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

摘要

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 N-2 physisorption. The methodology is to determine the porosity using helium pycnometry, the pore size distribution (PSD) of pores between 0.1 mu m and 200 mu m using mercury intrusion, and the PSD of pores between 1 nm and 100 nm using N-2 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 gm 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-5 mu m) and the SSA is mainly from mesopores and micropores (<2 nm); (3) even though the PV percentage of megapores (5-200 mu m) 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 N-2 physisorption, the PSD, SSA and PV of pores between 1 nm and 200 mu m can be determined for shale, which is critical to understand the shale pore structure and its effects on gas storage and flow mechanisms. (C) 2015 Elsevier Ltd. All rights reserved.