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This study integrates different geochemical analyses (chemical compositions, carbon isotopes) and a physical mixing model to quantitatively identify gas sources and migration mechanisms in the Nanchuan No.2 coal mine. Results reveal distinct isotopic signatures: gases from mine-out area exhibit intermediate δC 1 values (− 55.1‰ to − 49.6‰) between coal-seam methane (avg. δ 13 C 1  = − 61.0‰) and Yanchang oil-type gas (avg. δ 13 C 1  = − 49.7‰), indicating mixed origins. The mixing model quantifies oil-type gas contributions to goaf emissions as 74.3% (methane) and 75.5% (ethane), dominated by vertical migration from Triassic Yanchang Formation source rocks through mining-induced fractures. Low coal-seam gas content (avg. 0.97 m³/t) further supports external hydrocarbon influx. Structural heterogeneity in oil-type gas distribution correlates with spatial variations in contribution ratios (52.7–100%). Mining disturbances disrupt caprock integrity, creating pressure-relief pathways that drive gas migration via fracture networks. This work establishes a framework for optimizing gas control strategies in coal-oil-gas symbiotic systems, emphasizing the critical role of isotopic tracing in hazard mitigation.

With the help of mechanical models and numerical calculations, the research obtained: (1) the range of compressive shear stress in well bore is in the shape of “dam body” with the dip angle of a reverse fault, also, the magnitude of the compressive shear stress is related to the load factor and the maximum compressive principal stress which have an increasing relationship; (2) the superposition stress of lateral abutment stress and SZZ of a working face is σA, which is related to the distance between reverse faults; (3) the closer the distance to the reverse fault and the greater the vertical well displacement and deformation as the advancement length of the working face increases, the sensitivity to the effects of the reverse fault and mining is: XDISP > ZDISP > YDISP, and the sensitivity to the boundary is such that: ZDISP > YDISP > XIDSP; (4) the closer the distance to the reverse fault and the larger the length of the working face, the greater the displacement and deformation of the vertical well. In addition, the sensitivity to the effects of reverse faults and mining is: XDISP > ZDISP > YDISP; (5) when the working face continues to be mined, the shear stress on the well bore and the circumference of the hole is sensitive to the influences of reverse faults as follows: SXZ > SYZ > SXY; (7) the density of strain energy at the well bore is most sensitive to the lateral distance to the working face strike mining line. Based on these results, it is proposed to arrange large-diameter pressure relief boreholes around the hole and arrange layers to eliminate the influence of the well bore boundary and eliminate the accumulation of shear strain energy around such a well bore.

Coal–rock combination is an important structure for understanding the mechanism of coal and gas outburst, rock burst and coal roadway deformation. In the study, theoretical analysis and physical similarity simulation were adopted to investigate the influencing law and mechanism of coal thickness on the compressive strength and prepare five types of coal–rock combination specimens. Uniaxial compression tests were conducted to obtain the characteristics of the final failure state and the variation law of the compressive strength of the specimens. Results showed that the failure pattern and compressive strength of the specimens were quite different from the rock and coal samples. The following final failure state characteristics of the combination appeared as the proportion of coal thickness increased: “H”-type tensile failure, “hyperbola”-type co-shear failure and “X”-type shear failure. The values of the compressive strength and longitudinal wave velocity of the combination were between rock and coal and presented a negative exponential function with increasing coal thickness. The compressive strength of the rock body part weakened because of the interface in the coal–rock combination, whereas the coal body part strengthened. Further, a compressive strength calculation model was established for the combined body considering the dual effects of coal–rock size effect and interfacial effect. This study is helpful to explain the mechanical behavior of coal in mining engineering.

