Browsing by Subject "Compositional reservoir simulation"
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Item Development of a multi-formulation compositional simulator(2013-05) Santos, Luiz Otávio Schmall dos; Sepehrnoori, Kamy, 1951-Compositional simulation is a complex task that involves solving several equations simultaneously for all grid blocks representing a petroleum reservoir. Usually, these equations are separated into two groups: primary and secondary equations. Similarly, the unknowns of the system are also separated into primary and secondary variables. Considering the large number of unknowns, there are many ways to separate such variables in order to deal with the primary variables. This work aims at comparing a number of formulations for compositional reservoir simulation. It also aims at enhancing the formulations with new features not provided in the original publications. To accomplish these objectives, various formulations prevailing in the literature are implemented in The University of Texas at Austin in-house fully implicit simulator named GPAS (General Purpose Adaptive Simulator) and their performances were compared. Subsequently, some of the formulations were enhanced and tested for various applications. The comparison of the formulations studied indicated differences in efficiency for each approach. These differences come from the fact that when one is solving for a different set of primary variables, the manipulation of the equations is analogous to the use of a preconditioner applied to a linear system of equations. Furthermore, unlike a preconditioner, changing the primary variables affects the non-linear solver. Therefore, differences in terms of the number of Newton-Raphson iterations, used for solution of nonlinear equations resulting from discretization of nonlinear partial differential equations representing fluid flow in the reservoir, are expected. In addition to these differences in the non-linear solver, many formulations explore the fact that a reduced number of equations need to be solved implicitly, thus considerably reducing the CPU time dedicated to the linear solver. Finally, new features not provided in the original published formulations such as three-phase flash calculation, physical dispersion, and unstructured grid were implemented and verified. Additionally, it was demonstrated that, in certain situations, these enhancements are essential to properly model the physical phenomena occurring in oil and gas reservoirs.Item Modeling of multiphase behavior for gas flooding simulation(2009-08) Okuno, Ryosuke, 1974-; Johns, Russell T.; Sepehrnoori, Kamy, 1951-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.