A Laboratory Investigation of Miscible Displacement by Carbon Dioxide

Abstract We have conducted displacements of reconstituted reservoir fluids in Boise outcrop sandstone cores of 6 to 42.5 ft in length using CO2 at various pressures. Our studies indicate that a miscible displacement may be achieved by CO2 injection at pressures well below those necessary when methane is the injection fluid. Miscibility by CO2 is generated through multiple contact equilibria in which the CO2 is progressively enriched with intermediates progressively enriched with intermediates (ethane through hexane) from the oil. The miscibility pressure for CO2 and a given reservoir fluid is difficult to determine because CO2 exhibits highly efficient swelling and vaporization of the oil. We also found it impractical to operate below the rates where differences in density between displaced and displacing fluids prevent viscous fingering in vertical cores. We were able to define the miscibility pressure using the anticipated increasing recovery as a function of core length in miscible displacements in combination with effluent fluid visual cell observations and chromatographic data. Introduction Miscible displacement processes in petroleum reservoirs fall generally into two classes: (1) processes in which the injected fluid and in-place fluid form a single-phase solution for all compositions and (2) processes in which the injected fluid and in-place fluid do not on a single equilibrium contact form a single-phase solution over most of the range of possible compositions, but which may generate a zone of contiguous single-phase compositions by multiple contact mass transfer of components between the injected and in-place fluids. Processes in Class 1 are sometimes characterized as having first contact miscibility; one example of such a process is the propane slug process. There are two important types of Class 2 processes, the enriched gas-drive process and the high-pressure gas process. In the enriched gas-drive process the injected gas contains methane and intermediate hydrocarbons, usually ethane, propane and butanes. These intermediate hydrocarbons dissolve in the oil, enriching it to the point where it becomes miscible with the injected gas. Miscibility in the high-pressure gas process is generated by transfer of intermediates, ethane through hexane, from the reservoir fluid into the injected lean gas. The enriched gas-drive process and the high-pressure gas process are both characterized at pressures where miscibility is possible by an initial period of immiscible displacement until sufficient mass transfer of intermediates has occurred to satisfy the requirement for miscibility.