Petrophysical and Mechanical Properties of Complex Structure Coalbed Methane Reservoir: A Laboratory Investigation

The presence of complex structure coal seam, containing undeformed coals (UC), tectonically deformed coals (TDC), partings and roof-floor, has a significant effect on the safety of coal mining and the effectiveness of coalbed methane reservoir fracking. This research attempts to study the complex structure CBM reservoir physical properties and rock mechanical behaviors. Coals, partings and roofs collected from the same coal mine in Qinshui basin with four coal–rock combinations made for modeling different coal seam structure were used for experiments. It has been proved that the average macropore ratio of UCs and TDCs accounts for 18.6 and 30.1%, respectively, the ratio was positively correlated with permeability, and macropores and fractures are not developed in parting and roof. The average compressive strength of TDCs, UCs, partings and roofs are 4.94, 11.06, 23.63 and 41.56 MPa. Before peak stress, average total energy density of TDCs is the lowest, those of UCs and partings are similar, and that of roofs is the highest. Stress–strain curves in elastic deformation stage of coal–rock combinations were divided into several segments, and post-peak curve will go through one or two resistance stages. Compressive strengths of CR1, CR2, CR3 and CR4 are 25.62, 14.80, 13.57 and 4.79 MPa, respectively. After loading, only the roof and floor of CR4 remain intact, UCs are broken into lumps, TDCs are almost fragmented. The accumulated acoustic emission energy of four combinations from low to high is CR2, CR1, CR3 and CR4, and strong AE signal ratios of CR1, CR2, CR3 and CR4 are 63.13, 60.98, 54.75 and 33.48%, respectively, indicating the dominant structural damages of TDCs are developed macropores and multi-directional micro-fractures. These results can convincingly demonstrate that slightly brittle deformation is beneficial for CBM reservoir hydraulic fracturing.