Mechanistic modeling of an underbalanced drilling operation using carbon dioxide has been developed in this research. The use of carbon dioxide in an underbalanced drilling operation eliminates some of the operational difficulties inherent with gaseous drilling fluids, such as generating enough torque to run a downhole motor. The unique properties of CO2, both inside the drill pipe and the annulus are shown in terms of optimizing the drilling operation by achieving a low bottom hole pressure window. Typically, CO2 becomes supercritical inside the drill pipe at this high density; it thus can generate enough torque to run a downhole motor. As the fluid exits the drill bit it will evaporate to a gas, hence achieving the required low density for underbalanced drilling.

The latest CO2 equation of state to calculate the required thermodynamic fluid properties is used. In addition, a heat transfer model that takes into account varying properties of both pressure and temperature has been developed. A marching algorithm procedure is developed to calculate the circulating fluid pressure and temperature, taking into account the varying parameters. Both single phase CO2 and a mixture of CO2 and water have been studied to show the effect of produced water on corrosion rates. The model also is capable of handling different drill pipe and annular geometries.

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