Abstract
The characterization of gas migration through low-permeability clay formations has been a focus of R&D programs for radioactive waste disposal, which is also of great importance for shale gas exploration, cap-rock behavior of hydrocarbon reservoirs, and hbox {CO}_{2} sequestration. Laboratory tests have been performed on Opalinus Clay, a Mesozoic claystone that is being investigated in Switzerland as a potential host rock for the storage of nuclear waste. The laboratory program included specific water and air injections tests, as well as oedometer and isotropic compression tests. Undisturbed core samples have been retrieved from a shallow borehole in the Mont Terri Underground Research Laboratory (URL) and from a deep borehole in northern Switzerland. For the shallow cores from Mont Terri URL, largely linear-elastic deformations associated with the gas injection test could be inferred and the change in void ratio was accounted for by the pore compressibility. The corresponding change in permeability was obtained from the results of the water tests, indicating a log-linear relation between permeability and porosity. The derived porosity change and the corresponding change in permeability were implemented in the standard TOUGH2 code, which reproduced the measured gas test results using fitted water retention data derived from laboratory measurements. Similar air injection tests performed on Opalinus Clay cores from the borehole at greater depth showed overall similar behavior, but at lower porosities, lower permeability values, and lower compressibility. These cases indicated nonlinear behavior which was implemented using an effective stress-dependent porosity change and the associated change in permeability. In addition, the anisotropy associated with the bedding planes of the clay formation was considered by assuming different properties for “soft” and “hard” layers to account for storage capacity for the injected gas prior to gas breakthrough. The computed change in the overall porosity could be compared to the measured axial deformation during the gas injection test and was used for calibration of the parameters describing the relationship between the effective stress and porosity, as well as the corresponding change in permeability and capillary pressure.
Highlights
The characterization of gas migration through a low-permeability clay host rock for repositories is important because significant amounts of waste-generated gas mainly produced by anaerobic corrosion of metals and degradation of organic materials are expected to migrate from low- and intermediate-level waste (L/ILW) and high-level waste (HLW) repositories into the surrounding host rock (Nagra 2004, 2008)
Previous investigations of gas flow in the Opalinus Clay (OPA) focused on estimating the gas-entry pressures from gas tests on core samples (NAGRA 2002) and from packer tests in the Benken borehole at a depth of about 600 m bgl (Marschall et al 2005)
In order to account for some variability in hydraulic properties of the core sample, the core was represented by a two-dimensional vertical cross-section model corresponding to the height (25 mm) and diameter (50 mm) and where the thickness was adjusted to yield the volume of the core sample
Summary
The characterization of gas migration through a low-permeability clay host rock for repositories is important because significant amounts of waste-generated gas mainly produced by anaerobic corrosion of metals and degradation of organic materials are expected to migrate from low- and intermediate-level waste (L/ILW) and high-level waste (HLW) repositories into the surrounding host rock (Nagra 2004, 2008). The National Cooperative for the Disposal of Radioactive Waste (Nagra), Switzerland has developed a comprehensive program to characterize gas flow in the Opalinus Clay (OPA), a Mesozoic claystone formation and one of the host rocks for a deep geological repository, through laboratory tests to determine the relevant hydraulic, geomechanical and two-phase properties, and to develop appropriate constitutive models through numerical analyses of the laboratory tests (Nagra 2009). Similar gas tests with preceding hydrotest were carried out in shallow boreholes at the Mont Terri Underground Research Laboratory (URL) (Marschall et al 2003) that indicated dilatancy controlled gas flow (or “pathway dilation”, terminology after Horseman et al 1996) This dilatancy (expansion) involved an increase in the pore space or microfracturing at rising gas pressures, which resulted in an increase in intrinsic permeability. In an extension of the previous analyses, the current analyses of the air injection tests from the deep Schlattingen borehole used an updated test configuration and took into account explicitly the measured deformation of the core sample during the air injection tests
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