Chapter 22: The Impact of Oil Viscosity Heterogeneity on Production from Heavy Oil and Bitumen Reservoirs: Geotailoring Recovery Processes to Compositionally Graded Reservoirs
Ian D. Gates, Jennifer J. Adams, Steve R. Larter, 2010. "The Impact of Oil Viscosity Heterogeneity on Production from Heavy Oil and Bitumen Reservoirs: Geotailoring Recovery Processes to Compositionally Graded Reservoirs", Heavy Oils: Reservoir Characterization and Production Monitoring, Satinder Chopra, Laurence R. Lines, Douglas R. Schmitt, Michael L. Batzle
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Most of the world’s petroleum resources are contained in heavy oil and oil-sands reservoirs. Average recoveries from heavy oil and oil-sands reservoirs are typically low ranging from 5% to 15% for cold heavy-oil production and from 30% to 85% for steam-based in situ processes. Two reasons account for this: (a) geologic heterogeneity in the form of variable rock and rock-fluid properties, and (b) fluid heterogeneities in the form of variable fluid composition. Geologic heterogeneities refer to spatial variations of porosity, permeability, relative-permeability curves, shale, and mud layers, etc. Fluid heterogeneities refer to spatial variations of the fluid composition and properties such as viscosity and density. We will show that the controlling variable on recovery of these resources is fluid compositional variations.
Figure 1 displays the three axes that define a recovery process for oil sands: tolerance to geologic and fluid heterogeneity (geotolerance), environmental impact (gas emissions and water use), and energy efficiency. A recovery process such as mining is geotolerant; that is, heterogeneity does not matter because all of the oil sand is processed and the oil recovery factor is high — typically higher than 90%. However, mining is only suitable for shallow resources, is very costly, and has high carbon dioxide emission and other environmental penalties. In situ processes to produce viscous and poor-quality oils rely on high-pressure primary production, as in cold heavy-oil production, or thermal and/or solvent-based methods to mobilize the oil by reducing its viscosity. The key problem of these processes is that they are not very geotolerant; that is, their performance is adversely affected by the reservoir geology and fluid heterogeneity. Also, profit margins are small because of high capital and operational costs. Furthermore, thermal processes produce large amounts of carbon dioxide emissions and use huge volumes of water. We believe this geo-intolerance, the excessive emissions, the environmental impact, and the consequent energy losses can be reduced and are due to insufficient tailoring of recovery processes to geologic and fluid property variability commonly seen in heavy-oil and bitumen reservoirs.
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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.