In this Case Study, AFS examines the various challenges with modeling CO2-rich transport pipelines and provides a pathway for future modeling of these systems.
Major energy transition initiatives aiming to achieve net zero carbon targets have pushed the requirement to accurately model CO2 pipeline networks and solve flow assurance issues to the forefront. Specifically, Carbon Capture and Storage (CCS) captures carbon emissions and transports CO2 at elevated pressures through a pipeline to a permanent underground reservoir storage location. Current CO2 modeling challenges include Equation of State (EoS) selection, accurate representation of thermodynamic properties of CO2-rich fluids, and the limitations of current modeling methods.
AFS utilized its depth of knowledge in hydraulic analysis and Pressure Volume Temperature (PVT) modeling to perform a detailed study of CO2 projects and a literature review of CO2 modeling best practices, including:
– Quantifying the impact of contamination in CO2 and modeling of impurities in CO2. The addition of impurities to pure CO2 will result in a shift of the pressures and temperatures where phase changes occur due to differences in the vapor pressure of components (Figure 1).
– Reviewing state of the art EoS and how to apply to CO2 systems based on current technology. There is no ‘blanket’ solution for EoS selection and testing should be conducted with available EoS to best represent the fluid.
– Highlighting the limitations of the current EoS and modeling methods such as numerical instability around the critical point and the pathway for better predictions.
Figure 1: CO2 Phase Envelope with N2 Contamination
Caution must be exercised when performing CO2 thermodynamic and flow assurance modeling to ensure numerical stability. In particular, near the phase boundaries and the critical point, which is dependent on the carbon capture method and feed stream into the pipeline. Fluid behavior in these regions can change suddenly between gas-like and liquid-like, whereas traditionally, for oil and gas dominated networks, small variations in composition, pressure and temperature do not have as far-reaching implications for the safe and efficient operation of a pipeline. Therefore, having an accurate understanding of the CO2 impurities in the system is vital to ensure reliable hydraulic predictions.
Additionally, one of the ways to mitigate these potentially onerous modeling outcomes is that CO2-rich fluids should be defined thermodynamically with high-accuracy EoS such as GERG2008, which is the most widely adopted EoS in the CCS industry. However, sensitivity analysis should be performed with other available EoS to validate the system PVT behaviour.