Stress Sensitivity for the Occurrence of Coalbed Gas Outbursts: A Reactive Force Field Molecular Dynamics Study

Abstract

Coalbed gas outburst processes commonly occur in ductile shear zones and release gas quantities that far exceed the maximum gas adsorption capacity of coal seams. Although outburst accidents pose a serious safety hazard, this process has long remained poorly understood at a molecular level. In this work, we use reactive force field molecular dynamics simulations to analyze the influence of stress on the mechanochemical decomposition reactions and products of coal macromolecular models. The results indicate that a maximum of reaction amounts occurs upon changing the external force for the mechanochemical reactivity of bituminous coal under working conditions. This implies that gas generation readily occurs over a suitable stress-sensitive range, which can be used to qualitatively explain why coalbed gas outbursts typically occur in ductile shear zones. When a force is too large, it can cause brittle fracture and is less likely to initiate mechanochemical reactions that generate gas, whereas a too small of a force is insufficient to trigger chemical changes of the coal macromolecular structure. The anisotropic properties of coal must also be considered in terms of mechanochemical reactions. Coal molecules can generate different kinds of gases when driven by external forces in different orientations. The simulation results provide important insights into coalbed gas outburst characteristics and the anisotropic nature of coal. Only by comprehensively considering stress factors will it be possible to understand the occurring conditions of coalbed gas outbursts.