Permeability evolution during progressive deformation of intact coal and implications for instability in underground coal seams

We report measurements of deformation, strength and permeability evolution during triaxial compression of initially intact coals. Permeability is continuously measured by the constant pressure differential method, together with axial and volumetric strains for both water (H2O) and strongly adsorbing carbon dioxide (CO2) gas. Strength and Young’s modulus increase with increasing confining stress and permeability is hysteretic in the initial reversible deformation regime. As deviatoric stress and strain increase, permeability first decreases as pre-existing cleats close, and then increases as new vertical dilatant microcracks are generated. Post-peak strength the permeability suddenly increases by 3–4 orders-of-magnitude. During loading, the inflection point where permeability begins to increase occurs earlier than the turning point of volumetric strain, which may be explained by the competing processes of axial crack opening and closure of oblique and transverse cracks. The generation of these vertical microcracks does not enhance gas migration in the horizontal direction but will accelerate the rate of gas desorption and weaken the coal.Based on this mechanistic observation, we propose a process-based model for bursting in underground coal seams. Horizontal and vertical stresses redistribute ahead of the mining-face immediately after the excavation and influence pore pressure, permeability, and desorption rate. Due to this redistribution, the zone closest to the mining-face may experience tensile failure. Interior to this zone a region may develop with gas overpressures induced by desorption and this may contribute to the occurrence of coal and gas outbursts. Beyond this, an overstressed zone may initiate shear failure driven by gas pressures if the desorption rate outstrips the rate of drainage. We discuss the implications of this on the instability of coal seams to CO2 injection and the potential for induced fault slip.

► We measure permeability and deformation evolution during triaxial compression of intact coals.
► Permeability begins to increase earlier than the turning point of volumetric strain.
► Results are applicable to coal seams under mining-induced stresses.
► We propose a process-based model for instability by bursting in underground coal seams.
► We discuss the implications of this on the instability of coal seams to CO2 injection.