The feasibility of tertiary nitrogen injection in waterflooded volatile oil reservoirs in the North Sea

Abstract

A systematic investigation has been carried out into the technica; feasibility of tertiary nitrogen flooding in water-flooded volatile oil reservoirs (such as the Statfjord reservoir of the Brent Field) in the North Sea. The work comprises both laboratory experiments and theoretical process-type calculations using a compositional simulator. The experiments included centrifuge tests for deriving three-phase relative permeabilities, multiple contact tests between nitrogen and reservoir fluid in a windowed PVT cell to check whether miscibility can develop, and core floods under reservoir conditions. The core floods confirm that tertiary gravity drainage by nitrogen injection is capable of accumulating waterflood residual oil in an oil/water bank and moreover leads to a high recovery of the lightest hydrocarbon components. The simplified process simulations confirm the crucial role of phase behaviour and three-phase relative permeabilities in this complicated reservoir process. It so happens that phase behaviour and three-phase relative permeabilities tend to be favourable in the Statfjord reservoir.