As part of geological carbon storage, caprocks of lower permeability play an important role to restrict CO₂ escaping from the storage complex. However, some unexpected geological faults may exist in in the storage complex or the overburden and can be possibly reactivated due to the CO₂ injection. Understanding the role of faults and fractures as fluid pathways, through overburden strata, to the surface is critical to ensure storage safety. Fault flow behaviour has been studied in the context of hydrocarbon development, supported by observations from wells drilled through faults, but such observations are rare in geological carbon storage projects.

Here, we focus on faults that pre-date CO₂ injection. These faults can form barriers to across fault flow and conduits to along-fault flow. Precisely how faults will impact on fluid flow is dependent on a number of factors, including the fault history and geometry and host and fault rock properties, the thermodynamics conditions of the reservoir and fluids and the composition of the fluids. 

Leakage from natural analogue CO₂ stores occurs along faults (e.g. Miocic et al., 2016, Roberts et al., 2019). Studies of these natural analogues find that faults are complex and channel fluids heterogeneously at depth and towards the surface (Roberts et al., 2015) and so understanding of how faults might affect CO₂ migration is not straightforward.

In this work, we review the current understanding of fault leakage risk associated to pre-existing faults systems and how field experiments can complement desktop study and support fault leakage risk assessment.