Effect of Water on Effective Pore Structures for Medium-Rank Coal: Based on the Prefreezing Nitrogen Adsorption-Desorption Experiment

Water is ubiquitous in coal reservoirs, and its distribution can have a remarkable influence on the effective pore space of methane. This study conducted the combination experiments of moisture equilibrium and prefreezing nitrogen adsorption-desorption to explore the adsorption behavior of water in coal pores and thus to reveal the distribution characteristics of water in pores with different scales as well as the influence of water on pore structures. The results showed that the adsorption mechanism of water vapor undergoes a transition from monolayer to multilayer to condensation with the increase in relative humidity (RH). The occurrence characteristics of adsorbed water in coal pores are controlled by the RH and pore size. When the RH is increased from 0 to 98%, the nitrogen adsorption capacity, specific surface area, and effective pore volume of the samples were all decreased significantly due to the different adsorption modes of water, which is more significant in pores with d < 10 nm. Additionally, the relative pressure corresponding to the branching position of the nitrogen adsorption-desorption curve will be changed with the increase in moisture content. Based on this, it is calculated that the adsorbed water will change the smoothness of the pore wall and the complexity of the pore structure.