Gas Network Development in Compact Bentonite: Key Controls on the Stability of Flow Pathways

Abstract

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.

Modelling gas fracturing in saturated clay samples using triple-node zero-thickness interface elements

“…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).…”

“…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).…”

“…At time 1.0 Ms, when the gas injection pressure reaches 𝑝 𝑔 ≈ | | 𝜎 𝑖𝑛𝑖 | | = 4.5 MPa, the crack is almost completely closed and, consequently, the dissipation of the gas overpressure slows down significantly, driven only by the gas transport processes in the continuum elements. Self-sealing of gas pathways following reduction of the injection rate/pressure has been reported in different gas migration tests by Horseman et al (1999), Graham et al (2016), Daniels and Harrington (2017), Harrington et al (2019) and Gutiérrez-Rodrigo et al (2021).…”

Phenomenological Model of Nonlinear Dynamics and Deterministic Chaotic Gas Migration in Bentonite: Experimental Evidence and Diagnostic Parameters

“…The results of a series of laboratory experiments in Mx80 bentonite (Cuss et al 2014;Graham et al 2012Graham et al , 2015Harrington et al 2019) and Callovo-Oxfordian claystone (COx) (Cuss et al 2011;Harrington et al 2017a, b) showed that the advective transport of repository gas in bentonite causes the development of new porosity and propagation of dilatant pathways, resulting in the generation and evolution of a gas network. It was also found a strong coupling between stress, gas pressure and flow in bentonite, which takes place during multiple discrete propagation events, demonstrating spatial variability and timedependency of bentonite permeability.…”

“…It was also found a strong coupling between stress, gas pressure and flow in bentonite, which takes place during multiple discrete propagation events, demonstrating spatial variability and timedependency of bentonite permeability. Harrington et al (2019) concluded that characterization 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.…”