Evaluation of coal mines’ rock mass gas permeability in the equivalent stress zone

Abstract

Purpose. Based on a comparative analysis of the internal mechanism of shape change of rock samples, which were loaded in specified deformations mode, and geomechanical and gas-dynamic processes in coal mass, to establish a causal link between these phenomena. To qualitatively characterise their gas permeability as a function of the rock’s volume expansion. To justify the possibility of using a full “stress-strain” diagram as a technogenic methane deposit formation model and its spatial localisation. Methodology. Theoretical research is based on using the solid mechanic constitutive principles and results of studying the rock samples failure in the mode of specified strains. Findings. The ability to use a full “stress-strain” diagram for detecting and localising methane reservoirs during the coal seams excavation was proved during the research. It was found that the compaction threshold coincides with the bearing pressure maximum in front of the longwall face. This area corresponds to the rock mass with minimal porosity and minimal filtration, which allows considering it as an envelope of an artificial gas deposit. Regularities that connect the three-dimensional equivalent stress state with the final gas permeability of the gas-saturated coal mass were obtained. These data allow creating a predictive numerical geomechanical model of methane migration paths. Originality. The ability to use a full “stress-strain” diagram in the controlled strain mode for numerical modelling of gas permeability of a methane-saturated coal mass during the mining of coal seams and the determination of technogenic gas deposit boundaries are justified. Dependences of the current and final gas permeability on the rock’s mechanical characteristics in a post-peak strain state are obtained. Practical value. Functional dependencies based on geomechanical models are obtained that allow the identification and localisation of technogenic methane reservoirs in mines during coal seam excavation, with subsequent utilisation of the extracted gas. Furthermore, methane removal enhances mining safety by reducing the risk of gas dynamic phenomena while decreasing gas emissions into the atmosphere contributes to reducing the greenhouse effect.