Most of oil reservoirs operated by Total in West African deep offshore are multi layered highly unconsolidated sands. Burial depths, below the mud line, ranges from 500 to 2500mTVD. Sea water injection is done for pressure maintenance in deviated and horizontal wells equipped with sand control. Polymers injection for EOR and PWRI are also practiced.
In theory, the very high permeability of soft sand reservoirs should allow injecting in matrix mode at high rates with small differential pressure. In reality, declining injectivity over time requires injecting in non-matrix mode, at much higher pressures than anticipated, hence the concerns about the confinement of the injection. The mechanisms of injectivity restoration by increasing the pressure in soft sands and the subsequent risks of cap rock failure have to be understood. Their incorporation in modeling tools is crucial for predicting the operating pressure range and designing the surface and wells facilities accordingly. Standard tensile fracturing models commonly applied to consolidated reservoir rocks do not seem to be adapted to soft materials which behavior is dominated by shear failure and resulting in dilation. This paper will present modeling approaches aimed at predicting the responses of soft sands and of the cap rock to produced water injection taking into account the coupling between geomechanics, flow and formation damage. Field observations are presented and confronted with model predictions.