The Effect of an Initial Gas Content on Thermal EOR as Applied to Oil Sands (includes associated papers 18399, 18568 and 18743 )

Abstract

Summary This paper discusses the results of a number of physical model experiments on thermal EOR. Seven experiments were performed, three in which a "dead" heavy oil was used, and four in which the oil sand was saturated with methane. For oils without an initial gas content, coinjection of CO2 with steam was capable of improving oil recovery over that obtained with steam alone. When an initial dissolved gas was present, coinjection of CO2 was not beneficial. Injection of CO2 or CH4 slugs just before steam injection was beneficial in increasing oil recovery for experiments where an initial dissolved gas was present. Introduction Numerous researchers have reported results on thermal EOR in heavy oil. These results refer to field experience and to numerical modeling studies. Little experimental work is available on the significance of dissolved gas in thermal recovery processes. This work was done to provide data on the effects of dissolved gas on cyclic steam stimulation. The work consists of seven large physical model experiments with and without an initial dissolved gas content. Meldau et al. reported on a field application of air injection with steam in a reservoir containing 11° API [0.99-g/cm3] oil. They also performed a numerical study that indicated that injecting gas with steam produced substantial improvements in oil production. Their model assumed a dead oil and therefore did not study the effect of an initial gas saturation. They found that the mechanisms responsible for improved oil recovery were trapping of gas below the critical saturation value, increased gas drive of oil near the wellbore, movement of heat into the upper layers of the reservoir, and higher reservoir pressure during drawdowns. Hong and Ault performed a numerical simulation of a heavy-oil and a light-oil steamflood. They compared steam-only injection to the injection of steam with a noncondensable gas. They assumed a 14° API [0.97-g/cm3] heavy oil in 4,000-md sand and no initial gas saturation in their oil. Coinjection of CO2 or N2 with steam improved the early oil recovery from a steamflood of a heavy-oil reservoir. However, the ultimate oil recovery was the same as that for a steam-only injection. The enhanced early recovery was a result of the sweep of the reservoir by injected gas.