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.

The recent increase in oil prices during the recent years has led to re-investing in reservoirs that were previously not economical. In addition, most of the reservoirs had been partially depleted, and the current industry trend is to infill drill and/or sidetrack abandoned reservoirs, seeking new reserves. The existence of such reservoirs has led to the extensive use of underbalanced drilling (UBD), in an effort to minimize formation damage. UBD is the best available technology for low pressure and/or depleted reservoirs. A UBD operation is considered a success when it achieves the required underbalanced pressure. Alternate UBD techniques may not achieve the required wellbore pressures. For example two-phase drilling fluids have been used extensively, but these tend to generate high bottom hole pressure. In many situations, the high pressure generated is not the best solution for UBD. In cases such as deep wells with low bottomhole pressure, the use of these fluids will not achieve the required circulating downhole pressure.

The use of gases as drilling fluids may achieve the required circulating pressure but generate other problems. One such problem, the circulating gas density in the drill pipe, is not able to rotate down-hole motors. Recently, super critical carbon dioxide (SC-CO2) has been used in a few applications of interest. The unique features of SC-CO2 make it an ideal candidate for a UBD drilling fluid, since at higher pressure and temperature it will become super critical, which gives it both gaseous and liquids properties. Recent authors have showed the benefits of using SC-CO2 in drilling operations[i],[ii]. Proper hydraulic modeling will optimize the drilling operation in terms of optimum pressure control and project design. In addition, developing a complete hydraulic model, which takes into account a detailed study of the thermodynamic properties of CO2, is needed, since the operation is very sensitive to these parameters.

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