PVT reports often lists separator corrected Gas/Oil Ratios (GORs), Gas and Oil Formation Volume Factors (FVFs), etc. together with measured Differential Liberation (DL) and Constant Volume Depletion (CVD) data. The separator corrected black oil properties listed in PVT reports are calculated using empirical correlations.
But, what does separator corrected black oil properties stand for and why are they useful?
Figure 1 sketches a DL PVT experiments in a PT diagram. The Differential Liberation GOR is the volume at standard conditions of the gas released from the subsequent pressure stages divided by the volume of the residual oil. If the oil from one of the DL stages was instead flashed through a separator train, the GOR would be different, and that is the separator corrected GOR. It is also called surface GOR and gives a better idea of the ratio of produced gas and liquid at standard conditions than the DL GOR.
Figure 1 Differential Liberation process in a PT diagram
Among the updates in PVTsim Nova 2 released in November 2015 is the option to output separator corrected DL and CVD data. PVTsim does not use empirical correlations, but calculates the separator corrected black oil properties from single or multi stage separator simulations.
Figure 2 shows a sketch of a CVD experiment, which experiment can be seen as a “laboratory tank model” of a gas condensate reservoir. The gas depleted from the experiment will have approximately the same composition as the well stream produced from a gas condensate reservoir at the same pressure. The liquid phase found in the cell at pressures below the saturation pressure reflects the condensed liquid in a depleted reservoir and this liquid is unlikely to be produced. That results in a decreasing condensate-gas-ratio (CGR) of the produced fluid with time. A separator simulation on the depleted gas phase as sketched in Figure 2 will give an approximate idea about the produced CGR as a function of reservoir pressure.
Figure 2 CVD experiment with separator correction of gas phase properties.
Table 1 shows the standard output from a CVD simulation in PVTsim.
Table 1 Standard CVD simulation output
Table 2 shows the separator corrected CVD data comprising total density, liquid GOR and FVF, gas phase FVF (or Bg) and produced CGR.
Table 2 Separator Corrected CVD simulation data
As can be seen from the plot in Figure 3 the CGR has dropped to around 50% of its original value when the reservoir has decreased from the saturation point of 5520 psia to around 3000 psia. That means the liquid production at 3000 psia is only half of the CGR when production started.
Figure 3 CGR dependence on reservoir pressure