Nanopore Structural Characteristics and Their Impact on Methane Adsorption and Diffusion in Low to Medium Tectonically Deformed Coals: Case Study in the Huaibei Coal Field

Abstract

Coupled with isothermal adsorption and the Steele potential function, the characteristics of nanopores and their impact on methane adsorption and diffusion in low- to medium-rank tectonically deformed coals (TDCs) were revealed by high-pressure mercury intrusion and low-pressure N2/CO2 gas adsorption. The specific surface area (SSA) of low to medium TDCs is mainly provided by micropores (<2 nm, 96.64–99.56%), and the pore volume is mainly provided by macropores (>50 nm, 99.68–99.91%). The fractal characteristics of nanopores can be divided into four groups, i.e., D1 (>100 nm), D2 (<100 nm), D3 (>8 nm), and D4 (<8 nm). For primary coals and brittle deformed coals, D1 > D2, indicating that the heterogeneity of seepage pores is stronger than that of adsorption pores. For scaly coals, D2 ≈ D1, demonstrating the close heterogeneity and connectivity in adsorption and seepage pores, which are beneficial for coalbed methane (CBM) desorption and diffusion. However, D2 > D1 for wrinkle and mylonitic coals, indicating a stronger heterogeneity in adsorption pores than seepage pores, especially for mylonitic coals. D4 gradually increases with the enhancement of tectonic deformation, and D3 shows a sharp increase in wrinkle coals. D2, D4, and SSA (<8 nm) all have a better positive correlation with the maximum adsorption capacity (R2 = 0.57, 0.54, and 0.76, respectively), indicating that pores <8 nm in size have a dominant role in the adsorption capacity. With abundant activated desorption pores (0.7–1.5 nm), the content of schistose coals is between the contents of configuration diffusion pores (0.5–0.7 nm) and Knudsen diffusion pores (>1.5 nm). Schistose coals are good CBM reservoirs, followed by the scaly coals. Lacking activated desorption pores and Knudsen diffusion pores, wrinkle and mylonitic coals have a high incidence of coal and gas outburst.