Modeling Methane Leakage in Shallow Aquifers: The Role of Contact Angle Variability and Capillary Heterogeneity

Methane transport in shallow aquifers is governed by complex, multiscale processes, with influences ranging from molecular-scale wettability to field-scale flow dynamics. Reliable prediction of methane leakage and migration under low-pressure, shallow subsurface conditions is critical for evaluating environmental risks and informing mitigation efforts. However, current modeling approaches often rely on simplified capillary pressure scaling methods that consider heterogeneity in porosity and permeability, while neglecting spatial variations in contact angle. To address this limitation, we present a multiscale modeling framework that couples molecular dynamics (MD) simulations with large-scale reservoir flow models. The study focuses on a fluvial-dominated aquifer system composed of quartz- and kaolinite-rich facies. MD simulations are used to compute the contact angles of methane–water systems on mineral surfaces representative of these two lithofacies. These contact angles are then incorporated into capillary pressure curves to reflect realistic wettability conditions in the flow model. Comparative simulations, with and without molecularly informed contact angle data, demonstrate that variable wettability significantly influences methane migration patterns, saturation distributions, and the extent of methane leakage to the atmosphere. Differences in contact angle between the quartz- and kaolinite-dominated units lead to altered plume behavior and migration pathways. These findings underscore the importance of accounting for mineral-specific wettability variations in capillary pressure scaling. Failure to do so may result in underestimated leakage risks and misinformed assessments of methane mobility in shallow groundwater systems.