Multiphase flow and underpressured shale at the Bruce nuclear site, Ontario, Canada

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 gasphase 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.

Glacially Induced Hydromechanical Coupling in Shale May Have Caused Underpressured Water in the Eastern Michigan Basin Despite the Possible Presence of Gas Phase Methane

“…For the zones in which gas phase was introduced, the van Genuchten 1/P 0 parameter was increased by a factor of 2, as justified by the uncertainty analysis conducted by Plampin and Neuzil (2018), which showed that varying this parameter by a factor of 2 in either direction from the fitted value effectively captured the spread in the measured capillary pressure characteristics of samples from the relevant Table 1 Porous Media Properties Used in the iTOUGH2-EOS7C Simulations Note. The three layers of the model consisted of the "tight" region in the middle, bounded above, and below by the Guelph and Cambrian aquifers.…”

“…This was done by developing a simple representation of the site that extends from 0.37 to 0.86 kmBGS-the entire region of the subsurface in which anomalous pressures have been observed-to which the rock properties listed in Table 1 were assigned. The domain was initially homogeneous, with most of the rock property values set equal to those of the "tight" region of the model developed by Plampin and Neuzil (2018). The boundaries of the model domain were applied at the locations of the Guelph and Cambrian aquifers, as indicated by the horizontal dashed gray lines in Figure 1a.…”

“…For the zones in which gas phase was introduced, the van Genuchten 1/P 0 parameter was increased by a factor of 2, as justified by the uncertainty analysis conducted by Plampin and Neuzil (2018), which showed that varying this parameter by a factor of 2 in either direction from the fitted value effectively captured the spread in the measured capillary pressure characteristics of samples from the relevant…”

“…These multiphase flow parameters are reported consistently with the TOUGH2 documentation (Pruess et al, 2012) in Table 1. The model also differed from that of Plampin and Neuzil (2018) in that the geomechanical properties K and ν (drained bulk modulus and Poisson Ratio, respectively) were assigned to the rocks, based on calculations by Neuzil and Provost (2014), to allow for hydromechanical coupling to be represented.…”

“…Calder et al (2015) showed that gas phase could have dampened the effects of loading on water pressure at the site due to its relatively high compressibility compared to that of water, but they used an earlier paleoclimate reconstruction model for the glacial loading inputs, and their model did not generate significant underpressures. Normani and Sykes (2012) suggested that the presence of gas phase could have caused the underpressures, but Plampin and Neuzil (2018) provided a counterargument to this claim by demonstrating that multiphase flow alone did not seem capable of causing the underpressures, absent other forcing terms. This finding does not indicate that gas phase precludes underpressure development entirely, however, rendering the potential cause(s) of underpressures in multiphase systems still uncertain.…”