Abstract
AbstractPhase equilibrium calculations require experimental lab data to constrain component properties in an equation of state (EOS) model. In this work, we evaluated the miscibility behavior of four different injection gases for a reservoir onshore Abu Dhabi containing medium-light oil.The extensive experimental program included swelling and slimtube experiments using CO2 as well as three hydrocarbon gases, in addition to conventional black-oil experiments. Based on the large company PVT database, gas additions in the swelling experiment were carefully designed to reduce uncertainty on the transition point where the swollen fluid type changes from oil to gas. We developed several fully tuned EOS models by gradually lumping down while monitoring the match quality. Due to the richness of the injection gases, C2 (15 mol%), C3 (15 mol%), C4 (10 mol%) and CO2 were kept as separate components resulting in a 10-component final EOS description. The compositional data showed a distinct tail effect above C20, which we believe is a lab artifact. The plus fraction was therefore lumped back from C36 to C20 to characterize the composition from C20 up to C80. This step improved the match of the dead oil properties.While the swelling data were matched well, the slimtube recovery data yielded some uncertainty on the MMP estimate, especially for the CO2 test. We made use of the reported effluent gas composition to show that one of the points denoted as immiscible in the lab report is most likely miscible. We tested several EOS-based MMP algorithms and found them to give similar MMP estimates for the hydrocarbon gases whereas the CO2 MMP was predicted with a much larger spread. Our view is that for CO2 flooding, cell-based simulations capture the dynamic behavior of miscible displacement better than the key tie-line approach.The tuned EOS model reproduced the single-phase density very accurately, which is important for the saturation estimation in the transition zone. Several injection gases appear suitable for miscible flooding and further screening studies are now ongoing.