Events

Title: Advanced Equation of State Modeling for Compositional Simulation of Gas Floods

Supervisors: Kamy Sepehrnoori, Russell T. Johns

Multiphase behavior is 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 theamount of gas available to contact oil. This is more important in CO2 flooding as thesolubility of CO2 in brine is much higher than hydrocarbons. In this research, wedeveloped efficient and fast multi-phase equilibrium calculations algorithms to modelphase 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. To reduce the additional computational time of the four-phase flash calculations, we used a reduced flash approach. The results show a significant speedup 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 analysis without loss of accuracy.