Immiscible Nitrogen Flooding in Bentheimer Sandstones: Comparing Gas Injection Schemes for Enhanced Oil Recovery

摘要

Gas injection is a widely applied enhanced oil recovery method. However, poor vertical and areal sweep efficiency result in inefficient oil displacement. For improving gas mobility control, Water-Alternating- Gas injection has often been applied. The goal of this study was to compare several immiscible nitrogen injection schemes and to investigate how rock-fluid and fluid-fluid interactions control the immiscible flooding process. Well-controlled core-flood experiments were performed in Bentheimer sandstone cores. Nitrogen was injected into cores saturated with n-hexadecane at connate water saturation at constant pressures (5 and 10 bar) and while varying backpressure (5 to 60 bar). Nitrogen was also injected at residual oil to waterflood and a Water-Alternating-Gas injection scheme was assessed. Coreflood results clearly demonstrated the beneficial effects of Water-Alternating-Gas injection over continuous gas injection. The findings in this study suggest that a) an increase in pressure favours oil recovery slightly during continuous nitrogen injection at connate water saturation, b) residual oil saturation for immiscible nitrogen flooding is lower under three-phase flow compared to two-phase flow and c) the relatively high oil recovery, i.e. lower ultimate residual oil saturation, by Water-Alternating- Gas injection is most likely related to an increase in trapped gas saturation.