Gas condensates are often saturated with water at reservoir conditions. A PVT laboratory will remove the water from a gas condensate sample before carrying out any experimental studies. Measured liquid dropouts in routine CME or CVD experiments are therefore for the water free fluid.
The question is often brought up what impact it would have on the liquid dropout if the study was carried out on a gas condensate that was saturated by water as is often the case in the reservoir.
Wang et al.  have recently published a paper on the difference between the liquid dropout from a water free gas condensate and from a gas condensate saturated with water. The composition of the water free gas condensate (1) is shown in Table 1. This gas condensate fluid was contacted by an excess amount of water. After equilibrium was reached, the gas phase had the composition (2) in Table 1. An H2O amount of 0.67 mol% had been dissolved in the gas phase, but that was not the only change happened to the gas phase composition. The concentration of gas components (N2, CO2, C1, etc.) had decreased while the concentration of components heavier than C7 had increased. The gas condensate had lost some of its lighter components to the aqueous phase as can also be seen from the aqueous phase composition in Table 1. The loss of light gas components had reduced the GOR of the hydrocarbon phase from 3,000 to 2,879 Sm3/Sm3. CME and CVD experiments were carried out the original reservoir fluid (1) and on the water saturated gas condensate (2) after it had been separated from the free water phase. Because the latter gas condensate had been stripped for some of its lighter gas components, its liquid content had increased and so had the liquid volume percent of the dew point volume. This does not mean that a water saturated gas condensate will produce more liquid than a water free gas condensate. It simply means that removal of gas components increases the liquid fraction and thereby the volume percent liquid dropout in a pressure depletion experiment.
|Component||Water free Gas condensate (1)(mol%)||Gas condensate with water (2)(mol%)||Aqueous phase(mol%)|
Table 1: Fluid compositions considered
CME experiments for the water free gas condensate (1) and the gas condensate with water (2) in Table 1 have been simulated using PVTsim Nova 4.2. The resulting liquid dropouts are shown in Figure 1. Also shown is the simulated liquid dropout of the gas condensate that has been in touch with water (2) from which the H2O has been removed. As can be seen the presence of water (0.67 mole%) has very limited impact on the liquid dropout. The reason for the fairly large deviation between the liquid dropouts of the two gas condensates in Table 1 is that light gas components have been removed from the one that has been contacted by water. That has decreased the GOR and a higher fraction of the fluid will condense upon pressure depletion.
Figure 1: Liquid dropout from gas condensate compositions in Table 1, and from the one which has been contacted by an excess amount of water after removal of H2O.
An experimental program like the one described above does not give a correct picture of how water dissolved in a gas condensate at reservoir conditions will impact the amount of liquid formed by pressure depletion. The water phase beneath the hydrocarbon zone in the reservoir is saturated by gas. It will therefore not lose any gas components to the water phase.
A PVT laboratory could reproduce the amount liquid dropping out from a water saturated gas condensate in the field by following the below procedure:
- A gas condensate sample not to be used in PVT experiments is contacted by an excess amount of water to saturate the water by gas.
- The water phase from 1. is contacted with the gas condensate sample to be used in the PVT experiment. A small fraction of the water will dissolve in the gas phase. Since the water phase is already saturated by gas, no gas will be lost to the water phase.
- The water phase is discharged from the fluid in 2. and the PVT experiment carried out for the gas.
Such experiment will show that the water dissolved in the gas phase has practically no impact of the amount of hydrocarbon liquid dropout as illustrated by the orange and green lines in Figure 1.
Fortunately, Wang et al. have given enough experimental details to conclude that the higher percentage liquid dropout from the fluid with water was not caused by the presence of water in the gas phase. It was caused by the free water phase having stripped the gas for some of its lighter components. With fewer details, the data could easily have been misinterpreted and the erroneous conclusion reached that water will increase the liquid dropout from a gas condensate.
- Wang et al., Fluid behavior of gas condensate system with water vapor, Fluid Phase Equilibria 438, 2017, pp. 67-75.