Summary
Many casing- and screen-damage incidents have been reported in deepwater oil
and gas fields in the Gulf of Mexico and other locations around the world. We
reviewed historical casing/well failure events in a highly compacting sandstone
field and performed a comprehensive geomechanics analysis of various casing-
damage mechanisms (tension, axial compression, shear, and bending) related to
large reservoir depletion. Among five wells that experienced mechanical
well-integrity issues, two of them showed casing restrictions in the caprock at
intervals approximately 1,000- to 1,600-ft true vertical depth (TVD) above the
top of the depleting (main) reservoir. A multi-finger caliper log obtained from
one of the wells indicates that the overburden casing failure occurred at a
highly geopressured, thin sand layer approximately 1,100-ft TVD above the top
of the compacting reservoir. The remaining casing-failure events occurred near
(less than 200-ft TVD) or within the compacting reservoir interval.
A 3D nonlinear finite-element-method (FEM) model has been developed for
simulating stress changes in the overburden and the reservoir intervals and
evaluating the effect of lithological anomalies on casing stability. The
simulation results indicate that large tensile and shear strains could develop
within a thin, weak-strength layer in the overburden and at the interface
between caprock and the depleting reservoir interval. Casing damage by
bending/shear could also occur at these thin-layered sands saturated with
overpressured gas.
In the reservoir interval, shear stresses acting on the screens can be
relatively high because of the difference of the movements between the internal
base pipe and the external shroud and gravel. Screen failure may also occur at
the welded points. If casing failure occurs in the unperforated sand layer just
above the compacting reservoir, it induces localized high-velocity flow on the
upper part of the screen, causing potential screen erosion. Casing failure
caused by fault slip near the reservoir occurs only if a fault has sealing
capability while maintaining a large pressure differential across the fault
plane.
The numerical-analysis results presented in this work help engineers
understand possible casing- and screen- deformation and -failure mechanisms
experienced in highly compacting sandstone fields. On the basis of the study
findings, we also present some completion-design guidelines to avoid or
mitigate compaction-induced casing damage in both the overburden and reservoir
intervals.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
4 November 2011
- Meeting paper published:
31 October 2011
- Revised manuscript received:
21 February 2012
- Manuscript approved:
2 March 2012
- Published online:
24 May 2012
- Version of record:
11 June 2012
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