The first recorded deliberate attempt to stimulate recovery from an oil reservoir by hydrocarbon gas injection was in the Macksburg field, Washington County, OH, ' long before water injection was used for secondary recovery purposes. For almost 60 years, most secondary recovery projects included some form of immiscible gas injection, and its use continued even after the advent of new methods and materials. In spite of this, it was the late 1940's before serious attempts were made to develop quantitative techniques for describing reservoir performance under gas-injection operations, especially with regard to depleted oil reservoirs. Before then, such efforts were directed primarily toward describing the water displacement process. As a result, techniques used to describe the performance characteristics of immiscible gas injection consist of modifications to methods originally developed for describing performance of water-injection operations, even though there is a fundamental difference in the basic displacement mechanisms of the two fluids.