Summary
A wellbore cement sheath is expected to provide zonal isolation and borehole
integrity during well construction and well life. Mechanical interactions of
the cement sheath to existing and operationally induced stresses, along with
other elements in proximity to the wellbore, have increasingly large technical,
economic, and environmental ramifications.
Staged-finite-element procedures during well construction consider
sequentially the stress states and displacements at and near the wellbore. The
model replicates complicated stress states arising from simultaneous action of
far-field stresses, overburden pressure, cement hardening and shrinkage,
debonding at the interfaces, and plastic flow of cement sheath and rock
formation. At present, temperature, flow, and poroelasticity effects are not
included. The technique tracks the time-dependent behavior of cement slurry,
curing (with or without shrinkage), and hardened cement during the critical
period after slurry placement.
Material models for casing, cement, and rock formation and failure criteria
for cement, formation, and interface bonds were calibrated using published
information and experimental data. Calculations were conducted for various
loading and unloading scenarios, geometric configurations, properties of rock
formations, and cement-slurry formulations. Results are discussed in terms of
field implication;, for example: (1) Interface microchannels may or may not
develop, depending upon shrinkage magnitudes; and (2) simplifying modeling
assumptions that are often used, such as 2D stresses and/or deformations, may
obscure critical casing, cement, and formation behavior in the wellbore region
and in the producing horizon.
This paper, part of a series quantifying the interacting physical components
and processes at and near the wellbore region, initiates useful comparisons of
analytical results and field realities. The series illustrates and compares
results and practical implications from simple to increasingly complex, but
more realistic, assumptions, such as isotropic/directional-stress states and
isotropic/anisotropic casing, cement, and formation-material parameters.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
2 February 2007
- Meeting paper published:
31 March 2007
- Revised manuscript received:
2 April 2008
- Manuscript approved:
28 April 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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