Pore structure complexity and its significance to the petrophysical properties of coal measure gas reservoirs in Qinshui Basin, China

The pore structure of continuous unconventional reservoirs (CURs) in coal measures was investigated using different technologies for 29 samples (9 coal samples, 9 shale samples, and 11 sandstone samples) from Qinshui Basin, China. Results show that coals have relatively high porosities and permeabilities ranging from 4.02% to 5.19% and 0.001 to 0.042 mD, respectively. Micropores (< 2 nm) are well-developed in coals and contribute to the majority of pore volume (PV) and specific surface area (SSA). The porosities and permeabilities are between 1.19%–4.11%, and 0.0001–0.004 mD of sandstones with a predominance of macropores (> 50 nm). However, shales are characterized by poorly petrophysical properties with low porosity and permeability. Macropores and mesopores (2–50 nm) are well-developed in shales compared with micropores. For coals, abundant organic matters are expected to promote the development of micropores, and clay minerals significantly control the performance of mesopores. For shales and sandstones, micropores are mainly observed in organic matters, whereas clay minerals are the important contributor to mesopores. Moreover, micropore SSA significantly determines the adsorption capacity of CURs and sandstones have the best pore connectivity. The permeability of CURs is positively associated with the macropore PV since macropores serve as the main flow paths for gas seepage. Additionally, we also proposed that effective porosity has a significant effect on the permeability of CURs. The findings of this study could enhance the understanding of the multiscale pore structure of CURs and provide insights into the mechanisms that control gas storage, transport, and subsequent co-production for continuous unconventional natural gas (CUNG) in the Qinshui Basin and other coal-bearing basins worldwide.