The bubble point pressure is determined by an experiment called “Constant Composition Expansion” (CCE), also called: “flash liberation”. The device used to perform this experiment is the PV cell, as shown and described in the article “Constant Composition Expansion “. The Differential Liberation (DL), discussed in this article, is experimentally performed in a similar PV cell.
The main difference between these two types of experiments is that in the Constant Composition Expansion (or flash expansion) no gas is removed from the PV cell. But instead, the gas remains in equilibrium with the oil. As a result, the overall hydrocarbon composition in the cell remains unchanged.
In the differential liberation experiment, however, pressure gradually decreases in steps and any liberated gas is removed from the oil. All depletion stages are performed at the same reservoir temperature. Therefore, there is a continual compositional change in the PV cell, the remaining hydrocarbons becoming progressively richer in the heavier components, and the average molecular weight thus increasing.
The differential liberation experiment starts at the bubble point pressure determined from the CCE (since above this pressure the flash and differential experiments are identical).
The following example guides you on how to use and interpret the data from Differential Liberation test. The reservoir temperature is T= 200 °F and the bubble point pressure is 3330 psia.
The essential data obtained from the differential liberation experiment, performed on the same oil sample as CCE test, are listed in the following table: