Ovalization of casing strings has been detected in the tectonically active
foothills of Colombia, South America, initially through operational
difficulties, and subsequently, by extensive monitoring. The effect of
this ovalization on cross-sectional collapse has been numerically and
experimentally analyzed in a previous publication (Pattillo et al.
2004). The current study emphasizes the broader context to which those
detailed results were applied. Here, the emphasis will fall on
characterizing the deformation, assessing its impact on well integrity, and
developing a strategy for managing the consequences to achieve improved
business and operational decision making.
Casing deformation (ovalization) has been observed in many re-entered wells
in BP’s Colombian acreage, and some operational problems have been attributed
to this—primarily, restricted access and failures caused by reduced collapse
resistance. While drilling in the area has been plagued by problems associated
with severe wellbore instability (Last et al. 1996), there is now compelling
evidence that the somewhat unusual rock stresses resulting from the tectonic
environment are also causal to the observed deformation of casing strings after
wells have been completed; analogues have not been found.
Following early detection of ovality in wells, a monitoring policy was
initiated to provide both a basis on which to characterize the changing
geometry and key information for assessing well access and integrity.
Conventional collapse formulas predict a dramatic reduction in collapse
resistance as ovalization increases, but the field experience in Colombia
refutes this. An earlier companion study (Pattillo et al. 2004) addresses the
physics of this apparent inconsistency. Numerical simulations, validated
by laboratory testing, provide both insight and new collapse-resistance
adjustments for ovalized pipe.
The improved understanding was used to create an operational strategy for
managing the impact of casing ovalization. Implementation of the strategy
provides both a statistical, risk-based means for planning wells and
intervention, and important input to assessments of well and field
© 2006. Society of Petroleum Engineers
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- Original manuscript received:
25 October 2004
- Revised manuscript received:
1 December 2005
- Manuscript approved:
30 December 2005
- Version of record:
20 June 2006