Numerical approximation based on dif-ferent forms of the governing partial differentialequation can lead to significantly different results fortwo-phase flow in porous media. Selecting the properprimary variables is a critical step in efficientlymodeling the highly nonlinear problem of multiphasesubsurface flow. A comparison of various forms ofnumerical approximations for two-phase flow equa-tions is performed in this work. Three forms ofequations including the pressure-based, mixed pres-sure—saturation and modified pressure—saturation areexamined. Each of these three highly nonlinearformulations is approximated using finite differencemethod and is linearized using both Picard andNewton—Raphson linearization approaches. Modelsimulations for several test cases demonstrate thatpressure based form provides better results comparedto the pressure—saturation approach in terms ofCPU_time and the number of iterations. The modi-fication of pressure—saturation approach improvesaccuracy of the results. Also it is shown that theNewton—Raphson linearization approach performedbetter in comparison to the Picard iteration linearization approach with the exception for in the pressure—saturation form.
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