Assessment of Injection Pressure Margin in Deep Water Injection Well Design

Abstract

Waterflooding is a proven secondary recovery mechanism for hydrocarbon production while maintaining reservoir pressures. To achieve the desired voidage replacement ratio, each injector well is designed to inject at a certain rate to support its paired producer. From a safety perspective, it is essential to avoid 1) injection above shale fracture pressure that could result in top seal failure and 2) injection above fault propagation pressure that could result in fault reactivation/propagation. The potential consequences of these hazards are out of zone injection or loss of containment. Following recent major oil spill incidents around the world (e.g. Macondo disaster in the Gulf of Mexico), regulations have become more stringent on well designs pertaining to wellbore stability and reservoir geomechanics. This paper will showcase an integrated effort at assessing the injection pressure margin and the impact on the detailed injector well designs for a deepwater turbidite field development in the Gulf of Guinea (Nigeria). The technique applied requires a good understanding of the fracture pressures, rock strength, failure mechanism and good drilling and completion practices. To test different scenarios, one well injecting to a deep reservoir and another well injecting to a shallow reservoir were selected for this assessment. Pore pressure, fracture pressure, and injection pressure along the wellbore were analyzed to ascertain the margin between the downhole injection pressure and fracture pressures above and at the proposed casing shoe setting depths. The results of this study provided basis for injector well designs, justification for full field geomechanics modeling, and recommendations for data acquisition while drilling and well reservoir monitoring throughout the injection/production period.