Summary
Casing integrity in shallow marine sediments could be challenging if natural
gas hydrates exist in the sediments. Elevated wellbore temperature during
drilling of deeper sections of deep offshore wells can cause in-situ gas
hydrates to dissociate, thereby increasing pore pressure and altering the
mechanical properties of the sediments. Gas hydrate can also dissociate during
setting and/or cementing, causing gas release, which could result in delaying
completion of the wellbore because of the flow of gas around the casing
(conductor pipe) or affecting the casing integrity or casing stability by
creating voids (channels) in the cement sheath leading to nonuniform stress
loadings.
In this communication, a numerical model is developed using a finite-element
code to simulate the stability of casing in gas-hydrate-bearing sediments by
considering the interaction between the formation, the casing, and the cement
with coupling the thermodynamic stability of the hydrates to hydraulic,
mechanical, and heat transfer terms. The mechanical and hydraulic terms are
fully coupled and the coupling between mechanical and thermal terms is modeled
through staggered technique (one-way coupling).
To model the worst-case scenario, the permeability of gas-hydrate-bearing
sediments is assumed very low; as a result, the gas and water generated during
gas-hydrate dissociation cannot flow and will increase pore pressure. The
mechanical property degradation of formation caused by hydrate dissociation is
represented in the model by cohesion softening as a function of dissociated gas
hydrate saturation.
The developed numerical model is found to be very useful in understanding
the behavior of wellbores drilled in gas-hydrate-bearing sediments, which will
help the determination of the resultant stress fields and enable a more
accurate determination of the required casing strength.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
22 February 2008
- Meeting paper published:
9 June 2008
- Revised manuscript received:
31 October 2008
- Manuscript approved:
5 November 2008
- Published online:
30 July 2009
- Version of record:
23 December 2009
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