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., D
1 (>100 nm), D
2 (<100
nm), D
3 (>8 nm), and D
4 (<8 nm). For primary coals and brittle deformed coals, D
1 > D
2, indicating
that the heterogeneity of seepage pores is stronger than that of adsorption
pores. For scaly coals, D
2 ≈ D
1, demonstrating the close heterogeneity and
connectivity in adsorption and seepage pores, which are beneficial
for coalbed methane (CBM) desorption and diffusion. However, D
2 > D
1 for wrinkle
and mylonitic coals, indicating a stronger heterogeneity in adsorption
pores than seepage pores, especially for mylonitic coals. D
4 gradually increases with the enhancement of
tectonic deformation, and D
3 shows a sharp
increase in wrinkle coals. D
2, D
4, and SSA (<8 nm) all have a better positive
correlation with the maximum adsorption capacity (R
2 = 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.
