Research On The Phase Behavior In Gas Injection EOR And The Prediction Of Minimum Miscibility Pressure

Environment and energy shortage are two severe problems during the progress of human sustainable development in the 21 century, of which the global warming caused by green house effect has become a top problem and greatly drawn people's concern. The chief culprit of green house effect is superfluous CO2 gas in the atmosphere. At present, injecting CO2 gas into the oil reservoir to enhance the oil recovery is considered as a solution both to the environmental and energy problems. Because the implement of CO2-EOR not only can improve the production of reservoir oil, but can also sequestrate the CO2 in the depleted reservoir to mitigate the green house effect.CO2 miscible displacement is one of the most efficient method to enhance oil recovery, during which, exact and reliable estimation of minimum miscibility pressure in CO2-oil system is very significant for the design of miscible displacement. In this study, phase behavior simulation technology was applied to research the property of the fluid after injecting CO2 binding the interaction mechanism between CO2 and the reservoir oil composition under the reservoir condition. The phase nehavior model was built by tuning the parameter of EOS in rgressing the PVT experiment data and the simulation result. Meanwhile, the minimum miscibility pressure of CO2-oil system was obtained through multi contact calculation.In this study, the PVT simulator WinProp was applied to reproduce the phase behavior and tune the EOS parameter during regressing the PVT experiment data and simulation result based on a developed 10-component equation of state (EOS) model for the Peng-Robinson EOS. Both routine and gas injection PVT experiments were matched including differential liberation, constant composition expansion and swelling test. The parameters used for regression include critical temperature, critical pressure, acentric factor, volume shift and binary interaction coefficients(Tc, Pc, ?, kij) between the non-hydrocarbon components (i.e. CO2, N2) and the lumped pseudo-components. During the regression process, the characterization of heavy component is focused since the effect on the saturation pressure. Also, the correlation among parameters was monitoring in case over-regressing of one particular parameter. Besides, the changing rule of gas-oil system after CO2 injection was also discussed. With the mole fraction of CO2 increase during the injection, all of the data including gas oil ratio (GOR), saturation pressure, oil formation volume factor and swelling factor increase, and the viscosity and density of the reservoir decrease. Also, with the mole fraction of CO2 increase during the injection, bubble point pressure, dew point pressure and critical pressure increase, on the contrary, the critical temperature decreases, and the two-phase region on the phase diagram expands. According to the comparison of the results of the tuned EOS with experiment values, it can be seen that the newly developed EOS model has a good agreement with the experimental values. Also, the tuned EOS was applied to predict the CO2-oil system MMP, the predicted result was 18.0 MPa and the slim tube experimental result was 17.8 ±0.2 MPa.