A Method for Predicting Pressure Maintenance Performance For Reservoirs Producing Volatile Crude Oil

Abstract

Published in Petroleum Transactions, AIME, Vol. 213, 1958, pages 59–64. Paper presented at 32nd Annual Fall Meeting of SPE in Dallas, Texas, Oct. 6–9, 1957. Abstract When dry gas is injected into a reservoir containing a volatile crude oil, a significant amount of the reservoir liquid phase may become vaporized. The resultant rich gas phase, when subsequently produced, contributes to tank oil production. This contribution assumes greater importance, the more volatile the oil involved. Oil recovery may be substantially greater than that predicted by conventional frontal-drive methods, which do not consider the vaporization equilibrium between the reservoir oil phase and the injected gas. A calculation method has been developed to account for vaporization of the reservoir liquid phase during gas-injection operations, and for the tank oil production which results from this factor. Recovery performance calculations are presented for a reservoir containing a highly volatile oil. Tank oil recovery is calculated to be about twice that predicted by the use of the conventional frontal-drive equations. In contrast to usual pressure maintenance performance results, in which the gas-oil ratio rises at an increasing rate after gas breakthrough, the predicted gas-oil ratio rises rapidly to about 12,000 scf/bbl and then rises much less rapidly. During gas injection, most of the reservoir liquid phase contacted is evaporated by the dry injection gas. The gas-oil ratio during this period is dependent upon reservoir pressure. The higher the operating pressure, the lower the gas-oil ratio. The predicted behavior is in accordance with laboratory PVT tests made to simulate the vaporization behavior. In addition to recovery performance predictions, results of the calculation procedure include complete wellstream composition data of value in the design of gasoline plant facilities often used in connection with gas-injection operations.