Properly defining the classical vaporizing and condensing mechanisms when a gas is injected into a crude oil

In this paper, real crude oils are dealt with which are modeled with an ave rage of 30 components. On the selected examples, neither the initial tie li ne, i.e., the tie line that extends through the original oil composition, n or the gas tie line, i.e., the tie line that extends through the injected g as composition, controls the miscibility process. In any case, the miscibil ity process is controlled by one of the (nc - 3) crossover tie lines, if nc is the number of components in the crude oil. However, it is shown that th e miscibility process may be a pure vaporizing gas drive mechanism (VGDM). To be more precise, it is shown that a mixed condensing/vaporizing mechanis m may, with increasing pressure, turn into a pure VGDM. This means that the classical definition of a VGDM, i.e., the lowest pressure at which the ini tial tie line is critical, must be changed. This is the main point of this paper. In the case of a VGDM controlled by a crossover tie line, the MMP (t he lowest pressure at which the key crossover tie line becomes critical) ma y still be computed by a one cell simulation algorithm. Another aim of this paper is to explain what really happens during a one cell simulation and t o give proof that such an algorithm may not make critical the initial or th e gas tie line.