One of the key factors to achieve favorable oil recovery efficiency by gas flooding is the miscibility of the injected gas in the reservoir fluid, which is affected by the gas and oil compositions, reservoir temperature and pressure. The miscibility is often characterized by the minimum miscibility pressure (MMP), commonly defined at which oil recovery reaches 90% at 1.2 hydrocarbon pore volume of gas injection. This study examines the phase equilibrium and miscibility of CO2 in a waxy crude oil from a Tensleep reservoir in Wyoming to elucidate the tertiary recovery, effect of flooding at pressure below MMP and the effect of impurity in CO2 on miscibility. Constant composition expansion tests at the reservoir temperature, saturation pressures and viscosities were measured for oil with different concentrations of CO2. The MMP was measured for CO2 flooding by a slim tube (60 ft., 35.6%, 5.84 D). Fluid compositional analysis was done in GC equipped with FID detector. During the slim tube tests, when the system pressure was above the MMP, oil recovery reached over 90% after gas breakthrough and almost no new oil was produced. When the system pressure was around and below the MMP, lighter colored oil was produced continuously as long as CO2 was injected after the gas breakthrough. This observations backed by gas chromatography analysis indicated that at pressure around and below the MMP, CO2 continuously evaporates the light components from the residual oil. The remaining oil in the reservoir could potentially become heavier and more difficult to recover. Also, when 5 mol% of nitrogen was present in the injected CO2 stream, the MMP of gas mixture in oil increased by nearly 15%. This study provides quantitative measure for the effect of CO2 purity on enhanced oil recovery efficiency.

Phase equilibrium and miscibility study of CO2 and mixed gas in Tensleep oil for enhanced oil recovery

249th National Meeting (2015)

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