Project title: The impact of fault geometry and rock properties on fault reactivation
Researcher: Dr Janis Aleksans
Reactivation of geological faults due to induced changes in stresses in the subsurface is a significant threat to the viability of many projects that involve injection of fluids into subsurface reservoirs. Standard methodology for assessing such risks involves a rather simplistic approach and does not account for the geometrical complexity of the faults. We are using novel Rigid Block Spring Network Model (RBSN), in which rock is simulated as an assembly of elastic blocks that are bonded together by elastic and breakable bonds. The RBSN is ideal for this project as it can model the two key processes, slip on a pre-existing fault surface and failure of the wall-rock.
This project aims to construct numerical models of faults that will be based on the same mechanical principles as the slip tendency approach but will go beyond this approach by allowing slip to actually occur on faults when the Coulomb slip criterion is satisfied. The main advantage in doing this is that slip at a point on a fault surface will be controlled, not only by the orientation and frictional properties at that point, but will also reflect the geometry of the fault on a range of scales so that slip will occur within the larger fault geometrical context. In addition, stress variations due to the variable elastic properties of the wall rock will arise as the effective stress deviates from the starting regional stress field so that resolved shear stresses will deviate from those that would be estimated from either the regional stress field or from a planar fault surface. We use Rigid Block Spring Network Model to simulate a fault of a particular geometry at a given depth and regional stress field that is subject to a change in pore pressure replicated by a reduction in confining pressure. The models will be run until slip occurs at a point on the fault surface. The point at which slip initiates will be recorded and the reasons for slip occurring at this location will be investigated.