Water-bearing characteristics and their effects on the nanopores of overmature coal-measure shales in the Wuxiang area of the Qinshui Basin, north China

In this study, a group of overmature coal-measure shale core samples was collected in situ from an exploration well located in the Wuxiang area of the Qinshui Basin, north China. The pore water contents (C-PW) of the shales under as-received conditions, equilibrium water contents (C-EW) of the shales under moisture equilibrium conditions (relative humidity: 100%), and nanopore structures of the shales under both as-received and dried conditions were measured. The results indicate that the C-PW values of these shales are much lower than their C-EW values, which implies that the bulk pore systems of these shales have low water-bearing extents. In addition, approximately half of the total pore volumes and surface areas of the as-received shales are occupied by pore water, and the effects of pore water on shale nanopores with various pore types and widths are different. The average water-occupied percentages (P-W) are 59.16%-81.99% and 42.53%-43.44% for the non-micropores and micropores, respectively, and are 83.54%-97.69% and 19.57%-26.42% for the inorganic-matter hosted (IM) and organic-matter hosted (OM) pores, respectively. The pore water in shales not only significantly reduces the storage of shale gas by occupying many pore spaces, but also causes the shale gas, especially the absorbed gas, to be mostly stored in the OM pores; meanwhile, the IM pores mainly store free gas. Therefore, the water-bearing characteristics and their effects on the pore structures and gas-bearing properties of coal-measure shales should be noted for the evaluation and exploration of shale gas in the Qinshui Basin.