Pore structure characteristics of coal-bearing shale using fluid invasion methods: A case study in the Huainan–Huaibei Coalfield in China

Abstract

Organic shale in coal-bearing strata, which was deposited in marine-continental transitional and lacustrine environments, is well developed in China and contains a large amount of shale gas. However, the present research concerning shale predominantly focuses on marine shale. In this research, the pore structure characteristics of coal-bearing shale in the Huainan–Huaibei Coalfield in southeastern North China Block were analyzed using fluid invasion methods, including helium including helium pycnometry, mercury intrusion porosimetry and N2 physisorption. The methodology is to determine the porosity using helium pycnometry, the pore size distribution (PSD) of pores between 0.1 μm and 200 μm using mercury intrusion, and the PSD of pores between 1 nm and 100 nm using N2 physisorption. It is worth noting that the accuracy decreases for pores less than 1.5 nm. The specific surface area (SSA) and pore volume (PV) for pores between 1 nm and 200 μm are calculated from interpreted PSD data. In addition, the Brunauer–Emmett–Teller (BET) model was used to calculate the SSA of shale pores. The results indicated that (1) silt-shaped pores are dominant, followed by cylindrical and spherical pores; (2) coal-bearing shale PV is primarily from mesopores (2–50 nm) and macropores (50 nm–5μm) and the SSA is mainly from mesopores and micropores (<2 nm); (3) even though the PV percentage of megapores (5–200 μm) is typically less than 2%, the connection of megapores notably affects the shale permeability, and the pores larger than 80 nm have the largest correlation coefficient with shale permeability; and (4) shale gas adsorption predominantly occurs in micropores and mesopores. By combining mercury intrusion and N2 physisorption, the PSD, SSA and PV of pores between 1 nm and 200 μm can be determined for shale, which is critical to understand the shale pore structure and its effects on gas storage and flow mechanisms.