Heavy-Oil Waterflooding: Back to the Future

Abstract

Abstract Waterflooding of conventional oil fields has been going on since the dawn of oil and gas production. In the 1940s and '50s, researchers in the oil community undertook a successful effort to understand the mechanisms underlying waterflooding, in order to improve its application. One of their findings was that as the mobility ratio increases the breakthrough time decreases exponentially. Furthermore, at high mobility ratios (> 500), the recovery factor does not increase much after breakthrough even after further water injection. Despite this conventional wisdom about waterflooding, it is gaining popularity around the world as a potential technology for recovering heavy oil. For example, in western Canada heavy oil reservoirs with a dead oil viscosity of up to 2,000 mPa.s have been targeted for waterfloods. Some surprising results, which appear to be at odds with conventional theory, include: extended periods of oil production at very high water cuts; and, much higher recovery factors than could be predicted from conventional theory. Mechanisms that have been proposed include: pressure support; unstable displacement and creation of water channels; water imbibition from those channels; viscous drag; emulsification; solution gas drive; apparent swelling of the oil; increasing gas saturations in the water channel causing a reduction in the water relative permeability; and, fracturing. Unfortunately, the speculation about the potential mechanisms involved in heavy oil waterflooding has not yet coalesced into a more tangible understanding of the role of each mechanism and the interplay between them. The relative importance of the mechanisms involved in heavy oil waterflooding, and the capability to enhance them at different stages in the operational life cycle of the waterflooding process, may provide a key to improving the recovery factors in heavy oil reservoirs that are being exploited by waterflooding. This paper will discuss the effort required to examine the mechanisms involved in heavy oil waterflooding, from an experimental perspective. It will address the different laboratory scales that can be employed to investigate these mechanisms, from the pore scale via micro-models to the semi-field scale using large experimental cells. Pathways incorporating these mechanisms in numerical models will be also discussed.