Evaluation of a top hole full return drilling system applying a concentric dual drill string and an integrated pump

Abstract

This thesis evaluates the possibility for a full mud return, top hole drilling system, applying a concentric dual drill string and an integrated pump. Top holes are usually drilled without mud return, leaving the cuttings on the sea floor. Sea water with barite and other additives are employed as drilling fluid and is released to the sea when used. By employing a dual drill string and a down hole pump to lift the return to top side facilities, full return is enabled. This facilitates the use of high performance mud, which have several advantages, including primary well control before the BOP is set, improved hole stability, elimination of a pilot hole to check for shallow gas influx and extended top hole sections. Possible solutions to obtain a complete and functioning new system have been analyzed. Based on existing technology and its current limitations, two alternative systems are developed on a conceptual level. The first system includes one integrated return pump, the second employs multiple integrated return pumps. The design base case is set to 1000 meter water depth and 500 meter deep well, of which 100 meter is drilled with a 36” drill bit, and 400 meter is drilled with a 26” drill bit. This base case covers most of the top holes drilled on the Norwegian sector. System pressure estimates are presented, and a mud level regulation solution is developed and analyzed. The mud level regulation system allows the mud level in the well to be controlled to keep the well balanced and stabilized, and to prevent mud discharges to sea floor. The level regulation solution is theoretically proved, and enables reliable regulation of the mud level in the well based on existing technology. Predictions of the system behavior are made, and the limitations of the systems are presented. The developed systems drilling capacities are analyzed and found not capable of fulfilling the base case requirements, due to the limitations of the selected dual drill pipe. The low flow rate of the pipe limits the ROP, due to high cutting generation with large drill bit diameters. The hydraulic horsepowers at the drill bit nozzles are also too low, due to the lowered available pressure drop, low flow rate, and large drill bit. However, the available pressure drop at the drill bit nozzles are estimated to over 80 bar. It is recommended to employ a larger dual drill pipe, with increased pressure capacity. Then the drilling capacity of the system would be comparable to other full return top hole drilling systems. The systems impact on cost and drilling parameters are discussed and found to be comparable with other innovative solutions for full return top hole drilling. There are uncertainties of both developed systems. The uncertainties regarding the system employing only one return pump concerns the design limitations of the chosen return pump type, a progressive cavity pump. The uncertainties regarding the multiple return pump system, concerns the system behavior with several return pumps distributed throughout the drill string. A full return top hole drilling system employing a concentric dual drill string and an integrated pump is found feasible. But due to existing technology limitations, a mud motor is chosen to power the return pump, this demands a drill pipe with a higher capacity than what exists today, to obtain comparable drilling capacity to other top hole drilling systems. The development of an electric conducting dual drill pipe would expand the possibilities much further, and improve the overall drilling capacity of the system.