Permeability and meso-structure evolution of coking coal subjected to long-term exposure of triaxial stresses and high-pressure nitrogen

Permeability and meso-structure evolution of coal are crucially important to the profound understanding of coal-gas interactions in the enhanced coalbed methane recovery and in controlling gas outburst. Very few studies focus on the effect time has on these interactions We performed a series of experiments on the permeability evolution of coking coal subjected to the long-term exposure of triaxial stresses and nitrogen pore pressure. The coking coal seam belongs to a low permeability reservoir with an initial permeability of 10(-16) m(2). The change tendency of the permeability was divided into types I and II. In type I, coal permeability increased at first then decreased and stabilized at a lower level while only decreasing and stabilization were present in type II. There existed a threshold value (2.0 x 10(-19) m(2)) about the initial permeability to control the tendency transition from type I to type II. The natural logarithm of both the maximum permeability in the increasing phase and the stable permeability in the stabilization phase were closely linear to the natural logarithm of initial permeability. The tendency corresponded to a change of coal structure, which was broken into many small-sized grains by the integrative action of long-term stresses, gas pressure, and gas adsorption. A simple elastic-plastic model was established to explain the destroyed mechanism of the coal macromolecular structure. The results will be helpful in understanding the mechanism of gas outburst in coal mines and gas-coal interactions in enhanced coalbed methane recovery.