This paper discusses the reduction of heavy oil viscosity by carbon dioxide injection for improved oil recovery (IOR). Carbon dioxide injection is recognized as a viable injection gas for reservoir stimulation in light oil (30° API or higher) reservoirs. Carbon dioxide is also applicable in heavy oil (10 to 25° API) reservoirs since it lowers oil viscosity, swells the oil, provides component extraction, and is soluble in both oil and water. Carbon dioxide sources include natural reservoirs, extraction of carbon dioxide from industrial processes, generation by oxygen combustion in reservoirs, and thermo-converter generation at the surface. The surface-combustor provides Carbon Dioxide in areas which lack sufficient supplies and require the transportation of Carbon Dioxide by pipeline or high pressure trucks. Research and development of this versatile surface and downhole combustors continue today at several U.S. Universities.
The benefits of using Carbon Dioxide include crude oil and water swelling, component extraction, oil viscosity reduction, and reservoir pressuring.
An article by Simon and Graue(1) in 1965 indicates that carbon dioxide solution in crude oil lowers the viscosity to approximately 0.02 to 0.12 of the oil's original viscosity at 120°F. They studied 5 oils which were below 20°API and had viscosities from 58.6 to 1300 cp at 120°F. The result of their study indicates viscosity reductions from 0.025 to 0.115 at a pressure of 1000 psia.
A 1986 topical report by Chung (2) reported on the viscosity reduction in 7 heavy oils which ranged from 10°API to 21.6°API. The oil viscosities ranged from 49 to 1484 cp at various temperatures (75 to 200°F). Chung observed viscosity reductions from 0.03 to 0.15 of the original oil viscosity.
