Ryosuke Okuno

Three–Phase Flash in Compositional Simulation Using a Reduced Method

CO2 flooding at low temperatures often results in three or more hydrocarbon-phases. Multiphase compositional simulation must accurately simulate such gas floods. Drawbacks of modeling three hydrocarbon-phases are the increased computational time and convergence problems associated with flash calculations. Use of a reduced method is a potential solution.

Application of Gibbs Ensemble Monte Carlo to Phase Equilibria of CO2/Hydrocarbon Mixtures

Gibbs Ensemble Monte Carlo (GEMC) is a molecular simulation method to predict phase behavior of fluids, such as crude oil and natural gas. It enables us to visualize microscopic structures of fluid phases. In this study, we apply GEMC to phase behavior of CO2/oil systems, where the CO2-rich liquid (L2) phase can coexist with the oil-rich liquid (L1) phase and vapor (V) phase. The L2 phase can have comparable density as the L1 phase. When the L2 phase is dense enough to extract light and intermediate hydrocarbons, CO2 flooding can achieve high displacement efficiency of more than 90%. A clear understanding of the L2 phase will help us design CO2-solvent injection processes. However, little is known about its microscopic structures and corresponding dynamic properties (i.e., viscosities and diffusion coefficients), apart from densities and phase compositions. The GEMC method is used to calculate phase equilibria of CO2/C16H34 mixtures at different pressures at 305 K, which are close to the critical point of CO2. Vapor-liquid phase equilibrium is predicted at pressures lower than 75 bar and liquid-liquid phase equilibrium at higher pressures up to 170 bar, where the density of the L2 phase is around 0.868 g/cm3 and that of L1 phase around 0.871 g/cm3. These results are in good agreement with the previous experimental data. By further adding CH4 and C2H6 into the mixture, liquid/liquid/vapor equilibrium was observed. The mole fraction, density and structural properties such as pair distribution function (which is directly related to the X-Ray diffraction), molecular clustering, and aggregation status of the three different phases are presented. Currently, we are extending our calculations to other CO2/hydrocarbon systems and calculating viscosities of two different liquid phases using Molecular Dynamics simulations with the atomic model used for GEMC.

Use of a Reduced Method in Compositional Simulation

Simulating gas injection processes requires a compositional model to predict the fluid properties resulting from mass transfer between reservoir fluid and injection gas. A drawback of compositional simulation is the efficiency and robustness of phase equilibrium calculations. Reduced methods for phase equilibrium calculations have been studied as a potential solution to improve the efficiency of compositional simulation. However, most of those studies have been performed only in stand-alone calculations, and the robustness and efficiency of a reduced method has not been confirmed in compositional simulation. In this research we develop a robust and efficient algorithm for a reduced method and validate it in compositional simulation. We examine the efficiency and convergence property of the conventional algorithm for a reduced method, and solve several implementation problems in a compositional simulator. The reduced method is implemented in UTCOMP, a compositional IMPEC simulator, to demonstrate the performance for various numbers of components and degrees of miscibility. The results show that the reduced method enables significant saving in execution time of compositional simulation without loss of accuracy, compared to standard methods. Also, we observe that the reduced method exhibits improved robustness especially for miscible processes where composition paths go near critical regions.