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The determination of effective diffusion coefficients of gases or solutes in the water–saturated pore space of mudrocks is time consuming and technically challenging. Yet, reliable values of effective diffusion coefficients are important to predict migration of hydrocarbon gases in unconventional reservoirs, dissipation of (explosive) gases through clay barriers in radioactive waste repositories, mineral alteration of seals to geological CO₂ storage reservoirs and contaminant migration through aquitards. In this study, small angle and very small angle neutron scattering techniques have been utilized to determine a range of transport properties in mudrocks, including porosity, pore size distributions and surface and volume fractalmore » dimensions of pores and grains, from which diffusive transport parameters can be estimated. Using a fractal model derived from Archie's Law, we calculate effective diffusion coefficients from these parameters and compare them to laboratory–derived effective diffusion coefficients for CO₂, H₂, CH₄ and HTO on either the same or related mudrock samples. The samples include Opalinus Shale from the underground laboratory in Mont Terri, Switzerland; Boom Clay from a core drilled in Mol, Belgium and a marine claystone cored in Utah, USA. The predicted values were compared to laboratory diffusion measurements. The measured and modeled diffusion coefficients show good agreement, differing generally by less than factor 5. Furthermore, neutron or X–ray scattering analysis is therefore proposed as a novel method for fast, accurate estimation of effective diffusion coefficients in mudrocks, together with simultaneous measurement of multiple transport parameters including porosity, pore size distributions and surface areas, important for (reactive) transport modelling.« less

Storage of anthropogenic CO₂ in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO₂ escaping. Although natural CO₂ reservoirs demonstrate that CO₂ may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO₂-bearing brines. The resulting uncertainty poses a significant challenge to the risk assessment of geological carbon storage. We describe mineral reaction fronts in a CO₂ reservoir-caprock system exposed to CO₂ over a timescale comparable with that needed for geological carbon storage. Moreover, the propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions andmore » carbonate precipitation, which reduces its penetration into the caprock to ~7 cm in ~10⁵ years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO₂.« less