Evaluating Subsea Capping Stack Usage for CO2 Blowouts

Abstract Given the challenges posed by CO2's distinct thermophysical properties (i.e., its expansive behavior and attendant risks of causing hydrate, dry ice, and/or water ice formation), a dynamic multiphase flow simulator is employed to evaluate the effectiveness and suitability of using a subsea capping stack to respond to a CO2 well blowout. A dynamic multiphase flow simulator is used to investigate a capping operation for CO2 well blowouts. Following the typical sequence of a capping procedure and applying a soft shut-in, different primary bore sizes and choke line configurations are considered. Additionally, different reservoir flow rates, fluid types (CO2 and CH4) and water depths are investigated – all with the intention of understanding what differentiates a CO2 blowout from that of methane under varying conditions. This study reveals that significant hydrate formation on the exterior of a capping stack is most probable for shallow-water, high-rate CO2 blowouts – while the risk of dry ice is non-existent for all scenarios evaluated. Fluid temperatures can plummet to as low as 20° F across the capping stack's body and choke lines, suggesting frozen-water deposition on their exterior. In the context of the CH4 blowouts modeled, there's an absence of the temperature drop necessary to form significant hydrates on the capping stack's exterior. Further, while dimensions of the primary bore and choke lines certainly influence temperature drops, their overall impact is marginal when compared to CO2's supercritical phase transition. This paper delves into the underexplored realm of using subsea capping stacks for CO2 well blowouts, presenting new challenges and insights not widely discussed in existing literature. For engineers, the insights are useful, highlighting the differential risks associated with CO2 blowouts versus traditional hydrocarbon ones.