Gas diffusion in water-saturated underground porous media: From micro to macro scale study

Design and implementation of underground storage of H2 and CO2 require reliable and calibrated simulation models capable of predicting injected fluid behavior and its interaction with geological formation and native fluids at field scale over geological timescale. Molecular diffusion of gas through formation water in reservoir, aquifer, and caprock influences various mechanisms related to the dissolution, such as the activation of microbial activity and the trapping phenomena and, in particular, the long-term containment of the stored or sequestered gas. This project focuses on characterizing the diffusive term, which describes the molecular diffusion of dis- solved gas through water-saturated porous media. Here, the bulk diffusion coefficient adopted at the microscale is rescaled to obtain the effective diffusion coefficient representing the porous medium at the macroscale through pore space morphology description based on tomographic images. Two approaches are adopted and compared: an established one involving a pore-scale numerical simulator to assess diffusion tortuosity, which is then used in a correlation to estimate the effective diffusion coefficient; an innovative workflow involving fitting the pore- scale numerical solution of the diffusion–transport equation to the analytical solution of Fick's second law for a homogeneous representative elementary volume (REV). To this end, a 1D solution of Fick's second law is derived under appropriate initial and boundary conditions. Due to the lack of nano-CT caprock data, the methodology was applied to low-porosity, tortuous rocks. The provided results should be regarded as conser- vative estimates; the methodology must be reapplied to nano-CT images of actual caprocks for field-scale predictions.