Summary
The traditional plug-and-abandonment (P&A) method of exploration wells
in the North Sea is to set a series of cement plugs to isolate the pressurized
zones from each other and from surface. This paper describes a North Sea
P&A field case. In this case, an alternative method was used with a
Bingham-plastic unconsolidated plugging material with high solids
concentrations. This alternative method addresses well-integrity issues such as
those caused by shrinking of cement or gas migration during setting, fracturing
after setting, or long-term degradation by exposure to heat and chemical
substances in the well.
The gas-tight well-barrier element described here does not set up after
placement and does not shrink. Furthermore, it cannot fracture even when shear
forces exceed its strength. When this happens, the material floats and shear
forces are reduced below yield strength, causing the plug to reshape. Because
this is a purely mechanical process, the transition between solid and fluid
phase is repeatedly reversible (in principle, an infinite number of times).
The plug is thermodynamically stable because its sealing property is decided
by the solids particle-size distribution (PSD) and bound water only. The
closely packed particles and absence of free water mean that the entire column
is kept homogeneous and no internal redistribution of particles may occur.
Hence, the permanent gas-tight barrier will prevent influx through the
wellbore.
In the field case, a successful implementation of the technology was
obtained. The field case shows how the fast and efficient placement of the plug
contributes to overall cost reduction. The paper explains how the well-barrier
element complies with Norwegian requirements for permanent P&A; these
requirements also apply to the UK sector (NORSOK D-010 2004; Oil and Gas UK
2009). Operational procedures are also presented in some detail.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
29 June 2010
- Meeting paper published:
6 October 2010
- Revised manuscript received:
24 December 2010
- Manuscript approved:
4 January 2011
- Published online:
21 July 2011
- Version of record:
15 September 2011
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