On how water injection may induce fault reactivation and slippage: A numerical method

During the water injection process in oil fields, the original stress equilibrium of faults might be disrupted by the large influx of fluids into the reservoir, resulting in movements. Fault slip may lead to the leakage of underground oil and gas, in addition to impairing the integrity of wellbore and casing. To explicate the mechanism of fault slip caused by fluid injection, as well as quantify the fault slippage, A geomechanical finite element model on a reservoir scale is established. This model combines the information regarding the geological structural geometric characteristics and the mechanical properties of rocks at varying depths. Based on the TSL (traction-separation law), we utilize the cohesive contact method to depict the cohesive mechanical strength of the fault gouge and its damage evolution process. Consequently, we simulate and examine the reactivation and slip laws of the fault, which induced by the reduction of cohesive strength and frictional strength post water injection. The research results illustrate that the relatively stable state of the fault primarily relies on the cementation of fault gouge. As the volume of fluid invading the fault area increases, the shear cementation strength of fault gouge diminishes, resulting in the complete activation of the fault. Afterwards, the fault starts slipping. The continued slip post-reactivation of the fault is influenced by the frictional strength of the fault plane. As the friction coefficient of the fault plane drops, the average slip distance of the fault rises. This investigation offers important insights into the impact of fluid injection on fault behavior and can guide the design of injection operations in oil fields.