Summary Three hydrocarbon phases can coexist at equilibrium at relatively low temperatures in many carbon dioxide (CO2) floods. The formation of an aqueous phase in contact with hydrocarbon phases is inevitable in almost all recovery processes, because of the permanent presence of water in the reservoirs either as injection fluid or as initial formation water. As the number of phases increases, flash calculations become more difficult and time-consuming. A possible approach to reduce the computational time of the phase-equilibrium calculations is to use reduced methods. This paper presents a general strategy to model the phase behavior of CO2/ hydrocarbon/water systems in which four equilibrium phases occur by use of a reduced-flash approach. The speedup obtained by a reduced-flash algorithm compared with the conventional-flash approach is demonstrated for a different number of components and phases. The results show a significant speedup in the Jacobian-matrix construction and in Newton-Raphson (NR) iterations by use of the reduced method when four phases are present. The computational advantage of the reduced method increases rapidly with the number of phases and components. The developed four-phase reduced-flash algorithm is used to investigate the effect of introducing water on the phase behavior of two west Texas oil/CO2 mixtures. The results show changes in the phase splits and saturation pressures by adding water to these CO2/hydrocarbon systems.

Oilfield problems owing to asphaltene precipitation are well known. Asphaltenes can block pore throats or change the formation wettability and thereby reduce the hydrocarbon mobility. Simulation of asphaltene precipitation during gas injection requires a comprehensive thermodynamic model, which accounts for the complex phase behavior of asphaltenes. In this paper, PC-SAFT EOS is implemented for the first time in a compositional reservoir simulator to model asphaltene precipitation. The additional computational time of PC-SAFT compared to the cubic equations-of-state such as Pen-Robinson (PR EOS) is decreased by improving its root finding algorithm. A deposition and wettability alteration model is then integrated with the thermodynamic model to simulate the dynamics of precipitated asphaltenes. Different gas injection scenarios are modeled to show the effect of gas injection on apshaltene precipitation and deposition. Simulation results show that the profile of the damaged area by asphaltene deposition is governed by the shape of the asphaltene precipitation envelope for the reservoir fluid. The damage caused by asphaltene deposition, through plugging and wettability alteration, was revealed as a decline in productivity index curves. Results indicate that a reservoir fluid can have precipitation risk during gas injection even if it does not exhibit precipitation in the primary recovery life of a field. The computational time of the simulations using PC-SAFT EOS were compared to those using PR EOS for different number of components. Results of these comparisons show the feasibility of using PC-SAFT in compositional simulations.

Three hydrocarbon phases can co-exist at equilibrium at relatively low temperatures in many CO 2 floods. Formation of an aqueous phase in contact with hydrocarbon phases is inevitable in almost all recovery processes, because of the permanent presence of water in the reservoirs either as injection fluid or as initial formation water. Successful modeling of CO 2 flooding requires accounting for the presence of four phases. However, as the number of phases increase, flash calculations become more difficult and time-consuming. A possible approach to reduce the computational time of the phase equilibrium calculations is to use reduced methods. This paper presents a general strategy to model the behavior of CO 2 /hydrocarbon/water systems where four equilibrium phases occur using a reduced flash approach. The speedup obtained by a reduced flash algorithm compared to the conventional flash approach is demonstrated for a different number of components and phases. The results show a significant speedup in the Jacobian matrix construction and in Newton-Raphson iterations using the reduced method when four phases are present. The computational advantage of the reduced method increases rapidly with the number of phases and components. The developed four-phase reduced flash algorithm is used to investigate the effect of introducing water on the phase behavior of two West Texas oil/CO 2 mixtures. The results show significant changes in the phase splits and saturation pressures by adding water to these CO 2 /hydrocarbon systems.

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