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The initialization procedure proposed by Michelsen for locating stationary points in the tangent plane distance function was applied to several cases considered as challenging in the literature concerning phase equilibrium calculations. For most of the systems studied in the literature, such a simple procedure is enough to detect the phase instability; thus it is unnecessary to resort to more robust, more elaborated, and more time-consuming initialization schemes. For the conditions and systems where problems were found, a simple procedure was proposed to extend Michelsen's original scheme with the aim of increasing the probability of obtaining a negative value of the tangent plane distance function. Two algorithms for phase-split calculations were used. Both algorithms use a relaxation parameter to increase their convergence regions. No difficulties were found to calculate the equilibrium distributions in contrast with several convergence problems reported, in which other approaches were applied to the same two-phase systems.

During the last 20 years, it has been a very large discussion about different production mechanisms to explain the anomalous production behavior observed in some heavy oil reservoirs especially in the Orinoco Belt in Venezuela and Lloydminster in Canada. This behavior has been characterized by high production rates and/or high primary oil recovery, besides good pressure maintenance. A special phenomenon associated to the solution-gas drive mechanism, known as "foamy" or "bubbly oil", seems to be responsible for the recovery of a much higher fraction of the original oil in place. However, the "foamy oil" behavior cannot clearly explain the mechanism of the observed oil mobility enhancement.Considering the physical principles of multiphase flow through porous media, the momentum transfer or viscous coupling appears as a hidden driving mechanism to explain the improved oil mobility. A capillary model is used to assess the importance of lubrication in two-phase flow and to determine the effect of the viscous coupling on relative permeabilities.Different cross-sectional geometries are analyzed. The problem is solved analytically even for the unequal viscosity case. This is the first attempt known to these authors not only of using this analytical method of solution, but also of using the concept of contact angle for determining the fluids distribution inside a non-circular channel.The model results show that the viscosity ratio does affect the relative permeabilities, especially in systems involving heavy oil.