Carbon Dioxide Modeling in Underbalance Drilling as a Supercritical Drilling Fluid

Mechanistic modeling of an underbalanced drilling operation using carbon dioxide has been developed in this study. The use of carbon dioxide in an underbalanced drilling operation eliminates some of the operational difficulties that arise with gaseous drilling fluids, such as generating enough torque to run a downhole motor. The unique properties of carbon dioxide, both inside the drill pipe and in the annulus are shown in terms of optimizing the drilling operation by achieving a low bottomhole pressure window. Typically carbon dioxide becomes supercritical inside the drill pipe at this high density;it will generate enough torque to run a downhole motor. As the fluid exits the drill bit it will vaporize and become a gas, hence achieving the required low density that may be required for underbalanced drilling. The latest carbon dioxide equation of state to calculate the required thermodynamic fluid properties is used. In addition, a heat transfer model taking 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 carbon dioxide and a mixture of carbon dioxide 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.