Multiscale pore characterization of coal measure reservoirs and gas storage and transport behavior in Yanchuannan gas field of China

Gas coproduction is a promising technology for maximum gas production from coal measure formations. A comprehensive reservoir assessment for all the targeted formations, including coal and its adjacent tight sandstone formations, is required for gas coproduction planning. This study focused on the pore characterization of coal and tight sandstones in the Yanchuannan gas field in the Ordos Basin in China. Quantitative experiments and a multiscale imaging characterization were employed to probe the pore characteristics. The results indicate that multiscale pores occur both in coal and tight sandstones; coal has a bimodal pore size distribution, and sandstone is evenly distributed. For micrometer-scale pores, the tight sandstone is dominated by intergranular pores (5-80 μm) and intracrystalline pores (2-5 μm). For nanoscale to submicrometer-scale pores, the sandstone consists of primary (<200 nm) and secondary intragranular dissolution pores (200-500 nm) and microfractures. In comparison, coal is characterized by relatively large-scale microfractures, plant tissue pores, primary intergranular pores, and smaller nanosized pores. The gas storage capacity and mode of coal are affected by pore size and bound water. The CH4 molecules are mainly adsorbed within 2 nm from pore walls. The gas storage mode and required diameter in coal should be between two extreme modes with and without considering H2O as proposed in this paper. For the tight sandstone, the pore-throat lower limit of gas storage is approximately 12-15 nm. Moreover, gas transport behavior in coal measure reservoirs has a spatial scale and a time scale.