Standard gas Enhanced Oil Recovery (EOR) Pressure-Volume-Temperature (PVT) program includes experiments such as solubility/swelling, multi-contact, slim tube, vapor-liquid equilibria (VLE) tests, and fluid property measurements. These tests are designed to determine the extent of gas miscibility and mixture phase behavior during gas injection in hydrocarbon reservoirs. These experimental programs are known to be expensive and time consuming. The degree of complexity increases as the industry move into conducting gas EOR PVT for high/ultra-high-pressure reservoirs. The focus of this paper is to demonstrate the challenges associated with these measurements and evaluate the merit, applicability, and usage of such data for fluid model development for high pressure gas EOR studies.
Associated challenges include utilizing gas concentrations up to 90% mole during swelling tests to determine critical mixture composition. Determination of dew point pressures by visual inspection or liquid build-up method proved inefficient. An interpretation of pressure-volume data showed good promise for determining both saturation pressure and liquid build-up curve for opaque dew point systems. VLE tests were designed at gas concentrations close to critical mixture composition to generate phases with increased affinity for mass transfer. Measured Minimum Miscibility Pressure (MMP) for all studied oil and gas systems were less than 6000 psia, except for N2 gas. Such relatively low MMP values suggest that development of full miscibility is not a concern for these high-pressure fluid systems. Such relatively low MMP values suggest that generated miscibility is not a concern for these high-pressure fluid systems. Instead the focus shifted to determine and effectively model the first contact miscibility pressures for these fluid systems (if it existed at pressures less than initial reservoir pressure). Measured MMPs were assigned a low weight factor in EoS model optimization process. Swelling test data for gas concentrations lower than 50% mole was of little value for EoS model optimization. Presence of precipitated asphaltenes challenged accurate measurement of liquid phase density and viscosity, as capture and analysis of a representative sample was very difficult. A knowledge of asphaltenes phase envelope for mixtures of reservoir fluid and injection gas proved to be invaluable.
A robust gas EOR PVT database was generated for mixtures of 6 injection gases and 7 deep water Gulf of Mexico Wilcox Trend reservoir fluids. Tests were carried out at pressures as high as 20000 psia and temperatures ranging between 230-265 °F. New high-pressure testing capabilities were developed, and modified data interpretation techniques were implemented. Lessons learned during the measurement, interpretation and application of these high-pressure gas EOR PVT data helped us design an effective measurement program. The developed fit-for-purpose experimental program leads to reduced cost (elimination of unnecessary tests) and increased reliability of high pressure phase equilibria and fluid property data for gas EOR reservoir simulation studies.
