Introduction

The final reservoir type is the saturated oil reservoir and is distinguished by the presence of both liquid and gas in the reservoir. The material balance equations for undersaturated oil reservoirs, apply to volumetric and water-drive reservoirs in which there are no initial gas caps. However, the equations apply to reservoirs in which an artificial gas-cap forms, owing either to gravitational segregation of the oil and free gas phases below the bubble point or to the injection of gas, usually in the higher structural portions of the reservoir. When there is an initial gas cap (i.e., the oil is initially saturated), there is negligible liquid expansion energy. However, the energy stored in the dissolved gas is supplemented by that in the cap, and it is not surprising that recoveries from gas-cap reservoirs are generally higher than from those without caps, other things remaining equal. We will begin with a review of the factors that affect the overall recovery of saturated oil reservoirs and the application of the material balance used throughout the text. Drive indices, introduced in are revisited, as they are most applicable to these types of reservoirs and quantitatively demonstrate the proportional effect of a given mechanism on the production. The Havlena-Odeh method will be applied to provide a tool for early prediction of reservoir behavior, followed by tools to understand and predict gas-liquid separation. We will conclude with a discussion of volatile reservoirs and the concept of a maximum efficient rate (MER).