To explore the dynamic characteristics and energy dissipation laws of coal fracture instability under varied impact loads, using an improved Hopkinson bar (SHPB) test system was used to conduct one-dimensional impact load, impact load and axial static load coupling dynamic tests. The dynamic characteristics of coal samples under different impact speeds and axial static loads were studied, analyzing the macroscopic fracture morphology and pore evolution of coal samples were analyzed, the mechanism of coal rock fracture instability from the perspective of energy dissipation. The research results indicate that the stress-strain curves of coal samples under different impact load disturbances all include three stages: linear elasticity, plasticity, and plastic softening. Under one-dimensional impact load, the peak strength and peak strain of coal samples exhibit significant strain rate effects. As the strain rate increases, the peak strength and peak strain of coal samples gradually increase; Screening statistics show the mass distribution of broken fragments with different particle sizes. As the strain rate increases, the mass of larger particles decreases while the mass of smaller particles increases; The incident energy, reflected energy, and dissipated energy of coal samples gradually increase with the increase of impact load, and the energy dissipation density increases exponentially; Under the coupling effect of impact load and axial static load, a low-speed impact load of 5.54 m/s is set. As the axial static load increases, the peak strength continues to increase and the peak strain linearly weakens; Based on the characterization of pore structure using nuclear magnetic resonance (NMR) experiments, the internal micropores of coal samples continuously develop with the increase of axial static load, and the expansion trend of cracks along the axial direction increases; The incident energy of coal samples remains stable with the continuous increase of axial static load, while the reflection energy decreases and the dissipation energy gradually increases. The energy dissipation density increases linearly. According to the energy dissipation mechanism of coal rock, the initiation, expansion, and penetration of pores induce the occurrence of fracture and instability in coal; In the initial stage of axial static load, more dissipated energy was used for the development of internal micropores and crack expansion. Under the instantaneous disturbance of impact loads, the formation of macroscopic fracture within the coal is induced, which ultimately results in large-scale fracture instability. Under the instantaneous disturbance of impact load, it will induce the formation of macroscopic fracture surfaces in the coal body, resulting in large-scale fracture instability.

The displacement of methane by injecting nitrogen is an effective method for improving extraction efficiency in low-permeability coal seams. By conducting the experiments on uniform/pulsed pressure nitrogen injection for methane displacement (UPNI-MD/PPNI-MD), the coal deformation characteristics are quantitatively characterized, and the coal deformation mechanism is discussed. The results show that the technology of methane displacement by PPNI-MD is more effective in enhancing the permeability of low-permeability coal seams. The pressure of nitrogen injection and the amount of methane/nitrogen adsorption are crucial factors affecting the coal deformation during the process of UPNI-MD/PPNI-MD. The erosive effect of methane adsorption on the mechanical properties of coal decreases, while the erosive effect of nitrogen adsorption on the mechanical properties of coal shows a "wave-like" variation in the case of PPNI-MD. The application of PPNI-MD technology can effectively decrease the coal expansion deformation while undergoing the methane displacement process. Nitrogen injection to displace methane breaks the system energy balance of coal, resulting in deformation. And the coal deformation during nitrogen injection for methane displacement depends on the energy carried by the injected nitrogen, the energy change during gas competitive adsorption and the heat exchange between nitrogen and coal. The effective stress of coal depends on the change in methane/nitrogen pressure. The mechanical properties of coal depend on the amount of methane/nitrogen adsorption and the pore pressure. High-energy nitrogen damage to coal during the process of PPNI-MD is manifested in coal deformation caused by methane/nitrogen pressure, coal deformation caused by methane/nitrogen adsorption, and damage to coal mechanical properties caused by methane/nitrogen adsorption.

