Emerging challenges in phase behavior modeling of reservoir fluids at high-pressure high-temperature (HPHT) conditions

Abstract

Global energy demand is driving the oil and gas industry to explore uncharted areas leading to unconventional reservoirs at extreme temperature (T > 420 K) and pressure (P > 68 MPa) conditions where widely used equations of state (EOS) models fail to accurately predict properties of reservoir fluids and model their phase behavior. This work compares promising and adaptable EOS models for HPHT systems, highlighting their concomitant shortfalls in specified scenarios which need to be addressed by researchers in the quest for accurate predictive tools at extreme T and P reservoir conditions. Five EOS models were used for density prediction for n-heptane at 323 K and 423 K over a pressure range of 28–270 MPa where PC SAFT emerged the overall best. At 520 K however, VT PR-EOS and VT SRK-EOS performed better. For a binary system of C3/nC10, PC SAFT, which was highly promising, began to drop in accuracy with increase in temperature from 277 to 510 K. Furthermore, four EOS were tested for volume and z-factor prediction of pure systems (C1–C6) and their binaries. While PC SAFT is most promising, significant drawbacks are evident when applied to binary systems and are expected to worsen with increase in number of constituents. It was made clear that a pressure-dependent correction factor will significantly improve the accuracy of the PC-SAFT model. Suggestions on novel alternative routes for EOS model development and improvement are also given.