Mechanisms of Bubble Formation by Pressure Decline in Porous Media: A Critical Review

Abstract

Abstract The modeling of primary production of heavy oils by solution gas drive is an active area of research. All the models, either written at Darcy scale or at pore scale (capillary network, population balance), account for the following mechanisms: bubble formation, bubble growth and gas flow. The first stage of bubble formation, also called bubble nucleation, is still controversial. In this paper, we discuss the existing nucleation models and demonstrate that the preexistence of bubbles is the only theory that is justified physically and can explain the ensemble of experimental observations. The preexisting bubbles are stabilized either by surfactants (models used for cavitation studies) or capillarity in crevices (models used in boiling). In both models, a given number of bubbles are activated at a given pressure drop. The only adjustable parameter is the distribution of diameters of the preexisting bubbles. This distribution is a property of the rock/fluid system that can be experimentally determined. The other models used in literature are based on the formation of a stable nucleus by thermal fluctuations. They lead to the notion of nucleation rate that is in contradiction with experimental results. We also discuss the terminology used in recent papers. Especially the terms of "instantaneous nucleation" and "progressive nucleation" are irrelevant if a mechanism of preexisting bubbles is assumed. They are also misleading since they lump the mechanism for bubble formation (statistics or preexistence) and the mechanism for pressure decline (either step or constant rate).