The development and utilization of deep formation resources are easily disrupted by impact loads. To investigate what effect of impact on the pore structure and energy evolution of coal, the dynamic compression tests were performed by using the Split Hopkinson Pressure Bar (SHPB) test system. The fractal characteristics of macro cracks were analyzed by box dimension, the micro-pores structure and fractal features of coal samples were studied about nuclear magnetic resonance (NMR), which clarified the intrinsic relationship between fracture structure characteristics and energy dissipation. The results showed that with increasing impact velocity from 1.27 m/s to 4.90 m/s, the dynamic strength and peak strain increased by 85.11 % and 53.76 %, respectively. The fractal dimension of the cracks grew by 26.87 %, and the fractal dimension of pore network and full aperture decreases gradually. With increasing impact velocity, the fracture dissipation energy and energy dissipation rate of coal samples increase exponentially. As the energy dissipation rate increases, the cracks fractal increases in a quadratic function relationship and the pores fractal decreases continuously. Low-velocity impacts induce dislocation plugging between coal matrix crystals, while impact effect causes more dislocations to form stress concentrations at pore tips. When the energy accumulation reaches its maximum value, the content of mesopores and macropores together with the pore connectivity increases. Instantaneous disturbance creates more macroscopic fracture surfaces in the coal, resulting in large-scale fracture instability. This research findings will provide some theoretical foundations to understand the formation mechanism of dynamic disasters in deep mines.

Gas injection displacement is a widely used technique to enhance the recovery of coal methane (CH4) or oil reservoirs. Additionally, this method plays a crucial role in effectively developing CH4 resources in a closed goaf. In the present study, the distribution characteristics of fissures in a closed goaf were determined using a physical simulation test, and the connected fissure network was extracted and modeled. Further, CH4 extraction numerical simulation tests were conducted on the connected fissure network under gas injection conditions. The migration and distribution characteristics of CH4 were analyzed in the connected fissure network under different gas injection conditions, including gas injection position, gas injection rate, and gas injection type. The research also evaluated the impact of different gas injection conditions on CH4 production in vertical wells. Finally, a model was developed to determine characteristic parameters for CH4 production in vertical production wells, which were calculated and compared across different gas injection conditions. Results revealed a negative correlation between CH4 volume fraction and gas injection time in vertical wells under different gas injection conditions, contrary to the S-type growth curve. Gas injection positions significantly influenced the migration and distribution of gas within the connected fissure network, with higher CH4 productivity and production efficiency in the gas injection position P-2 compared to P-1. The increase in the gas injection rate enhanced CH4 production efficiency, albeit having little effect on CH4 productivity. Gas injection types yielded no significant influence on CH4 production efficiency, although CH4 productivity was lower with 100% CO2 injection compared to 100% N-2 and mixed gas. This investigation provides the foothold for enhancing the recovery of CH4 in closed goafs and contributes to the progress of carbon emission reduction technology in coal mining areas.

To realize the resource utilization of solid waste (coal slime) and further the dual carbon goals, utilizing coal slime and coal ash as adsorbates for CO2 capture is crucial. This study employed low-temperature N2 adsorption, low-pressure CO2 adsorption, X-ray diffraction, X-ray fluorescence, and isothermal adsorption tests to assess coal slime and coal ash’s pore/mineral composition characteristics. Subsequently, the influence on CO2 adsorption was analyzed to reveal the CO2 adsorption mechanisms of pores and clay minerals, and CO2 molecule adsorption behavior. The results showed that: (1) ashing led to reductions in total pore volume, specific surface area, micropore volume, and micropore specific surface area, accompanied by substantial decreases in micropores and mesopores; (2) ashing generated high-temperature stable mineral species, including quartz, andalusite, hematite, and gypsum, while all calcite decomposed into CaO; (3) coal slime exhibited greater CO2 adsorption capacity than coal ash, influenced by pore structure and clay minerals; (4) the adsorption behavior of coal slime and coal ash likely aligns with micropore filling theory, suggesting CO2 is adsorbed within the 0.30–1.47 nm pore structure. This research contributes to optimizing coal by-product utilization in mining areas and exploring adsorbate materials for CO2 sequestration in abandoned goaf.

