Finite Element Modelling of Casing in Gas Hydrate Bearing Sediments

Manoochehr Salehabadi, Min Jin, Jinhai Yang, Hooman Haghighi, Rehan Ahmed, Bahman Tohidi
2008 Europec/EAGE Conference and Exhibition   unpublished
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 due to the flow of
more » ... due to 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 non-uniform 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 modelled 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 due to 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 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. Introduction: Gas hydrates are ice-like crystalline compounds formed from mixtures of water and suitably sized 'guest' molecules and stable under low temperature and high-pressure conditions. Guest molecules in natural hydrates are either methane or a mixture of components comprising natural gas. Typically, they are found in sediments within a few hundred meters of the seafloor, in water depths of around 500m depending on seabed temperature, gas composition, and geothermal temperature gradient. An increase in the system temperature and/or a reduction in the system pressure could result in gas hydrates dissociation, and production of water and gas. As gas hydrates store large quantities of gas (around 172 vol/vol), their dissociation will result in the release of large amounts of gas. The presence of gas hydrate is one of the problems when developing conventional oil and gas fields in deepwater offshore. To-date gas hydrate bearing sediments have been drilled through without any major problems in numerous locations in the Canadian and Alaskan arctic, and Gulf of Mexico (Tan, et al, 2005) , (Smith, et al, 2005) . The techniques used to overcome drilling problems are reducing the drilling fluid temperature, increasing the hydrostatic mud pressure and chemically stabilizing the gas hydrate (Tan, et al, 2005) . Nevertheless the lack of a tool to predict the behaviour of the well drilled in gas hydrate bearing sediments, has resulted in a strategy of avoiding hydrate bearing sediments when locating deep offshore production platforms. This could increase the cost of development for deep offshore oil and gas fields. Casing stability is an important part of the well design, therefore it is necessary to develop a tool to predict casing behaviour for wells drilled in gas hydrate bearing sediments. Standard casing design ignores the interaction of casing-cementformation on the required strength of casing (Berger, et al, 2004) . Indeed, there is not a simple method available to determine the magnitude of this effect. In addition, the conventional casing design fails to account for the non-uniform loaded casing.
doi:10.2118/113819-ms fatcat:zulsf6477vfqvfsjzshvuphwzu