Gas migration in coal is strongly controlled by surface diffusion of adsorbed gas within the coal matrix. Surface diffusion coefficients are obtained by inverse modelling of transient gas desorption data from powdered coals. The diffusion coefficient is frequently considered to be dependent on time and initial pressure. In this article, it is shown that the pressure dependence can be eliminated by performing a joint inversion of both the diffusion coefficient and adsorption isotherm. A study of the log–log slope of desorbed gas production rate against time reveals that diffusion within the individual coal particles is a multi-rate process. The application of a power-law probability density function of diffusion rates enables the determination of a single gas diffusion coefficient that is constant in both time and initial pressure.

The injection of supercritical carbon dioxide (Sc-CO₂) into coal seams remarkably changes the physical-chemical structures of coal mass and thereby improves coal mechanical properties. In this study, a series of unconfined and triaxial compressive strength tests were performed on anthracite coal samples under the pressure of 8 MPa at the temperature of 35°C for 24 h and 48 h, respectively. Besides, experiments with longer Sc-CO₂ treatment time were carried out on coal mass without stress constraint to observe the damage mode of coal. The results show that Sc-CO₂ treatment obviously alters time-dependent mechanical properties of anthracite coal. The coal samples treated with Sc-CO₂ for 240 h and 960 h show different damage modes from limited fractures to a complicated fracture network. The time-dependence of the mechanical weakening model on Sc-CO₂-treated coal was proposed to explain the link between CO₂ flow and mechanical weakening effect by means of physical-chemical effects on time scales.

Degassing a coal seam with in‐seam boreholes is an important method for mitigating the gas hazards in the underground coal mine. However, the low strength of the outburst‐proven coal limits the borehole sealing performance and borehole space maintaining, and thus influences the drainage performance of in‐seam boreholes. This study was conducted to seek a method to improve the sealing performance and borehole space maintaining for high‐efficiency CMM drainage. A visco‐elastic plastic model involving the plastic softening and dilatancy features for soft coal was proposed, and the deformation, shrinkage, and fracture characteristics of the coal surrounded a borehole and a roadway were analyzed. An enormous amount of connective fractures generated in the failure zone and the plastic zone, and the plastic zone develops timely for the creep behavior of coal, which aggravates the difficulties of borehole sealing. A comprehensive approach including the theoretical method and the technical method was developed to determine the proper sealing depth which can significantly influence the sealing performance. The sieve tubes made of high‐density polyethylene has been used to maintain the borehole space during the scheduled pre‐drainage period. A series of field tests were conducted in an outburst‐proven coal seam to verify the feasibility of the comprehensive approach and the borehole space maintaining method. Field tests showed that the comprehensive approach to determine the proper sealing depth and the installing of sieve tubes to protect the borehole space can provide favorable conditions for maximum CMM pre‐drainage from outburst‐prone coal and maximum utilization of the in‐seam boreholes.

Coalbed methane (CBM) is an important resource of energy. For CBM recovery, sorption‐induced coal deformation can cause significant reservoir permeability change. Moisture content and coal pore morphology affect the gas adsorption capacity and can alter the coal deformation of the coal seam. Therefore, it is crucial to establish a coal gas moisture‐coupled model for CBM production prediction. However, there are currently insufficient data available for quantitative analyses. In this paper, a series of typical sorption‐induced strain experiments were carried out during methane adsorption and desorption on coal samples with different metamorphic degrees and moisture content. The pore morphology and adsorption capacity of coals were measured to analyze the reason for different deformation of coals with various pore structures and moisture. Results show that the adsorption capacity and deformation is corresponding to the specific surface area of micropore, which first decreases and then increases with coal ranks. The deformation and adsorption gas content of dried coals is greater than that of natural moisture coals, which means that moisture can reduce the sorption capacity of coals, resulting in the decrease of gas adsorption‐induced coal deformation. There is also residual deformation of coal samples after gas desorption caused by the residual adsorbed gas in coals. This paper quantitatively investigates the effects of pore characteristics and moisture on coal deformation and the internal mechanism. This work will provide essential information for building out a fully cross‐coupled model of coal gas‐moisture relationships for CBM production prediction.

Coal and gas outburst is an extremely serious dynamic phenomenon involving the comprehensive action of many factors, and remains a major disaster that needs to be solved in coal mine production. Considering the significant influence of moisture content on coal and gas outburst, it is necessary to carry out experimental research on coal and gas outburst under different moisture content conditions. The coal of the serious outburst coal seam in the Luling coal mine, which has had several coal and gas outburst accidents, was selected as the experimental sample. Firstly, the gas desorption law and outburst index characteristics of coal under different moisture content were studied, and the influence of moisture content on gas outburst parameters of coal was obtained. Then, the simulation tests of coal and gas outburst under different moisture content were carried out by using a triaxial coal and gas outburst simulation test system. Based on the above experimental research, the influence of moisture on coal and gas outburst was summarized, and the energy calculation and prevention countermeasures of coal and gas outburst under different moisture content conditions were carried out. With the increase of moisture content, the adsorption constant a, the initial velocity of gas diffusion Δp, and the gas desorption index of drill cuttings K₁/Δh₂ decrease, but the Protodyakonov coefficient f increases, all of which have an exponential relation to moisture content. Meanwhile, with the increase of moisture content, the threshold of coal and gas outburst pressure increases, and the intensity and energy of outburst decrease. At 0.45 MPa pressure, the outburst of 1.47% moisture content is the most serious, the outburst of 5% moisture content is weakened, while the outburst of 10% moisture content is not triggered. Five percent moisture content can be used as a critical index of hydraulic measures to prevent coal and gas outburst in No. 8 coal seam of the Luling coal mine. This research can provide new insights into the theoretical study of coal and gas outburst in serious outburst coal seams and the control of coal and gas outburst by hydraulic measures.