Geomechanical Aspects of Waterflooding: Setting Strategies and Operational Limits for Containment

Abstract Waterflooding is often planned for unconsolidated sand reservoirs for pressure support or production enhancement. Injections may be carried out under "matrix" or "fracture" conditions. Despite the typically high permeability and high porosity nature of unconsolidated sand reservoirs, global experiences suggest that water injection in such formations is not trouble free. Under matrix injection significant injectivity decline due to formation plugging could be encountered within the first few years or even months of the injector start up, particularly in the absence of fine filtering of water. Injection under fracture conditions may be considered to avoid formation plugging and injectivity decline. However field evidences suggest that "fracture" water injection in unconsolidated soft reservoirs could lead to high pressures, potential loss of containment and failure of sand control. The consequence of severe injectivity loss for either injection strategy is early injector abandonment or replacement of expensive wells and production deferment or loss of waterflood reserves. Considering the above limitations for different injection strategies, it is paramount that geological settings, formation type, rock mechanical properties and consolidation levels are considered in selecting water injection strategy, well completion and topsides filtration design in field development planning. Even with these considerations it is often not possible to confidently predict the performance of water injectors in unconsolidated sand, particularly under fractured conditions. There is no widely accepted theoretical understanding of the fracturing process in soft formations and therefore the predictive capabilities for modelling injectivity are quite limited. This paper presents the geomechanical factors considered for selecting the waterflooding strategy for a particular deepwater, unconsolidated soft sand reservoir in offshore Sabah, Malaysia. The actual decision making process was based on an integrated approach between various subsurface and engineering disciplines. However, due to the unconsolidated formation settings, as well as the faulted nature of the caprock and reservoir, geomechanics played a critical role in devising the waterflood deployment strategy and setting operational limits to ensure containment. The work involved comprehensive geomechanical modelling of the waterflooding process in the reservoir using both analytical and 3D numerical modelling. Global experiences of waterflooding in unconsolidated formations were also invaluable and were integrated in selection of completions and development strategy.