Modeling Fault Reactivation Triggered by Fluid Injection

With increased focus on CO2 storage, hydraulic fracturing, and steam injection operations in recent years, understanding of fluid induced seismicity has drawn a lot of attention amongst researchers and practicing engineers. Reactivation of existing dormant faults due to injection operations can lead to flow through or along the fault from the reservoir to undesirable zones. It poses a serious challenge that requires careful study so that measures could be taken to avoid such occurrence. In this paper, a method is presented where geomechanical response is used to compute the slip tendency when the stress changes in a reservoir due to fluid injection. The slip tendency is considered as the main variable to determine whether the fault is reactivated or not. It is computed based on the effective stress normal to the fault surface, and the tangential stress. When the effective stress on a fault surface is reduced due to the increase of pore pressure in grid-blocks adjacent to the fault, it can potentially make the slip tendency exceed the critical limit. In such a case, the transmissibility of grid-blocks on both sides of the fault are increased to allow the fluid to flow into the fault and subsequently along the fault. When the fluid leaks from the reservoir to another zone through the fault, the effective stress on grid-blocks adjacent to the fault increases as the pore pressures in those grid-blocks decrease. This will in turn reduce the slip tendency. Therefore, the transmissibility in this case will also decrease to a value smaller than the one when the fault reactivated. The algorithm allows the fault to be reactivated or deactivated to cope with the pressure change in the reservoir. The slip displacement at the fault is also estimated. The method is implemented in a multidimensional, multiphase flow simulator to demonstrate the advantages of using geomechanics for predicting fault reactivation, which can lead to leakage of fluids from the reservoir to other zones or to the surface. Three examples, two synthetic and one field, are presented to illustrate application of the procedure. The proposed method of fault reactivation modeling is suitable for implementing in any fluid flow simulator with geomechanics capability. It is intended for studying and designing of safe injection strategies that avoid undesirable fault reactivation.