Uniaxial compression experiments were conducted on coal samples with different initial damage degrees, and the acoustic emission (AE) signals of the whole deformation and destruction process were captured synchronously. The distribution patterns of the acoustic emission signals at different loading stages were discussed. The chaotic system was introduced to process the acoustic emission series. The results indicate that the acoustic emission responses of coal samples with different damage levels had obvious stage characteristics during the whole destruction process. According to the information dimension fitting curves, the coal samples had good fractal characteristics in the whole loading process, and the value of information dimension decreased with the increased of threshold value.The information dimension of the acoustic emission energy signals showed a dynamic evolution law of “increase-decrease-increase” with time, and the rapid decreased of the information dimension could be used as a stable and effective precursor for predicting the macro-damage of coal.

With the rapid development of Carbon Capture, Utilization and Storage (CCUS) technology, it is necessary to explore the feasibility of coal slime as a porous carbon material for CO2 capture. In this paper, scanning electron microscopy (SEM) was used to observe the morphological characteristics of coal slime samples with different metamorphic degrees, and the pore structure of coal slime was explored by low temperature N2 adsorption and low-pressure CO2 adsorption experiments. The pore distribution characteristics were analyzed, and the adsorption law of different metamorphic degrees were summarized through CO2 isothermal adsorption experiments. The results showed that: The specific surface area (SSA) and pore volume (PV) of the mesopores of the coal slime exhibited a U-shaped distribution with coal rank, and are much smaller than that of its micropores. Micropores less than 2 nm are the main adsorption space of coal slime, its PV accounted for 59%, 60%, 71%, and SSA accounted for 92%, 93%, 95%, obviously, which are dominant at all stages. The linear correlation fitting coefficients R2 between the limiting adsorbed amount a of CO2 and the micropores PV and the SSA were up to 0.830 and 0.887, respectively. The coal slime has good adsorption performance for CO2. Based on the Langmuir model to fit the limit adsorption amount, a-value can reach 41.774 cm3 g-1, 32.072 cm3 g-1, 38.457 cm3 g-1 at 303 K with the increase of Rmax. Studying the impact of coal slime on CO2 adsorption performance provides a theoretical basis for the subsequent preparation of energy storage materials and is of great significance for the safe, efficient and economic capture and sequestration of CO2, to alleviate the serious situation of the environment and realizing the dual-carbon goal.

The resource endowment characteristics of "abundant coal, scarce oil, limited methane" in China determine that coal will remain the fundamental energy source, maintaining its predominant position. Realizing the green and low-carbon operation of the precise methane extraction technology system in the entire life cycle of coal mining is one of the key tasks for the sustainable development of the coal industry. In response to the new situation requirements of the "carbon peak and carbon neutrality" background and the technical challenges of carbon emission reduction at the source of methane in coal mines, with "quality improvement and efficiency enhancement, carbon emission reduction" as the core, and "precise methane extraction, green and low-carbon" dual objectives as the guide. With "process technology and equipment intelligent innovation" as the main line, based on the technical aspects of the complete process of coal mine methane extraction, the technology system of precise methane extraction in the entire life cycle of coal mining has been actively explored and constructed. It has realized the coordinated coupling of precise methane extraction, and green and low-carbon. A carbon emission reduction technology system has been formed at the source of coal mine methane with reliable technology - low-consumption equipment - safety and intelligence. Subsequently, five key technical issues were investigated: precise prediction of methane content - methane emission volume - methane "sweet spot" resource areas, precise pre-extraction of methane in the mining coal seam by the "time-sharing + partitioning" method, precise extraction of pressure relief methane from mining by the "regional segmentation + directional" method, precise extraction of methane from the "sweet spot" resource areas in abandoned mine, and "dynamic + intelligent" precise control of coal seam methane extraction. Finally, the future development directions of the precise methane extraction technology system for coal mining throughout its life cycle under the goal of "carbon peak and carbon neutrality" are elaborated, including the deep integration of "dynamic - transparent" coal geology and "intelligent - precise" methane pre-extraction, the deep integration of "dynamic - transparent" transport and storage area of pressure relief methane and "precise - regional segmentation" directional extraction, enhanced methane extraction technology of the methane "sweet spot" resource area through CO2 absorption and storage in abandoned mine, the integrated platform of "precise prediction - intelligent design, precise monitoring - intelligent assessment, precise diagnosis - intelligent regulation", and "Smart+" technology combining precise methane extraction - energy saving and emission reduction for equipment - green and low-carbon for mines. By implementing the precise methane extraction technology system for the entire life cycle of coal mining under the goal of "carbon peak and carbon neutrality", new ideas are provided for achieving methane precise extraction, green and low-carbon goals in China's coal industry.

