Experimental Study on Gas Emission under Pressure from the Coal Matrix: Understanding Gas Transport Patterns from an Engineering Scale

Abstract

Gas transport within the coal matrix is key to realizing mass exchange between the fracture and matrix systems. However, considerable controversy remains regarding gas transport within the coal matrix system. In this study, the applicability of several typical models was evaluated based on the proposed criteria, mainly through gas emission under pressure experiments, and it is found that the Darcy seepage model describes gas transport behavior within the coal matrix more scientifically. Subsequently, the validity of the seepage model was re-evaluated through pore structure analysis, and the gas transport patterns and underlying mechanisms of the dual-porosity and dualpermeability models were discussed. The results suggest that gas transport within the matrix is dominated by the Fick diffusion flow only when methane molecules within the coal matrix are located in the micropore filling section. When the pore size is located in the outer pore section, gas within the matrix will be transported outward in a free state driven by pressure. In addition, the proposed gas transport model and field data were used to validate the analysis. It is concluded that in view of the low efficiency of gas transport in the micropore filling section, the dual-porosity and doublepermeability models can be used to simplify gas transport in coal seams.

Dynamic Response of Gas Recovery Enhancement Efficiency in Coalbed Methane Displacement by Hot Flue Gas: From the Perspective of Thermo–Hydro–Mechanical–Chemical Coupling

“…The majority of the gas cannot be drained out of coal seams owing to their low permeability, difficulty in gas desorption and flow, and low drainage rates of gas. This constitutes the main technological difficulty in coal seam mining. − In the process of coal mining, a great deal of residual gas surges into the mining space due to strong mining disturbance, causing gas overrun and even coal and gas outburst accidents. This seriously threatens the safe production in mines. − Subsequently, the gas released into the mining space will be discharged into the atmosphere along with the mine airflow, which can result in the greenhouse effect (the greenhouse effect of gas is about 22–28 times that of carbon dioxide).…”