The mechanical properties of hydrate bearing sediments may deteriorate as a result of hydrate dissociation due to heating of the wellbore casing, with possible adverse effects on the stability of the casing. The authors have created a numerical model in plane strain in order to simulate the stability of a well-bore supported with a casing. The model captures the interaction between the formation, casing, cement and cement-casing bond. The simulation results demonstrate that the safety factor of the casing is very dependent on the quality of the cement, for with strong cement, it is multiples of the safety factor of the casing with weak cement. The safety factor for the casing with weak cement, in the presence of hydrates in the formation, reduces due to hydrate dissociation and the subsequent deterioration of the mechanical strength of the formation. This paper shows that in this case an effective cement placement around the casing becomes crucial in order to keep the casing factor within acceptable limits.
Casing deformation resulting from reservoir compaction and surface subsidence during the primary production stage has been analysed and reported by many investigators 1,2,3. Generally the following effects can be considered:
Pressure depletion in the formation, which can lead to subsidence and possible shearing of the casing
Heating the formation, which can generate high stresses in the formation
Deterioration of the sediment mechanical properties due to heating as for hydrate bearing sediments, which can induce large stress in the casing.
Casing integrity for well drilled in hydrate bearing sediments presents a challenge that is explored in this paper.
Gas hydrate, or clathrates, are ice-like deposits containing a mixture of water and gas, the most common gas being methane. Other gases of small molecular size such as CO2, H2S, ethane and higher hydrocarbons are also found in natural gas hydrates. Gas hydrates are stable under high pressure and low temperature conditions, and hence they can be found in deepwater settings at relatively shallow depths below the seafloor and in permafrost regions 4. The dissociation of gas hydrates can take place due to a reduction in pressure and/or an increase in temperature, or due to the injection of certain chemicals that may affect hydrate stability. Once melted due to the circulation of hot fluid in the casing. Hydrate bearing sediments will suffer degradation in their mechanical properties (e.g, strength and stiffness), accompanied by overpressure development5. Moreover the gas released by hydrate dissociation may undermine the quality of the cement supporting the casing (if dissociation takes place before the cement is fully cured) and hence reduce the collapse resistance of the casing6.
This the well by the surrounding sediments in response to heating. Some of the sediment considered are hydrate bearing sediments (HBS). The cements examined could be strong cement (foam cement) or weaker cement (hard cement). The cement-casing bond strength is investigated in the modelling. The modelling is performed using FLAC3D, which is a continuum code developed by Itasca Consulting Group 7,8.