered based on damage degree threshold. Instead, a fixed number of cycles is used for control: the crack initiation stage, crack propagation and penetration stage, and crack stabilization stage are executed 2, 4, and 2 times respectively, before mechanically transitioning to the next stage. All other steps and parameters regarding gradient parameters, inert gas accompaniment, pulse injection, and directional control are completely consistent with Example 1.

[0075] Comparative Example 4: The only difference in Comparative Example 1 is that it employs a conventional liquid nitrogen-thermal steam simple alternating fracturing method. Specifically, it does not involve the injection of inert gas; there is no multi-stage gradient, and the parameters are fixed throughout: liquid nitrogen pressure 3.0 MPa, steam temperature 250°C; liquid nitrogen is injected continuously, not pulsed; there is no damage feedback control, and the alternating cycles are fixed at 6 times; there is no directional fracturing control, and all branch holes are always open. Only the basic drilling layout, alternating medium injection, and operation completion steps are retained.

[0076] Experiment 1: Coal Seam Permeability Test This experiment was conducted in accordance with GB / T29119-2012 "Method for Determining Permeability of Coalbed Methane Reservoirs". The test objects covered the fractured coal seam areas corresponding to Examples 1-3 and Comparative Examples 1-4. All tests were conducted in parallel test areas of the same target coal seam to ensure that the geological conditions such as coal seam thickness, burial depth, original permeability, and coal body composition were consistent in each test area. Before the test, the original permeability benchmark value of each test area was determined. Three test points at different depths were arranged in each test area, namely the top, middle, and bottom of the target coal seam. The steady-state method was used for permeability testing, and the test pressure was set at 2 MPa. After the pressure stabilized, each test point was continuously monitored for 15 minutes, and the stable flow rate data was recorded and the original permeability was calculated. Subsequently, the fracturing operation was completed according to the process of each example and comparative example. After fracturing, the coal seam was allowed to stand for 24 hours to allow the coal seam stress to recover and stabilize. Then, the permeability of the fractured coal seam was tested again using the same test point arrangement, and the test parameters were consistent with the original permeability test. During the experiment, detailed data such as test pressure, stable flow rate, and test time were recorded for each test point. The permeability values ​​before and after fracturing in each test area were calculated according to the formula specified in GB / T29119-2012.

[0077] Experiment 2: Slit Network Feature Testing This experiment was conducted in accordance with DZ / T0288-2015 "Methods for Investigation and Characterization of Rock Fracture Networks," corresponding one-to-one with the test areas of Experiment 1, and simultaneously covering all embodiments and comparative examples. High-definition borehole cameras were used for testing, with observations conducted in the central main borehole and three representative radial branch boreholes in each test area. The observation depth range covered the entire thickness of the target coal seam, with an observation interval of 1 meter. Three high-definition images were acquired at each observation point from different angles. During the experiment, the number of fractures, fracture length, fracture width, fracture orientation, and the connectivity between fractures and boreholes were recorded at each observation point. Image analysis software was used to process the acquired images and extract fracture network characteristic parameters, including fracture penetration rate, average fracture density, and maximum fracture width.

[0078] Experiment 3: Stability Test of the Fracture Initiation Process This experiment was conducted in accordance with MT / T1087-2008 "Technical Specification for Acoustic Emission Monitoring of Coal Seams". Tests were performed synchronously throughout the entire fracturing process in each embodiment and comparative example. The test system used acoustic emission monitoring sensors and temperature monitoring sensors compatible with each embodiment and comparative example, and the sensor placement and number were consistent with the deployment requirements in each fracturing process. Acoustic emission signal parameters and temperature field data were collected and recorded in real time throughout the fracturing process. Noise reduction and validity assessment of the acoustic emission signals were performed according to the requirements of MT / T1087-2008. The number of times the acoustic emission signal amplitude was below 100 μV for more than 5 minutes, the number of times the temperature deviation exceeded 50°C, and the number of unstable events such as sudden pressure increases and decreases were statistically analyzed in each embodiment and comparative example to quantitatively characterize the stability of different fracturing processes.

