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Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity and reduced sealing performance. The results of four experiments investigating the effect of temperature on the permeability of a bentonite backfill are presented. Bentonite is a clay commonly proposed as the backfill in repository designs because of its high swelling capacity and very low permeability. The experiments were conducted in two sets of purpose-built, temperature controlled apparatus, designed to simulate isotropic pressure and constant volume conditions within the testing range of 4-6 MPa average effective stress. The response of bentonite during thermal loading at temperatures up to 200 • C was investigated, extending the previously considered temperature range. The results provide details of bentonite's intrinsic permeability, total stress, swelling pressure and porewater pressure during thermal cycles. We find that bentonite's hydraulic properties are sensitive to thermal loading and the type of imposed boundary condition. However, the permeability change is not large and can mostly be accounted for by water viscosity changes. Thus, under 150 • C, temperature has a minimal impact on bentonite's hydraulic permeability.

One of the most challenging aspects of understanding the flow of gas and water during testing in clay-rich low-permeability materials is the difficulty in visualizing localized flow. Whilst understanding has been increased using X-ray Computed-tomography (CT) scanning, synchrotron X-ray imaging and Nuclear Magnetic Resonance (NMR) imaging, real-time testing is problematic under realistic in situ conditions confining pressures, which require steel pressure vessels. These methods tend not to have the nano-metre scale resolution necessary for clay mineral visualization, and are generally not compatible with the long duration necessary to investigate flow in such materials. Therefore other methods are necessary to visualize flow paths during post-mortem analysis of test samples. Several methodologies have been established at the British Geological Survey (BGS), in order to visualize flow paths both directly and indirectly. These include: (1) the injection of fluorescein-stained water or deuterium oxide; (2) the introduction of nanoparticles that are transported by carrier gas; (3) the use of radiologically tagged gas; and (4) the development of apparatus for the direct visualization of clay. These methodologies have greatly increased our understanding of the transport of water and gas through intact and fractured clay-rich materials. The body of evidence for gas transport through the formation of dilatant pathways is now considerable. This study presents observations using a new apparatus to directly visualize the flow of gas in a kaolinite paste. The results presented provide an insight into the flow of gas in clay-rich rocks. The flow of gas through dilatant pathways has been shown in a number of argillaceous materials (Angeli et al., 2009;Autio et al., 2006;Cuss et al., 2014;Harrington et al., 2012). These pathways are pressure induced and an increase in gas pressure leads to the dilation of pathways. Once the gas breakthrough occurs, pressure decreases and pathways begin to close. This new approach is providing a unique insight into the complex processes involved during the onset, development and closure of these dilatant gas pathways.

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faulting/fracturing in the same basin will have formed during the depletion of the reservoir. 39Therefore in many geological settings both natural and induced discontinuities will have 40 formed. 41Fluid flow in argillaceous materials, whether through the bulk rock or along discontinuities, 42is closely related to the mechanical state of the caprock. In particular, the role of faults and 43 fractures as potential conduits or barriers to fluid flow is likely to be of critical importance to 44 seal integrity in Carbon Capture and Storage (CCS) sites. In addition, recent studies [Zoback 45 & Gorelick, 2012] and on-going developments relating to induced seismicity [Green, et al. 46 2011] in other industries have also highlighted the importance of a thorough understanding of 47 the potential for, and controls on, fault reactivation behavior. 57Faulted geological settings are complex systems that are borne out of multiple episodes of 58 deformation, in the form of faulting, subsidence and exhumation, and altered stress regimes. 59This means that faults cannot be viewed as static features over geological time. Nor can they 60 be considered static on CO 2 injection time scales, as complex pore-pressure histories and 61 chemical alteration-driven deformation may also have an impact on caprock systems. As 62 such, time is a significant factor in fault sealing. 63On the long time-scales of interest in CCS, cross-reservoir fluid migration may lead to 64 changes in stress-state long after injection ceases. The response of new or previously-sealed 65 discontinuities exposed to these dynamic conditions, may be significant. Noy et al. [2012] 66 demonstrated that pore-pressure perturbations, resulting from the injection of CO 2 , may 67 persist for significant periods (~300 years) after the injection phase. These perturbations are 68 likely to be particularly large in magnitude within the immediate vicinity of injection, but are 69 also demonstrably of concern 'a considerable distance outside the CO 2 footprint at the end of under an elevated pore-pressure condition, (ii) critically oriented faults with the potential for 74 reactivation (as compared to those far from critically stressed), or faulting with the potential 75 for infrequent but significant seismicity. 76Additionally, both near-and far-field discontinuities may be exposed to a range of changing 77 fluid chemistries during the evolution of a storage site, from CO 2 -rich fluids to the migration 78 of brines at the periphery of the pressure pulse. In contrast to reservoir rocks, the 79 phenomenon of clay swelling is of major importance to the sealing behavior of argillaceous 80 cap-rocks [Horseman et al., 2005; Tsang et al., 2005], with the potential to notably affect 81 transmissivity of discontinuities. CO 2 has been shown to markedly impact on the swelling 82 properties of clays [Espinoza & Santamaria, 2012], but there is a paucity of data relating to 83 the impact on shale swelling properties and, in particular, the potential effects for fault 84 seal...