The study on the evolution characteristics of coal permeability is of great significance for rationally determin-ing gas extraction parameters and increasing gas extraction efficiency.In order to study the effects of different coal stresses and gas pressures on coal permeability,the experiment on the deformation of stress-loaded coal and gas adsorption-diffusion was conducted,the segmented dynamic model of coal permeability was established,and the rationality of the es-tablished model was verified by the experimental results.The results shown that the gas adsorption amount and coal de-formation both shown a Langmuir-type with the increasing gas pressure,and the dynamic diffusion coefficient of gas de-creased exponentially with time.As the gas pressure decreased,the expansion deformation of the stress-loaded coal de-creased,and the permeability increased gradually.The permeability and expansion deformation of stress-loaded coal gradually decreased with the increasing stress.The coal permeability shown a"V"shape with continuous stress loading,and it reached the smallest at the stress peak.The coupling between matrix and fracture deformation caused by gas adsorp-tion,the dynamic diffusion of gas in matrix,and the mass exchange between matrix and fracture were all considered in the established permeability model of coal.The rationality of established segmented model of coal permeability was verified by the experimental results.The permeability model of coal based on elastic deformation can reflect the permeability evol-ution at the stage of elastic deformation.Within the experimental range,the absolute error between the experimental test and numerical simulation results of coal permeability was-0.135×10-15～0.296×10-15 m2,and the absolute error of volu-metric strain of the coal due to gas seepage was-0.327×10-5～2.026×10-5.The permeability model considering plastic de-formation can also reflect the permeability after stress peak.The error between experimental and numerical results was-0.435×10-15～0.997×10-15 m2.

The pore structure of coal plays a key role in controlling the storage and migration of CH 4 /N 2 . The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement effect of N 2 injection. The deformation characteristic of coal during adsorption–desorption of CH 4 /N 2 is an important factor affecting CH 4 pumpability and N 2 injectability. The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. Subsequently, a simultaneous test experiment of CH 4 /N 2 adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. Finally, the applicability of low-permeability coal to N 2 injection for CH 4 displacement technology was investigated. The results show that the micropores and transition pores of coal samples are relatively developed. The pore morphology of coal is dominated by semi-open pores. The pore structure of coal is highly complex and heterogeneous. Transition pores, mesopores and macropores of coal have good connectivity, while micropores have poor connectivity. Under constant triaxial stress, the adsorption capacity of the coal for CH 4 is greater than that for N 2 , and the deformation capacity of the coal for CH 4 adsorption is greater than that for N 2 adsorption. The axial strain, circumferential strain, and volumetric strain during the entire process of CH 4 and N 2 adsorption/desorption in the coal can be divided into three stages. Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-difference. A strain model for the adsorption–desorption of CH 4 /N 2 from coal was established by considering the expansion stress of adsorbed gas on the coal matrix, the compression stress of free gas on the coal matrix, and the expansion stress of free gas on micropore fractures. N 2 has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas flow, which can significantly improve the effectiveness of low-permeability coal seam gas control and promote the efficient utilization of gas resources.

