As an oilfield goes mature, an increased water cut can significantly decrease the maximum fluid production rate or even stop the production entirely. Therefore, separating produced water from the wellstream as early as possible is a potential way to maximize oil production. A novel inclined gravity downhole oil-water separator concept has been introduced and patented by ABB Research Ltd., which combines gravitational separation with distributed water tapping along the incline separator tube. The concept depicts that the downhole separator can be installed somewhere above the production packer and below SCSSV (surface-controlled subsurface safety valve). Gravitational forces create a separated water or water rich layer at the lower side of the pipe. This segregated water rich layer is drained using distributed tapping points along the separation tube and then flow to surface via annulus, whilst the oil rich layer flow through the tubing continue up to surface. Several experimental tests have been performed and this paper describes how to use the experimental results into a well performance simulator to predict how the inclined gravity downhole oil-water separator modifies the performance of high production rate wells. The study includes the well performance effect of separator setting depth, setting inclination, tubing size and tubing configuration. Well performance sensitivity due to water cut and separation efficiency is also discussed. The simulation results show that inclined downhole oil-water separation is very beneficial and able to increase oil production up to 82% for the selected wells with 81–87% water cut. Introduction Conventionally, oil and water are separated at the surface using gravity-driven separators, where the size of the separator is a function of flow rate and the required retention time. The gravity separators often occupy large portions of the space on the offshore platform. In the mid 80's hydrocyclones and centrifuges have been introduced to treat produced water before disposal. Around the 90's, tests of separation facilities with hydrocyclones as a bulk separator has been carried out successfully. These technologies have directed the industry towards the size reduction of separation facilities at surface. However, a major further step is to separate the bulk of the water in a downhole in-line arrangement 15. Applying downhole oil-water separation (hereafter, called as DOWS) could de-bottleneck the production plant on platform and reduce the space on board, eliminate future need of new constructions to increase water handling capacity. By separating water in the downhole, the liquid density of the wellstream and the back pressure on the formation are reduced. Hence, increasing the drawdown pressure which enhances production (Fig. 1). Three basic types of downhole oil water separator have been classified based on the separation system utilized6. The first type using hydrocyclones, the second based on gravity forces and the third type using membrane separation technology, which is yet to be developed and applied in the field but has been investigated through simulation studies. A new concept in the gravity type separation is inclined gravity downhole oil-water separator with distributed water tapping where the drained water can be controlled effectively. Some advantages of this type of downhole separator are its simplicity, robustness in structure and the little sensitivity to the accuracy of installation angle. Due to its simplicity components, it also has a long-life potential. In the case where the separator fails to perform well it can be kept and used as ordinary tubing without the need of workover cost to pull it out. Some of the challenges to this concept are the design of the instrumentation that can regulate the drainage rate to achieve the best separation and the potential well integrity issue with regards to flowing HC in the A-annulus.
This paper describes observations on water separation from wellstream in an inline separator using the concept of distributed water withdrawal from inclined tube section. The Physical observations were conducted in a laboratory using a 7″ 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 high degree of agitation, turbulence and phase dispersion. Observations demonstrated that in 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 upwards or downwards 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 in high degree of turbulence and phase dispersion. The mechanistic understanding of the separation allows a degree of predictability and guideness for separator design. Introduction 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 inclined tube with distributed water withdrawal. It appears to be effective, with simple or no internals, robust in structure and with little sensitivity to the accuracy of installation angle. It was initially conceived for down-hole applications1 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.
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.
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