Summary
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: 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 subhydrostatic 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 was 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. Before 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's
first boat-spooling operation of concentric coiled tubing (CCT); rigorously
reassessing deck loadings; and running the operation completely
self-sufficiently, with a maximum of 10 personnel onboard.
Finally, this paper provides a detailed description of the successful CCTVT
operations that recovered a total of 2,950 lbm of proppant from three wells to
expose the perforations and unload the wells of liquid. Furthermore, CCT was
used to mill out tubing-profile nipples and install a packer to hang off 1,200
ft of flush-joint tail pipe. Innovative thinking and close collaboration
between operator and service companies were required, because the ability to
perform multiple operations on CCT 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 newly unlocked
opportunities in deep depleted gas wells worldwide.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
9 May 2011
- Meeting paper published:
5 April 2011
- Manuscript approved:
23 June 2011
- Published online:
24 January 2012
- Version of record:
28 February 2012
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