Impact of inherent moisture on the methane adsorption characteristics of coals with various degrees of metamorphism

Gas and water coexist in coal reservoirs. Many scholars have examined the effect of moisture on the gas adsorption property of coal. In this paper, the oxygen-containing functional groups and pore characteristics of five coal samples with different degrees of metamorphism were studied by Fourier transform infrared (FTIR) spectroscopy and CO2 adsorption method, and methane adsorption tests were carried out on coal samples with different inherent moisture contents. The water-holding capacities of different rank coals are related to their oxygen-containing functional groups and micropores characteristics, and the former play a decisive role. This is probably the main reason why low rank coals have stronger water-holding capacity than medium and high rank coals. The Langmuir volume tends to decrease with increasing inherent moisture content for all of the samples, which can be attributed to hydrogen bonding between the coal macromolecules and water molecules. The relationship between the amount of adsorbed methane and the inherent moisture content can be characterized well using the linear and exponential models. And it can be found that the moisture effect coefficient is dependent on the coal rank. Further analysis indicates that the influence of inherent moisture on the methane adsorption capacity is more prominent for low rank coals, which is probably due to its greater hydrophilia and water-holding capacity. Whereas, the reduction in methane adsorption capacity is mainly controlled by the pore-blocking effect of adsorbed water for high rank coals, and the extent of the influence of inherent moisture is a little.