Gas Breakthrough Mechanisms in Reconstituted Geomaterial

Subsurface energy and waste disposal in geo-reservoir environments rely on the sealing potential of clay-rich geological formations to act as physical barriers to long-term anthropogenic influences and minimise the risk of catastrophic leakage from storage facilities. Clay-rich materials are favourable sealing materials due to them characteristically consisting of small pores providing high capillary entry pressures, preventing the intrusion of non-wetting fluid (i.e., CO₂, H₂). The common assumption is that the gas penetrates the barrier due to capillary breakthrough, i.e., the menisci forming at the interface between the gas and pore-water reach the receding contact angle as gas pressure increases and can no longer sustain the unbalance between the gas and pore-water pressures.

However, capillary breakthrough is not the only possible mechanism. Developing a better understanding of the mechanisms controlling gas sealing is vital for the long-term successful deployment of subsurface energy and waste disposal in geo-reservoir environments. This study aims to investigate the contribution of different mechanisms controlling gas breakthrough in clay-rich barriers.

Previous experimental campaigns have demonstrated that gas breakthrough occurs through localised pathways (e.g., fissures) across the sealing barrier. Capillary breakthrough could be facilitated by gas penetrating the pore-water by diffusion and can ‘drain’ towards pre-existing gas cavities in the pore-space and expand them, a mechanism known as ‘cavitation’. Expanded gas cavities can merge and lead to the formation of the localised pathway. This mechanism implies that gas breakthrough is time-dependent, which is not considered in the ‘on/off’ capillary breakthrough mechanism. Additionally, there might be other time-dependent mechanisms contributing to the deformation of the menisci and/or the deformation of the clay (creep) leading to localised pathway formation. A second gap in the literature consists in the lack of information on the effect of particle shape, mineralogy, and material compressibility on gas breakthrough. This is key information to inform the selection of candidate clay-rich barriers.

Experiments that tested natural material commonly had pre-existing fissures, and therefore, tested the breakthrough pressure of these discontinuities. In this study reconstituted clayey materials are tested with the aim of distinguishing the mechanisms of gas pathway formation. An experimental apparatus was setup to allow 1D consolidation of reconstituted samples at a pre-consolidation stress of 10 MPa, representative of in-situ conditions, followed by the injection of gas (non-wetting fluid) at constant sample volume (i.e., constant effective stress). 1D mechanical consolidation ensures samples are ‘intact’ prior to gas injection, i.e., no pre-established discontinuities. The materials tested include bentonite clay, kaolinite clay, muscovite mica silt, silica (quartz) silt and mixtures of the materials with varying mass fractions. The use of different fluid electrolyte concentrations were chosen to investigate the effect of mechanical behaviour of the material compressibility and density on gas breakthrough pressure. Different pressure increase strategies showed the effect of diffusion on breakthrough mechanisms. Furthermore, that water flow (e.g., drained vs undrained conditions) is controlling the deformation and displacement of the meniscus and hence the breakthrough pressure.