Uncertainty in fault seal parameters: implications for CO₂ column height retention and storage capacity in geological CO₂ storage projects

Faults can act as barriers to fluid flow in sedimentary basins, hindering the migration of buoyant fluids in the subsurface, trapping them in reservoirs and facilitating the build-up of vertical fluid columns. The maximum height of these columns is reliant on the retention potential of the sealing fault with regards to the trapped fluid. Several different approaches for the calculation of maximum supported column height exist for hydrocarbon systems. Here, we translate these approaches to the trapping of carbon dioxide by faults and asses the impact of uncertainties in i) the wettability properties of the fault rock, ii) fault rock composition, and iii) reservoir depth, on retention potential. In similarity to hydrocarbon systems, uncertainties associated with the wettability of a CO2-brine-fault rock system for a given reservoir have less of an impact on column heights than uncertainties of fault rock composition. However, the wettability of the carbon dioxide system is highly sensitive to depth, with a large variation in possible column height predicted at 1000m and 2000m depth, the likely depth range for carbon storage sites. In contrast to hydrocarbon systems higher phyllosilicate entrainment into the fault rock may reduce the amount of carbon dioxide that can be securely retained. Our results show that if approaches developed for fault seal in hydrocarbon systems are translated, without modification, to carbon dioxide systems the capacity of carbon storage sites will be inaccurate, and the predicted security of storage sites erroneous.