2021-09-20 08:26:34
Constant Composition Expansion (CCE):
For an oil sample, the CCE experiment is used to determine bubblepoint pressure, undersaturated-oil density, isothermal oil compressibility, and two-phase volumetric behavior at pressures below the bubble point.
Table 6.9 presents data from an example CCE experiment for a reservoir oil.
Fig. 6.3 illustrates the procedure for the CCE experiment. A blind cell (i.e., a cell without a window) is filled with a known mass of reservoir fluid. Reservoir temperature is held constant during the experiment. The sample initially is brought to a condition somewhat above initial reservoir pressure, ensuring that the fluid is single phase. As the pressure is lowered, oil volume expands and is recorded. The fluid is agitated at each pressure by rotating the cell. This avoids the phenomenon of supersaturation, or metastable equilibrium, where a mixture remains as a single phase even though it should exist as two phases. Sometimes supersaturation occurs 50 to 100 psi below actual bubblepoint pressure. By agitating the mixture at each new pressure, the condition of supersaturation is avoided, allowing more accurate determination of the bubblepoint.
Just below the bubblepoint, the measured volume will increase more rapidly because gas evolves from the oil, yielding a higher system compressibility. The total volume, Vt, is recorded after the twophase mixture is brought to equilibrium. Pressure is lowered in steps of 5 to 200 psi, where equilibrium is obtained at each pressure.
When the lowest pressure is reached, total volume is three to five times larger than the original bubblepoint volume.
The recorded cell volumes are plotted vs. pressure, and the resulting curve should be similar to one of the curves in
Fig. 6.4. For a black oil (far from its critical temperature), the discontinuity in volume at the bubblepoint is sharp and the bubblepoint pressure and volume are easily read from the intersection of the p-V trends in the single- and two-phase regions.
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