A robust drift-flux model for two-phase CO2 pipe flow

Abstract

CO2 transportation via pipelines and injection wells serves as the interface between carbon sources and storage sinks and thus plays a vital role in large-scale CCUS. CO2 is generally designed to be transported as dense-phase or supercritical fluids, but liquid-gas two-phase CO2 flow may occur during various scenarios such as pipeline start-up, shut-in, depressurization, capacity change, and subsurface injections. Despite the widespread existence of multiphase CO2 pipe flow during CO2 transportation, multiphase CO2 flow models are relatively scarce in the body of publicly available literature and existing models are only moderately accurate. In this work, we developed a robust drift-flux model capable of predicting the liquid holdup and pressure drop in two-phase pure CO2 pipe flow. The system of constitutive equations was solved by the marching method. The thermodynamic properties of liquid and vapor CO2 phases were determined by the highly accurate Helmholtz-type equation of state (EOS) developed by Span and Wagner. Supercritical CO2 friction correlation was employed to estimate the frictional pressure gradient of two-phase pure CO2 flow. A comprehensive verification against available two-phase CO2 flow loop test data shows that the model has superior performance over existing models in both horizontal and downward two-phase CO2 flow, with a maximum of 5% error in liquid holdup prediction and a maximum of 10% error in pressure drop prediction. This model may serve as a valuable tool for the design and optimization of CO2 pipelines in ongoing and future CCUS efforts worldwide.