Xiaojie Fang

Borehole sealing quality plays a critical role in coal seam gas extraction efficiency, while the mechanical mismatch between conventional cement-based sealing materials and surrounding coal masses often leads to interface damage and sealing failure under mining-induced stress conditions. To address this issue, a modified static chemical agent (MSCA) was developed by incorporating fly ash (FA) and hemihydrate gypsum (HG) into a static chemical agent (SCA) system. The hydration behavior, expansion characteristics, mechanical properties, and engineering applicability of MSCA were systematically investigated through laboratory experiments, microscopic analyses, numerical simulations, and field industrial tests. The results indicate that HG significantly accelerates the hydration reaction and promotes the early development of expansion pressure of MSCA. The maximum expansion pressure of MSCA increases linearly with SCA content and quadratically with HG content. XRD and SEM analyses reveal that the hydration products mainly consist of Ca(OH)2, calcium silicate hydrate (C–S–H), calcium aluminate hydrate (C–A–H), dihydrate gypsum (DG), and ettringite (AFt). Increasing FA and HG contents facilitates the formation of C–S–H gel and interwoven AFt crystal networks, resulting in a denser microstructure and improved mechanical performance. The splitting strength of MSCA increases significantly with decreasing SCA content and increasing HG content, whereas the expansion pressure exhibits an opposite trend. Among all tested formulations, the D1MSCA achieves the optimal balance between expansion pressure and mechanical strength, with a splitting strength of 1018 kPa and an expansion pressure of 5.7 MPa, which are close to the mechanical properties of coal seams and the field grouting pressure conditions. Numerical simulation results demonstrate that the mechanical compatibility between the sealing material and the coal seam is a key factor controlling interface damage under mining-induced stress. A sealing material with an elastic modulus close to that of the coal seam can effectively alleviate stress concentration around the borehole and suppress crack propagation. Field industrial tests further confirm that MSCA-sealed boreholes maintain significantly higher gas extraction concentrations than conventional cement-sealed boreholes during the entire mining process. Under strong mining-induced stress conditions, the gas extraction concentration of MSCA-sealed boreholes remains 31%–50% higher than that of conventional cement-sealed boreholes. The proposed MSCA provides an effective approach for improving borehole sealing quality and gas extraction performance in underground coal mines under mining disturbance conditions.

Inertinite-rich coals are widely distributed, in which fusinite (semifusinite) constitutes the dominant maceral component of inertinite. Studying the pore characteristics of fusinite-rich lignite and their impacts on coal reservoir permeability is key to gaining an objective assessment of lignite reservoir properties. Based on this, this study selects the lignite of the Yimin Formation in the Hailaer Basin to carry out nuclear magnetic resonance experiments and permeability testing, combined with electron microscope observation and maceral quantification, coupling maceral quantitative results with coal reservoir pore structure and permeability results, and analyzing its geological control mechanism. The results demonstrate that the Yimin Formation lignite of the Hailaer Basin is inertinite-rich coal. Porosity decreases with rising huminite content but increases with higher inertinite proportions. The geological control of the above phenomena lies in the small pores developed in the huminite and the mesopores and macropores developed in the inertinite. Consequently, huminite content correlates negatively with movable water saturation and permeability, whereas inertinite content shows positive correlations. Fusinite and semifusinite emerge as the primary controls on permeability variations within the inertinite group. However, fusinite-rich lignite exhibits nonlinear porosity–permeability relationships, and part of the samples show the characteristics of “high porosity and low permeability.” This anomalous behavior stems from the complex pore networks of the fusinite-rich lignite, which are characterized by cellular structures with either singular or multiple apertures. Different pore structures limit fluid mobility, which is intuitively manifested as permeability differences. Therefore, reservoir optimization necessitates a differentiated evaluation of high-porosity intervals versus high-permeability zones.

The multilayer development of coalbed methane (CBM) is affected by many factors, and the gas production contribution of each seam is unclear. In this study, 12 geological and 10 engineering parameters were adopted to reveal the key factors affecting the combined‐layer drainage productivity of CBM wells before using numerical simulation to clarify the contribution of each coal seam. The results show that the coal reservoir in the Laochang mining area exhibited the advantages of high gas content, intact coal body structure, and large thickness of the combined coal seams, and it was deficient with low saturation and ratio of critical desorption pressure to reservoir pressure. The medium permeability after fracturing and the influence of a small range of fracturing leads to the steep and deep pressure drop funnel of the coal reservoir, and most of the reservoir pressure drop is ineffective; this may be the reason for the unstable production and rapid attenuation of most CBM wells in the study area. The gas production of the Nos. 13 and 18 coal seams is stable and makes a significant contribution, and they can be used as the key objects of follow‐up CBM exploration and development in this area. Geological factors, such as coal body structure, ratio of critical desorption pressure to reservoir pressure, thickness of combined coal seams, and gas saturation, as well as engineering factors, such as permeability and half length of fractures after fracturing, are the main factors affecting the productivity of the CBM wells in the Laochang mining area.

The current in situ stress regime is of great significance to the exploration and development of coalbed methane (CBM). In this study, the vertical stress (), maximum horizontal principal stress (), and minimum horizontal principal stress () in the Liupanshui Coalfield were studied. Variations of the maximum and minimum envelopes and trend lines of the lateral pressure coefficient () with depth were obtained, and the non‐monotonic decrease of permeability with burial depth was determined. On this basis, the effect of in situ stress on the coal reservoir permeability was evaluated. The results show that the average vertical stress gradient of the main coal seam in the Liupanshui Coalfield is 0.024 MPa/m. Generally, there are two stress regimes of > > (44.23%) and > > (46.15%); > > accounts for a smaller proportion (9.62%), and only occurs in relatively shallow coal seams ( > is dominant; > > is dominant between 600 and 800 m; and between 800 and 980 m, the coal reservoir is affected by both > > and > > , representing a transition zone between the two stress states. With the increase of burial depth, the permeability of the coal reservoir shows a complex non‐monotonic decline, and the variation of the permeability of the coal reservoir differs under the influence of different stress regimes. Further analysis shows that under the background of stress regime transformation, permeability is mainly affected by the stress value, horizontal principal stress difference, stress regime, and coal cleat, indicating that in situ stress is the main controlling factor of coal reservoir permeability in the Liupanshui Coalfield.