The Effects Of Injection Pressure On Condensing Gas Drive Recovery
Authors: Lofton Larry Keith , Morse Richard A. 
Published: December 2007

Abstract
This study presents criteria for developing an efficient condensing gas drive. Results are based on computer model simulation of a vertical, undersaturated reservoir with gas being injected into the crest and oil being produced from the base of the structure. Fractional oil recovery at gas breakthrough proved to be less sensitive to changes in oil withdrawal rates as the gas injection pressure was increased.
The validity of the model was established by accurately simulating several low-pressure gas drives conducted in the laboratory. Oil recoveries at gas breakthrough using the model compared closely with those recoveries determined in the laboratory and calculated by using the Buckley-Leverett fractional flow theory.

Introduction
Gas drive can be a very efficient scheme of improved oil recovery where gravity forces play an important part in oil production from a reservoir. For example, very high oil recovery can be achieved at low production rates through crestal gas injection from pinnacle reefs or other structures with significant relief using gravity drainage. In 1950, Whorton and Kieschnick introduced the concept of high-pressure gas injection. They stated that separator gases injected at high pressures improved oil recovery over low-pressure gas injection. Solubility of the gas in the oil was varied by operating at different reservoir pressures. Stone and Crump, however, showed that gas injection could improve oil recovery for a constant pressure by changing the gas composition, thus increasing the solubility. This process of enhanced oil recovery by soluble gas injection is called condensing gas drive.
Maximum use of gravity will result when gas is injected into the crest and oil produced from the base of a vertical reservoir. Terwilliger et al. demonstrated that fractional oil recovery at gas breakthrough is inversely proportional to the oil withdrawal rate. In other words, any attempt to increase rates will decrease oil recovery at gas breakthrough. Results were determined by a laboratory investigation using displacement by a gas with essentially no solubility in the displaced liquid.
The purpose of this study is to determine how gas injection pressure for a condensing gas drive affects the relationship between withdrawal rate and fractional oil recovery at gas breakthrough. Included in the investigation are explanations of recovery differences using soluble as compared with nonsoluble gas injection. The study was restricted to a vertical, undersaturated reservoir, simulated with a one-dimensional, two-hydrocarbon-phase computer model having variable bubble-point capability.
In order to substantiate simulation results, it was necessary to establish the validity of the computer model. A condensing gas drive was set up in the laboratory. The properties of the porous medium and fluids used in the laboratory were defined precisely in order that the experimental gas drive precisely in order that the experimental gas drive could be simulated with the model. A fluid system that had a very high gas solubility at low pressures was desirable for laboratory work. Freon R-12 gas and decane were used for this reason. The gas drive was conducted in the laboratory for several gas injection pressures and oil withdrawal rates. Using the same pressures and rates, the gas drives were simulated with the computer model. Decane recovery from the porous medium at gas breakthrough in the laboratory was compared with recovery obtained from simulation techniques. Using laboratory properties, the Buckley-Leverett analytical theory for fractional flow also was used to evaluate decane recovery at gas breakthrough.

PROCEDURE
Solubility effects of gas injection into a vertical, undersaturated reservoir were analyzed with respect to oil withdrawal rates, gas injection pressures, and fractional oil recoveries at gas pressures, and fractional oil recoveries at gas breakthrough.