CO2 Two-Phase Flow Operation in a CCS Transport Pipeline Network: Can We Trust Existing Multiphase Flow Simulators?

The current wide-industry practice is that CO₂ transport via pipelines should be carried out in single-phase, either as gas, liquid, or supercritical state. The transport in any particular state has its pros and cons. However, the general rule-of-thumb is that is more efficient to transport CO₂ in liquid or supercritical form over gaseous phase. For optimal transport of CO₂ in supercritical/liquid phase, the minimum operating pressure should exceed the critical pressure for CO₂ mixture to avoid phase-splitting (two-phase flow). On the other hand, gas phase operation is an option at early life of the project when injecting into a depleted reservoir in order to minimize Joule-Thomson cooling. As pressure in the store increases, the system would then need to transition to high pressure operation to avoid entering into the two-phase region.

Conceptually, allowing two-phase flow operation during normal operation would offer certain advantages. It would potentially be the most cost-effective solution for the transition period and beyond (i.e. once gas phase operation is no longer possible). In this work, we present a conceptual study of an offshore transport system operating under two-phase flow regime for an industrial CCS hub located in North West England. The aim of the study is to try to answer two fundamental questions: Is the two-phase flow pipelines possible in a safe, manageable and predictable manner? Are there operational benefits in allowing pipelines to operate in a two- phase zone? In addition, we present a perspective on the current limitations of existing multiphase oil & gas flow models for CO₂ multiphase flow.