CH4 adsorption capacity of coalbed methane reservoirs induced by microscopic differences in pore structure

Abstract

The difference in porous structure significantly impacts the CH4 adsorption capacity in the coalbed methane (CBM) reservoir. Herein, a series of experiments, including maceral and mineral test, N2 adsorption/desorption, proximate analysis, are conducted for 8 coal samples collected from 5 basins, to compare the effect of coal composition, moisture content, and ash yield on pore structures of various sizes. Subsequently, the comprehensive analysis of the above factors on CBM adsorption capacity is explored via the results of CH4 isothermal adsorption experiments. The results show that the vitrinite in the organic macerals has the greatest influence on the pore content of different sizes, followed by the exinite, and the inertinite has a minor influence. The mineral content has a positive effect on the micropores and macropores, while it could weaken the content of mesopores. The rising moisture content will reduce the content of micropores and macropores while promoting mesopores. Besides, the microporous specific surface area slightly rises with increased ash yields, while the proportion of mesoporous specific surface area decreases due to mineral filling. The ash yield has little effect on the macropores. Due to the integrated effect of moisture and ash, the influence on CH4 adsorption capacity varies from pores with different sizes. The content of micropores and macropores promotes adsorption capacity, while mesopores have an inhibitory effect. Observations here could benefit the understanding of the interaction of coal with methane.