Pressure controlling method for managed pressure drilling with supercritical carbon dioxide as the circulation fluid

Heat transfer along the wellbore was analyzed, and then a closed mathematical model, which fully couples the hydrostatic pressure, temperature, physical properties of CO2 and friction, was established to keep bottom-hole pressure constant during drilling process. Based on the pressure profile in wellbore achieved for a certain surface back pressure, a pressure controlling method for managed pressure drilling with supercritical carbon dioxide was presented. The influences of mass flow rate, well depth and inlet temperature on the annulus pressure profile and surface back pressure were investigated. The results show that, the pressure profile is almost in linear correlation with well depth in the annulus, which provides convenience for well control. The needed back pressure (applied by surface choke) decreases with increasing mass flow rate and decreasing well depth. The impact of inlet temperature on the annulus pressure profile, surface back pressure and flow friction is negligible. It also shows that the density of CO2 increases significantly and abruptly at a critical pressure. It is suggested that the storage pressure of CO2 in surface tank be larger than the critical pressure for a certain temperature.