Abstract

Ravenspurn North is a mature gas field in the Southern North Sea with 42 wells, drilled and many hydraulically fractured in the late 1980s. By 2006 more than half of the wells had ceased to flow and many were flowing intermittently. With ailing wells and consequently falling production rates, the longer term future for Ravenspurn North field was uncertain.

A wireline campaign suggested a common failure mode for many wells that large amounts of proppant had accumulated in the wellbore. Modeling of the potential range of static and dynamic pressure losses caused by this proppant supported a significant enhancement to the remaining gas potential. However, the depths of the wells combined with sub-hydrostatic pressure conditions and large diameter lower completions made achieving cleanout challenging. Furthermore, the target wells were located on unmanned offshore installations with minimal facilities, deck space and deck loading.

This paper details how each of these obstacles were successfully tackled to reinstate a target set of wells. It describes the various cleanout options that were considered, and why Concentric Coiled Tubing Vacuum Technology (CCTVT) was ultimately selected. Prior to this project CCTVT had never been deployed in the North Sea and nowhere in the world at these reservoir depths. The operation was delivered on the small unmanned installation by conducting the world-first boat spooling operation of Concentric Coiled Tubing; rigorously re-assessing deck loadings; and running the operation 100% self sufficient with a maximum of 10 personnel on board.

Finally this paper culminates with a detailed description of the successful CCTVT operations which recovered a total of 2,950 lbs of proppant from three wells to expose the perforations, and unload the wells of liquid. Furthermore, Concentric Coiled Tubing was used to mill out tubing profile nipples and install a packer to hang off 1,200 ft of flush joint tailpipe. Innovative thinking and close collaboration between operator and service companies was required as the ability to perform multiple operations on Concentric Coiled Tubing was in itself pioneering.

Overall this campaign has pushed the boundaries of intervention technology to deliver an extremely challenging project. This has resulted in a more certain future for the Ravenspurn North field, as well as unlocking opportunities in deep depleted gas wells worldwide.

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