Phase Behavior of CO2-Rich Live Oil Samples From High Pressure Reservoirs

Abstract

Abstract Oil production activities are strongly dependent on the phase behavior of produced fluids. Among the usual phase changes, those that lead to the appearance of one or more solid phases are among the main focus of flow assurance studies. The flocculation of asphaltenes, the heaviest and most polar fraction of crude oil, usually occurs downhole at high pressure and temperature conditions. To assess the live oil phase behavior, four samples were evaluated using a high pressure microscope (HPM). The results obtained from HPM isothermal depressurization experiments revealed the existence of four different phase behavior patterns, suggesting the existence of a liquid-liquid equilibrium at elevated temperatures and pressures in CO2-rich fluids. The pictures and videos acquired from the HPM showed that the the second liquid phase droplets hardly coalesce, suggesting either a low interfacial tension between the two coexisting liquids or the presence of a natural surfactant at the interface. This unusual phase behaviour is not promptly antecipated by thermodynamic simulations, rendering unavoidable the experimental characterization of live oil samples. In light of this new scenario, techniques that only evaluate the appearance of a different phase like the solid detection system (SDS) analysis based on the light scattering in the NIR region are not conclusive when used independently. HPM analysis or the combination of different techniques like SDS and isobaric filtration, become indispensable in order to assess solids formation in live oil samples above their bubble points.