MULTIPHASE FLOW, HYDROMECHANICS AND UNDERPRESSURED GROUNDWATER AT THE SITE OF A PROPOSED DEEP GEOLOGIC NUCLEAR WASTE REPOSITORY IN CANADA

Many countries are planning or constructing deep geologic repositories in sedimentary basins to isolate nuclear waste from possible interaction with the biosphere and groundwater resources. Assessing the efficacy of these solutions requires understanding of long-term hydrogeologic processes that involve large spatiotemporal scales and complex, often coupled, phenomena. Matters are complicated further when there is evidence for other fluid phases besides water (e.g., methane gas) within the subsurface system, because repository design requires low-permeability rocks for which multiphase flow fundamentals are poorly understood. Such rocks conduct fluids at extremely slow rates, causing the results of geologic processes to be preserved over long periods of time and thus allowing field sites to be used as natural laboratories for investigating fundamental processes. A thick sequence of low-permeability shale and limestone at the site of a proposed repository for low- and intermediate-level nuclear waste in southern Ontario, for instance, exhibits distinctly underpressured groundwater and potential evidence for gas-phase methane in situ, which raises questions regarding the relationship between these two conditions. While previous research has shown that processes such as the retreat of ancient glaciers or exhumation of previously overlying sediments may account for the observed underpressures, the role of gas-phase methane, if any, in these hydromechanical processes is unknown. This research helps to address these questions by performing numerical simulations, using two different codes, of long-term multiphase flow and hydromechanical coupling during glacial cycles within a simplified representation of the hydrogeologic environment at the proposed Canadian repository site. The Results of our research indicate that the presence of a gas phase alone is unlikely to have caused the underpressures, but also that its presence does not preclude the development of the underpressures via glacially-driven hydromechanical phenomena.