Comparative analysis of nanopore structure and its effect on methane adsorption capacity of Southern Junggar coalfield coals by gas adsorption and FIB-SEM tomography

Abstract

Nano-scale porous structure has a significant influence on methane adsorption in coal seam. This work presents a comparative analysis of different experimental techniques for evaluating pore structure and the relationship between pore surface area and roughness with methane adsorption capacity. This sorption capacity subsequently affects the accuracy of total gas-in-place estimates and feasibility of CO2 injection for enhanced natural gas recovery. To evaluate methane adsorption characteristics of nanopores (<100 nm), various laboratory analyses are presented. Surface area, porosity, pore volume and fractal analysis of adsorption pores were determined in 13 coal samples (maximum vitrinite (huminite) reflectance <1.0%) using gas (CO2, N2 and CH4) adsorption and focused ion beam scanning electron microscopy (FIB-SEM) tomography. FIB-SEM images indicate a clustered distribution of multi-scale adsorption pores with similar orientation. The macropores connect some mesopores, which enhance gas flow and have a positive influence on coal permeability. The percent of connected pores compared to the number of total nanopores is ∼2% in selected samples, as calculated by FIB-SEM. Surface area and pore volume distribution determined from FIB-SEM are consistently higher than those obtained from N2 adsorption methods. In general, the FIB-SEM technique can detect both isolated and connected mesopores and macropores but cannot measure micropores. Surface area, pore volume, and porosity that estimates from N2 adsorption tests are prone to underestimate actual conditions. Langmuir volume (9.92–24.42 m3/t) are seemingly independent of maceral composition and coal ranks in this set of samples, having no direct correlations. Methane adsorption capacity increases with increasing the adsorption pore surface area and fractal dimensions and follow a moderate straight-line relationship. Hence, methane adsorption capacity is directly influenced by micropore surface area and micropore roughness. These results are significant for understanding the interaction of coal with gases.