Effect of water occurrences on methane adsorption capacity of coal: A comparison between bituminous coal and anthracite coal

In this study, an NMR fluid typing method was applied to two typical anthracite and high volatile bituminous coals to identify different occurrence states of water (adsorbed and non-adsorbed water), and multiphase methane (adsorbed, porous medium confined, and bulk methane) and investigate the effect of the water occurrence states on methane adsorption capacity. First, results of isothermal adsorption analysis conducted using the NMR method was compared to those from conventional volumetric method to verify the precision of the NMR method. Then the influence of adsorbed water and non-adsorbed water on methane adsorption was examined based on two parallel treatments. For bituminous coal, the adsorbed water remarkably reduced the Langmuir adsorption volume, whereas adding non-adsorbed water had no effect on gas adsorption. In contrast, for anthracite coal, the methane adsorption capacity revealed a downward trend with increase in both adsorbed water and non-adsorbed water. The explanation is that adsorption competition between water molecules and methane gas molecules can constrain further adsorption of methane on the coal matrix. Thus, adsorbed water negatively impacts gas adsorption for both anthracite and bituminous coal. In terms of non-adsorbed water, due to the superior pore conductivity and hydrophilia of bituminous coal, methane gas can easily pass through the pores in the presence of non-adsorbed water, compared to anthracite, where water blocking and the Jamin effect tend to occur. Even water droplets formed because of anthracite's hydrophobicity can utterly block the pore throats, resulting in a continuous decrease in methane adsorption capacity. The implications of this study are important for better understanding the influence of various occurrence states of water on the adsorption/desorption processes in unconventional reservoirs.