Pore-Scale Study of Water Adsorption and Subsequent Methane Transport in Clay in the Presence of Wettability Heterogeneity

Clay minerals are abundant in shale, characterized by a lamellar structure and dimensions smaller than a micron, giving rise to nanometer-scale pore sizes and large specific surface area. They are commonly associated with water. However, the spatial distribution of the unsaturated water in clay is not very well understood, which significantly affects the subsequent shale gas flow capacity. Wettability heterogeneity in the presence of hydrocarbons further complicates the water distribution in clay. In this study, we use a 3-D lattice Boltzmann model to study water adsorption and condensation in a reconstructed clay microstructure based on broad-ion-beam scanning electron microscopy images. Three wettability conditions are considered including a water-wet case, a water-repellent case, and a mixed-wet case. The spatial distribution of the condensed liquid water phase is visualized, and the connectivity of the remaining open pore spaces is characterized. For the water-wet case, water preferentially condenses in the small and narrow pore spaces, while the water-repellent case reports the opposite trend. The mixed-wet case results in mixed behavior, and connectivity of the pore space is bound by the former two cases. We then study the subsequent methane flow capacity with the presence of water for these three cases, and the calculated methane relative permeability curves show a percolation threshold for the water-wet case, which is in good agreement with available experimental data in the literature measured on geosynthetic clay liners.