Multiple hydrocarbon phases are observed during miscible gas floods. The possible phases that result from a gas flood include a vapor phase, an oleic phase, a solvent-rich phase, a solid phase, and an aqueous phase. The solid phase primarily consists of aggregated asphaltene particles. Asphaltenes can block pore throats or change the formation wettability, and thereby reduce the hydrocarbon mobility. The dissolution of injected gas into the aqueous phase can also affect the gas flooding recovery because it reduces the amount of gas available to contact oil. This is more important in CO₂ flooding as the solubility of CO₂ in brine is much higher than hydrocarbons. In this research, we developed efficient and fast multi-phase equilibrium calculation algorithms to model phase behavior of asphaltenes and the aqueous phase in the compositional simulation of gas floods. The PC-SAFT equation of state is implemented in the UTCOMP simulator to model asphaltene precipitation. The additional computational time of PC-SAFT is substantially decreased by improving the root finding algorithm and calculating the derivatives analytically. A deposition and wettability alteration model is then integrated with the thermodynamic model to simulate dynamics of precipitated asphaltenes. Asphaltene deposition is shown to occur with pressure depletion around the production well and/or with gas injection in the reservoir domain that is swept by injected gas. It is observed that the profile of the damaged area by asphaltene deposition depends on the reservoir fluid. A general strategy is proposed to model the phase behavior of CO₂/hydrocarbon/water systems where four equilibrium phases exist. The developed four-phase reduced flash algorithm is used to investigate the effect of introducing water on the phase behavior of CO₂/hydrocarbon mixtures. The results show changes in the phase splits and saturation pressures by adding water to these CO₂/hydrocarbon systems. We used a reduced flash approach to reduce the additional computational time of the four-phase flash calculations,. The results show a significant speed-up in flash calculations using the reduced method. The computational advantage of the reduced method increases rapidly with the number of phases and components. We also decreased the computational time of the equilibrium calculations in UTCOMP by changing the sequential steps in the flash calculation where it checks the previous time-step results as the initial guess for the current time-step. The improved algorithm can skip a large number of flash calculation and stability analyses without loss of accuracy. / text

Miscible gas flooding is a common method for enhanced oil recovery. Reliable design of miscible gas flooding requires compositional reservoir simulation that can accurately predict the fluid properties resulting from mass transfer between reservoir oil and injection gas. Drawbacks of compositional simulation are the efficiency and robustness of phase equilibrium calculations consisting of flash calculations and phase stability analysis. Simulation of multicontact miscible gas flooding involves a large number of phase equilibrium calculations in a near-critical region, where the calculations are time-consuming and difficult. Also, mixtures of reservoir oil and solvent such as CO₂ and rich gas can exhibit complex phase behavior at temperatures typically below 120°F, where three hydrocarbon-phases can coexist. However, most compositional simulators do not attempt to solve for three hydrocarbon-phases because three-phase equilibrium calculations are more complicated, difficult, and time-consuming than traditional two-phase equilibrium calculations. Due to the lack of robust algorithms for three-phase equilibrium calculations, the effect of a third hydrocarbon-phase on low-temperature oil displacement is little known. We develop robust and efficient algorithms for phase equilibrium calculations for two and three phases. The algorithms are implemented in a compositional reservoir simulator. Simulation case studies show that our algorithms can significantly decrease the computational time without loss of accuracy. Speed-up of 40% is achieved for a reservoir simulation using 20 components, compared to standard algorithms. Speed-up occurs not only because of improved computational efficiency but also because of increased robustness resulting in longer time-step sizes. We demonstrate the importance of three-phase equilibrium calculations, where simulations with two-phase equilibrium approximations proposed in the literature can result in complete failure or erroneous simulation results. Using the robust phase equilibrium algorithms developed, the mechanism is investigated for high efficiency of low-temperature oil displacements by CO₂ involving three hydrocarbon-phases. Results show that high displacement efficiency can be achieved when the composition path goes near the critical endpoint where the gaseous and CO₂-rich liquid phases merge in the presence of the oleic phase. Complete miscibility may not be developed for three-phase flow without considering the existence of a tricritical point. / text