The paper highlights the limitations in the use of cubic equations of state (CEOS), for modelling reservoir behavior of liquid phase (black and volatile oil), in highly undersaturated reservoirs. This is important in high shrinkage volatile oils for which complex dependence of fluid behavior with composition is found.
Vaca Muerta formation fluids run from medium black oils to very dry gas, according to source rock maturity. High shrinkage volatile oil and very rich gas condensate are found and compositional variation is areally continuous. Despite this huge difference in fluid nature, where saturation pressures are present, more than 200 kg/cm2 undersaturation are reported.
Due to the complex dependence of fluid behavior with composition for near critical gas condensate and volatile oil, the use of compositional modelling is mandatory.
Reservoir pressure evolution with production and recovery calculations rely largely on compressibility, which itself is not a fitting parameter for usual CEOS. A very good fit for liquid volumes in CEOS, i.e. less than 5 % maximum error in volume, leads to unacceptable differences in compressibility (over 30 %).
To compare the consequences in production of this difference, a black oil model was run with different fluid scenarios: one with experimental compressibility and other two with compressibility obtained from CEOS. In the model, all formation, fracture and other fluid properties are the same, as well as the BHP imposed.
The results from numerical simulation show that rates and cumulative production from CEOS compressibility scenarios are half of those obtained with experimental compressibility. Part of this difference may be explained from fluid behavior alone, and the remaining is due to poral compressibility and fracture conductivities related to the different trends in pressure.
The CEOS are the only ones implemented up to now in the commercially available compositional numerical simulators. In this paper we show the limitation of these CEOS for the description of highly undersaturated volatile oil reservoirs.