We examine in this paper the feasibility of using a concentric jet pump to create an underbalanced condition while drilling subnormally pressured reservoirs, using only single phase injected fluids, eliminating the need for gas. A complete technical basis for the performance and design of jet pumps for underbalanced drilling applications is presented. The problem is handled in four parts: reservoir to jet pump suction, the jet pump itself, annular flow from jet pump exit to surface, and power fluid flow. Multiphase flow theory, viscous flow theory and jet pump flow theory are used to analyze the problem. The design goal is to find the correct combination of jet pump location, geometry, power fluid density and rate such that the jet pump exit energy is adequate to lift the mixed fluids and solids to surface. Other design considerations such as tolerance of varying reservoir conditions and depth, cavitation, sonic velocity limitations, and power fluid injection pressure constraints, are also taken into account in the design procedure. Jet pump design envelopes unique to its application in underbalanced drilling are developed to aid selection and design of the pump. A design program that implements the procedure has been developed. The procedure is illustrated with two examples: an oil well and a gas well. We find that the jet pump can successfully create an underbalanced state for a wide range of conditions. In general, for reservoir pressures in the range of 4 to 7 ppg equivalent, the jet pump appears to enable creation of an underbalanced condition without requiring injected gas, regardless of whether the reservoir is oil bearing or gas bearing. Operational and practical considerations that need to be addressed in proving the design are also discussed. Based on this work, we conclude that the concentric jet pump is a unique device technically suitable for a wide range of uncerbalanced drilling applications.

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