Feasibility of drilling with pure supercritical carbon dioxidernto serve the needs of deep underbalanced drilling operationsrnhas been analyzed. A case study involving underbalancedrndrilling to access a depleted gas reservoir illustrates the needrnfor such a study. For this well, nitrogen was initiallyrnconsidered as the drilling fluid. Dry nitrogen, due to its lowrndensity, was unable to generate sufficient torque in therndownhole motor. Mixture of nitrogen and water, stabilized asrnfoam, generated sufficient torque, but made it difficult tornmaintain underbalanced conditions. This diminished thernintended benefit of using nitrogen as the drilling fluid.rnCO2 is likely to be supercritical at downhole pressure andrntemperature conditions, with density similar to that of a liquidrnand viscosity comparable to a gas. A computational modelrnwas developed to calculate the variation of density andrnviscosity in the tubing and the annulus with pressure,rntemperature and depth. A circulation model was developed torncalculate the frictional pressure losses in the tubing and thernannulus, and also calculates important parameters such as thernjet impact force and the cuttings transport ratio. An attemptrnwas made to model the temperatures in the well using anrnanalytical model. Corrosion aspects of a CO2 based drillingrnsystem are critical and were addressed in this study.rnThe results show that the unique properties of CO2, which isrnsupercritical in the tubing and changes to vapor phase in thernannulus, are advantageous in its role as a drilling fluid. It hasrnthe necessary density in the tubing to turn the downhole motorrnand the necessary density and viscosity to maintainrnunderbalanced conditions in the annulus. The role of a surfacernchoke is crucial in controlling the annular pressures for thisrnsystem. A carefully designed corrosion control program isrnessential for such a system. Results of this study are alsornimportant for CO2 sequestration and CO2 based enhanced oilrnrecovery operations.
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