Abstract
Abstract Hydraulic testing has revealed dramatic underpressures in Paleozoic shales and carbonates at the Bruce nuclear site in Ontario. Although evidence from both laboratory and field studies suggests that a small amount of gas-phase methane could be present in the shale, previous studies examining causal linkages between the gas phase and the underpressure have been inconclusive. To better elucidate processes in such a system, we used a highly simplified 1D representation of the site to test, by using iTOUGH2-EOS7C, the effects of various factors on the evolution of gas-phase methane and pressures within the system. Heterogeneity was represented by three stratigraphic regions with slightly different capillary pressure characteristics and, in one case, three thin distinct zones with very different characteristics. Underpressure occurred only when gas pressures set as an initial condition required it, and even in this case it was geologically short-lived. We conclude that the presence of multiple fluid phases is unlikely to explain the underpressure at the site; we suggest that the influence of gas-phase methane on porewater flow is minimal. This is consistent with prior conceptualizations of the underpressured section as a thick aquiclude, in which solute transport occurs extremely slowly, bounded by aquifers of significantly higher permeability.
Highlights
Anomalous subsurface fluid pressures, interpreted as isolated highs or lows in pore fluid potential or hydraulic head, are generally thought to signal hydraulically isolated regions. They delineate large volumes of geological media with unusually low permeability (Neuzil 1995; Ingebritsen et al 2006) and reveal formation-scale properties not otherwise measurable. Such features have been documented in Paleozoic shales and carbonates at the Bruce nuclear site in southern Ontario, Canada, where hydraulic heads as much as 200 m below sea level and extremely large hydraulic gradients (4 m m−1) between underpressured and overpressured zones have been observed (Intera Engineering Ltd 2011; Beauheim et al 2017, 2013)
We examined the role that gas phase may play at the Bruce site by using simple conceptual models of the site and its history to frame multiphase flow simulations performed with the code iTOUGH2-EOS7C (Oldenburg et al 2004; Finsterle et al 2017)
We suspected that gas-phase formation and concurrent migration with co-resident groundwater could not entirely explain the underpressure observed at the Bruce site; we considered it desirable to further explore this possibility
Summary
Anomalous subsurface fluid pressures, interpreted as isolated highs or lows in pore fluid potential or hydraulic head, are generally thought to signal hydraulically isolated regions By extension, they delineate large volumes of geological media with unusually low permeability (Neuzil 1995; Ingebritsen et al 2006) and reveal formation-scale properties not otherwise measurable. They delineate large volumes of geological media with unusually low permeability (Neuzil 1995; Ingebritsen et al 2006) and reveal formation-scale properties not otherwise measurable Such features have been documented in Paleozoic shales and carbonates at the Bruce nuclear site in southern Ontario, Canada, where hydraulic heads as much as 200 m below sea level and extremely large hydraulic gradients (4 m m−1) between underpressured and overpressured zones have been observed (Intera Engineering Ltd 2011; Beauheim et al 2017, 2013).
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