Permeability evolution in fractured anthracite: The roles of crack geometry and water-content

We report laboratory experiments that examine the roles of crack geometry and water-content on the evolution of permeability in fractured coals. Experiments are conducted on 2.5 cm diameter, 5 cm long cylindrical anthracite samples from Pennsylvania. To explore the permeability evolution due to coal swelling, we conduct experiments on a sample containing multiple embedded cracks and a fully cracked sample, under both dried and water saturated conditions. Under constant total stress and with pore pressure increases, we find the presence of low gas pressure permeability reduction for the intact sample under dried condition, and the absence of permeability reduction for the split sample under dried condition. This observation is congruent with the need for connected bridges to be present within the split sample to cause the observed swelling-induced reduction in permeability. Under water saturated condition, the initial permeabilities for all gases are nearly two orders lower compared with those under dried condition, and all permeabilities increase with increasing pore pressure for both samples. Results suggest that the presence of water in the samples prevents coals from swelling. We also find the sorption capacities and swelling strains are significantly small for water saturated samples.