An investigation of interaction of drilling fluids with gas hydrates in drilling hydrate bearing sediments

• Thermal Stimulation causes natural gas hydrate dissociation which may lead to wellbore instability.
• Mud gasification, partial washout or caving and casing subsidence are some form of instability due to the NGH dissociation.
• Different additives in mud formulation may help wellbore stability and prevent NGH dissociation.
• By monitoring temperature linear distribution in THF-hydrate, the higher performance mud can be found.
• Using high performance mud and cooling it will help wellbore stability during drilling operation in NGHBS.

Natural gas hydrates, preserved in deep ocean sediments, are supposed to be the future hydrocarbon source of energy. The possibility of gas production from natural gas hydrate bearing sediments (NGHBS) has been scrutinized by many researchers. Thermal stimulation, depressurization and use of thermodynamic inhibitors are three main proposed approaches to produce gas from the hydrates. When drilling through NGHBS, these mechanisms may cause wellbore instability and other drilling hazards such as severe mud gasification, low quality logging and cementing, casing collapse due to high pressure gas accumulation behind the casing, casing subsidence due to NGHBS failure and consequently instability of the ocean floor. In this study, the mechanism of thermal stimulation was studied. An experimental set up was designed and manufactured to investigate hydrate behavior when it comes in contact with warmer drilling fluids. Several muds with different additives were tested to investigate which properties are responsible for wellbore integrity maintenance. For this intention, linear temperature distribution in the hydrate was recorded for near distance from hydrate–mud contact and the hydrate dissociation rate was calculated. The purpose was to reduce the heat flux of the drilling fluid into the hydrate using relevant additives or by altering the mud circulation rate. Experimental results were validated by a hydrate dissociation model to obtain viable assessment for designing prospective exploration wells.