In the recovery of oil from oil bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. These supplemental techniques, commonly referred to as "enhanced oil recovery", involve the injection of a fluid, or series of fluids, into the reservoir through an injection system comprised of one or more wells. As the injected fluid moves through the reservoir, it acts to displace the oil therein, to a production system composed of one or more wells through which the oil is recovered.
One type of enhanced oil recovery procedure involves miscible flooding in which an oil miscible solvent is injected into the formation. The solvent is forced through the formation by a suitable driving fluid, which may be gaseous or liquid depending upon the nature of the solvent, and acts to displace the oil to the production system. The miscible displacement mechanism may involve "first contact miscibility" or "conditional miscibility". In first contact miscibility, the miscible displacing agent normally takes the form of a light hydrocarbon slug such as pentane, propane or butane, or mixtures of such hydrocarbons, commonly termed "liquified petroleum gas" (LPG). The injected liquid hydrocarbon is capable of forming a single phase with the reservoir oil at the reservoir conditions immediately upon contact therewith. In conditional miscibility, a miscible transition zone is formed in the reservoir as a result of repeated contact of the reservoir oil by an injected high pressure gas. The transition zone of conditional miscibility may be formed of light hydrocarbons present in injected gas, referred to as enriched gas, or light hydrocarbons may be stripped from the reservoir oil by an injected lean gas.