Abstract A UK repository concept currently under consideration for the disposal of intermediate-level radioactive waste and some low-level waste not suitable for surface disposal involves using large quantities of cementitious materials for construction, grouting, waste containers, waste isolation matrix and buffer/backfill. CO 2 generated from the degradation of organic material in the waste will result in cement carbonation and associated mineralogical changes. Hydraulic and gas permeability tests were performed on Nirex Reference Vault Backfill (NRVB) cement at 40 °C and either 4 or 8 MPa. Carbonation reactions using CO 2 gas halved the permeability of the NRVB under simulated repository conditions. A greater decrease in permeability (by three orders of magnitude) was found during carbonation using dissolved CO 2 . Mineralogical changes were found to occur throughout the cement as a result of the reaction with CO 2 . However, a narrow zone along the leading edge of a migrating reaction front was associated with the greatest decrease in porosity. Fluid pressures increased slightly due to permeability reductions but fluid flow still continued (albeit at a lower rate) preventing the build-up of overly high pressures. Overall, the observed reductions in permeability could be beneficial in that they may help reduce the potential for fluid flow and radionuclide migration. However, continued carbonation could lead to potential issues with regards to gas pressure build-up.

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas build-up and subsequent migration through an engineered barrier system (EBS). Lasgit has a substantial experimental dataset containing in excess of 26 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose and quantify difficult to observe phenomena in large, non-uniform datasets, has been developed and applied. Presented results focus on the investigation and interpretation of second-order events occurring in close proximity (temporally and spatially) to a known macro-scale gas flow event that occurred during the second gas injection test. The similarity of the investigated event to dilatant flow observed in laboratory experiments is noted, as is the evidence for localized flow pathways in the bentonite EBS. The sensitivity of the toolkit’s ability to highlight second-order events is also evaluated.

Abstract The Swedish concept for geological disposal of radioactive waste involves the use of bentonite as part of an engineered barrier system. A primary function of the bentonite is its ability to swell when hydrated by its surroundings. One particular uncertainty is the impact on this function, resulting from deviations in pore-water pressure, p w , from expected in situ hydrostatic conditions. We present results from a series of laboratory experiments designed to investigate the form of the relationship between swelling pressure and p w , for compacted Mx80 bentonite, from low to elevated applied water pressure conditions. The experiments were conducted using constant volume cells, designed to allow the total stresses acting on the surrounding vessel to be monitored (at five locations) during clay swelling. The results demonstrate that swelling pressure reduces non-linearly with increasing p w , becoming less sensitive to changes at elevated pressures. After cyclic loading a marked hysteresis was also observed, with swelling pressure remaining elevated after a subsequent reduction in applied water pressure. Such behaviour may impact the mechanical and transport properties of the bentonite and its resulting performance. However, such hysteric behaviour was not always observed. Further testing is required to better understand the causes of this phenomenon and the controls on such behaviour.

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas buildup and subsequent migration through an engineered barrier system. Lasgit has a substantial experimental dataset containing in excess of 21 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose difficult to observe phenomena, has been developed and applied to the dataset. The preliminary application of the toolkit, presented here, has resulted in a large number of phenomena being indicated/quantified, including highlighting of second-order events (small gas flows, perturbations in stress/pore-water sensors, etc.) and quantification of temperature record frequency content. Localized system behaviour has been shown to occur along with systematic aberrant behaviours that remain unexplained.

Abstract In a Swedish repository for the disposal of heat-emitting waste, the long-term thermal stability of the bentonite engineered barrier forms a key component of the safety case. Central to such consideration is the evolution of hydraulic permeability and a potential degradation of hydraulic properties, in response to prolonged thermal exposure of the clay. To address this issue, a detailed programme of laboratory-based experiments has been undertaken at both the British Geological Survey and Studiecentrum voor Kernenergie/Centre d’Etude de L’Energie Nucleaire, in order to examine the hydraulic behaviour of bentonite that had previously been exposed to elevated temperatures. Hydraulic properties were calculated from both steady-state pressure gradients and from analysis of the pressure transients. Inspection of the data found no significant difference in hydraulic behaviour between the virgin material and clay samples taken from the Canister Retrieval Test. Based on these observations, the authors find no evidence for an adverse increase in hydraulic conductivity of bentonite as a result of prolonged thermal exposure to temperatures of 80 °C.