[0079] The coal seam permeability test data are shown in Table 1.

[0080] Table 1:

[0081] Note: All test areas are parallel test zones of the same target coal seam, with consistent original permeability, conforming to the conventional value range of low-permeability coal seams.

[0082] The test data for the characteristics of the fracture network are shown in Table 2.

[0083] Table 2:

[0084] Note: The fracture penetration rate is the ratio of the length of the penetrated fracture to the total fracture length, and the average fracture density is the number of fractures around a unit length of borehole. Both indicators directly reflect the integrity and uniformity of the fracture network.

[0085] The stability test data of the cracking process are shown in Table 3.

[0086] Table 3:

[0087] As can be seen from Examples 1-3 and Comparative Example 1, and Tables 1 and 2, when pure saturated steam is used for thermal excitation, the phase change behavior and heat transfer process of steam in coal fractures are altered due to the lack of dilution and carrying effect of inert gas medium. This affects the uniformity of thermal stress distribution and transfer efficiency in the coal body, and thus restricts the uniform propagation and network connectivity of cracks.

[0088] As can be seen from Examples 1-3 and Comparative Example 2, and Tables 1 and 2, using fixed parameters for crack initiation cycles failed to adjust the energy input intensity and medium ratio according to different development stages such as crack initiation, propagation, and stabilization. This resulted in a mismatch between energy distribution and the dynamic development requirements of cracks, which affected the formation efficiency and structural characteristics of the crack network and may induce excessive fragmentation or insufficient propagation in local areas.

[0089] As can be seen from Examples 1-3 and Comparative Example 3, and Tables 1 and 3, when a fixed number of cycles is used instead of feedback control based on damage degree, the stage switching and progress advancement of the fracturing process no longer depend on the real-time characterization of the coal seam fracturing state. This leads to a disconnect between process execution and actual coal seam response, which increases the uncertainty and blindness of process control and affects the stability and repeatability of fracturing effect.

[0090] Combining Examples 1-3 and Comparative Example 4 with all experimental data, it can be seen that, in the absence of conventional alternating methods that lack several key features of this scheme, such as inert gas accompaniment, multi-stage gradient, pulsed microcirculation injection, damage feedback regulation, and directional fracturing control, the various process steps are simply superimposed. Effective synergy is not formed between dimensions such as medium properties, energy input, process control, and spatial guidance, resulting in systemic limitations in terms of permeability improvement, fracture network construction, and operational stability.

[0091] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam, characterized in that: Includes the following steps: S1. Construction and System Layout of Borehole Network: Construct a central main borehole and multiple branch boreholes arranged around it in the target coal seam to form a connected borehole network; install independently operable electrically controlled dynamic sealing valves at the entrance of each branch borehole; lay out liquid nitrogen injection pipelines for injecting atomized liquid nitrogen into the borehole network, and steam injection pipelines for injecting hot steam mixed with inert gas. S2. Multi-stage gradient cyclic fracturing: Execute a fracturing process that includes at least a crack initiation stage and a crack propagation and penetration stage, each stage including one or more alternating cycles of liquid nitrogen cold shock and hot steam excitation; wherein, the liquid nitrogen shock pressure and hot steam temperature used in the crack propagation and penetration stage are both lower than the corresponding values ​​used in the crack initiation stage. The crack initiation process also includes a crack stabilization stage performed after the crack propagation and penetration stage. The liquid nitrogen impact pressure and hot steam temperature used in the crack stabilization stage are both lower than the corresponding values ​​used in the crack propagation and penetration stage. S3. Damage Feedback and Dynamic Control: During the fracturing process, the acoustic emission signals and temperature field data of the coal seam are monitored in real time. Based on the cumulative energy of the acoustic emission signals, the cumulative number of acoustic emission events, and the non-uniformity index of the temperature field, the damage degree characterizing the degree of coal body fracturing is calculated. When the damage degree reaches a preset threshold, the switch from the crack initiation stage to the crack propagation and penetration stage is triggered. The dynamic control also includes: based on the spatial distribution of the acoustic emission signal monitored in real time, controlling the opening of the electrically controlled dynamic sealing valves corresponding to the branch boreholes at different locations to guide the crack to expand in a preset direction; S4. Operation Completion: After the fracturing operation is completed, stop the injection of medium and carry out pipeline purging and borehole sealing.

