Phase Behavior of Several CO₂/West-Texas-Reservoir-Oil Systems

The design of miscible CO₂ recovery methods and the evaluation of laboratory CO₂ coreflood and pilot field studies require knowledge of the phase behavior encountered in such processes and the ability to make reliable predictions. Because of the complexity of CO₂/hydrocarbon phase behavior, experimental measurements are necessary as a basis from which to develop an understanding. In this paper, measured phase equilibria and volumetric properties are reported for several west-Texas-reservoir-oil/CO₂ systems. Reservoir oils studied were from the San Andres, Grayburg, and Devonian Chert formations. Both static (single-contact) and multiple-contact measurements have been conducted in a visual fluid property cell. Static data cover a wide range of CO₂ compositions and provide a general understanding of CO₂/reservoir-oil phase behavior. Multiple-contact measurements in which the CO₂-rich phase is repeatedly contacted with recombined reservoir oil (oil cycling) simulate phase behavior occurring at the flood front. Multiple-contact measurements in which the oil-rich liquid phase is repeatedly contacted with pure CO₂ (CO₂ cycling) simulate the phase behavior exhibited by residual oil near a CO₂ injection well. The multiple-contact data cover narrow compositional paths encountered in displacement processes and serve as a basis for equation-of-state (EOS) evaluation. Phase behavior trends common to all systems are discussed.