Produced Water Re-Injection in a Deep Offshore Environment - Angola Block 17

Abstract

Abstract Angola Block 17 is composed of several deep water offshore fields, producing oil, water and gas from unconsolidated Miocene and Oligocene sands. In these reservoirs, pressure maintenance and sweep efficiency are achieved by injection of desulfated sea water mixed with an increasing proportion of produced water. This reservoir management policy perfectly fits with Total's commitment to reduce discharges to the environment during its operations. However, Produced Water Re-Injection (PWRI) comes with its own challenges, mainly because of well injectivity impairment linked to the presence of oil and solids in the injected water. Water treatment facilities are designed to meet water discharge specifications in terms of oil in water content and to remove solids. Yet it is difficult to accurately monitor the concentrations of solids present in the injected water at surface and when it reaches the reservoir wellbore interface. Reservoir management (and more precisely voidage replacement management) strongly depends on the ability to monitor, analyze and manage the injectivities of wells. The objective of the present work is to describe the methods used to set injectivity index references and follow their evolution in time then to provide an insight into the injectivity decline and enhancement mechanisms. Different injectivity decline models based on instantaneous proportion of produced water in the injected water or cumulative injected volumes of produced water and an injectivity enhancement model based on plastic shear strains which take place around the injector wells are also presented, discussed and compared to field data. In addition, methods for improving or restoring injectivity (injection of sea water batches or chemical products) are illustrated. Finally, a method for defining maximum allowable injection pressures for each well is described.