Reservoir Condition Measurements of Compositional Effects on Gas-Oil Interfacial Tension and Miscibility

摘要

Minimum miscibility pressure (MMP) is an important design parameter for enhanced oil recovery processes involving gas injection (carbon dioxide, nitrogen, or hydrocarbons). Hence an experimental approach called "vanishing interfacial tension (VIT)" has been developed, to determine gas-oil miscibility, based on the fundamental definition of zero gas-oil interfacial tension (IFT) at miscibility. In this study, gas-oil interfacial tensions were measured between CO2 and a live crude oil from a depleted Louisiana oil field at varying pressures and at reservoir temperature (238oF) using the pendant drop and capillary rise techniques to determine the MMP. The gas-oil ratios have also been varied (both on molar and volumetric basis) widely in the high-pressure and high-temperature optical cell to examine the compositional path effects on IFT and miscibility. Detailed compositional measurements of both vapor and liquid phases were carried out using a gas chromatograph, and densities of both phases were measured using a digital densitometer. The experimental results indicated that the gas-oil interfacial tensions measured at various gas-oil ratios at reservoir temperature, although displaying different relationships with pressure, converged to almost the same end-point pressure at zero-IFT. In spite of the large variations in the initial mixture compositions, only negligible changes in VIT miscibility pressures, from 6142 to 6216 psig (1.2% maximum difference), were observed thereby establishing the compositional path independence of the VIT technique. The compositional analyses of gas and oil phases at different gas-oil ratios showed that the gas phase is predominantly CO2, while the CO2 in the liquid phase continuously increased with pressure, which indicates condensing drive characteristics of CO2 to be responsible for miscibility development in the selected reservoir case. The density-pressure plots showed the evidence of approach to critical mixture formation near VIT miscibility. This experimental study confirms the robustness of the VIT technique for accurate, quick, and cost-effective determination of gas-oil miscibility conditions.