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Offtake levels, under natural flow, from Thistle and Deveron wells A46(45) and A51(03) are both constrained by relatively poor pressure support. It was decided to install jet pumps in the existing completions to test the reservoir performance under artificial lift. If enhanced offtake levels could be supported permanent gas lift completions would be installed at a later date. In addition to this interim increases in the production rates would be achieved prior to workover.
A wireline retrievable jet pump, complete with a integral sub-surface safety valve (SSSV), has been designed for installation in the (open) sliding side door (SSD) of the existing completions. Power fluid, supplied from the water injection system, is routed via the casing-tubing annulus and returns are produced up the tubing string. Downhole isolation of both the tubing and the annulus is achieved by means of the annulus pressure operated SSSV which is located immediately below the jet pump.
The design procedure for the selection of the optimum nozzle/throat combination is described. Operating procedures have been developed to ensure successful operation of the device and details are given of both the downhole and surface hardware.
Results of actual field performance are presented and comparisons made with predicted values. Finally details are given of design modifications which were implemented in response to problems which developed during field trials.
The Thistle field, located in block 211/18a of the Northern North Sea, produces oil via a single, centrally located platform. Area 6 is located to the north of the main field and production is achieved via a single well A46(45). The Deveron field has separate field status and is located immediately to the west of Thistle. Deveron is produced via wells A51(03), A44(07) and A48z(23) drilled from the Thistle platform (Fig. 1).
Generally the Thistle main field has adequate reservoir pressure support to facilitate production under natural flow. However, the performance of certain areas of the main field, Area 6 and the Deveron field has indicated that artificial lift would be beneficial in specific wells. A previous study identified gas lift as the optimum artificial lift method and two wells are currently being gas lifted. Current plans envisage that a total of 7 wells will be gas lifted including Area 6 and Deveron wells A46(45) and A51(03).
Prior to committing to the installation of permanent gas lift completions in these wells, it was considered prudent to perform short term artificial lift trials to confirm that enhanced offtake levels could be sustained. The simplest and most cost effective way of achieving this was to employ jet pumps installed by wireline in the existing completions. If the trials indicated that enhanced production rates could indeed be sustained it would be beneficial to continue jet pumping in the interim pending installation of the gas lift completions. For this interim production period downhole closure was required. In response to this an integral jet pump/SSSV was developed which could be installed into the (open) SSD's of the existing completions.
The method adopted for pump sizing and details of an integral jet pump/SSSV assembly are presented in this paper. Field experience on two wells is described and comparisons made between actual and simulated performance. Details of the surface hook-up and jet pump operating procedures are also presented.
The theory of jet pumps has been adequately documented (Ref. 1). In summary the jet pump is a fluidic device.