Fault Reactivation as Mechanism of Early Water Production in Unconsolidated Sandstones Reservoirs

Abstract There are several case studies showing that water injection apparently induces fault reactivation in oil and gas fields. However, there are not many publications in literature relating water production increasing with fault reactivation caused by depletion. The unexpected water production by fault reactivation related to depletion, promotes a huge impact on an oil field production, reducing the reservoir drainage efficiency and causing premature aging of operating plants, in some cases generating production bottlenecks due to produced water treatment limitations. This paper presents parameters that influence fault reactivation mechanism and their importance in early water production. In order to evaluate the phenomenon, a failure stability parameter that can be monitored during reservoir production time life is proposed and its behavior can be analyzed by comparison with produced water rate. The methodology is based on Mohr-Coulomb failure criterion using a simplified approach. Basically, the method considers the change in the state of effective stresses, vertical and horizontal, during production and its ratio (σh/σv) called K0.The relationship of effective stress necessary for the Mohr circle touches the envelope of rupture for a given frictional angle, called K1, is also considered. Thus, it is possible to observe the stress state changing as field production occurs using flow simulation. Fault reactivation analysis is obtained from the difference K0 minus K1, being K0 less or equal to K1, the condition at which depletion can induce faults in the reservoir. The method has been successfully applied on an unconsolidated sandstone reservoir from Campos Basin, explaining the evolution of anomalies in BSW (basic sediment and water) values that could not be predicted ignoring the theory of fault reactivation by depletion. Recognizing that fault reactivations contribute to the unexpected water production is an important changing of paradigm in oil industry. The determination of the parameter of reactivation by depletion constitutes an important mark to indicate the right moment for beginning waterflooding. Many mature fields that present high Water Cut values, previously thought to have occurred due to rising of water-oil contact, may be revisited and restudied using the theory of fault reactivation by depletion. The approach can help to identify undrained-overlooked areas where infill drilling projects could be implemented, extending oil production and increasing oil recovery factors.