2. The method for increasing permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: In step S2, during the crack initiation stage, the liquid nitrogen impact pressure is 4-5 MPa, the hot steam temperature is 280-300℃, and the volumetric injection ratio of inert gas to steam is 1.0:4.5-5.

5.

3. The method for increasing permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold shock and hot steam as described in claim 2, characterized in that: During the crack propagation and penetration stage, the liquid nitrogen impact pressure is 2-3 MPa, the hot steam temperature is 220-250℃, and the volumetric injection ratio of inert gas to steam is 1.0:2.8-3.

2. During the crack stabilization stage, the liquid nitrogen impact pressure is 1-2 MPa, the hot steam temperature is 150-180℃, and the volumetric injection ratio of inert gas to steam is 1.0:0.9-1.

1.

4. The method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: In step S2, the liquid nitrogen cold shock is performed in a pulsed microcirculation manner, and each microcirculation includes an injection period and an intermittent period; the microcirculation cycle of the crack initiation stage is shorter than the microcirculation cycle of the crack stabilization stage.

5. The method for increasing permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: In step S3, the damage degree is calculated by weighted summation of the normalized cumulative acoustic emission energy, the temperature field inhomogeneity index, and the cumulative number of acoustic emission events. The weighting coefficient of the cumulative acoustic emission energy is 0.4-0.6, the weighting coefficient of the temperature field inhomogeneity index is 0.2-0.3, and the weighting coefficient of the cumulative number of acoustic emission events is 0.2-0.3, and the sum of the three weighting coefficients is 1.

6. The method for increasing permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: In step S3, when the calculated damage degree reaches the first threshold of 0.15-0.25, the switch from crack initiation stage to crack propagation and penetration stage is triggered; when the damage degree reaches the second threshold of 0.55-0.65, the switch from crack propagation and penetration stage to crack stabilization stage is triggered.

7. The method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: Step S3 also includes dynamically adjusting the process parameters based on the real-time monitored acoustic emission signal amplitude and temperature data, specifically including: When the acoustic emission signal amplitude is monitored to be below 100µV for more than 5 minutes, increase the liquid nitrogen or steam injection pressure at the current stage by 0.5-1.0MPa and reduce the liquid nitrogen atomization particle size by 20-50µm. When the temperature deviation in the corresponding area is monitored to exceed 50℃ for an extended period, reduce the steam injection flow rate at the corresponding location by 2-3 m³ / h and increase the inert gas injection ratio by 0.5-1.0 times.

8. The method for fatigue-induced fracturing and permeability enhancement of coal seams using alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: In step S1, the diameter of the main borehole is 150-200mm, the diameter of the branch borehole is 100-150mm, the branch borehole extends along the direction of the maximum principal stress of the coal seam at an angle of 30-45°, and the length is 20-50m; in step S4, the pipeline purging uses dry nitrogen gas at a pressure of 1-2MPa and continues to purge for 30-40min.

9. The method for increasing permeability of coal seams through fatigue fracturing caused by alternating liquid nitrogen cold shock and hot steam as described in claim 1, characterized in that: In step S4, the borehole sealing is performed using a high-strength polyurethane sealant. The initial curing time of the sealant is 15-30 minutes, and after complete curing, the compressive strength is ≥25MPa, and the temperature resistance range is -50℃ to 200℃.

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