In the differential liberation process, the solution gas that is liberated from an oil sample during a decline in pressure is continuously removed from contact with the oil, and before establishing equilibrium with the liquid phase. This type of liberation is characterized by a varying composition of the total hydrocarbon system.
The experimental data obtained from the test include:
The differential liberation test is considered to better describe the separation process taking place in the reservoir and is also considered to simulate the flowing behavior of hydrocarbon systems at conditions above the critical gas saturation. As the saturation of the liberated gas reaches the critical gas saturation, the liberated gas begins to flow, leaving behind the oil that originally contained it. This is attributed to the fact that gases have, in general, higher mobility than oils. Consequently, this behavior follows the differential liberation sequence. The test is carried out on reservoir oil samples and involves charging a visual PVT cell with a liquid sample at the bubble-point pressure and at reservoir temperature. As shown schematically in above Figure, the pressure is reduced in steps, usually 10 to 15 pressure levels, and all the liberated gas is removed and its volume is measured at standard conditions. The volume of oil remaining VL is also measured at each pressure level. It should be noted that the remaining oil is subjected to continual compositional changes as it becomes progressively richer in the heavier components. The above procedure is continued to atmospheric pressure where the volume of the residual (remaining) oil is measured and converted to a volume at 60°F, Vsc. The differential oil formation volume factors Bod (commonly called the relative oil volume factors) at all the various pressure levels are calculated by dividing the recorded oil volumes VL by the volume of residual oil Vsc, or:
The differential solution gas-oil ratio Rsd is also calculated by dividing the volume of gas in solution by the residual oil volume. Relative total volume Btd from differential liberation as calculated from the following expression:
Btd = Bod + (Rsdb - Rsd) Bg
Btd = relative total volume, bbl/STB
Bg = gas formation volume factor, bbl/scf
The gas deviation z-factor of the liberated (removed) solution gas at the specific pressure and these values are calculated from the recorded gas volume measurements as follows:
V = volume of the liberated gas in the PVT cell at p and T
Vsc = volume of the removed gas at standard condition
Gas formation volume
It can be expressed by the following equation:
Bg = gas formation volume factor, ft3/scf
T = temperature, °R
p = cell pressure, psia
Tsc = standard temperature, °R
psc = standard pressure, psia
It should be pointed out that the differential liberation test represents the behavior of the oil in the reservoir as the pressure declines. We must find a way of bringing this oil to the surface through separators and into the stock tank. This process is a flash or separator process.
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