Modeling subsea gas-liquid separation using proxy for phase separation for pre-salt reservoirs with high GOR

Abstract

Brazilian pre-salt oil fields include ultra-deep water reservoirs with high CO2 content and high gas/oil ratio (GOR). For these cases, large volumes of CO2-rich gas reach the topside facilities that present limited gas processing capacity. In this context, the production of the oil field is restricted to the maximum gas production capacity of these facilities. The use of a subsea gas-liquid separation (SGLS) and reinjection system may be a solution to boost oil production, as this strategy allows reinjecting part of produced gas directly from the seabed. In this study, we developed a methodology for modeling the subsea separation and reinjection process, which is integrated with a compositional reservoir simulator. A proxy based on an equation of state (EOS) was used for volumetric phase separation in the subsea separator. This approach eliminates the necessity of flash calculation at the SGLS conditions. The methodology makes it possible to assess the potential impact of implementing this technology from a reservoir management perspective. The modeling methodology is applied to a benchmark case (synthetic simulation model representative of a pre-salt reservoir) and evaluated the impact of the SGLS over total production. The results show that the SGLS technology can successfully boost oil production in the context of platform gas production restriction. As soon as the SGLS was implemented, an extended oil production plateau was achieved due to increased gas production capacity. We observed a significant GOR increment in produced fluids during the simulation as a result of gas recycling strategy. This increasing GOR caused the field production to be restricted by maximum gas production constraint most of the time, despite the SGLS implementation. A higher GOR also affects the SGLS performance, increasing the proportion of gas that becomes available for separation at subsea separator conditions. An economic analysis is presented, showing that the SGLS has the potential to increase the financial return of the project. The introduced methodology will enable the inclusion of the subsea separation process in future analyses, paving the way for feasibility assessments and optimization processes for cases in which the SGLS may be an alternative.