Well Control Considerations for CO2 Wells Based on Multiphase Flow Simulations

CCUS (Carbon Capture Utilization and Storage) is expected to be an important contributor to the ambition of reaching a net zero objective. Drilling and workover of wells to be used as CO2 injectors, pose a well control risk that needs to be properly understood and minimized. The paper will address the significant differences between CO2 wells and traditional hydrocarbon wells. Due to differences in phase behavior, both well design and operational procedures would be impacted and needs to be revised. Traditional well control procedures are not necessarily applicable in CCUS wells without modifications. A generic well was used to study the differences between a CO2 well and a hydrocarbon (methane) well with respect to design and procedural changes in case of a well control incident. A well control simulator capable of transient multiphase modeling of hydrocarbon and CO2 systems was used to evaluate the differences. Important operational and design limiting factors like volume, pressure and temperature responses, hydrate formation, phase transition as well as mitigation strategies and contingencies were evaluated. The model showed that with both SOBM and WBM, the CO2 influx entered the well in its supercritical form. Under static conditions, the CO2 remained in its supercritical form and showed no migration in SOBM, making its detection more challenging than for a methane influx. Driller's method was applied while simulating circulating the CO2 influx. For both mud types, the model showed greater liquid velocities compared to methane, which would reduce the time to react and put greater demands on surface equipment. The model also showed a greater drop in temperature than for a methane influx. Due to J-T cooling effect, the model shows that supercritical CO2 expansion near the surface can lead to significant temperature reduction, which in turn can lead to formation of hydrates and block the circulation. This effect also needs to be considered in the well design as the physical properties of the casings and surface pipes can be changed due to variation in temperature. Based on these results, measures to mitigate CO2 well control risks are discussed. This study will provide new knowledge regarding well control incidents related to CO2 wells. Improved understanding can be used to optimize operational procedures and potentially lead to new mitigation techniques.