Heterogeneous Wettability Alters Methane Migration and Leakage in Shallow Aquifers

Capillary heterogeneity is increasingly recognized as a first-order control of gas plume migration and trapping in aquifers and storage formations. We show that spatial variability in the water-methane contact angle, set by mineralogy and salinity, reshapes capillary entry pressures and, in turn, migration pathways. Using molecular dynamics simulation, we estimate contact angles on quartz and kaolinite under fresh and saline conditions and embed these results in continuum-scale multiphase flow simulations via a contact-angle-informed Leverett J-function, mapping wettability directly onto the flow properties at continuum scale. Accounting for contact-angle heterogeneity changes the methane behavior: mobile and residually trapped methane in aquifers decrease by up to 10%, while leakage to the atmosphere increases by as much as 20%. The magnitude of this effect is scaled with the permeability contrast, leakage rate, salinity, and facies proportions. By coupling molecular-scale wettability to continuum-scale flow and transport, the cross-scale framework provides a more physically grounded basis for groundwater protection and risk assessments and yields more reliable emissions estimates. The approach can be generalized to other subsurface gas transport problems including hydrogen and carbon dioxide storage as well as natural releases such as methane from permafrost thaw.