PHASE ENVELOPE CONSTRUCTION FOR MIXTURES USING HIGHLY ACCURATE HELMHOLTZ ENERGY EQUATION OF STATE

Phase envelope construction for mixtures is very important in the oil and gas industry. The most widely used model for phase envelope construction is Peng — Robinson equation of state (PR-EOS) due to its simplicity. In order to construct phase envelopes of natural gas, a highly accurate Helmholtz energy equation of state GERG-2008 was proposed. In this work, the accuracy of phase envelopes calculated by a highly accurate equation of state GERG-2008, simplified GERG-2008 and traditional cubic PR-EOS was analyzed. The pressure-based algorithm is used to calculate phase envelopes. Phase envelopes of the methane—ethane mixtures were constructed and compared with reference data. The results show that phase envelopes can be constructed with GERG-2008 in high accuracy. PR-EOS has good accuracy in phase envelope construction under low pressure. The simplified GERG-2008 also works well under low pressure; however, as pressure increases, it performs worse than the simpler PR-EOS, especially in the vicinity of critical point. Besides, a modified density solver for the complicated GERG-EOS was proposed. Calculations show that the proposed density solver can provide reliable results.