Fault zones are widely recognised as being important to the secure long term storage of CO2 as they could provide a leakage pathway out of the target reservoir. Fault characterisation within reservoirs, especially where they extend into caprock, and other overlying formations, needs to be thoroughly understood as part of any risk assessment for CO2 storage. The aim of this study is to review what is known about the permeability of fault zones in order to highlight under what circumstances faults may impact overall storage integrity.
The behaviour of fault zones in relation to sub-surface fluid migration is important to many industries and consequently has been comprehensively documented in the literature. CO2 operations involve the injection and pressurization of reservoirs usually resulting in changes to the state of in-situ stresses which may modify fault properties. Instability could lead to slippage along pre-existing faults or fracture systems, which may be associated with seismicity. In addition, the movement of faults, and the generation of factures within the damage zone adjacent to the core, may create conduits that lead to the leakage of fluids to the surrounding overburden or even to the surface.
In 2015 IEAGHG published a study reviewing the geomechanical stability of faults during pressure build up which provided a helpful background to the behaviour of faults in stress regimes relevant to CO2 storage. This study is designed to build upon the previous work and provide a significantly broader review of the current state of fault zone permeability and also to investigate what mitigation options may be available to CO2 storage operations if leakage was to occur.