TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractFeasibility of drilling with pure supercritical carbon dioxide to serve the needs of deep underbalanced drilling operations has been analyzed. A case study involving underbalanced drilling to access a depleted gas reservoir illustrates the need for such a study. For this well, nitrogen was initially considered as the drilling fluid. Dry nitrogen, due to its low density, was unable to generate sufficient torque in the downhole motor. Mixture of nitrogen and water, stabilized as foam, generated sufficient torque, but made it difficult to maintain underbalanced conditions. This diminished the intended benefit of using nitrogen as the drilling fluid. CO 2 is likely to be supercritical at downhole pressure and temperature conditions, with density similar to that of a liquid and viscosity comparable to a gas. A computational model was developed to calculate the variation of density and viscosity in the tubing and the annulus with pressure, temperature and depth. A circulation model was developed to calculate the frictional pressure losses in the tubing and the annulus, and also calculates important parameters such as the jet impact force and the cuttings transport ratio. An attempt was made to model the temperatures in the well using an analytical model. Corrosion aspects of a CO 2 based drilling system are critical and were addressed in this study. The results show that the unique properties of CO 2 , which is supercritical in the tubing and changes to vapor phase in the annulus, are advantageous in its role as a drilling fluid. It has the necessary density in the tubing to turn the downhole motor and the necessary density and viscosity to maintain underbalanced conditions in the annulus. The role of a surface choke is crucial in controlling the annular pressures for this system. A carefully designed corrosion control program is essential for such a system. Results of this study are also important for CO 2 sequestration and CO 2 based enhanced oil recovery operations.