Feasibility of Supercritical Carbon Dioxide as a Drilling Fluid for Deep Underbalanced Drilling Operation

Abstract

Abstract Feasibility 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. CO2 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 CO2 based drilling system are critical and were addressed in this study. The results show that the unique properties of CO2, 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 CO2 sequestration and CO2 based enhanced oil recovery operations. Introduction Supercritical carbon dioxide (SCCO2) has attracted the attention of researchers because of its unique properties attainable for some drilling applications. The study by Kolle[1,2] demonstrated that SCCO2 can provide better jet-erosion and mechanical drilling rates than water based-fluids. The coiled-tubing (CT) drilling using SCCO2 holds potential for drilling ultra-slim lateral drainage holes with good hole-cleaning and down-hole pressure control[2]. The present work addresses the potential advantages and feasibility of using SCCO2 as a drilling fluid in underbalanced drilling of depleted/ sub-normally pressure formations. In addition to its jetting characteristics, the possible phase change across the nozzle may prove to be beneficial for this application. Example Case Study A potential application where use of supercritical CO2 as a drilling fluid may be of great value is highlighted by the following case study which refers to a severely depleted gas well where depletion of the reservoir over time had led to an extremely low pressure of 700 psi at a depth of 14,340 ft.It is believed that the casing had collapsed due to such extreme pressure imbalance.[9] In order to regain production from this depleted gas reservoir, the operator decided to drill a sidetracked well branch, from the existing completed well.After the sidetrack operation, conventional drilling through the depleted sixty feet thick reservoir section was unacceptable as it would have caused lost circulation problems and differential sticking due to excessive overbalance resulting from the large hydrostatic pressure exerted by a tall column of mud.This could also cause severe reduction in productivity due to potential water-blockage and formation damage. To overcome these problems, it was decided to set casing just above the objective formation and drill the depleted zone with nitrogen as a drilling fluid, in order to maintain wellbore pressure below the reservoir pressure while drilling.Coiled tubing drilling was the preferred method as it provided pressure control while tripping and allowed continuous operation without the time consuming and expensive tubing-pulling operation.