In order to further study the temporal and spatial characteristics of coal deformation during the interaction between loaded coal and CH 4 /N 2 . The dynamic test of adsorption capacity and adsorption deformation of raw coal adsorbing CH 4 /N 2 and its binary mixture under triaxial stress loading were carried out. The results showed that the adsorption capacity of CH 4 in raw coal was greater than that of N 2 under constant triaxial stress. The adsorption capacity of binary mixed gas was between that of pure gas. There was anisotropy in the adsorption expansion deformation of raw coal under triaxial stress. Under different gas injection pressure conditions, the axial/circumferential strain of raw coal presented the same evolution law with the adsorption time. The axial strain of the gas injection end could be divided into 5 stages, and the axial/circumferential strain in the middle and exhaust ends of the coal sample could be divided into 3 stages. Under the same gas injection pressure, when raw coal adsorbed gas, the time order of axial strain changed from negative to positive in each measuring point was CH 4 , CH 4 /N 2 mixed gas, N 2 . When the raw coal adsorbed CH 4 , N 2 and CH 4 /N 2 mixed gas, the axial/circumferential strain of raw coal increased with the increase of adsorption capacity in the order of exponential function, unary linear function and unary quadratic function. After triaxial stress loading, the axial/circumferential initial strain of raw coal was different, and the initial strain of the raw coal was distributed was small at both ends and large in the middle. When raw coal adsorbed CH 4 under triaxial stress, the expansion deformation ability was stronger, it had large reduction of surface free energy, and obvious reduction of coal strength, which made it more prone to deformation and failure.

Overburden strata fracture evolution is critical to dynamic disaster prevention and gas-relief drainage, so it is important to accurately determine the evolution relationships with mining disturbance. In this paper, experiments and numerical simulation were adopted jointly to characterize the time-varying fracture area of overlying strata. The experimental results showed that the roof strata gradually broke and collapsed with coal mining, which indicated the fractures of overburden strata developed in an upward direction. The fracture development causes were explained by numerical simulation, which showed that stress increase exceeded the strength of coal and rock strata, and fractures were formed and expanded. Both experiments and numerical simulation results showed the two sides and the top of fracture areas provided channels and spaces for gas migration and reservoir, respectively. In addition, the breaking angle of overburden strata and the height of fracture areas were analyzed quantitatively. Through microseismic monitoring at the mining site, the fracture scales and ranges of overburden strata were verified by the energy and frequency of microseismic events, which were consistent with the support of maximum resistance. The position of drainage boreholes was considered based on the results of overburden strata fracture evolution. Our study is aimed at promoting coal mining in safety and improving gas drainage with a sustainable approach.

CH4 adsorption-desorption-seepage characteristics of coal is an important indicator to measure the gas extraction capacity of coal seam. The deformation of coal caused by CH4 adsorption-desorption-seepage is an important factor affecting the extraction ability of CH4. Synchronous test experiments of CH4 adsorption-desorption-seepage and whole-process coal deformation under different gas pressures were carried out by using the self-developed comprehensive test experimental system of gas-solid coupling for coal measure gas. The timeliness characteristics of CH4 adsorption-desorption-seepage and coal deformation were analyzed. The influence of gas pressure on CH4 adsorption-desorption-seepage and coal deformation was clarified. The quantitative relationship between CH4 adsorption-desorption-seepage and coal deformation was discussed. A whole-process strain model of coal for CH4 adsorption-desorption-seepage was established. The results showed that increasing gas pressure could promote CH4 adsorption-desorption-seepage of coal. The higher the gas pressure was, the greater the capacity of CH4 adsorption-desorption-seepage in coal was. Increasing gas pressure could promote the expansion/contraction deformation caused by CH4 adsorption-desorption-seepage in coal. The higher the gas pressure was, the stronger the deformation ability of CH4 adsorption-desorption-seepage in coal was. The expansion or contraction deformation caused by CH4 adsorption-desorption-seepage in coal was simultaneously affected by coal bedding structure and external stress. The axial/circumferential strain caused by CH4 adsorption-desorption-seepage in coal showed exponential function, quadratic function, and exponential function growth relationship with adsorption capacity, desorption capacity, and seepage capacity of CH4, respectively. The stress in the process of CH4 adsorption-desorption-seepage in coal covered the expansion stress of adsorbed CH4 to coal matrix, the compression stress of free CH4 to coal matrix, and the expansion stress of free CH4 to coal microporous fracture. Considering three kinds of deformation effects by CH4 in two states, the whole process strain model of CH4 adsorption-desorption-seepage in coal was established, which could effectively reflect the deformation timeliness characteristics of the whole process of CH4 adsorption-desorption-seepage in coal. This research provides some theoretical support for the efficient extraction of coal seam gas.

