Development and Application of an Equation of State Compositional Simulator

A new, three-dimensional, isothermal, compositional simulator has been developed for miscible gas flooding. The maximum of four phases which may flow simultaneously are the aqueous phase, oil phase, gas phase and a second, nonaqueous liquid. An equation of state is used to model the nonaqueous phase behavior, and a sequential phase behavior algorithm is employed to compute up to three nonaqueous-phase compositions. Other features include physical dispersion, capillary pressure, interfacial tension and relative permeability. The overall computational procedure of the simulator is sequential and noniterative over a time step. The solution scheme is analogous to IMPES, but the total concentration of each component in moles is solved for explicitly instead of saturations. A third-order finite-difference method is used to solve the material balance equations. Comparisons of the results of this third-order scheme with those of one-point and two-point upstream weighting show that this method is effective in reducing numerical dispersion and minimizing grid orientation effects. Computational time can be reduced significantly for the same accuracy compared to both one-point and two-point upstream weighting. The simulator has been applied to the study of carbon dioxide process mechanisms. The purposes were to investigate the effects of phase behavior on processes and to identify the dominant flow patterns as a function of heterogeneity for carbon dioxide flooding. First, one-dimensional runs were made to examine the physical properties at two-, three-and four-phase flow conditions for carbon dioxide flooding. Next, both first-contact and multiple-contact miscible displacements were simulated with a fine mesh to investigate the transition from gravity override to viscous fingering dominated displacement. Then, the effects of gravity, physical dispersion, capillary pressure, phase behavior and heterogeneity were combined and simulated for carbon dioxide flooding on a field scale using statistically generated permeability fields. These simulation results show that the displacements are dominated by viscous fingering only when the gravity number and dispersion are small, and the permeability fields have low heterogeneities and very short correlation lengths. The dominant flow patterns were found to be channeling, gravity override and spreading under all other conditions. Thus, viscous fingering was not found to be a dominant flow pattern for field scale carbon dioxide flooding under the condition,s studied. Previous fine mesh simulation studies of fingering have stressed first-contact miscible displacements where fingering occurs much more readily. The more realistic studies of this work were the result of considerable enhancement of both the physical and numerical accuracy of the simulations over previous efforts and this has led to an important new conclusion.