Use of Supercritical Carbon Dioxide in Underbalanced Drilling Operation

Underbalanced drilling (UBD) is defined as the practice of drilling a well with the wellbore fluid gradient less than the natural formation gradient. It differs from conventional drilling in that the bottom holes circulating pressure is lower than the formation pressure, thereby permitting the well to flow while drilling proceeds. Underbalanced drilling technology is a valuable method for minimizing formation invasion related problems. Because the majority of hydrocarbons today are found in existing fields with depleting pressures, or in complex and low quality reservoirs, the economical use of UBD becomes more and more popular. This technology can save the industry millions of dollars by increasing the amount of recoverable oil within a shorter time frame. Historically, most underbalanced drilling (UBD) projects were undertaken to eliminate drilling problems and cost. However, recently, the reduction of formation damage has become the main focus for underbalanced operations. This has the greatest potential in directly increasing the profit to the operating company. Potential benefits include increasing of production rate, the ultimate recovery, and enabling accelerated production. Underbalanced technology, while still on a sharp growth curve, is finally becoming accepted as a normal method for handling the drilling and completion of wells. Use of supercritical carbon dioxide (CO2) in underbalanced drilling operation has been investigated in this paper. 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 down holes motor. The unique properties of CO2, both inside the drill pipe and in 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 will generate enough torque to run a down holes motor. As the fluid exits the drill a bit, it will vaporize and become a gas, hence achieving the required low density that may be required for underbalanced drilling. Both single phase CO2 and a mixture of CO2 and water have been studied to show the effect of produced water on corrosion rates.