Summary
Numerous casing and production-liner deformation/failure problems have been
reported in high-porosity chalk formations in both the overburden and the
reservoir sections, causing costly operation problems that prevent workovers
and recompletions. This paper presents the results of studies performed to
investigate stability of an openhole, cemented liner and uncemented-liner
completions in a highly compacting chalk formation. The effects of critical
cavity dimensions caused by various acid-stimulation techniques were also
investigated.
On the basis of the review of historical caliper-survey data, we ascertain
that axial-compression collapse is a major liner-deformation mechanism in the
reservoir zones. Axial-compression collapse has been found in both low-angle
wells (also in buildup sections of horizontal wells) and horizontal laterals.
The casing deformation in low-angle sections is a result of reservoir
compaction (i.e., change in the vertical formation strain), while the
deformation in horizontal sections is primarily induced by increased axial
loading because of cavity deformation. The current completion practice using
cluster perforations and high-volume acid treatments causes vertically enlarged
cavities, resulting in poor radial constraint.
A series of laboratory triaxial tests was performed on selected reservoir
chalk samples to measure the stress/strain and failure behavior of the chalk
formation considering a wide range of porosity and water saturation and
different levels of confining pressures. Using the chalk-failure criteria and
constitutive relations developed from the analysis of laboratory
triaxial-compression-test data, a 3D nonlinear poroelastic/plastic
finite-element-method (FEM) model was developed for the openhole stability
analysis. The simulation results show that the abnormally high ductility of
chalks after pore collapse around a borehole could actually enhance borehole
stability, with a magnitude beyond expectation.
In this study, analytical and numerical models are also developed for
evaluating cavity-induced axial- compression collapse of production liners.
Model results indicate that the risk of the cavity-induced axial- compression
collapse substantially increases for short perforated intervals stimulated with
large acid treatments. However, increasing the perforation-interval lengths
along the entire liner axis results in more-uniform acid distribution and will
greatly reduce the chance of axial-compression collapse caused by localized
cavity deformation. On the basis of these analysis results, key completion
design criteria and stimulation strategies were developed for wells completed
in highly compacting chalk reservoirs to reduce risk of casing and liner
mechanical problems.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
30 July 2009
- Meeting paper published:
5 October 2009
- Revised manuscript received:
20 March 2010
- Manuscript approved:
12 April 2010
- Published online:
28 October 2010
- Version of record:
23 December 2010
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