Finite element modelling of casing in gas hydrate bearing sediments

Casing integrity in shallow marine sediments can be a challenge if natural gas hydrates are present in 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 due to the flow of gas around the casing (conductor pipe) or affecting the casing integrity by creating a void (channel) in the cement sheath leading to non-uniform stress loadings In this communication, a model is developed using a finite-element code to simulate the integrity of casing in gas hydrate bearing sediments by considering the interaction between the formation, the casing, and the cement. The developed model couples the thermodynamic stability of the hydrates to mechanical deformation and heat transfer. The coupling between mechanical and thermal terms is modelled through staggered coupling. To consider the worst case scenario, the permeability of gas hydrate bearing sediments is assumed to be very low such that the gas and water generated during gas hydrate dissociation cannot flow and will increase pore pressure. As hydrates in the pore space are dissociated, the mechanical property degradation is represented in the model as a function of dissociated gas hydrate in the pore space by progressive loss of cohesion. The developed numerical model is found to be very useful in understanding the behaviour 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.