Visualization Study During Depletion Experiments of Venezuelan Heavy Oils Using Glass Micromodels
Authors: Lago M. , Huerta M. , Gomes R. 
Published: March 2010

Abstract
A series of pressure depletion experiments was conducted in a glass micromodel using five Venezuelan oils with the objective of studying their microscopic mechanism during primary production under a solution gas drive process. Photographs taken from a large area of the micromodel during depressurisationreveal a higher number of bubbles for heavy oils than for light oil. Image processing results show differences in the number of activated nucleation sites among three heavy oils that have similar degrees of critical supersaturation. Three steps in the heavy oil flow are also established.

Introduction
There are some Venezuelan and Canadian heavy oil reservoirs with a high recovery factor during primary production under a solution gas drive process. Recently, Sheng et al.(1) published a review on this unexpected behaviour, which is different from conventional gas drive concepts. These authors stated that the most plausible microscopic mechanism to explain the gas mobility reduction in heavy oil reservoirs is the creation of a gas-in-oil dispersion, but further investigation on the gas bubble size and number in this dispersion needs to be done.
Experimental work on the understanding of the solution gas drive mechanism from a microscopic point of view can be found in the literature(2–7). Pressure depletion tests using glass micromodels(2–5) have been a useful visualisation tool to study nucleation, growth, coalescence, break-up, and flow of gas bubbles. Also, depletion tests performed in core samples using a visual core holder(6, 7) have been conducted to observe the gas bubbles on the core surface. Firoozabadi and Aronson(6) reported a high number of bubbles for heavy oils, and relate this result to a gas mobility reduction and, consequently, a better solution gas drive efficiency.
In this paper, we report a series of depletion experiments in a glass micromodel, using five Venezuelan oils. The main objective of this study was to compare the behaviour of these oils during primary production under a solution gas drive process. During the depressurisation, in contrast to previous glass micromodel studies(2–5), photographs of a large area of the micromodel (~31cm2) were taken in order to obtain statistical information regarding the gas distribution. These experiments allowed us to observe a larger view of the gas distribution, close to that seen in transparent core holder experiments(6, 7), but keeping the details of the gas bubbles.

Experimental Set-Up
The experimental apparatus designed by PDVSA-Intevep is composed of a glass micromodel inside a high-pressure cell, a fluid injection system, and an image-capturing device.
The micromodel is made of two glass plates of 10 cm x?10 cm x?4 mm in size. The upper plate has two holes for injecting and retrieving the fluids. The pattern, shown in Figure 1, was chemically etched on the lower plate following a known procedure(8). The obstacles consist of cylindrical grains of 1.0 mm diameter, arranged in a regular 2D network with a minimal separation distance of 0.5 mm.