Importance of thermo-osmosis for fluid flow and transport in clay formations hosting a nuclear waste repository

Three osmotic processes have been identified in clay-rich media. Electro- and chemo-osmosis – flows of water caused by salinity and electrical potential gradients – have so far attracted almost exclusive attention. But, despite the recurring concern about the importance of thermo-osmosis – a flow of water driven by a temperature gradient – in argillaceous media, it remains largely neglected. Here we propose a new formalism for estimating the thermo-osmotic permeability based on a theoretical analysis at the molecular and pore scale, then upscaled. We show that the thermo-osmotic permeability can be estimated only from surface-charge density, temperature, pore size and salinity. The possible prominent role of thermo-osmosis in compacted shale layers with a temperature gradient is then exemplified. This first general estimate of the thermo-osmotic permeability can be used to improve our understanding of the influence on fluid flow and pressure fields of the natural geothermal gradient in sedimentary basins, which is required, e.g., for safety assessments of nuclear-waste repositories in shale layers.