Pressure Maintenance Needed Here
Authors: Crawford Paul B. 
Published: December 2007

Abstract
Detailed studies were made of the effect of injecting gas into reservoirs of differing volatilities. Pressure maintenance may consist of reinjecting all or part of the gas produced, and this may be initiated at any stage of reservoir depletion. Reservoir oils with high formation volume factors respond to gas injection, and recoveries may be increased substantially depending upon rock properties and composition of the oil. Initiating a full-scale gas return at some intermediate stage in the depletion may show greater recoveries than partial pressure maintenance initiated at a higher pressure. Several studies are presented to determine the role of nitrogen on depletion performance of volatile reservoir fluids, and to determine the possibility of recovering additional oil by starting a pressure maintenance program early in the life of the reservoir.

Introduction
Reservoir oils with substantial quantities of light hydrocarbons have very high shrinkage. Even a small reduction in pressure below the bubble point may result in a 5 to 20 percent shrinkage, which is sufficient to permit a high GOR and rapidly deplete reservoir energy. If a program of pressure maintenance can be initiated at an early stage in the reservoir life, much higher oil recoveries can be expected.
Cook, Spencer and Bobrowski provided a method of estimating performance of highly volatile reservoirs using special laboratory analyses of reservoir oil samples. The method included calculating the volume and composition of hydrocarbon liquids recovered by processing produced gas in a natural gasoline plant.
Jacoby and Berry showed that properties of volatile reservoir fluid were such that conventional depletion calculations may no longer be valuable. Laboratory differential vaporization test procedures do not adequately represent the reservoir depletion process; in such cases the recovery of stock-tank oil per unit of pressure decline can be predicted only from reservoir compositional material balance and separator recovery calculations. Calculations by Jacoby and Berry indicated that for a volatile crude oil the compositional material balance method may indicate a tank oil recovery by primary depletion to be more than twice that predicted for ordinary black crude oils by conventional methods. Their paper emphasized the difference between equilibrium flash vaporization when all gas remains in contact with the oil phase, and differential vaporization when all gas is withdrawn as soon as it evolves. They pointed out that the actual vaporization sequence occurring in each reservoir is determined by the production process itself. They outlined a method for calculating fluid compositions for volatile-type reservoirs.
In 1956 Reudelhuber and Hinds described depletion of volatile oil reservoirs using compositional material balance techniques. They based their recovery calculations on laboratory depletion data. For pressure maintenance Jacoby and Berry presented a calculation method to account for vaporization of the reservoir liquid phase during gas injection operations and for the additional stocktank oil production resulting from this factor. Recovery performance calculations were presented for reservoirs containing highly volatile oil. Calculated tank oil recovery was about twice that predicted by using the conventional frontal drive equations. Most of the reservoir liquid phase contacted during gas injection is evaporated by the dry injection gas. The GOR during this period is dependent upon reservoir pressure. The higher the operating pressure the lower the GOR. Predicted behavior was in accordance with laboratory PVT tests simulating vaporization behavior.
Jacoby and Berry pointed out that gas injection into crude oil reservoirs displaces oil to the producing well and at the same time partially or fully maintains reservoir pressure. Oil shrinkage that would occur upon pressure reduction is thereby minimized or eliminated. They showed that volatile oil vaporizes when repeatedly contacted with dry gas, and it may be adequately predicted with multicomponent flash calculations that consider individual vaporization behavior of the normal hydrocarbons. They also indicated that conventional frontal drive methods are inadequate to predict recovery performance of volatile oil reservoirs being subjected to gas injection and that conventional frontal drive methods will underestimate total oil recovered by gas injection. Efficiency of oil recovery by gas injection into volatile oil reservoirs falls off with increasing rapidity with a decrease in operating pressure.
JPT
P. 1449ˆ