In deep underground mine engineering, the critical warning signals before the sudden failure of coal are crucial to predict coal or rock dynamic catastrophes and to help the coal industry grow sustainably. Therefore, with the objective of accurately identifying the precursor signals of coal fracture, a uniaxial compression test was adopted. Tests were performed on multiple sets of raw coal samples, and acoustic emission (AE) technology was used to capture the deformation and destruction courses of the coal samples. Furthermore, the signal intensity of AE energy was discussed. Based on the critical slowing down theory, the AE energy sequence was processed. The results indicate that there are significant discrepancies in the strength of coal affected by initial pore fissures. During the whole loading process, the AE energy signals showed obvious stage characteristics, and there was a high risk of rapid coal energy storage during the unstable rupture development (URD) stage, which predicted the imminent destruction of the coal. The variance mutation point that was not affected by the lag step selection was easier to identify than that of the autocorrelation coefficient, and the precursor points were all in the URD stage, which is more accurate than using the AE cumulative energy curve slope.

为揭示不同温度下瓦斯吸附-解吸-渗流全过程煤体变形的差异性,应用 自主研发的煤体瓦斯流固耦合试验系统,研究三轴应力加载下瓦斯吸附-解吸-渗流及全过程煤体变形随温度变化的响应特征.试验结果表明:瓦斯吸附阶段,煤体变形量与吸附时间呈Langmuir型上升变化;瓦斯解吸阶段,煤体变形量与解吸时间呈指数型衰减趋势;瓦斯渗流阶段,煤体变形量与时间呈幂函数上升趋势.瓦斯吸附量、渗透率及过程中煤体变形量均随温度升高而降低,瓦斯解吸率随温度升高而增大;煤体变形量与瓦斯吸附量、解吸量、渗透率呈正相关关系.温度效应对全过程煤体变形具有显著影响.

Most coal mining areas in China have a low coal seam permeability. Gas extraction technology by gas injection displacement for increasing flow is one of effective methods to improve the efficiency of gas extraction in low-permeability coal seams. In this study, the gas occurrence characteristics of low-permeability coal seam in China were reviewed and classified. Five characteristics were summarized, including high resource reserves, low gas saturation, coexistence of multiple reservoir pressures, low-permeability, and complex partitioning. Based on the method of difference confidence, grey correlation and confidence interval calculation, the applicability of coal seam gas in China to the gas extraction technology by underground gas injection displacement for increasing flow was analyzed. Six direct factors affecting the effect of gas injection displacement were obtained. The influence degree was as follows: Platts coefficient ＞ Gas pressure ＞ Permeability ＞ Ash content ＞ Gas content ＞ Langmuir pressure constant. A screening and evaluation method of coal seam parameters was established which was suitable for the gas drainage technology by underground gas injection displacement for increasing flow. According to the different mechanisms of increasing gas flow in gas injection displacement, the gas flow increasing mechanism of gas injection displacement was clarified from two aspects: gas factor increasing flow（carry CH 4 with filling energy to increase flow, promote CH 4 desorption by sharing pressure to increase flow, promote CH 4 diffusion by diluting to increase flow, and competitive adsorption to increase CH 4 flow） and energy injection modification increasing flow（enlarge pores with filling energy to increase CH 4 flow, enlarge pores with promoting desorption to increase CH 4 flow, expand pores with promoting desorption to increase CH 4 flow, and damaged pore structure/spread fracture network by pulse gas flow to increase CH 4 flow）. The influence of gas source and process parameters on the gas injection displacement effect was summarized. A brief introduction to the key technology and equipment, application mode and application effect of the underground gas extraction technology by gas injection displacement for increasing flow. The future development trend of the underground gas extraction technology by gas injection displacement for increasing flow in China was prospected, including multidisciplinary deepening the theory of gas extraction by gas injection displacement for increasing flow, multimethod collaborative the technology of gas extraction by gas injection displacement for increasing flow, precision of the parameters of gas extraction by gas injection displacement for increasing flow, intellectualization of the equipment of gas extraction by gas injection displacement for increasing flow, coordination of injection-pumping-excavation-mining connection layout in coal seam.

