Evolution of permeability in coal to sorbing gases - A preliminary study

We report laboratory experiments to investigate the permeability evolution of a bituminous coal as a function of applied stresses, pore pressure, and deviatoric stress. Experiments are conducted on 2.5 cm diameter, 2.5-5 cm long cylindrical samples, from the Powellton coal seam, West Virginia. We use a triaxial apparatus at confining stresses ranging from 6 to 10 MPa and axial stresses of 6 -18 MPa. A pressure transient method is used to measure permeability to an inert gas (Helium), a slightly adsorbing gas (Nitrogen), and a significantly adsorbing gas (Carbon dioxide). When pore pressure is kept constant, increasing effective stress causes a reduction in permeability attributed to cleat closure. An increase in deviatoric stress can either reduce or enhance the permeability depending on whether its magnitude is high enough to generate new fractures or low enough to only close the existing cleats. Under invariant total stress, increasing pore pressure increases the permeability to the inert gas (He) but decreases permeabilty to adsobing gases at low pressures and increases permeabilities at high pressures - this behavior is attributed to the competition between coal swelling and changes in effective stress.