Fundamentals of Fluid Flow in Porous Media


Chapter 5

Miscible Displacement

Fluid Properties in Miscible Displacement: Factors Affecting Displacement Efficiency

Displacement efficiency at microscopic (pore level) and macroscopic level in a miscible displacement process are less than 100%. The magnitudes of efficiencies depend on a number of factors, including whether a displacement is secondary or tertiary recovery process.

Microscopic Displacement Efficiency

In a miscible displacement conducted as a secondary recovery process (assume there is not mobile water in the system), the IFT between displaced (oil) and displacing (solvent) phases is zero. According to the Capillary number (Nc) definition, Nc become infinity as IFT goes to zero:
Capillary Number (Nc) become infinity as IFT goes to zero

The residual saturation in the portion of the rock contacted by the displacing phase should be essentially zero as Nc → ∞.

Experimental studies of first contact miscibility process show that the residual saturation of the displaced phase is very small when the solvent continuously is injected. However, when the solvent is injected as a small primary slug followed by a secondary slug, recovery can be poorer as a result of dispersion and mixing of different slug materials

[1]. Significant mixing can result in loss of miscibility at either leading or training edge of the primary slug displacement. Laboratory studies of MCM processes have shown that recoveries are somewhat poorer than for FCM processes. There are different reasons for the reduced recoveries at the microscopic level in MCM processes. One is that a finite distance of travel is required in the process before miscibility is achieved. In a vaporizing gas drive process a small amount of the liquid phase drop out can occur as a result of mixing effects when compositions are in the vicinity of the bimodal curve. Bypassing in the flow process at the microscopic level owning to small-scale heterogeneities or dead-end pores can cause mixing. The mixing process can result in overall compositions that are within the two phase region, which would lead to the trapping of a residual liquid phase, although at a small saturation. Mass transfer by dispersion in a displacement process also can lead to reduced displacement efficiency. Dispersion causes the composition path that occurs during a displacement to move into the multiphase region of the phase behavior diagram. This results in formation of a liquid phase which remains as a trapped phase because of its low saturation. However a small part of the trapped liquid could be re-vaporized to the flowing gas phase, but the recovery by a re-vaporization process occurs at a relatively slow rate. Gardner et al[2] showed that the higher the dispersion level, the poorer the calculated recovery efficiency. They concluded that displacement efficiencies achieved in their slim tube experiments at the pressure well above the MMP were limited by the levels of dispersion in the experiments. Dispersion phenomenon will be explained separately in the following chapter.


[1] “Miscible slug process”, Koch, Jr., Slobod, R.L., Trans.., AIME (1957) 210, 40-47

[2] “The Effect of Phase Behavior on CO2 Flood Displacement Efficiency”, Garder, J.W., Orr Jr., F.M., and Patel, P.D., JPT, Nov. 1981


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