Gas migration experiments in bentonite: implications for numerical modelling

Abstract

In the Swedish KBS-3 repository concept, there is potential for gas to be generated from corrosion of ferrous materials under anoxic conditions, combined with the radioactive decay of the waste and radiolysis of water. A full understanding of the probable behaviour of this gas phase within the engineered barrier system (EBS) is therefore required for performance assessment. We demonstrate key features from gas transport experiments on pre-compacted Mx80 bentonite, under laboratory and field conditions, and discuss their implications in terms of a conceptual model for gas migration behaviour. On both scales, major gas entry is seen to occur close to the sum of the porewater and swelling pressures of the bentonite. In addition, gas pressure at breakthrough is profoundly sensitive to the number and location of available sinks for gas escape. Observations of breakthrough can be explained by the creation of dilatational pathways, resulting in localized changes in the monitored porewater pressures and total stresses. These pathways are highly unstable, evolving spatially and temporally, and must consequently influence the gas permeability as their distribution/geometry develops.Such observations are poorly embodied by conventional concepts of two-phase flow, which do not fully represent the key processes involved. Although dilatancy based models provide a better description of these processes, the paucity of data limits further development and validation of these models at present.

A Mathematical Model of Gas and Water Flow in a Swelling Geomaterial—Part 1. Verification with Analytical Solution

“…In addition, a number of mathematical models have been developed to simulate the gas transport processes observed through these laboratory and field-scale studies with some success [9][10][11][12]. However, no studies to date have been able to determine the exact mechanisms which control gas entry, flow, and pathway sealing [4,6,8,[13][14][15][16][17]. Development and use of numerical models are of key importance in understanding the processes involved and their use in long-term safety assessments.…”

Gas Transfer Through Clay Barriers

“…Field scale observations at an underground research facility in Sweden confirmed these results, showing clear hydromechanical coupling during gas flow (Harrington et al 2007;Cuss et al, 2010Graham et al, 2012).…”

Extended two-phase flow model with mechanical capability to simulate gas migration in bentonite

“…Finally, the persistence of a preferential pathway after their formation is a point that should be considered with caution, as some experiments have shown the instability of the pathways (e.g. Horseman 1999; Graham et al 2012). Here, it is simply considered that the pathway remains open once it is formed (the porosity and the permeability continue to follow the constitutive laws of the pathway dilation model).…”