Finite element modeling of casing in gas-hydrate-bearing sediments

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. Copyright © 2009 Society of Petroleum Engineers.