The Effect of Phase Behavior on CO2-Flood Displacement Efficiency

Abstract

Summary The relationship between phase behavior and displacement efficiency (meaning here, recovery efficiency in the absence of any bypassing) as regards the displacement of Wasson crude oil by CO2 is examined at two different pressures--the lower pressure the one at which three coexisting hydrocarbon phases are encountered. At both pressures, experimental phase behavior data comprising primarily phase volume fractions observed in both single- and multiple-contact experiments are presented. Pseudoternary representations of the phase behavior are incorporated into a simple, one-dimensional, finite-difference simulator to relate the phase behavior to displacement efficiency. At the appropriate dispersion level, displacement efficiencies computed with the model are consistent with high oil-recovery efficiencies obtained experimentally in slim-tube displacements, and indicate that the displacement efficiency of the process (again, in the absence of bypassing) should be high in certain consolidated media as well. Introduction Probably as much research effort has been spent on phase-behavior aspects of CO2 flooding as on any other feature of the process. Undoubtedly the most rigorous way of examining the importance of this factor is by means of a fully compositional simulator containing dispersion terms and employing an equation of state capable of describing in detail phase equilibria related to the process. But modeling the phase behavior of CO2-crude oil mixtures is not easy, particularly in the case of low-temperature (i.e., less than, say, 120 degrees F) systems where three (four, counting precipitate) coexisting hydrocarbon phases are encountered. And even though three-phase single-contact data can be matched, as shown by Fussell, who used a Redlich-Kwong equation of state, it has yet to be demonstrated that an equation of state "tuned" to single-contact data and using pseudocomponents will describe adequately the multiple-contact process that is CO2 flooding. Furthermore, the complexity of the equation-of-state approach presents problems. it often results in black-box usage in the simulations, which inhibits the investigator from obtaining any basic insight into the overall process mechanism. In addition, if one wanted to examine correctly the relationship between phase behavior and recovery efficiency in situations where viscous fingering occurs--by explicitly simulating fingers, for example (not practical in every-day reservoir scale simulations)--it is quite possible that the time and cost of such computations that use an equation of state would be extremely high and possibly prohibitive.Partly because of these problems with the equation of state approach and partly because the experimental data reported here may be of some interest, we present a simpler but much less general alternative approach to examine the importance of phase behavior in CO2 floods involving crude oils. In this approach, single-contact phase equilibrium experiments are augmented by multiple-contact experiments where the composition of the crude is varied in a manner qualitatively similar to what might take place during an actual displacement. Data from both these experiments then are combined to construct a ternary diagram applicable at one specific pressure. (We also advocate the use of multiple-contact data in determining equation-of-state parameters.) JPT P. 2067^