The Influence of Subsea Natural Gas Hydrate Dissociation on the Safety of Offshore Drilling

When subsea natural gas hydrate (NGH) dissociation is encountered, offshore drilling safety is notably influenced. Gas production performances of NGH were analyzed and gas production after hydrate deposition was estimated under different drilling conditions according to the dynamics and thermodynamics of NGH dissociation. The calculations were based on the fundamental characteristics of subsea NGH, which provided the basis for evaluating quantitatively the influence of natural gas hydrate dissociation on the safety of offshore drilling. The results showed that as drilling proceeded through hydrate bearing zones, gas production rate and cumulative gas production of hydrate dissociation tends to rise, and that the gas production was in a squared exponential relationship with the NGH dissociation radius under certain NGH saturation. NGH dissociation rate rose exponentially with the increase of the temperature difference between drilling fluid and NGH sediment, and drilling through hydrate layers in shallow water tended to break it down and as water depth and wellbore pressure rised, NGH dissociation became more difficult. Increasing the drilling rate was preferable for reducing NGH dissociation when drilling through hydrate layers. It was demonstrated that NGH control during offshore drilling should focus on NGH prediction before drilling, and that drilling fluid density and temperature adjustment and essential well control measures should be taken to ensure drilling safety under the condition of certain gas hydrate decomposition