The fracture radius surrounding the borehole is a key parameter for assessing the effectiveness of acidizing and fracturing in coal seams. To determine this radius, a self-developed physical experimental platform for CO 2 foam acidizing and fracturing in low-permeability coal seams was designed. Fracturing fluid erosion tests were carried out on raw coal samples, and the stress-strain relationship of the acidified coal around the borehole was analyzed. The effect of acidization on the intrinsic constitutive parameters of the surrounding coal was clarified, and an improved constitutive model for the coal body around the acidized fracturing borehole was established. It was found that increasing the erosion time of the fracturing fluid leads to reorganization of the coal body’s physical structure, an increase in free volume, and enhanced compressibility and brittleness. Additionally, increasing the erosion period intensifies the chemical reactions within the minerals inside the pore fissures of the coal body, leading to more intense chemical reactions and significant brittle damage in the saturated samples. The improved damage constitutive model can accurately characterize the stress-deformation behavior of low-permeability coal seams under acid fracturing. Using this model, the effective fracturing radius of the 226 comprehensive working face was determined to be 5.2 m, and the “three-flower hole” borehole arrangement was proposed accordingly. This study provides theoretical guidance and significance for the borehole arrangement in acid fracturing of low-permeability coal seams.

To rationalize the setting of joint parameters, model size, and initial value of vertical stress in simulation of mining of steeply inclined coal seams, a fault tree analysis method of discrete element numerical simulation was used and a mathematical model was proposed. A method of eliminating the influences of size-effect errors on the parameters of coal and rock samples was obtained based on previous work. Furthermore, the constitutive equation and eigenvalue determination formula of a joint discontinuity surface were established, and a method of determination of the joint parameters was proposed, forming the complete “coupling chain” between parameters for numerical simulation. In addition, a formula for the initial value of vertical stress was constructed by way of the compression and shear model of the element body. Also, the minimum dimension was determined by means of strength factor analysis of fracture mechanics. Taking the research literature as an example, the model size and initial value of vertical stress were calculated. On this basis, the physical parameters of coal samples, the physical parameters of coal rocks considering the influence of the size effect and the calculated coal rock joint parameters considering the influence of size effect were directly used to comparatively analyze the displacement and stress fields, thus verifying the reasonability and correctness of the mathematical model.

Collaborative prediction model of gas emission quantity was built by feature selection and supervised machine learning algorithm to improve the scientific and accurate prediction of gas emission quantity in the mining face. The collaborative prediction model was screened by precision evaluation index. Samples were pretreated by data standardization, and 20 characteristic parameter combinations for gas emission quantity prediction were determined through 4 kinds of feature selection methods. A total of 160 collaborative prediction models of gas emission quantity were constructed by using 8 kinds of classical supervised machine learning algorithm and 20 characteristic parameter combinations. Determination coefficient, normalized mean square error, mean absolute percentage error range, Hill coefficient, mean absolute error, and the mean relative error indicators were used to verify and evaluate the performance of the collaborative forecasting model. As such, the high prediction accuracy of three kinds of machine learning algorithms and seven kinds of characteristic parameter combinations were screened out, and seven optimized collaborative forecasting models were finally determined. Results show that the judgement coefficients, normalized mean square error, mean absolute percentage error, and Hill inequality coefficient of the 7 optimized collaborative prediction models are 0.969–0.999, 0.001–0.050, 0.004–0.057, and 0.002–0.037, respectively. The determination coefficient of the final prediction sequence, the normalized mean square error, the mean absolute percentage error, the Hill inequality coefficient, the absolute error, and the mean relative error are 0.998%, 0.003%, 0.022%, 0.010%, 0.080%, and 2.200%, respectively. The multi-parameter, multi-algorithm, multi-combination, and multi-judgement index prediction model has high accuracy and certain universality that can provide a new idea for the accurate prediction of gas emission quantity.

This study aims to investigate the mechanical properties and energy characteristics of coal-rock combinations during uniaxial compression, specifically focusing on variations in interface inclination angles. Each of the stress–strain curves of the CRC (Coal-Rock-Like Material Combination) with varying interface inclination angles were obtained using the DYD-10 universal testing machine and a high-definition camera system. Subsequently, the energy of the CRC was calculated using a specific formula. The experimental results demonstrated that when the interface inclination angles increased, the compressive strength and elastic modulus of the CRC showed a monotonous decreasing trend. The storage of elastic energy mainly occurred in the elastic stage and the plastic fracture stage, and the growth of dissipated energy increases obviously in the post-peak failure stage. As the inclination angles of the interface experience a rise, the energy storage limit (ESL) of the specimen decreases obviously, the CRC was more prone to fracture as the rate of dissipated energy conversion gradually increased. In order to understand the process of coal and rock mass disasters and prevent coal and gas outbursts, the mechanical features and fracture energy characteristics of the CRC under various interface dip angles were examined in this article.

