The immiscible carbon dioxide flooding process has considerable potential for the recovery of moderately viscous oils, which are unsuited for the application of thermal recovery techniques. Approximately 95% of Saskatchewan's heavy oil formations are less than 10m thick, and often have an underlying water sand. Under these conditions, thermal methods are inefficient and uneconomical due to excessive vertical heat loss and steam scavenging by the bottom water. This provides the motivation for searching an alternative to thermal recovery techniques for thin, moderately heavy oils.
Laboratory research conducted in the 1950s identified several aspects of carbon dioxide flooding such as viscosity reduction, oil swelling, miscibility effects, and solution gas drive. Both laboratory and field studies have been conducted to determine the effectiveness of the carbon dioxide process for heavy oil recovery. This paper concentrates on the laboratory and field studies conducted in the past as well as the future of the immiscible carbon dioxide flooding process for the recovery of heavy oils.
Moderately viscous heavy oils lack the necessary extractable hydrocarbons [C5 - C30 ] for miscible conditions with carbon dioxide to be economically attained. In some cases, moderately light oils [25–35 °API] are displaced immiscibly because the high pressures required to achieve miscibility with carbon dioxide would lead to formation fracturing. This is undesirable in that it leads to gas channeling and early carbon dioxide breakthrough.
Both laboratory and field studies have been conducted to determine the effectiveness of the immiscible carbon dioxide process. Laboratory studies are used to determine and optimize the recovery process mechanisms. Field studies, both pilot and conventional, have been conducted in two modes, namely: primary and tertiary. Primary recovery methods have been the most successful to date while tertiary methods have helped greatly in reducing water and gas cuts in late flood life projects1. The objectives of this paper are to give a resume of the dominant mechanisms in the immiscible carbon dioxide displacement process, and to analyze field data in order to develop the minimum criteria for process selection.
How does the carbon dioxide mix with the reservoir fluids, namely: oil and water? Three mass transfer mechanisms are discussed in this section. Solubility is the most important mechanism of carbon dioxide transport in the reservoir. Diffusion and dispersion also affect, to a lesser extent, the transport of carbon dioxide.
The most important property of heavy oil-carbon dioxide, systems is carbon dioxide solubility. "Solubility of one substance in another depends fundamentally upon the ease with which the two: molecular species are able to mix. 2 Klins3 stated: that for low pressure application [<7 MPa), the major effect would be the solubility of carbon dioxide in crude oil. The solubility of pure carbon dioxide in Lloydminster Aberfeldy 115-l7 °API] oil at 5.5 MPa and 20.6 °C is approximately 70 sm3/sm3 of oil. Solubility is a strong function of pressure, and to a lesser degree, temperature and oil composition. Solubility increases with pressure and decreases with temperature and reduced API gravity.