Dynamic kill modeling of subsea CO2 and CH4 blowouts: Differentiating factors and their implications for offshore carbon sequestration

This study focusses on understanding the physics of halting subsea CO₂ blowouts using dynamic-kill methods. A computational fluid dynamics (CFD) model that replicates the thermophysical properties of CO₂ is used. We also analyze analogous CH₄ scenarios, juxtaposing the impacts of both fluids.

We simulate sixteen blowout/dynamic-kill scenarios, representing all combinations of water depth (305 or 762 m), blowout rate (2.83 or 11.34 MMm³/d), relief well intercept depth (1220 or 1830 m TVD BML), and reservoir fluid (CO₂ or CH₄). This defines a sufficiently broad simulation space for gaining insight into the heretofore undefined aspects of dynamically killing subsea CO₂ blowouts – and how they differ from CH₄ blowouts.

CO₂‘s thermophysical properties generally reduce dynamic kill pumping rates to one-third of that required for CH₄ blowouts when using 1801-kg/m³ kill fluid. CO₂ phase change at elevated pressures drives most of the difference. During a dynamic kill, increases in well pressure can cause CO₂ density to jump from 68 to 904 kg/m³, while CH₄ exhibits a modest 22 to 192 kg/m³ range. This radical difference in density-vs-pressure behavior results in CO₂ blowouts being generally easier to kill than CH₄ blowouts. Other differences in the dynamic killing of CO₂ and CH₄ blowouts, like multiphase flow behavior, are detailed in the paper to explain their impacts.

We also find that CO₂ blowouts in shallower water can generate sub-freezing temperatures at the wellhead, resulting in ice and/or hydrate formation – a result that can introduce unintended complexity into the overall response to a subsea blowout.