Moisture Retention and Multi-mechanistic Transport Behavior in Nanoporous Coal

Coal-water interactions induced by water retention in coals control the performance of coalbed methane reservoirs and coal utilizations. Experimental measurements on Illinois coal samples using X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, low-temperature N₂ adsorption, low-pressure CO₂ adsorption, and dynamic water vapor sorption were carried out. A mechanism-based isothermal model of water vapor sorption was proposed to estimate the water adsorption capacity at varied relative humidity, which implicitly considered both monolayer and multilayer adsorptions and capillary condensation. The analytical models for quantifying the stage-based diffusion coefficients as well as the apparent diffusion coefficients at different relative humidities were proposed and well validated. The contributions of different diffusion regimes to the total mass flow were discussed. At the first stage, both free water vapor diffusion and surface diffusion of adsorbed water molecules contribute to the total mass flow whereas the apparent diffusion at this stage is dominated by latter flow regime; during the second stage, the contribution of free water vapor flow to the apparent flow can be neglected and the mass transfer at this stage is still dominated by the surface diffusion flow; upon reaching the critical relative humidity, the flow in capillary condensation will dominate the total mass flow.