Miscibility and Miscible Displacement
Authors: Holm L.W. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.

Summary
Miscible displacement has been developed as a successful EORprocess in the past 25 years. Two types of miscible displacement exist: processin the past 25 years. Two types of miscible displacement exist: first-contactand multicontact miscible displacement. An understanding of the definition ofmiscibility and how injected fluids react with reservoir fluids is necessary tounderstand these processes.

Miscibility
Through research over the past 25 years, miscible-phase displacementprocesses that use certain gases as injectants have been developed assuccessful means for increasing oil recovery from many reservoirs. Tounderstand these processes it is first necessary to provide a definition of"miscibility," particularly as provide a definition of "miscibility,"particularly as distinguished from "solubility." Solubility is defined as theability of a limited amount of one substance to mix with another substance toform a single homogeneous phase. Miscibility is defined as the ability of twoor more substances to form a single homogeneous phase when mixed in allproportions. For petroleum reservoirs, miscibility is defined as that physicalcondition between two or more fluids that will permit them to mix in allproportions without the existence of an interface. If two fluid phases formafter some amount of one fluid is added phases form after some amount of onefluid is added to others, the fluids are considered immiscible. An interfacialtension (IFT) exists between the phases when they are immiscible. When asubstantial IFT (greater than 0.1 dynes/cm) exists between phases in a porousmedium, capillary forces prevent the complete porous medium, capillary forcesprevent the complete displacement of one of those phases by the other. Asubstantial residual oil saturation remains in a porous medium after aninjected immiscible fluid is used to displace oil from the medium, as inwaterflooding. Figs. 1 and 2 illustrate the differences between immiscible andmiscible conditions for certain fluids.

Miscible Displacement
Miscible displacement implies that with the IFT between the oil anddisplacing fluid eliminated (IFT=O), the residual oil saturation will bereduced to zero in the swept region. There are basically two types of miscibledisplacements: first contact and multicontact. The term first contact meansthat and, amount of the solvent can be injected and will exist as a singlephase with the oil in the reservoir. Low-molecular-weight hydrocarbonssuch aspropane, butane, or mixtures of liquefiable petroleum gas (LPG)or heavierhydrocarbonssuch as gasoline fractionshave been used as solvent forfirst-contact miscible flooding. In practice, solvents for first-contactmiscibility are usually too expensive for continuous injection. Instead, attempts have been made to inject a limited solvent volume, or slug, whichconceptually can be miscibly displaced with a less expensive fluid, such asmethane, nitrogen, or flue gas. This chase fluid, in turn, is immisciblydisplaced by water, leaving a residual saturation of the chase fluid. Multicontact or dynamic miscible displacements are of two mechanistic types:vaporizing-gas drive and condensing-gas drive. In the vaporizing-gas-driveprocess, a lean gas (i.e., methane. nitrogen, or flue process, a lean gas(i.e., methane. nitrogen, or flue gas) is injected, and as it travels throughthe reservoir, it vaporizes methane through LPG components from the reservoiroil. When the leading edge of the displacing gas front has vaporized sufficienthydrocarbons, it becomes miscible with virgin reservoir fluid. A similarmechanism occurs when CO2 is injected as liquid or critical fluid. However, highly compressed CO2 extracts heavier (gasoline-range) hydrocarbons from thereservoir oil, which allows the displacement front to become miscible at lowerpressures than those required for the lean gas. In the condensing-gas-driveprocess, an enriched gas (containing hydrocarbons heavier than methane) isinjected, and as it travels through the reservoir, it gives up heaviercomponents to the oil. When the oil becomes sufficiently enriched, it becomesmiscible with freshly injected enriched gas. Both types of multicontactmiscible displacements require a transfer of hydrocarbon components between theinjected and the reservoir fluid under dynamic conditions.

Measurement of Miscible Fluid Conditions
Miscibility between the reservoir oil, solvent, and gas is a function of thecomposition of these fluids and the pressure and temperature in the reservoirduring the displacement process. At a given reservoir temperature, the pressurerequired to achieve miscible displacement can be estimated from publishedcorrelations. First-contact miscibility can be determined experimentally with ahigh-pressure visual cell. The reservoir oil and solvent are combined in thecell at reservoir temperature. First-contact miscibility is achieved at thepressure when the interface between the fluids disappears.
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