Insight into the micromechanisms of gas breakthrough in water-saturated clay-rich geomaterials – Implications for CO2 sequestration

The successful deployment of carbon dioxide (CO₂) geological sequestration in porous media is reliant on the sealing efficiency of the overlying, clay-rich caprock to act as a physical barrier. Clay-rich caprock formations are considered favorable materials to act as a seal due to them characteristically consisting of small pores providing high capillary entry pressures, hence preventing the intrusion of a non-wetting fluid.

The juxtaposition and availability of deep seated (buried) caprock-reservoir systems to carbon capture and storage clusters may not be available. Therefore, the assessment of shallow seated, weakly consolidated caprock-reservoir systems (e.g., Sleipner) will be required. Our experimental campaign tests analogous caprock geomaterials which have relatively high compressibility, representative of shallow seated (buried or less indurated) clay-rich caprocks.

Past experimental campaigns demonstrate that CO₂ breakthrough is dominated by the creation of very localized channels across the sealing barrier which occur at pressures far lower than the one predicted by the Laplace’s equation [1]. However, limited data characterizing these pathways exists. Furthermore, the physical indicators of susceptibility which underly the micro-mechanisms of failure (e.g., fracturing), are still only postulated for clay-rich geomaterials.

where Pc* is the capillary breakthrough pressure [kPa], ψ, reflects pore shape [-], T_s, interfacial tension between water and gas (e.g., CO₂), and θ, represents wettability [°].

An innovative experimental set-up which allowed for the onset of surface crack formation to be captured during gas injection (representing the non-wetting fluid in CO₂ geological sequestration) into intact clay-rich geomaterials is presented. This allowed for the investigation of physical indicators of susceptibility to gas breakthrough via localized pathways.

Results on different fracture patterns when non-wetting gas (i.e., air) is injected into consolidated clay show the formation of large fractures that nucleate from within the sample. Upon air pressurization, before fracture formation, the sample undergoes volumetric deformation (i.e., consolidation), as the resulting action of the vertical stress applied at the air-water interface (menisci). Once a fracture forms deformation stops and breakthrough occurred at lower pressures than traditionally recorded. The mechanisms of air intrusion are expected to be of a similar nature as CO₂ intrusion. Post-mortem assessment of the internal nature of these localized pathways was then visualized using xCT imaging.

As a continuum mechanics framework will not predict fracture formation under our test conditions, it appears that the experimental evidence support the underlying hypothesis that disjoining pressure governs the mechanisms that ultimately control fracture formation and thus, eventually CO₂ breakthrough. The disjoining pressure is governed by the electrostatic double-layer interactions, van der Waal’s dispersion forces, structural forces, and solvation forces.

If the pore size distribution is such that high gas pressures are required to overcome capillarity, the gas pressure will force single clay particles apart, displacing water form adjacent interparticle spaces. This represents a localized failure mechanism at the clay interface, resulting in fracture nucleation. It is expected that clay displaying large swelling pressures will subsequently display high gas entry pressure, termed “pathway dilation”. Therefore, pressurized gas will enter a dilated pathway at lower pressures than anticipated.