The low-gas permeability area of a fully mechanized up-dip working face was quantitatively studied using a physical similarity simulation test and theoretical analysis under varying dip angles of rock strata. Based on the theory of fractal geometry, this study obtained the fractal dimensions of the low-gas permeability area, the boundary area of the low-gas permeability region, and various layer areas of the low-gas permeability area by increasing the dip angle of rock strata. The findings reveal that the goaf’s high penetration area moved from a symmetrical shape to an asymmetrical one as the dip angle of rock strata increased. The high penetration area on the open-off cut side is notably larger than that on the working face side, due to the effects of advancement at the working face. In the goaf, the lateral length of the cavity decreases as the rock strata’s dip angle increases, while the longitudinal width expands and then contracts until it vanishes because of sliding. In the goaf, the lateral length of the cavity decreases as the rock strata’s dip angle increases, while the longitudinal width expands and then contracts until it vanishes because of sliding. In the goaf, the lateral length of the cavity decreases as the rock strata’s dip angle increases, while the longitudinal width expands and then contracts until it vanishes because of sliding. Moreover, the low-gas permeability area has a larger fractal dimension. The fractal dimension of the area with low gas permeability steadily decreased as periodic weighting emerged, ultimately reaching values of 1.24, 1.27, and 1.34. Moreover, the area’s fractal dimension was greater on the open-off cut side in comparison to the working face side. As the distance from the rock strata floor decreased, the fractal dimension of the area with low gas permeability increased. According to the gradient evolution law, the low-gas permeability area may be divided from bottom to top into three areas: strongly disturbed, moderately disturbed, and lowly disturbed. Based on the theory of mining fissure elliptic paraboloid zones and experimental findings, a mathematical model has been developed to analyze the fractal characteristics of low-gas permeability areas that are influenced by the rock strata’s dip angle. Finally, this study established a dependable theoretical foundation for precisely examining the development of cracks in the area of low gas permeability and identifying the storage and transportation region of pressure relief gas, which is affected by various dip angles of rock strata. It also offered assistance in constructing a precise gas extraction mechanism for pressure relief.

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. Highlights 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. 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. The applicability of low-permeability coal to N 2 injection for CH 4 displacement technology was investigated.

This paper aims to address the issue of hydraulic support crushing accidents or support failures in the retracement roadway (RR) that frequently occurs when a fully mechanized mining face is retraced during the end-mining stage. The deformation and instability mechanism of surrounding rock in the RR during the end mining of a fully mechanized mining face at the Hanjiawan Coal Mine located in the northern Shaanxi mining area is explored through field measurement, theoretical analysis, similar simulation, and numerical simulation. The results reveal that the stability of the remaining coal pillar (RCP) and the fracture position of the main roof are the main factors contributing to large-scale dynamic load pressure in the RR during the end-mining stage. The plastic zone width limit of the RCP is identified to be 5.5 m. Furthermore, the stress distribution within the RCP during the end-mining stage is determined, and the linear relationship between the load borne by the RCP and the strength of the coal pillar is quantified. A similar simulation experiment is conducted to examine the collapse and instability characteristics of the overlying rock structure during the end-mining stage. UDEC (v.5.0) software is utilized to optimize the roof support parameters of the RR. A surrounding rock control technology that integrates the anchor net cable and hydraulic chock is proposed to ensure RR stability. Meanwhile, a method involving ceasing mining operations and waiting pressure is adopted to ensure a safe and smooth connection between the working face and the RR. This study provides a reference for the surrounding rock control of the RR during end mining in shallow, closely-spaced coal seams under similar conditions.