Fault permeability and CO2 storage

Faults comprise zones of crushed, sheared and fractured rock that have the potential to influence the migration of stored CO2. Fault-zone permeabilities of 10(-9) to 10(-19) m(2) are controlled by many interdependent factors including; fault-zone architecture and rock types, mechanical strength and permeability of host rock, orientation and magnitude of in situ stresses, fracture aperture size and connectivity, fluid properties and burial history. Mitigating the risk of CO2 migration via faults to the atmosphere or into economically valuable resources requires an understanding of the conditions under which they promote fluid flow from the reservoir. In situ flow data from natural seeps indicate that faults can promote the upward flow of CO2, with flux rates being greatest where the highest densities of fractures occur. Flow simulation modelling suggests that low-permeability fault rock may compartmentalise reservoirs giving rise to increased pressures and promoting upward flow of CO2. Migration rates along faults of up to 1000 m/yr are possible and could produce leakage rates of up to 15000 t/yr at natural seeps. These rates are likely to be site specific and positively related to reservoir pressures. Present understanding of fault hydraulic properties is generally not sufficiently complete to predict when and where faults will influence CO2 migration. To improve understanding of fault hydraulic properties, studies of outcrop, analogue and numerical models are required. In situ flow measurements are critical for testing site-specific and generic fault fluid-flow models that are important in establishing guidelines for the inclusion of faults in risk assessment and determining what mitigation measures are most appropriate. (C) 2017 The Authors. Published by Elsevier Ltd.