Technical Report
Faults typically consist of two sub-structures: a fault core; and a wider fault damage zone. Faults in low porosity rocks tend to have a fine-grained fault core whereas faults in coarse-grained, high porosity rocks, usually have low porosity deformation bands that can develop into high permeable slip surfaces. Fault zone permeability increases with increasing fluid pressure but permeability varies both across and along faults. Hydraulic properties also vary between the damage zone and the core where gouge material is concentrated. This concentration of fine grained minerals also reduces the mechanical strength of faults. Mechanical failure or reactivation occurs either when shear stress exceeds normal strength or when hydraulic fracturing is induced. Fault deformation can be either brittle or ductile. The former leads to the formation of cataclastite (fine grained granular) and shear fractures which dilate under low effective normal stress that can cause permeability enhancement. With increasing shear deformation, fracture asperities are sheared off leading to gouge production and a reduction in permeability. Thus, in brittle deformation permeability will generally increase under low effective stresses and small displacements but decreases with increasing effective stress and magnitude of displacement. Shear fractures created in ductile deformation contract during shearing and tend not to lead to an increase in permeability. Reactivation of faults can be assessed using both analytical and numerical approaches, but assessment is usually based on the Mohr-Coulomb failure criterion. This method can be used to determine the critical injection pressure. Numerical modelling can provide predictions of fault stability at different scales and incorporate different parameters such as the geometry of different faults. Numerical methods can be effective for identifying leakage potential and seal failure especially where dilatancy and stress dependent permeability changes occur. Experimental tests on minerals and rock samples exposed to CO₂ tentatively indicate that the coefficient of friction is not radically changed, however, this conclusion is based on limited exposure to CO₂. There is limited observational data on stress regimes and direct pore pressure measurements from core samples from cap rocks and fault zones. Acquisition of key data would enhance stress regime modelling and fault behaviour.
