A series of pressure depletion experiments was conducted in glass micromodels using five Venezuelan oils with the objective to studying their microscopic mechanism during primary production under a solution gas drive process. Photographs taken from a large area of a micromodel during depressurisation reveal 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.
There are some Venezuelan and Canadian heavy oil reservoirs with a high recovery factor during the 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 gasin- 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 literature2–7. Pressure depletion tests using glass micromodels2–5 have been a useful visualization tool to study nucleation, growth, coalescence, break-up, and flow of gas bubbles. Also, depletion tests performed in core samples using a visual core holder6,7 have been conducted to observe the gas bubbles on the core surface. Firoozabadi and Aronson6 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 is to compare the behaviour of these oils during the primary production under a solution gas drive process. During the depressurisation, in contrast to previous glass micromodel studies2–5, photographs of a large area of the micromodel (∼31cm2) are taken in order to obtain statistical information regarding the gas distribution. These experiments allow us to observe a larger view of the gas distribution, close to that in transparent core holder experiments6,7, but keeping the details of the gas bubbles.
The experimental apparatus designed in 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 × 10 cm × 4 mm. The upper plate has two holes for injecting and retrieving the fluids. The pattern, shown in Figure I, was chemical etched on the lower plate following a known procedureX. 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. This type of network has been etched previously") and has a porosity of 59.7 %. The etching depth was close to 75 µm.