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Use of Minimum Effective Stress Mapping to De-Risk Waterflood Design and Field Development


Understanding the factors that control vertical fault leakage is essential for predicting and preventing such leakage for both conventional reservoir development and subsurface storage. This study details the controls on vertical fluid migration along brittle fault zones, from subsurface reservoirs to mudline or ground level. Using fluid pressures for a deepwater field from dynamic reservoir simulation forecasts, effective stress was mapped for 30 years into the future, quantifying the risk of loss of containment up faults or through top seals from the planned injection. As secondary recovery of hydrocarbons by water injection begins in a deepwater field, there was concern about hydrocarbons migrating out of the reservoir to shallower levels or the seafloor. These hydrocarbons could exploit pre-existing paths within the rock in the form of faults or fault damage zones, or create new paths through mechanical top-seal failure. In the study reservoir, fluids were known to be naturally escaping from shallower sands through a combination of flow along faults and mechanical top-seal failure. Known hydrocarbon migration from shallow reservoirs elevated concern of fluid migration from the deeper targets during waterflood. With the final models and field development plan, the effective stress of the overlying shale never reaches zero during production or subsequent secondary recovery injection, which is the point when vertical fluid migration could occur. The minimum effective stress of the shale is reduced most significantly adjacent to injector wells, but this pressure dissipates rapidly away from the well, and before reaching any faults. Modifications to the fault network from updated interpretation and fault transmissibility calculations may change the distribution of pressure in the field, but these are expected to increase transmissibility and improve dissipation of pressure fronts and increase distance to failure. From this work, the use of minimum effective stress has been demonstrated as an appropriate method to estimate risk of vertical fluid flow along pre-existing faults. Understanding of the local fracture gradient is used as a constraint for determining injection pressures and rates for all future injector wells.