CO2 Migration in the Overburden: Rates, Pathways and Barriers to Fluid Flow

Large-scale geological heterogeneities and their potential to enhance fluid flow or hinder migration in the subsurface are reviewed. Evidence for flow or fluid containment by faults and fractures, fluid flow features, submarine mass movement deposits, buried tunnel valleys, hydrofractures and glacitectonic features is presented. Wellbore-related flow paths and well integrity studies are summarised; the risk of wellbore leakage may be best understood by simulations of predicted long-term behaviour of well barriers. Rates of fluid flux at natural seepage sites and controlled experimental releases of carbon dioxide (CO2) are compiled and compared, and lessons learnt for monitoring of CO2 storage sites identified. The overburden sequences to five operational or planned CO2 storage sites are compared and those characteristics in common that promote subsurface containment of fluids are highlighted.