iTOUGH2-EOS7C model used to analyze multiphase flow and underpressured shale at the Bruce Nuclear Site, Ontario, Canada

Hydraulic testing and long-term monitoring have revealed significant overpressures and dramatic underpressures in Paleozoic shales and carbonates at the Bruce nuclear site on the eastern flank of the Michigan basin near Tiverton, Ontario. Although several lines of evidence from both laboratory and field studies suggest that a small amount of gas phase methane could be present in the shale, previous studies examining causal linkages between gas phase and the underpressure have been inconclusive. To better elucidate processes in such a system, we simulated multiphase flow in a highly simplified 1-D representaion of the site using iTOUGH2-EOS7C. Specifically, we examined the effects of various factors, including geologic heterogeneity as well as different boundary and initial conditions, on the evolution of gas phase methane and gas- and water-phase pressures within the system. Heterogeneity observed in core samples was represented using three stratigraphic regions with distinctly different capillary pressure characteristics and, in one case, a few thin, distinct zones. Significant underpressure occurred only when gas pressures set as an initial conditions required it, and even in this case the underpressure was geologically short-lived. We conclude that the presence of multiple fluid phases alone is unlikely to explain the underpressure at the site and suggest that the influence of gas-phase methane on pore water flow is minimal. This is consistent with prior conceptualizations of the underpressured section at the site as a thick aquiclude, in which solute transport occurs extremely slowly, bounded by aquifers of significantly higher permeability.