Sealing integrity and heterogeneity of possible seals and casing cements from Germany in context of CO2 storage: effects of confining stress and time on permeability

Carbon Capture and Storage (CCS) is increasingly utilized to achieve climate goals by reducing atmospheric carbon dioxide (CO₂) levels. The long-term security of CO₂ sequestration depends on the integrity of both the reservoir and the overlying sealing units. While reservoir properties have been well studied, the transmissive properties of multi-barrier sealing systems remain less well understood and heterogeneity is rarely captured. This study investigates the sealing properties of low-permeability lithologies and well-cement, a critical factor in assessing storage site integrity. Helium was used as the primary permeant for all samples, with CO₂ additionally tested on selected samples to evaluate permeant-specific effects. Klinkenberg-corrected permeability evolution was assessed under increasing and decreasing confining pressures, considering mineralogical influences. The pressure sensitivity coefficient (γ-values) was determined to quantify permeability changes at different confining stresses. To evaluate the time required for permeability stabilization under representative in-situ stress conditions, samples were subjected to 30 MPa confining pressure, corresponding to effective stresses at approximately 2 km depth, and measured daily for 5-8 days. The results indicate that ductile lithologies exhibit a permeability reduction of approximately one order of magnitude after 3-5 days, while casing cement permeability can decrease by two orders of magnitude in 3 days. These findings highlight the necessity of allowing caprock samples to equilibrate under targeted effective stresses for at least three days to obtain reliable permeability measurements.