Leakage from geologic energy and carbon storage: Multiphase migration of H2 and CO2 through variably saturated porous media

The shallow fate of hydrogen leaking from storage wells, the likeliest leakage pathway, is less studied than that of carbon dioxide. Matched five-year multiphase simulations of a wellbore defect leaking into an air-filled, variably saturated section were run with only the gas changed. They were extended by sweeps of permeability, water saturation, gas mobility, defect conductance, source depth, section size and source geometry, and by aquifer cases. In intermediate-permeability sand, both plumes are diffusive and unsorted. Density sorting appears when a buoyancy-to-diffusion ratio built on the harmonic-mean permeability exceeds about one. A wet zone lowers the threshold only while its gas phase stays mobile. Dry, partly saturated and wet pairs follow one approximate rising trend of this ratio, with one low-mobility exception. Below the water table, both gases rise. Absolute concentrations depend on section and source geometry, but the leak-rate and dissolution orderings of the two gases do not.