Olga Robust, Enhanced and Accurate CO₂ Handling (Olga CO₂ REACH)

Background
Transporting CO₂ by pipeline is the most established and widely used method. In 2023, 60% of the 67 projects in construction or operation are utilizing pipelines, and nearly 70% of the 300+ projects in development are planning to do the same (Global CCS Institute, The Global Status of CCS: 2023. Australia). Safe and cost-effective design and operation of pipelines rely on simulation tools like Olga.

The Olga CO₂ REACH project was initiated following an industry workshop on challenges related to transport and injection in CCS projects held in October 2020, with 35 users of Olga attending, representing 25 energy companies. The industry participants worked out a list of gaps in simulation capabilities. Improvements to enable robust and accurate simulation of multiphase CO₂ flow was considered the most critical gap to close, and it formed the basis for the Olga CO₂ REACH work scope.

Goal
The vision of Olga CO₂ REACH was to enable successful CCS projects through accurate and robust simulation of multiphase transport and injection of pure CO₂ and CO₂ dominated fluids.
The plan was to accomplish this through step change improvements to the multiphase flow simulator Olga.

Activities
The scope of Olga CO₂ REACH was grouped into four work packages:
Work package 1: Robust and stable simulations – Development of a new equation solver to improve numerical stability and accuracy.

Work package 2: Accurate thermal modelling – Introduction of a two-energy solver to account for separate thermal behaviours of gas and liquid layers.

Work package 3: Improved prediction of holdup, pressure drop, and flow regimes – Enhancements to the Olga flow model for better prediction of multiphase CO₂ flow.

Work package 4: Process equipment and specific flow simulations – Development of models for pumps, compressors, and directional leaks.

Four pre-commercial Olga project versions containing the outcomes of the project were delivered to the project partners.

Results
A major achievement was the development of a new implicit equation solver, tightening the coupling between pressure and enthalpy, which is critical for CO₂ systems with narrow phase envelopes. The new solver demonstrated superior stability and accuracy, particularly during rapid phase changes and transient scenarios like depressurization and shut-in with heating.

An implicit solver encompassing two energy conservation equations was developed to model distinct thermal behaviours of gas and liquid layers, especially during fast transients, where the gas and liquid layers are not in thermal equilibrium. The solver was validated against experimental data, showing improved accuracy in predicting energy exchanges and phase transitions.

Modifications to the Olga flow model improved predictions of flow patterns, pressure gradients, and liquid holdups. Validation against 904 experimental data points showed significant reductions in prediction errors (51% for pressure gradient, 31% for liquid holdup, and 18% for flow patterns).

An equilibrium model for pumps and compressors was developed, incorporating non-ideal gas and compressible liquid behaviour, particularly important near the critical point. A new model was introduced to simulate leaks in stratified flow regimes, accounting for the angular position, liquid level, and leak diameter. A patent for this functionality was filed.

Further Work
The new equation solver, with a tight coupling between pressure and enthalpy and utilizing one energy conservation equation, was made commercially available in Olga 2025.1. Subsequent updates are planned for Olga 2026.1.

Publications
1. N. Hoyer, Enable successful CCS projects through accurate and robust simulation of multiphase transport and injection of pure CO₂ and CO₂ dominated fluids, CLIMIT SUMMIT, 8th February 2023.
2. N. Hoyer, Performant CO₂ flow predictions with Olga for Drilling and Production, CLIMIT SUMMIT, 26th February 2025.
3. A. Brigadeau, A new solver for transient simulation of CO₂ multiphase flow in pipelines and wells, SPE workshop: Evolution of Flow Assurance – Learnings from the Past and Navigating Future Challenges, 29th – 30th April 2025, Kuala Lumpur, Malaysia.
4. R.H. Nemoto, A. Brigadeau, Z. Yang, L. Rønning, T. Vincent-Dospital, J. Ø. H. Bakke, Validation of a new transient solver for simulation of multiphase CO₂ flow in pipelines, 21st International Conference on Multiphase Production Technology, Lisbon, Portugal. MPT 2025.

Brigadeau, New CO₂ solver in the Olga simulator—background and improvements, webinar, 26. november 2024 (tilgjengelig på forespørsel).