Abstract:6 Pre-compacted bentonite has long been proposed as a primary component of an Experiments also show the potential for gas entry into the buffer to occur as a result of 23 declining pore-water pressure conditions. As such, the influence of significant deviations 24 from hydrostatic conditions (for example, resulting from glacial loading) should not be 25 neglected when considering gas interaction with the buffer over long timescales.26
“…Consequently, a sharp raise of the gas outflow is observed, mainly fed by gas previously stored in the bottom contact interface and in the crack. Qualitatively similar sharp break-through events were experimentally observed by Horseman et al (1999), Graham et al (2016), Daniels and Harrington (2017), Harrington et al (2017Harrington et al ( , 2019 and Gutiérrez-Rodrigo et al (2021).…”
Section: Gas Injection In a Clay Sample 621 Case Csupporting
confidence: 84%
“…From the mechanical point of view, the oscillations are associated to partial closure and reopening of the crack tip. Notably, oscillatory or intermittent outflows after break-through events are also observed experimentally (Graham et al, 2016;Daniels and Harrington, 2017;Harrington et al, 2017Harrington et al, , 2019. Moreover, a detailed analysis of stress perturbation events indicates that the intermittent outflows are correlated with the opening and closing of cracks (Harrington et al, 2019).…”
Section: Gas Injection In a Clay Sample 621 Case Cmentioning
“…Consequently, a sharp raise of the gas outflow is observed, mainly fed by gas previously stored in the bottom contact interface and in the crack. Qualitatively similar sharp break-through events were experimentally observed by Horseman et al (1999), Graham et al (2016), Daniels and Harrington (2017), Harrington et al (2017Harrington et al ( , 2019 and Gutiérrez-Rodrigo et al (2021).…”
Section: Gas Injection In a Clay Sample 621 Case Csupporting
confidence: 84%
“…From the mechanical point of view, the oscillations are associated to partial closure and reopening of the crack tip. Notably, oscillatory or intermittent outflows after break-through events are also observed experimentally (Graham et al, 2016;Daniels and Harrington, 2017;Harrington et al, 2017Harrington et al, , 2019. Moreover, a detailed analysis of stress perturbation events indicates that the intermittent outflows are correlated with the opening and closing of cracks (Harrington et al, 2019).…”
Section: Gas Injection In a Clay Sample 621 Case Cmentioning
“…Currently, these contributing factors cannot be uncoupled for a number of reasons. Unfortunately, with less than 6 sensors, it is not possible to locate the source of the events in 3 dimensions, and very little can be said about any changes in the bulk clay properties, although some stiffening is thought to occur during microfracture network development resulting from stimulation by mobile gas [34]. Additionally, the aspect ratio and orientation of gas pathways have yet to be characterised [17], meaning that the form of the resulting radiation pattern can only be speculated at.…”
Compacted bentonite is proposed as an engineered barrier material within facilities for the geological disposal of radioactive waste. Barrier performance and its interaction with a free gas phase must be considered as part of sound repository design. This study involved the long-term experimental examination of gas flow in precompacted bentonite, with particular consideration of gas network stability. Results demonstrate that the stress field experienced by the clay is strongly coupled with gas flow. For the first time, three controls on this behaviour are considered: (i) injection flow rate, (ii) constant vs. variable gas pressure, and (iii) stimulation of the microfracture network. A detailed stress analysis is used to examine changes in the gas flow network. The results indicate a degree of metastability despite these changes, except in the case of stimulation of the microfracture network by removal of the primary drainage route. In this case, a rapid redevelopment of the gas flow network was observed. As such, availability of drainage pathways will represent a key control on the generation of peak gas pressures and distribution of gas within the engineered barrier. The cessation of gas flow is shown to result in crack closure and self-sealing. Observations from this study highlight that characterisation of the gas network distribution is of fundamental importance in predicting gas dissipation rates and understanding the long-term fate of gas in radioactive waste repositories.
“…5 a), where gas pumping ceased, allowing pressure decay. Figure 7 Modification of a conceptual diagram after 16 , demonstrating that the polarity metric provides quantitative evidence of the change in gas pathway behaviour during gas injection of bentonite. The following phases are identified: ( a ) gas has entered the clay and pathways beginning to open whilst gas pressure continues to rise, ( b ) gas is able to escape, pathways become unstable and may close and reopen as gas pressure peaks, ( c ) gas pressure declines sufficiently for gas pathways to continue to close.…”
Section: Discussionmentioning
confidence: 99%
“…Multiple experimental programmes document the occurrence of gas flow in clays, above a critical threshold 8 , 10 – 17 , which has been shown to relate to the internal stress state of the clay, σ ij , 13 , 15 , 16 . This behaviour has been attributed to gas advection via deformation of the matrix (iii), also termed ‘pathway dilatancy’.…”
Geological disposal of radioactive waste is being planned by many countries. Bentonite clay is often included in facility design, providing a barrier to radionuclide migration. Gas, generated by the waste or corrosion of waste canisters, may disrupt the properties of the bentonite. Robust prediction of this interaction is, therefore, necessary to demonstrate safe facility evolution. In some cases, gas may deform the clay, resulting in localised flow; however, the nature of this deformation has been widely debated. Accurate numerical representation of this behaviour has been limited by a shortage of information on the degree/distribution of deformation. Using experimental data from gas injection tests in bentonite, we show that first order fluctuations in the stress field can provide this information. We show that hundreds of microdeformation events can be detected, with similar characteristics to established fracturing phenomena, including earthquakes and acoustic emissions. We also demonstrate that stress field disruption (i) is spatially localised and (ii) has characteristics consistent with gas pathway ‘opening’ and ‘closure’ as gas enters and exits the clay, respectively. This new methodology offers fundamental insight and a new opportunity to parameterise and constrain gas advection models in clays and shales, substantially improving our capacity for safe facility design.
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