A Numerical Assessment of Carbon-Dioxide-Rich Two-Phase Flows with Dense Phases in Offshore Production Pipelines

Abstract

SummaryOne-dimensional numerical simulations of carbon dioxide (CO2)-rich crude-oil flows were performed with a commercial simulator for a typical offshore production pipeline under steady-state scenarios. Mixtures with 20–50 mol% CO2 and gas/oil ratio (GOR) of 300–600 std m3/std m3 were thermodynamically modeled with the predictive Peng-Robinson (PPR78) equation of state (EOS) (Robinson and Peng 1978; Jaubert and Mutelet 2004), and fluid properties were tabulated in pressure/volume/temperature (PVT) lookup tables. Thorough analyses on the separate CO2 and GOR effects on several flow parameters (e.g., temperature drop, pressure gradient, and flow patterns) were performed. The occurrence of the simultaneous flow of liquid and an ambiguous dense phase was quantified and discussed in depth. The properties of those phases [e.g., Joule-Thomson coefficient, viscosity, interfacial tension (IFT), and gas/liquid-density ratio] along the pipeline for several mixtures and operational conditions were addressed as well. It was seen that the dense phase can be a problem for phase-identification criteria, which can affect the flow-simulation results. This was further analyzed in simple cases of horizontal and vertical flows of CO2-rich crude-oil mixtures, under key temperature/pressure conditions. Finally, comparisons were performed between the holdup and pressure-gradient results of those cases, obtained with different liquid/liquid- and gas/liquid-modeling approaches of a hydrodynamic point model of a commercial simulator.