Abstract Aseries of complex experimental histories have been performed on two specimens of Nordland Shale from the cap rock of the Sleipner CO 2 injection site in the North Sea. By simultaneously applying a confining back pressure, specimens were isotropically consolidated and fully water saturated under realistic conditions of effective stress. Ingoing and outgoing fluxes were monitored at all times. Multistep consolidation and hydraulic tests were performed prior to gas injection to determine baseline hydraulic properties. Both specimens were found to be relatively compressible with a general trend of reducing compressibility with increasing effective stress. Hydraulic permeability, anisotropy ratio, and specific storage were quantified by inverse modeling using an axisymmetric two-dimensional finite element model. Estimates for elastic deformation parameters were derived from the analysis of consolidation transients. Both specimens yielded comparable intrinsic permeabilities of around 4 × 10 -19 m 2 (43 × 10 –19 ft 2 ) perpendicular to bedding and 10 –18 m 2 parallel to it. Specific storage was found to vary with effective stress within the range of 2–6 × 10 –5 m –1 (0.6–1.8 × 10 –5 ft –1 ). Gas transport properties were determined by multistep constant pressure test stages, using nitrogen as the permeant. Analysis of the flux data indicates gas entry and breakthrough pressures under initially water-saturated conditions of 3.0 and 3.1 MPa, respectively. Using a stepped pressure history, flow rate through the specimen was varied to examine the underlying flow law and the possible effects of desaturation. With the injection pump stopped, gas pressure declined with time to a finite value, providing a measure of the apparent threshold capillary pressure, which ranged from 1.6 to 1.9 MPa. Numerical modeling of the gas data, using the TOUGH2 code, suggests that anisotropy to gas flow is greater than hydraulic flow. Fits to the pressure data were obtained, but matching the magnitude of the flux through the sample was not possible. Based on the data and subsequent model activities, standard concepts of viscocapillary (two-phase) flow are clearly inadequate to accurately describe the processes and mechanisms governing gas flow in the Nordland Shale. Evidence suggests that gas movement occurs through pressure-induced pathway flow, accompanied by a limited degree of viscocapillary displacement. The laboratory experiments support the time-lapse seismic observations that the cap rock is performing as an effective capillary seal. The experimental results also indicate that if gas flow is induced in this type of material, it is mainly via discrete pathways, instead of distributed Darcy flow. This is consistent with observed CO 2 flow patterns within the reservoir, although a satisfactory explanation for how such pathways develop remains elusive.

Abstract A programme of 143 simple gas injection experiments was performed on unconfined and initially water-saturated clay pastes at water contents between the plastic and liquid limits. The aim was to investigate the relationships between gas entry pressure, water content and plasticity for a range of clay types, to define the principal mechanisms of gas entry and flow by simple visual observations and to determine the effects of previous gas injection and residual gas content on entry pressure. Gas movement was found to be entirely through pressure-induced pathways, including highly-dilated tension fractures, flattened ellipsoidal cavities and bubbles. By examining entry mechanisms across the range of water contents, it was possible to delineate three zones of behaviour. Gas entry pressures in the region of the plastic limit were surprisingly large, particularly for clay types with high total specific surface and plasticity index. The highest individual entry pressure recorded in the study was 1810 kPa for Wyoming bentonite. There was no evidence in any test that gas actually penetrated, or flowed through, the intergranular porosity of the clay matrix. In all cases, gas made its own volume by pushing back the paste and lifting the free surface of the sample. After gas injection, remnant gas-filled voids and cracks remained within the clay. These were re-opened during repeated injections at pressures which were only a fraction of the entry pressures of the gas-free pastes. Gas entry at high pressures was audible and occasionally violent. The significance of these findings to gas migration modelling and the quantitative prediction of gas fluxes in clay formations is briefly examined.