Modeling of an Underbalanced-Drilling Operation Using Supercritical Carbon Dioxide

A mechanistic model of an underbalanced-drilling (UBD) operation using carbon dioxide (CO2) is developed in this study. The use of carbon dioxide in UBD operations 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 drillpipe and in the annulus, are shown in terms of optimizing the drilling operation by achieving a low bottomhole pressure range. Typically, CO2 becomes supercritical inside the drillpipe at this high density; thus, it can generate enough torque to run a downhole motor. As the fluid exits the drill bit, it evaporates to a gas, hence achieving the required low density for UBD. The latest CO2 equation of state (EOS) 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 drillpipe and annular geometries.