Methane sorption behavior on tectonic coal under the influence of moisture

Abstract

Water is widely distributed in coal under the in-situ reservoir conditions, which has a great impact on gas adsorption and migration within coal. In this work, methane sorption behavior on tectonic coal under the influence of moisture was quantitatively evaluated using the combination of high-pressure volumetric method, X-ray photoelectron spectrometer (XPS) and mercury intrusion porosimetry (MIP). Comparative analysis of the differences in microstructures and methane adsorption capacity was also made between primary coal and tectonic coal. The results show that a large number of micropores are contained in tectonic coal, leading to the increasing complexity of pore structure. Primary coal has less pores but the largest number of oxygen-containing functional groups. Methane sorption properties are affected by moisture content in varying degrees for primary coal and tectonic coal. Combined with XPS and MIP results, the influencing mechanism of water content on methane adsorption was revealed. CH4 adsorption in primary coal is more sensitive to water, and only 2% of moisture content leads to the 67% of reduction in the CH4 adsorption amount, which is attributed to the stronger hydrophilia and less pore space. Owing to the developed pore networks, the increase of water content only results in a slight reduction of adsorption capacity in mylonitic coal when moisture content less than 2.5%. The further increase of moisture content results in the pore-blocking effect, inhibiting the access of other gas adsorbing molecules to the pores, hence, methane adsorption capacity is greatly reduced in mylonitic coal. This work is of great significance for better understanding the adsorption/desorption processes of tectonic coal.