Reservoir Fluid Characteristics

Tracers have been used to label fluids in order to track fluid movement and monitor chemical changes of the injected fluid. Radioactive materials are one class of commonly used tracers. These tracers have several drawbacks. One drawback is that they require special handling because of the danger posed to personnel and the environment. Another drawback is the alteration by the radioactive materials of the natural isotope ratio indigenous to the reservoir— thereby interfering with scientific analysis of the reservoir fluid characteristics. In addition, the half life of radioactive tracers tends to be either too long or too short for practical use. 

For resenroir engineering calculations various properties of the crude oil and its associated gas and water must be known. It will be shown that theoretically many of these properties could be calculated by the methods presented in previous chapters, provided the composition of the system is known and complete equilibrium constant data for all of the components are available. However, since this information is seldom at hand, values of the reservoir fluid characteristics are usually experimentally determined or approximated by methods that experience has shown to be sufliciently accurate for most engineering computations. 

The Two-Phase Formation Volume Factor (u). In reseiwou engineering calculations it is sometimes convenient to know the volume occupied in the reservoir by one stock tank barrel of oil plus the free gas that was originally dissolved in it. This volmne is known as the two-phase fomation volume factor and is ven the symbol u. It is apparent that the value of u is determined by the values of the reservoir fluid characteristics previously described. Expressed mathematically M is defined by the following equation... 

Fig. 87. Diagrammatic representation of apparatus for the determination of reservoir fluid characteristics.

To aid the reader to understand the full significance of the reservoir fluid characteristics presented in the previous chapter, they will now be used to develop several important reservoir equations. These equations are frequently used in reservoir analysis calculations to predict the behavior of a reservoir at any time in its future life. 

A little consideration will show that each term in equation 1 can be written in terms of the reservoir fluid characteristics as indicated below. 

Equation 3 is known as the material balance equation for a constant volume reservoir with no initial gas cap and is a relation between the quantities of fluids in the reservoir and those produced. If the reservoir fluid characteristics are known or can be estimated the volume of oil remaining in the resenroir after a given production interval can be calculated, as is shown in the following example. 

Examfu . a constant volume reservoir with no gas cap had an original pressure of 2500 psia. The reservoir temperature is 150° F. 10 barrels of stock tank oil and 10 S.C.F. of gas were produced by the time the pressure had dropped to 2000 psia Calculate the initial oil in place and oil remaining in the reservoir after this production interval. The reservoir fluid characteristics have the following values at the original reservoir pressmn and at 2000 psia. 

Example. A constant volume reservoir without an initial gas cap has an original pressure equal to the saturation pressure of 2500 psia and a temperature of 180° F. The connate water saturation is 20% and no water is produced. There are 56 X 10 stock tank barrels of oil originally in place. If the reservoir fluid characteristics are those given in Table 12, and the values of Kg/Ko as a function of oil saturation are those given in Figure 102, calculate R and AN as a function of pressure. 

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