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.
Use of gas is common in most underbalanced drilling (UBD) operations, particularly at subnormal reservoir pressures where "flow drilling" (use of single-phase drilling fluid to create underbalanced conditions) is no longer possible. Several sources of gas, including membrane-generated nitrogen, cryogenic nitrogen, natural gas and exhaust gas are used. While safe and common, the use of gas has the following disadvantages:
Expense of special gas equipment (gas generation and compression).
Safety considerations due to presence of high pressure gas lines.
If gas is injected through drill pipe, compromise of conventional MWD equipment and possible need for alternate MWD methods.
Corrosion considerations, especially with the use of membrane-generated nitrogen and exhaust gas.
Clearly, reduction or elimination of gas requirements for UBD applications in low pressure reservoirs is attractive. The jet pump is one device that can potentially achieve this goal.
A jet pump is a rugged and simple hydraulic device with no moving parts that works on the principle of momentum transfer. Fast moving power fluid transfers momentum to slow moving well fluids. The mixture is expanded in a diffuser to recover some of the kinetic energy as potential (or pressure) energy. This recovery of pressure energy then enables the fluid to be transported to the surface. Owing to its simplicity, jet pumps have been used as artifical lift devices for decades. However, all of these have been circular jet pumps, used within a tubing or casing string. In order to extend this principle to UBD applications, a concentric jet pump is needed.