Gas extraction is the fundamental way to prevent and control gas as well as to obtain clean energy. Based on the analysis of the mechanism of conventional negative-pressure gas extraction technology and N2 injection enhanced gas extraction technology, the technology of pulse pressure N2 injection enhanced gas extraction is proposed. The experiments of CH4 displacement by uniform pressure/pulse pressure N2 injection are carried out. The migration pattern of gas components during the whole process of CH4 displacement by N2 injection is quantitatively characterized. The influence pattern and sensitivity of N2 injection pressure on the effect of CH4 displacement are analyzed. The economic benefits of N2 injection by uniform pressure/pulse pressure to displace CH4 are investigated. The results indicate that conventional negative-pressure gas extraction technology has disadvantages such as insufficient seepage power and gas extraction will soon enter the exhaustion stage. While N2 injection enhanced gas extraction technology not only has the flow increasing effect of gas itself factors but also has the flow increasing effect of injected energy for changing coal properties. During the process of displacing CH4 by uniform pressure/pulse pressure N2 injection, the CH4 volume fraction decreases and the N2 volume fraction increases as the N2 injection pressure increases. Increasing the injection pressure of two N2 injection methods can effectively shorten the T50% (the time when the CH4 volume fraction is equal to the N2 volume fraction is equal to 50%), but with a reduced degree of impact. Increasing injection pressure not only increases the amount of CH4 precipitation but also increases the amount of N2 precipitation, which is not conducive to the later utilization of the precipitated CH4. With the increase of N2 injection pressure, the sensitivity of CH4 displacement efficiency by uniform pressure N2 injection decreases, the sensitivity of CH4 displacement efficiency by pulse pressure N2 injection remains the same, and the sensitivity of CH4 displacement ratio by uniform pressure/pulse pressure N2 injection decreases. The CH4 prevent and control effect, N2 injection cost, and CH4 resource utilization of pulse pressure N2 injection are better than that of uniform pressure N2 injection. The technology of enhanced gas extraction by pulse pressure N2 injection not only improves the CH4 prevent and control effect but also reduces N2 injection cost. At the same time, it promotes the efficient utilization of CH4 resources. The research results have great significance and value for improving the level of gas disaster prevention and resource development.

To reveal fracture mechanism of gas-bearing coal subjected to complex geology environment, the impact dynamics experiments were conducted to study energy characteristics based on split Hopkinson pressure bar (SHPB) system. The incident, reflected and transmission strain were collected to calculate various energy. It was found reflected strain was always larger than transmission strain. Therefore, the reflected energy was generally higher than transmission energy under different loading conditions, but they were smaller than incident energy. With time evolution, both elastic deformation and dissipative energy experienced slow increase, rapid increase, peak point and decrease stage regardless of loading conditions. Before macro failure, micro-meso fractures had changed drasticly, which also involved intense energy conversion. So dissipative energy peak was earlier than that of elastic deformation energy. Due to pre-damage of static load and weakening effects of gas, the dissipative energy decreased with their increases (static load from 2.00 to 9.00 MPa and gas pressure from 0.25 to 1.50 MPa) during impact fracture process. However, at high confining pressure and dynamic load environment, the impact failure of gas-bearing coal exhausted massive energy. These energy characteristics will provide guidances to prevent and control disaster during coal mining and coal seam gas (CSG) exploitation in deep area. (c) 2021 Elsevier Ltd. All rights reserved.

