海底天然气水合物分解对海洋钻井安全的影响

为分析天然气水合物分解对海洋钻井安全的影响,根据海底天然气水合物特征,结合天然气水合物分解动力学和热力学条件,研究了不同钻井工况下天然气水合物分解产气规律,估算了天然气水合物分解后的产气量。结果表明,在钻进天然气水合物层过程中,天然气水合物分解产气速率和累计产气量逐渐增大;在天然气水合物饱和度一定的情况下,近井天然气水合物层内的天然气水合物完全分解产气量与井身轴向半径呈平方关系;随着钻井液与天然气水合物层温差增大,天然气水合物分解速率呈指数增长;浅水区钻遇天然气水合物层易导致其分解,随着水深增加或井筒压力增大,天然气水合物分解越来越困难。研究表明,钻穿天然气水合物层时,提高钻进速度可减少天然气水合物分解;钻井过程中应根据钻前预测结果调整钻井液温度和密度来控制天然气水合物分解,同时采取必要的井控措施,以保证在适当的天然气水合物分解产气条件下安全钻进。 

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