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). Introduction The mechanism of bubble formation is involved in many domains such as boiling1–6 and cavitation7–9 and an important bibliography exists either for applications in bulk10–18 or in porous media8,19–32. Nucleation is also important in petroleum applications for solution gas drive. In this process, a reservoir is depleted and oil is produced by expansion of the gas released from the crude. The process of solution gas drive can be described in three steps:Gas nucleation:corresponding to the release of the dissolved light components into a free gas phase when pressure is decreased below the bubble point.Bubble growth: corresponding to mass transfer by molecular diffusion of the dissolved light components to the free gas phase. Until equilibrium concentration is reached, the liquid is always supersaturated and the system tends to the equilibrium state by transfer to the gas phase of the dissolved light components.Gas mobilization: Above a given gas saturation, called critical gas saturation Sgc, the gas phase becomes connected and is produced preferentially because of its higher mobility in comparison of oil. In some cases, flow of dispersed gas is also considered. In heavy oil for example, high oil viscosity may maintain gas in a dispersed form. In this paper we will discuss the first step of nucleation. Two types of models exist to describe the mechanism of nucleation: models based on thermodynamics and models based on the preexistence of microbubbles. A lot of experimental works in different domains have shown that thermodynamic models are not relevant to describe nucleation processes in common laboratory experiments2,7,14,15,23–25. However, the thermodynamical approach is still used in the domain of oil production. A justification is sometimes given by the agreement between models and experiments. But generally a given experiment can be fitted by different models. A good agreement does not mean that the physics is relevant and the extrapolation at reservoir scale can be wrong.