Precipitation of solids, believed to be asphaltenes is common in most carbon dioxide floods. The oils in most carbon dioxide flood applications, however, are light crudes with low initial asphaltene content. Precipitation is common after carbon dioxide breakthrough in production equipment. It is believed that precipitation does not occur in the reservoir. The objective of this paper was to identify the reasons for this phenomenon. The mature carbon dioxide flood at Rangely field in Colorado was selected as an example. A high-pressure, high-temperature pressure-volume-temperature apparatus in conjunction with a high-pressure core flooding system was used to study carbon dioxide induced precipitation from the Rangely crude oil. Rangely crude contains about 1% heptane insolubles. It was determined that first-contact precipitation, at different carbon dioxide concentrations was negligible. Core flooding experiments at field conditions revealed that precipitation did occur at the core exit. This essentially showed that the precipitation phenomenon in crudes that are likely CO2-flood candidates is due to the multiple contact process and the associated compositional changes. Multiple contact experiments conducted to prove this hypothesis showed that multiple contact precipitation amounts were 3–5 times the first-contact precipitation values. The compositional changes (in the original crude) to cause widespread precipitation were established. The precipitates from the field and various laboratory experiments were analyzed using GC and SARA, which showed that field precipitates contained compounds other than original heptane insolubles and also exhibited some unique features. Introduction Asphaltenes by definition are a compound class. Even though precipitation has been observed in most CO2 flooding applications, there is uncertainty with respect to the thermodynamic conditions that lead to precipitation. The differences between asphaltenes or solids precipitated by different means are also not well established. Asphaltene content in the crude as determined by conventional methods is not a good indicator of asphaltene precipitation problems. It has been reported that a crude with more than 17 wt % asphaltenes did not have asphaltene problems, while a crude with only 0.1 wt % had serious asphaltene deposition problems1. Asphaltenes precipitate because of changes of pressure, temperature and composition and the thermodynamic instability that these changes entail. The use of carbon dioxide to increase oil recovery has received considerable attention. In United States, there were 60 active miscible CO2 projects in operation in 1996, whereas in Canada there were 40 hydrocarbon miscible active projects2. It has been projected that CO2 miscible flooding potentially could recover as much as 40% of the total oil to be recovered by all enhanced recovery methods3. Carbon Dioxide is highly soluble in oil and to a lesser extent in water. Carbon dioxide improves oil recovery by the following mechanisms: reduction in crude oil viscosity and increase in water viscosity; swelling of crude oil; reduction in oil density; and miscibility effects. The miscibility between the oil and CO2 or hydrocarbon solvents eliminates interfacial tension and capillary forces and could help recover, in theory, all of the residual oil.

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