For condensate reservoirs, experimental measurements are necessary due to drop-off liquid in the reservoir. As the liquid is dropped, the composition in the reservoir changes and so do the properties. It is difficult to predict these property changes without experimental data.
The most commonly conducted experiment for condensate reservoir is Constant Volume Depletion (CVD). The other two experiments that can be conducted for condensate reservoir fluid are Constant Composition Expansion (CCE) and Multistage Surface Separation (MSS). We will discuss these three experimental techniques. However, before discussing these techniques, it is worth remembering that unless we have a representative sample of the fluid, we may not be able to reproduce the behavior of the reservoir. This requires that we collect a sample during the early stages of production life and at a pressure above the dew-point pressure. Once the reservoir pressure or the bottom-hole pressure drops below dew-point, it is very difficult to collect a representative sample using either bottom-hole sampling or by recombining gas and liquid samples from the separator.
Constant Volume Depletion (CVD)
The Constant Volume Depletion (CVD) experiment is highly recommended for condensate reservoirs. The CVD experiment provides valuable information about the reservoir behavior.
The CVD experiment mimics the behavior in the reservoir. The experiment begins by collecting a representative sample and starting the experiment at initial reservoir pressure. From initial reservoir pressure until dew-point pressure, the reservoir remains under single-phase conditions. The volume of the pressure cell is maintained constant and, as the pressure is reduced, the excess volume due to expansion is removed. Once the pressure reaches dew-point, further reduction in pressure results in dropping of the liquid. Again,
the excess gas due to expansion is removed to maintain constant volume of the cell. The dropped liquid is never removed from the cell. The incremental removed gas at each step is measured and the composition is recorded. This experiment is repeated at several
pressure steps, typically at a decrement of 300 psi, until the standard conditions are reached. Except in the last step, where the temperature may be the standard temperature, throughout the process, the pressure is decreased at constant temperature. The dropped liquid as a percentage of cell volume is also measured and recorded.
In principle, the CVD experiment tries to mimic condensate reservoir depletion behavior. In reproducing the depletion behavior, we assume that the dropped liquid will never be produced at the surface. This assumption is very close to field observations for condensate reservoirs producing under pressure depletion only. Once the CVD data are collected, we can predict the reservoir production (gas and liquid) as a function of pressure. This information is valuable in understanding what percentage of gas and liquid is recovered at
the time of abandonment.
Constant Composition Expansion (CCE)
In the Constant Composition Expansion (CCE) experiment, a representative sample is collected in a cell at an initial pressure. The pressure is slowly reduced and the increase in volume at constant temperature is measured. The composition in the cell remains the same throughout the expansion process. The z-factor values are reported for pressures
above dew-point. CCE data are useful in calculating recoveries above dew-point. In addition, if a condensate reservoir is to be simulated using compositional simulation, it is important that data from the CCE experiment be matched to tune some of the parameters in the equation of state.
Multi-stage Separation Test (MSS)
The Multistage Separation (MSS) test is conducted for condensate reservoirs to provide a basis for recoveries as a function of reservoir pressure. This information is used along with CVD experiments. In MSS, the condensate reservoir sample at dew-point is flashed at first
separator conditions. The liquid from the first separator conditions is flashed at second separator conditions and so on until we reach the liquid at standard conditions. The separator conditions are based on the actual field conditions. The amount of gas collected at each separator, as well as the liquid collected in the last stage, is reported.
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