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.