A Practical Guideline for CO2 Mixture Modeling in Injection Wells

Abstract A practical and field-tested guideline for CO2 mixture modeling, mainly focused on injection wells but applicable to pipelines/flowlines is presented that describes the phase behavior, transport and thermal properties of CO2 mixtures for the optimal and economical Carbon Capture and Storage (CCS) design applications. Actual operating field data sets of flowing bottomhole pressure/temperature and wellhead pressure/temperature for 600 hours of stable operations are collected and used for calibration from seven CO2 injectors in the Gorgon project, Australia. Four Equations of State (Peng-Robinson 78A, CPA Infochem, GERG 2008 and EoS-CG) which are reported as conventionally used for CO2 modeling are tested and validated using commercial transient software. The study investigates the contributions from friction, hydrostatic, and acceleration terms in the estimation of pressure drop; overall heat transfer coefficient (conduction, convection, and radiation) and the Joule-Thomson coefficient in temperature estimation along the injection system. The results show that GERG-2008 and EoS-CG with a reasonable roughness factor (0.0018") give a good pressure match with the field measured pressures, while PR and CPA-Infochem show some discrepancies, even with a very small roughness factor (0.00039"). Analysis indicates that for CO2 mixture injection of 0.2 ~ 0.6 MTA (million tons per annum) in 4.5" tubing, the hydrostatic pressure drop is the main contributor to total pressure drop, while the frictional pressure drop contributes only 2 ~ 4%. All the above EOS's show good temperature matches (approximately 1 ~ 2 °C difference average) with field data when estimating overall heat transfer coefficient (U) from material heat properties and Joule-Thompson (JT) effect from the fluid PVT file – CPA Infochem calculates the most accurate temperature followed by PR, GERG, and EoS-CG in that order. The JT effect contributes approximately 70% of the total temperature change while the remaining 30% temperature change is due to heat transfer by conduction, convection and radiation. The JT effect is also validated with actual data across the choke proving all four EoS's accurately calculate the JT effect. This paper includes a practical engineering guideline for modeling phase behavior, transport, and thermal properties of CO2 mixtures for field applications, which would be beneficial for production engineers, reservoir engineers, and facility engineers in the optimal and economical design and operation of CCS facilities.