The impact of fault surface 3D geometry on risking fault reactivation

The reactivation of faults can occur when the effective stresses acting on them are perturbed. In some cases man-made changes in effective stress can result in fault reactivation that can have enormous impacts including loss of integrity of underground storage facilities. Current practical methods for making this assessment are generally based on shear stresses calculated over fault surfaces. Depending on the absolute or relative magnitudes of these resolved shear stresses, which depend primarily on the local orientation of the fault surface relative to the regional stress field, different faults or parts of faults are said to be closer or further from the Coulomb failure envelope and are therefore more likely to slip due to changes in effective stress. This proximity to failure/slip is referred to as the slip tendency or reactivation tendency.

Although the slip/reactivation tendency approach is firmly grounded in Coulomb theory and laboratory experiments, there may be issues applying it to the reactivation of irregular fault surfaces. A key assumption of the approach is that an area of a fault surface can be treated in isolation so that the slip tendency can be evaluated once its orientation and frictional properties are known. However, it is well established that fault surfaces are not planar but often have highly irregular geometries and fault rock distributions so that the likelihood that a particular part of a fault will reactivate must also depend, not only on the properties at that point but also on adjacent areas of the fault surface.

To investigate the significance of fault surface irregularity for the evaluation of fault slip/reactivation tendency, we conduct numerical modelling of fault reactivation resulting from an increase in pore pressure within a normal faulting stress regime. The modelling employs a form of the Discrete Element method that uses rigid blocks. This approach provides for both accurate representation of the geometry and frictional properties of the irregular slip surfaces and also failure in the surrounding wall-rock and is capable of modelling the variety of ways in which slip may initiate on, or adjacent to an irregular fault.