Advanced simulation solves CO₂ kick risks in offshore CCUS Well Control

SLB^®’s advanced multiphase simulation revealed that CO₂ kicks in oil-based mud can cause extreme sub-zero temperatures and hydrate-formation risks, while water-based mud mitigates cooling and improves operational safety.

The study provided Chevron and Equinor with critical guidance for fluid selection and surface equipment design in carbon capture, utilization and storage (CCUS) wells. These insights enable safer well planning and operational reliability for future CCUS projects.

As CCUS projects expand globally, operators face new challenges when drilling through CO₂-saturated formations—where conventional well control assumptions that were developed for hydrocarbon systems no longer apply.

Chevron and Equinor’s primary objective was to understand the behavior of CO₂ kicks during circulation in a subsea environment—particularly regarding impact on choke systems and mud gas separators. The next step was to determine whether existing surface equipment designs are suitable under these unique thermodynamic conditions.

Previous well control models and operational practices were based on methane behavior, and failed to account for the distinct properties of CO₂, such as its strong Joule-Thomson cooling effect, phase transitions, and solubility differences in drilling fluids.

These gaps created uncertainty in predicting temperature drops, hydrate-formation risk, and separator performance, prompting Chevron and Equinor to seek a rigorous, physics-based evaluation to support safer well planning and operational decision making.

SLB deployed Drillbench™ XD dynamic drilling simulation software, integrating advanced multiphase flow simulation and thermodynamic modeling with the Olga™ dynamic multiphase flow simulator and a refined equation-of-state framework to accurately represent CO₂ behavior.

Eight controlled simulation scenarios were executed to assess the influence on wellbore and surface-system performance of:

• Kick type.
• Kick volume.
• The drilling-fluid system.
• Circulation rate.

A kick-intensity simulation-driven approach provided a validated, physics-based method to replicate transient multiphase flow, thermal behavior, and phase transitions associated with CO₂ kicks—none of which are captured by conventional hydrocarbon-based models.

The study revealed that CO₂ kicks circulated with oil-based mud produce extreme Joule–Thomson cooling downstream of the choke, with temperatures dropping to triple point at the mud-gas separator inlet in larger kick scenarios. These conditions create a high risk of hydrate and ice formation along the choke line and near-surface equipment.

By contrast, water-based mud buffered the cooling effect, maintaining temperatures near or above freezing and eliminating hydrate risk under equivalent conditions. Quantifiable results showed that methane-based kicks maintained stable thermal profiles above 60°F, while CO₂ kicks in oil-based mud produced temperatures below triple point at the choke, depending on kick conditions.

Adjusting the circulation rate had negligible impact on cooling, while increasing surface back-pressure reduced gas breakout but did not eliminate temperature-related risks.

Chevron and Equinor gained critical insight into the limitations of conventional mud gas separator design and well control practices for CCUS applications. The findings provide clear guidance that fluid selection—specifically the use of water-based mud—and pressure management strategies are essential to improving safety and operational reliability.

These learnings establish a foundation for updating well control procedures, refining surface equipment design, and planning future CCUS wells with greater confidence.