Usan Field: Managing Gas Injection at Mechanical Seal Capacity

Abstract Usan is a highly-compartmentalized deepwater field offshore Nigeria with both structural and stratigraphic barriers across and within the compartments. The reservoirs were at their mechanical seal capacity based upon evaluation of reservoir and leak-off pressure trends. This complicates gas injection and pressure maintenance in the Usan field. Due to the lack of gas export channels, gas injection was adopted as part of the development plan for the field in addition to water injection. During the first 18 months of production, less than 40% of produced gas was reinjected due to reservoir pressure/leak-off constraints. A gas injection strategy was developed based on a number of uncertainties which included actual crestal depth, fracture pressures, and communication within and across compartments. To maintain or increase gas injection, the team needed to establish reliable crestal depths for each compartment, define safe injection pressure for each injector, and effectively monitor the pressure response of the reservoirs to ongoing gas injection. A novel approach was used in better determination of the crests using fracture and fluid gradients. Safe injection pressures were based on dynamic injection pressures rather than static reservoir pressure. With these estimated crest depths and fracture pressures for the gas injection compartments, volume-depth curves were used to determine gas column height and safe injection pressures were calculated for each gas injector. Due to the mechanical seal integrity concerns, a monitoring system was developed to avoid the risk of fracturing the reservoir with continued injection. The surveillance strategy utilizes real-time flowing well pressures, short duration shut-in pressures, and evergreen volume-depth curves. Ultimately, managing reservoir pressure levels and trends in each compartment allowed for increase in overall gas injection in comparison to previous methodology. A 30% increase in gas injection rate was immediately achieved. Optimal gas injection has also been maintained resulting in improved oil recovery. This paper focuses on the methodology of determining/applying safe gas injection targets and maximizing oil recovery within the known constraints and uncertainties.