Non-classical Nucleation Model for Cold Production of Heavy Oil
Authors: Meyer V. , Creux P. , Graciaa A. , Franco F. , Luck F. 
Published: March 2010

We examine the gas bubble nucleation phenomenon encountered in extra heavy oil during cold production. The nucleation model described in this work is based on so-called non-classical nucleation. Using this method, we show mesoporous cavities could be at the origin of the nanobubble trapping mechanism. The results obtained show this physical approach tends to demonstrate the pre-existence of gas bubbles in these crevices (surface roughness). The physics of capillarity used here is based on traditional Laplace's law and an original disjoining pressure expression.

We test for several wettabilities in our mathematical model. The first configuration envisaged is for oil-wet rocks, although the cavity is assumed to be gas-wet. Water wettability is considered a second time, taking into account a precursor water film between the rock and the entrapped bubbles.

The mix of these two configurations could represent nucleation in a global mixed-wet porous media. However, we show in the first part of this article that water presence does not affect the initial bubble radius. Nevertheless, a bubble growth model developed in the second configuration shows that bubble confinement could play an important role on gas bubble nucleation and the early first steps of its development.

Introduction

Macroscopic nucleation mechanisms have been the subject of many studies over the last decades because of both its fascinating underlying physics and its technological importance in many domains. The physics of gas bubble generation and growth in supersaturated solutions is a confusing subject in the literature. Jones et al.(1) proposed a classification system for the kinds of nucleation that occur experimentally. They define mechanistically four types of nucleation and place the specific forms of nucleation into a better defined context.

Several authors concluded that the heterogeneous nucleation form is the most plausible mechanism in porous media. Nevertheless, homogeneous or heterogeneous nucleation on molecularly smooth surfaces requires a very high level of supersaturation. During a depletion experiment with heavy oil in porous media, the supersaturations reached are low, so another type of nucleation has to be envisaged. The opposite of the classical bubble nucleation theory which requires very high levels of supersaturation, is one where the activation of the pre-existing gas cavities needs very low levels. Therefore, to study the nucleation phenomenon in heavy oil we assume the pre-existence of trapped gas bubbles in contact with the supersaturated liquid. Lubetkin(2) focused on this and claimed that theory predicts a need for a much higher supersaturation to cause bubble nucleation than is found experimentally. Moreover, the theory predicts that the nature of the liquid influences the conditions required for nucleation and experimental works show a strong influence with the chemical nature of the gas. Considering this, we try to take into account the influence of the physical and chemical properties of the liquid and gas in equilibrium in the crevice and develop a detailed net analysis of the various existing forces.

Most previous work relates to spontaneous bubble formations which occur after a rapid decompression of a liquid solution initially saturated with dissolved gas molecules.