Understanding the thermo-physical properties of CO2 mixtures and predicting them in an accurate and reliable fashion are essential to enable transport of CO2 via shipping. This transportation mode could have very different operating pressure and temperature conditions to the ones encountered in transportation via pipelines. Thus, it is important that thermodynamic modelling uses the appropriate Equation of State (EoS). Moreover, to ensure thermodynamic consistency, the basis for the thermodynamic modelling should take into consideration all the elements of full-chain and potential operating scenarios including offloading (direct injection) from ships to injection site, refrigeration and boil-off management, pumping and heating during unloading, etc. Here, impurities (particularly light gases) can have large impact, for example, substantially increasing the saturation pressure of the CO2 liquid making carrier transport infeasible due to the elevated bubble-point pressures at refrigerated temperatures. The aim of this paper is to propose a general-purpose thermodynamic modelling basis to describe the ship value chain for the CO2-rich fluid to be transported for direct injection to offshore wells.
The modelling framework consists of EoS capability in modelling range of impurities, EoS modelling accuracy comparison of thermo-physical properties relevant to CO2 shipping systems (density, solubility of water in CO2 and phase equilibria including solid-liquid-vapour three-phase locus), guidance on EoS selection and EoS availability in various software packages.
There is not a single EoS which is enough to accurately model all major, minor and trace impurities. The selection of an EoS model and the associated modelling options depends on the CO2 content specification, the shipping system conditions (Low Pressure, Medium Pressure or High/Elevated Pressure) and the range of impurities present. The EoS model selected across all thermodynamic, process and flow assurance software should be validated and fitted against experimental data as the same models implemented across different software platforms are not necessarily identical. This ensures that the thermodynamic calculation is consistent throughout the liquefication process, CO2 shipping and injection to storage site and maintain seamless transition from one software to another.
Moreover, it is found that there is a scarcity of data directly applicable to shipping conditions, as thermo-physical properties of carbon dioxide under liquid, refrigerated conditions, are expected to differ significantly from those typical of pipelines under gaseous or supercritical state. Although some EoS validation results are available in the gaseous and supercritical phase in many research reports that can provide an indication of the effect of certain impurities on thermodynamic properties, however they are unreliable to a certain extent for the planning of CO2 shipping operation. Under this context, in this work, we propose a thermo-physical property modelling framework, while identifying knowledge gaps and suggestions for future work and research.
