Summary
Loss of zonal isolation in a wellbore can be caused by mechanical failure of
the cement or by development of a microannulus. However, behavior of the
sealant is driven by specific boundary conditions such as rock properties.
Large-scale laboratory testing of the cement sheath in an annular geometry and
a confined situation was performed to simulate various downhole stress
conditions and evaluate the behavior of several sealants. Failure modes of the
cement sheath were determined as a function of cement mechanical properties,
loading parameters, and boundary conditions. Results were used to validate an
analytical model that predicts cement-sheath failure.
Introduction
Interzonal communication in a wellbore may lead to loss of reserves,
contamination of zones, production of unwanted fluids, or safety and
environmental issues. Remedial solutions exist to repair the problems, but for
technical or economic reasons, the well may be shut in or abandoned.
To maximize well life, the cement sheath must be chemically and mechanically
durable. Sealants resistant to aggressive formation fluids should also be
designed to withstand stresses exerted during production and well operations,
such as casing-pressure tests, stimulation treatments, or temperature changes
during production cycles. To achieve this design goal, a better understanding
of the mechanical behavior of different sealants under downhole conditions is
required.1,2
According to Thiercelin et al.,3 changes in downhole conditions can cause
mechanical damage (e.g., mechanical failure or creation of microannuli) to the
cemented annulus, which may lead to loss of zonal isolation. Thiercelin et
al.’s paper3 concludes that the complete mechanical system formed by the steel
casing, cemented annulus, and formation should be considered, rather than
sealant strength alone.
Increase of pressure and temperature in the wellbore first expands the inner
steel casing, which instantly imposes this deformation on the surrounding
cement sheath. This applies imposed displacements, rather than imposed
stresses, to the cement inner diameter (ID). Over the lifetime of the well, the
cement sheath must withstand multiple displacement cycles. Several authors4,5
have proposed numerical models to simulate sealant mechanical behavior and
predict initiation of failures according to known mechanical properties of the
complete system (i.e., steel, cement, and rock).
A large-scale laboratory test for sealants in an annular geometry has been
developed. This test simulates changes in well conditions that cause
contraction or expansion of the inner casing. It can also evaluate the
confining role of the formation or outer casing. Such an experiment enables the
evaluation of sealant mechanical responses under wellbore conditions. The
tensile and compressive stresses generated in the annulus are similar to those
the sealant must withstand in a real wellbore. Loading simulated in the
full-scale annular sealing test is close to real field conditions.
Several cement systems exhibiting different mechanical behaviors have been
tested, and the experimental results have been compared with predictions of a
numerical model.
© 2005. IADC/SPE Drilling Conference
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History
- Original manuscript received:
2 June 2004
- Revised manuscript received:
23 December 2004
- Manuscript approved:
29 January 2005
- Version of record:
15 March 2005
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