Single and multi-phase flow properties of top seals and fault rocks: implication for CO2 storage

Global efforts to mitigate the impacts of anthropogenic greenhouse gas emissions have positioned CO₂ storage in geological subsurface systems as a key long-term mitigation strategy. However, secure containment of injected CO₂ is constrained by two major challenges: risk of leakage through compromised caprock, and reservoir compartmentalisation, which may cause abnormal pore pressure build-up and fault reactivation during injection due to stress sensitivity. Understanding the single-and-multiphase flow properties of these barriers is therefore critical for safe and long-term CO2 storage. Despite this importance, current knowledge of flow properties of these systems in the context of CO₂ storage remains limited. Additionally, MICP measurements of mudrock properties do not reflect in-situ stress conditions, while stressed MICP applications remain rare and contentious due to compressibility of clay-mineral-rich mudstone fabrics, which restricts mercury access to matrix porosity. This study addresses these gaps by investigating the flow properties of faulted sandstone from the Vale of Eden and mudstones from the Sidmouth Formation, UK, using a combination of laboratory experiments and numerical modelling. A novel contribution of this study is the successful measurement and modelling of internal pressure and density fields during xenon flow in low-permeability mudstone, using X-ray CT imaging and numerical modelling to investigate dominant flow pathways. Results indicate that flow through the mudstone is matrix-dominated, with Peclet numbers on the order of 10⁻⁹. The approach also captured and modelled xenon CT attenuation profiles within the mudstone. Gas permeability exceeds brine permeability by approximately one order of magnitude in fault rocks and by up to two orders of magnitude in mudstones after Klinkenberg correction. Furthermore, stressed MICP effectively characterises fault rocks with very low clay content but fails to reliably characterise clay-mineral-rich mudstones, where Field’s metal, used as a proxy for mercury, preferentially invades microfractures. Relative permeability in mudstone is on the order of 10^(-5) to 10^(-3), with non-wetting phase mobility remaining strongly restricted until brine saturation falls below 85%. Overall, the findings provide critical insights and data to support predictive modelling of CO₂ migration and containment in geological storage systems.