Modeling of wellbore dynamics of a CO2 injector during transient well shut-in and start-up operations

A model of the wellbore dynamics for CO2 injectors may be essential for effective risk assessment and mitigation planning for CO2 injectors in CO2 geological storage and utilization projects. During well shut-in and start-up operations, wellhead pressure and temperature may change suddenly, which may induce icing and formation of hydrates in the injectors, especially when cold CO2 contacts water. This may also cause the cement or casing to crack. In this paper, the variations in pressure, temperature, and phase behavior of CO2 in the injector during well shut-in and start-up operations were predicted using a 1D+1D model that simulated liquid-gas phase changes of CO2, full well structures, and transient heat exchange with the surroundings. Simulation results suggest an “optimal shut-in period,” and the lowest temperature of CO2 would decrease even further with a shorter or longer shut-in period than the optimal shut-in period. Based on all results of this analysis, a rapid injection well start-up is recommended during well start-up operations. Finally, the maximum drop in temperature at the wellhead for various injection rates and temperatures within the range of most CCS demonstration projects was predicted.