Status of CO2 and Hydrocarbon Miscible Oil Recovery Methods
Authors: Holm L.W. 
Published: December 2007

In this state-of-the-art review, the basic procedures for applying CO2 and hydrocarbon miscible oil recovery, processes are explained. Some principal field tests and commercial applications of the processes are presented with a current assessment of the miscible flooding techniques.

Introduction
From studies of displacing oil from reservoir rock by water or gas, we have teamed that a portion of the oil is left as residual when immiscible conditions are present because interfaces are formed by capillary and viscous forges. Eliminating the interfaces allows complete displacement of the oil. Early in the search for ways to completely displace oil from rock, the idea of washing porous media with several volumes of solvent was porous media with several volumes of solvent was technically feasible but, because of the amount of solvent required, it was economically unattractive. The potential for achieving attractive economics was first reported potential for achieving attractive economics was first reported by Whorton and Kieschnich; they found that natural gas at sufficiently high pressures would miscibly displace crude oil. Actually, a miscible solvent was extracted by the injected gas from the crude being displaced.
Miscible displacement has been described as the displacement of crude oil from pore space in a rock using a solvent action that prevents formation of interfaces between the driven and the driving fluids. Koch and Slobod and Hall and Geffen showed that a slug of solvent could be driven through a porous medium by a less valuable driving agent; this technique permits small quantities of the expensive solvent to be used to remove large quantities of oil. In the laboratory, ratios of about 10 bbl of oil per barrel of solvent were achieved. Since this discovery, both laboratory and field tests have shown why this flooding technique may not be economical; maintaining the integrity of the slug in irregular porous rock and under otherwise hostile reservoir porous rock and under otherwise hostile reservoir conditions has been the main problem. Both longitudinal and transverse mixing occurs to disperse the small slug. Gravity segregation of fluids, unfavorable viscosity effects, and sand permeability variations tend to disperse it and to prevent it from contacting a large portion of the reservoir sand. Also, dead-end pores and water-shielded pore spaces trap part of the solvent, thereby deteriorating the slug. All these work to dilute the miscible slug and reduce its displacement efficiency.
This paper reviews (1) various miscible recovery methods that have been proposed, (2) problems associated with applying each method, (3) improvements developed to overcome the problems, (4) past and current field applications, and (5) current appraisal of miscible recovery.

The Basic Process and Variations
The basic miscible displacement processes described here use such displacing fluids as LPG (propane) - gas; enriched (condensing) gas; high-pressure (vaporizing) gas; carbon dioxide; and mutual solvents.

LPG-Gas
The LPG, or propane slug, process illustrates the miscible oil recovery technique. The process consists of injecting a small slug of LPG or propane as a liquid and driving it through the reservoir with less expensive natural gas (Fig. 1).
JPT
P. 76