Abstracts

ern flank of the Michigan basin have revealed abnormal vertical head distributions and
gradients inconsistent with hydrostatic conditions. Westbay pressure data from the DGR
boreholes at the Bruce site indicate that the Cambrian sandstone and the Silurian Guelph
Formation are significantly over-pressured relative to fluid density corrected hydrostatic
levels while within the intervening 360 m sequence of Ordovician limestone and shale,
heads are significantly under-pressured. This study investigates various physical processes
in the interpretation of the pressures as relevant to understanding formation scale hydraulic
properties on geologic timescales. Three physically based numerical models were developed
to investigate the evolution of pore fluid pressures: glacial loading without crustal
flexure, exhumation of post Devonian sediments, and the presence of a non-wetting gas
phase in the rock. Using the computational models FRAC3DVS-OPG and TOUGH2-
MP, the parameter estimation model PEST, and a one-dimensional conceptual model, all
models could describe the under-pressures. A fourth physically based numerical model was
developed to assess the effect of crustal flexure on the generation of in-situ pore fluid pressures
at the Bruce site. Both flexural stresses and vertical stresses, imposed by the weight of
an ice-sheet, affect the formation of in-situ pore fluid pressures through hydro-mechanical
coupling. A two-dimensional finite element poroelastic simulator was written to support
this analysis; verification to analytical solutions yielded an excellent match. Multiple lines
of evidence, including in-situ hydraulic testing (Beauheim, 2014), geochemical analyses
(Al et al., 2015; Clark et al., 2013), and numerical modelling (Sykes et al., 2011), support
the conclusion that the Cobourg Formation and the shales of the Ordovician age sediments
are of very low permeability and have sustained their very low permeabilities over
geologic time.
350 - Diffusion research in support of the Canadian nuclear waste
management program
Tom Al 1 , Mark Jensen 2 , & Ian Clark 1
1
Earth Sciences, University of Ottawa, Ottawa, Ontario, Canada
2
Nuclear Waste Management Organization, Toronto, Ontario, Canada
The requirement to understand the diffusive properties of rocks is fundamental to site
characterization for geologic repositories. Recent research in this area has resulted in the
development of new magnetic-resonance imaging (MRI) and radiation-imaging (RI)
methods for measurement of diffusion and reaction properties including pore- and effective-
diffusion coefficients (D p
and D e
), tracer-accessible porosity (φ tr
), cation-exchange
capacity (CEC) and ion selectivity coefficients (e.g. logK Cs_Na
). The RI techniques are rapid
and versatile, involving either an X-ray or a gamma source ( 241 Am) source. The experiments
are conducted principally with iodide as a conservative tracer and weakly sorbed
cesium for reactive transport experiments. Site investigations for the DGR proposed by
OPG for low- and intermediate-level waste at the Bruce nuclear site in Ontario provided
opportunity to apply the new measurement methods, and to investigate effects of specific
tracers, anisotropy, confining pressure and scale. One hundred and thirteen measurements
were conducted across 700 m of Paleozoic stratigraphy by a combination of X-ray radiography
and the conventional through-diffusion technique. With the exception of a few
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