Microstructural investigation of mudrock seals using nanometer-scale resolution techniques

Small angle neutron scattering (SANS) and nitrogen low-pressure adsorption (LPS) have been used to characterise the pore structure of two organic lean mudrocks, Opalinus Clay, Mont Terri, Switzerland and Carmel Claystone, Utah. This was done in order to obtain a better understanding of H2 and CO2 transport, reaction and sorption related to radioactive waste disposal and carbon storage, respectively. The pore structure information derived by SANS and LPS are comparable and the results have revealed a vast heterogeneity from 2 nm to 2 µm, which can be related to the high clay contents. Due to the high clay contents, pores smaller than 10 nm constitute a large fraction of total porosity (25-30 %) and most of specific surface area (up to 80 %) in the sample mudrocks. Accordingly, these interplays contribute to a pore network of few-to-several nano-Darcy permeability in which pore size dependent transport mechanisms can vary from high sorptive diffusional fluid flow in small pores to low sorptive slip flow regime at progressively larger pores.