Compositional Effects on Gas-Oil Interfacial Tension and Miscibility at Reservoir Conditions

• Main Author: Sequeira, Daryl Sean
• Other Authors: Stephen O. Sears
• Format: Others
• Language: en
• Published: LSU 2006
• Subjects: Petroleum Engineering
• Online Access: http://etd.lsu.edu/docs/available/etd-11072006-150203/

Minimum miscibility pressure (MMP) is an important optimization parameter for an enhanced oil recovery process involving Carbon Dioxide or hydrocarbon gas injection. Therefore an accurate experimental measurement is required to determine the MMP. The MMP for a gas-oil system is directly related to the interfacial tension between the injected gas and the reservoir crude oil. When CO2 gas contacts the reservoir oil at reservoir temperature, the interfacial tension between the fluid-fluid phases reduces as the miscibility is approached and the interface between the fluid-fluid phases eventually disappears at miscibility i.e. the interfacial tension becomes zero. Hence, a pressure condition of zero interfacial tension at reservoir temperature is the minimum miscibility pressure for a CO2-reservoir crude oil system. The Vanishing Interfacial Technique (VIT) technique to determine MMP is based on this principle. Therefore, this research project involves the measurement of gas-oil interfacial tensions for a CO2-live reservoir oil system at reservoir conditions using the pendant drop and the capillary rise techniques to determine the minimum miscibility pressure through the VIT technique. Gas-oil interfacial tension, being a property of the interface between crude oil and gas, is strongly affected by the compositional changes induced by the counter-directional mass transfer (vaporizing, condensing or a combination of the two) of the various components taking place between the CO2 and reservoir oil. This study hence investigates the mass transfer mechanisms involved in these dynamic gas-oil interactions responsible for miscibility development by performing detailed compositional analyses, and density measurements. All the measurements were carried out at different ratios of fluid phases in the feed mixture (both molar and volumetric) for various pressures at the reservoir temperature in order to also study the effects of the initial feed composition on IFT and the phase compositions. This study has experimentally demonstrated that the gas-oil interfacial tension measured at varying feed compositions (i.e., initial gas-oil molar and gas-oil volume ratios) at reservoir temperature, although showing different relationships with pressure, converged to the same endpoint of zero-interfacial tension or similar minimum miscibility pressures. The effect of gas-oil molar ratios and gas- oil volume ratios on the compositions of the equilibrium phases for this CO2-reservoir fluid system proved that the mechanism involved in the mass transfer of hydrocarbon components between the fluid-fluid phases was a condensing gas drive mechanism. This study has demonstrated that the MMP determined from the VIT technique is independent of the compositional path followed by the fluids during their continuous interaction prior to attaining mass transfer equilibrium.