深部煤炭开采受顶底板破断、断层活化、相邻采掘活动和工程爆破等扰动,引起煤体瓦斯短时间大量放散,进而造成采掘作业空间瓦斯超限.为研究扰动作用下煤体瓦斯异常放散规律,建立了含瓦斯煤分离式霍普金森压杆试验系统,进行了震动载荷下煤体瓦斯放散实验.结果表明:震动载荷下煤体瓦斯放散具有瞬时性,瓦斯放散持续时间在0～4s内,震动载荷作用瞬间,最大瓦斯放散量随着轴向静载和围压的增加而减小,分别由0.1710,0.2080 L/min降低到0.0944,0.0210 L/min,随着瓦斯压力和震动载荷冲击速度的增加而增加,分别由0.0250,0.0500 L/min增加到0.1380,0.0988 L/min;在瓦斯放散过程中,最大瓦斯放散速度随着轴向静载和围压的增加而减小,随着瓦斯压力和震动载荷冲击速度的增加而增加;为刻画瓦斯放散的难易程度,基于瓦斯渗流稳态法,提出瓦斯瞬时放散率,瓦斯瞬时放散率随着轴向静载和围压增大而呈线性减小,降低幅度分别达到44.80％和89.90％,随着瓦斯压力的增大呈非线性减小,减小幅度逐渐降低,随着震动载荷冲击速度增加而增大,增加幅度达90.89％.此研究将有助于揭示扰动诱导煤体瓦斯异常放散机制和预测不同开采环境中瓦斯放散趋势.

顺层钻孔预抽瓦斯技术是降低煤层瓦斯含量的有效方法之一,合理的布孔参数是提高煤层瓦斯预抽效果的关键.为提高顺层钻孔布孔精准性和科学性,建立了含瓦斯煤体流固耦合抽采模型,基于响应面法设计布孔参数优化方案,运用COMSOL Multiphysics模拟软件分析了地质因素(煤层瓦斯含量、透气性系数)和工程因素(抽采负压、钻孔直径、布孔间距)交互作用对钻孔预抽煤层瓦斯的影响规律,提出钻孔间最大瓦斯压力与达标压力比(Pmax/Pb)的布孔参数判定指标,创新了煤层"分时分区"式顺层钻孔预抽煤层瓦斯精准布孔方法,得到适用于不同煤层瓦斯赋存特征的最优钻孔布置参数,并进行现场试验.结果表明:预抽初期,相邻钻孔间抽采叠加效应不明显;随着预抽时间延长,抽采叠加效应越发显著,垂直钻孔方向的抽采达标区域逐渐由孤立向复合转变.不同预抽时间下,Pmax/Pb对各因素的敏感性依次为:布孔间距＞煤层瓦斯含量＞透气性系数＞钻孔直径＞抽采负压.不同预抽时间下,煤层瓦斯含量和透气性系数的交互作用响应等值线分布密集,2者对Pmax/Pb影响非常显著,并且对Pmax/Pb数值影响较大;工程因素中抽采负压与钻孔直径的交互作用响应等值线呈圆形分布,说明二者交互作用不明显;在布孔间距与地质因素交互作用下,响应等值线沿布孔间距方向更为密集,布孔间距对Pmax/Pb影响仍然是显著的,与单因素分析结果相互佐证.试验工作面采用"分时分区"式顺层钻孔精准布孔方法确定的合理布孔参数依次为:抽采负压24.0 kPa、钻孔直径113 mm、布孔间距6.0 m;经优化预抽钻孔参数后,现场瓦斯预抽效果良好且预抽达标检验合格,实现了煤层瓦斯精准预抽.