To verify successful long-term carbon dioxide storage, an improved understanding of geological leakage risks across the primary caprock but also the shallower overburden is critical. While diffusive and advective matrix leakage can be considered to take place at low to moderate rates, especially fault-related fracture networks require an improved understanding. Faults can act as flow barriers and traps in reservoirs but also as leakage pathways into the shallow overburden. Being able to fully characterise fault and fracture networks, in terms of fracture permeability, fracture density, connectivity, aperture size and stress regime, can allow us to more accurately identify, analyse and model the bulk properties (e.g. transport, strength, anisotropy) and, therefore sealing behaviour, of faulted and fractured geological storage sites. 

Here, we briefly discuss current knowledge and knowledge gaps of matrix leakage as well as a workflow to assess the likelihood as well as rates of fault leakage from storage sites. The workflow integrates laboratory-based fracture permeability measurements, outcrop-scale analysis of fault and fracture networks occurring in reservoir/caprock sections, linking transport properties to rock mineralogical and geomechanical properties as well as upscaling of findings to the reservoirs scale. Key is to develop a hydromechanical model to upscale laboratory tests to network scale and potentially to reservoir scale, verified against in-situ fault permeability data, where available. This allows calibration of fault flow against sensitivities of monitoring tools over timescales covering the injection and post-injection phases.