Chapter 19: Effects of Heavy-oil Cold Production on VP/VS Ratio
Duojun (Albert) Zhang, Larry Lines, Joan Embleton, 2010. "Effects of Heavy-oil Cold Production on VP/VS Ratio", Heavy Oils: Reservoir Characterization and Production Monitoring, Satinder Chopra, Laurence R. Lines, Douglas R. Schmitt, Michael L. Batzle
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Heavy-oil reservoirs are an abundant resource, particularly in Canada, Venezuela, and Alaska. By some estimates, heavy oils represent as much as 6.3 trillion barrels of oil in place. This matches available quantities of conventional oil. More than 50% of Canada's oil production is now from heavy oil (Batzle et al., 2006). Much of the heavy-oil recovery in Western Canada involves steam injection, called “hot production.” An alternative to thermal heavy-oil production in the field is known as “cold production,” which is a primary nonthermal process in which reservoir temperature is not affected. The cold production process has been economically successful in several unconsolidated heavy-oil fields in Alberta and Saskatchewan, Canada (Sawatzky et al., 2002). During the cold production process, sand and oil are produced simultaneously by progressive cavity pumps, generating high-porosity channels termed “wormholes.” The development of wormholes causes reservoir pressure to fall below the bubble point, resulting in dissolved gas coming out of solution to form foamy oil. Foamy oil and wormholes are believed to be two key factors in the enhancement of oil recovery (Metwally et al., 1995; Maini, 2004).
The development of wormholes and the formation of foamy oil will disturb fluid properties in the reservoir during heavy-oil cold production. Batzle et al. (2006) showed that the bulk modulus of heavy oil drops to near zero very quickly from approximately 2.6 GPa after pressure is lower than the bubble point line at approximately 2 MPa. This disturbance will probably be detectable for seismic survey.
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Heavy oil is an important global resource with reserves comparable to those of conventional oil. As conventional resources get thinner, attention is being focused on heavy oil and bitumen, which hold the promise of becoming useful fuels. Already more than 1 million barrels of oil are being produced from the oil sands in Canada; heavy oil represents half of California’s crude oil production in the United States and is a major production in Mexico. With demand for global energy soaring, heavy oil will undoubtedly be an important resource to be exploited in a big way in the near future.
The SEG Development and Production Committee held its Heavy Oil Forum in Edmonton, Alberta, in July 2007. This was a joint research forum cosponsored by the Canadian Society of Exploration Geophysicists (CSEG) and SEG and hosted by the University of Alberta. Preceding the forum, a field trip took the participants to the vast Athabasca Oil Sands region where they observed the outcrops, open pit mining, and steam injection operations, followed by a tour of the steam-assisted gravity drainage projects. Topics of the well-attended forum included the definition of heavy oil; where is heavy oil found; how it is produced; heavy-oil reservoir characterization; fluid and rock properties; electrical, tilt, and gravity techniques; borehole, surface seismic measurements; and microseismicity.