CO 2 reservoirs and high CO 2 content gas reservoirs in marine sediments generally have complex pressure system, which usually encounters supercritical CO 2 (SC‐CO 2 ) invasion into the wellbore during drilling process. The wellbore transient flow and phase transition along the wellbore during the drilling process has not been thoroughly investigated, because the CO 2 physical properties and the reservoirs petrophysical properties could affect the transient flow of mixed fluid and phase transition of CO 2 along the well. A numerical model is presented in this research to analyze the transient flow in wellbores and to predict phase transition of CO 2 during formation SC‐CO 2 invasion in the drilling process. The main considerations were the following: multiphase transient flow in wellbores was coupled with the non‐Darcy flow through the formation; wellbore temperature, pressure and the mixture of drilling fluid and SC‐CO 2 were calculated in the method of coupling; the invaded SC‐CO 2 could affect the mixed fluid physical properties in the annulus; the change of physical properties of CO 2 (eg, solubility, phase transition) was considered and integrated. The research results indicate that as the mixture of drilling fluid and SC‐CO 2 flows from the bottom to the surface, the invaded CO 2 is difficult to be detected at the early invasion stage because of high CO 2 density and solubility; the CO 2 would change from supercritical to gas phase and the dissolved quantity and density of CO 2 rapidly decrease near the wellhead, and further cause well‐control issues. Furthermore, the bottom‐hole pressure (BHP) would decrease and could not provide sufficient balance to the formation pressure, which could significantly increase the risk of well blowout. Sensitivity analysis was utilized to evaluate the effects of the choke pressure, drilling fluid pumping rate, initial pressure difference, reservoir permeability, and drilled reservoir thickness on the BHP and the depth of phase transition of CO 2 .
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