Evaluation of Fault Sealing Ability and Prediction of Fault Critical Reactivation Pressure in Water Flooding Reservoir

Abstract Fault reactivation induced by excessive reservoir water pressure in oil fields is suspected as one of the possible perpetrators that caused crude oil eruption to the surface. This can lead to significant financial losses related to environment cleanup and curtailed oil production. During fluid production and injection, changes in stress have a significant influence on reservoir behavior. Fault sealing ability is one of the key factors that control hydrocarbon preservation. If these faults are reactivated, their permeabilities will likely increase, facilitating fluid migration and potentially compromising the hydraulic integrity of the caprocks that seal the reservoir. Therefore, during a waterflooding process, it is necessary to limit the injection pressure in order to avoid fault reactivation. A traditional approach to fault reactivation prediction provides a deterministic critical reservoir pressure without proper regard to the uncertainties in the model input parameters and the predicted results. A probability distribution of the fault reactivation critical reservoir pressure provides a better means to calculate a risk weighted Expected Net Present Value for management to make better decisions on water flooding and to anticipate potential consequences. In this study, a geomechanical model was established for the BZ Field, Bohai Bay, China. Using the geomechanical model, first, a fault seal analysis was performed and indicated that all faults were sealed in sands under initial stress and pore pressure conditions. Second, fault reactivation reservoir pressures for major faults in the field were predicted assuming a frictional sliding frictional coefficient of 0.5, which was derived using frictional faulting theory by the geological events provider for the nearby PL Field. Monte Carlo simulations were run to consider the uncertainties of frictional sliding coefficient, and especially the the minimum horizontal stress among the input parameters.