Water Separation From Wellstream in Inclined Separation Tube With Distributed Tapping
- Rocio Milena Rivera (Norwegian University of Science and Technology) | Michael Golan (Norwegian University of Science and Technology) | John D. Friedemann (Vetco Gray Inc.) | Benjamin Bourgeois (Statoil ASA)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- March 2008
- Document Type
- Journal Paper
- 1 - 11
- 2008. Society of Petroleum Engineers
- 4.5.9 Subsea Processing, 4.1.2 Separation and Treating, 2.2.2 Perforating, 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 4.3 Flow Assurance, 4.3.4 Scale, 1.6.9 Coring, Fishing
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This paper describes observations on water separation from wellstream in an inline separator using the concept of distributed water withdrawal from an inclined tube section. The physical observations were conducted in a laboratory using a 7-in. observation section applying flow rates compatible to North Sea single-well rates. The distributed water withdrawal is achieved by placing a set of evenly spaced tapping points at the bottom of the tube section with facilities to control tapping rates. The observations were made in connection with a joint-industry project to validate and commercialize a new concept for downhole and seabed separations. The driver for the study is to verify possible water separation by distributed tapping from the bottom of the tube in high flow rates with a high degree of agitation, turbulence, and phase dispersion. Observations demonstrated that for a wide range of flow, comparable to the flow conditions in high-rate oil wells, the high turbulence and phase dispersion do not erode and disperse completely a continuous layer of water on the bottom of the tube. This layer, flowing upward or downward depending on the flow conditions, facilitates a reasonable water separation when it is tapped properly. The scope of the test intended to capture the effect of flow and system variables on the separation efficiency. The experiments and the analysis validated the distributed separation concept. They demonstrated good separation at a wide range of flow stream conditions. They explained the reasons for good separation even with a high degree of turbulence and phase dispersion. The mechanistic understanding of the separation allows a degree of predictability and guidance for separator design.
Inline water separation from wellstreams reduces the backpressure on flowing wells, improves flow in gathering systems, and reduces water separation loads in surface separators. It is widely recognized that separation close to the source (the payzone) presents both hydraulic performance gain and phase separation advantages. These rewards drive current development of several novel inline separation concepts and separator designs, the reported concept included. The discussed concept is based on gravitational separation in an inclined tube with distributed water withdrawal. It appears to be effective, with simple or no internals; it is robust in structure and has little sensitivity to the accuracy of installation angle. It was initially conceived for down-hole applications (Håheim 2003), but has promising prospects for seabed separation in offshore fields.
Published information on inclined multiphase flow, developed primarily for addressing pressure losses and transience in surface piping and wellbores, failed to provide the information needed to predict distributed water withdrawal from wellstream in inclined tubes. Therefore, a laboratory study has been launched to study the relevant flow and separation aspects and to validate the concept. The study included the design and construction of a dedicated test loop intended to study the separation phenomenon with minimal needs for scaling up the results to real well-stream conditions.
This paper presents the results of the laboratory test program, discusses the process of validation of the separation concept, explains the observations of the governing separation mechanisms, and assesses the importance of the various involved parameters. It also provides preliminary information needed to guide the design of a prototype for field trials.
The Separation Concept
The studied separation method uses an inclined separation tube where water or water-reach phase is withdrawn in a distributed manner from the lower part of the tube. Evenly distributed discrete tapping points located at the lower end of the separation tube provide draining or withdrawal of water from the higher pressure in the tube to a slightly lower pressure outside. Gravitational forces, combined with the inclination effect of the upward flow, create separated water or a water-reach layer at the lower side of the pipe. This segregated layer is the core of the separation process. Depending on the flow conditions, the layer can be relatively thick and flow upward or a thin film and slide downward. Yet in a wide range of flow conditions, the water strata is continuous and can be effectively tapped from the tube with careful and properly adjusted distributed tapping. Fig. 1 illustrates schematically the separation and tapping principle.
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