Simulating Flow of CO2 with Impurities in OLGA; Dealing with Narrow Phase-envelopes and the Critical Point

The dynamic multiphase flow simulator OLGA [1–4] uses pressure (P), temperature (T), and fluid composition, as independent variables, meaning that all thermodynamic and transport properties for the fluid are given as functions of P and T. Although this is a good approximation for typical multicomponent oil and gas systems, several problems are being faced using the same formulation for pure CO2 and for CO2 containing small amounts of impurities. A better approach is then to use pressure and enthalpy (H) as independent variables for thermodynamic equilibrium calculations and phase properties. This change in formulation is critical for accurate modelling of single component and near single component systems.A prototype P-H formulation has been implemented in OLGA as part of the R&D project CO2-VIP [5].The new calculation scheme is presented here, demonstrated through simple examples, and validated against experimental results. It has proven to give increased simulation accuracy and stability.