深水水合物钻井导管下深设计与地层安全承载研究

为保障深水天然气水合物开采的安全进行，降低钻井导管组合系统下沉和水合物地层失稳的事故风险，针对深水天然气水合物钻井作业过程中导管喷射到位解锁、表层套管固井和紧急脱离等3个阶段导管组合系统的下入深度设计与竖向承载力进行研究。结合桩基理论建立不同阶段作业工况下钻井导管竖向承载力的计算模型，确定不同作业阶段钻井导管的最小下入深度，然后基于TOUGH+HYDRATE和FLAC3D软件建立水合物地层稳定性数值分析模型，研究试采作业过程中水合物地层的稳定性，根据摩尔-库伦破坏准则，确定水合物地层的安全试采时间。以南海某天然气水合物喷射钻井作业为例，考虑浅层土壤的工程地质特征以及钻井作业过程中由于钻井液侵入导致水合物地层分解，给出导管安全下深设计的推荐值为98 m，为保证水合物地层的安全承载，建议水合物的安全试采时间不应超过60 d。研究结果可为深水天然气水合物钻井导管的现场作业提供技术参考。

The design on the setting depth of drilling conductor and the study on the safe bearing load of formation in deepwater hydrate exploitation

In order to ensure the safe exploitation of deepwater natural gas hydrate and reduce the accident risks of the sink of drilling conductor combination system and the instability of hydrate formation, it is necessary to study the setting depth design and vertical bearing capacity of conductor combination system in the three stages of drilling deepwater natural gas hydrate formation, i.e., jetting to the pre-set position and releasing of conductor, cementing of surface casing and urgent separation. According to the theory of pile foundation, the model for calculating the vertical bearing capacity of the drilling conductor under the working conditions in different stages was established to determine the minimum setting depth of the drilling conductor in different operation stages. Afterwards, based on the software TOUGH+HYDRATE and FLAC3D, the model for numerically analyzing the stability of hydrate formation was established to research the stability of hydrate formation in the process of production test. In addition, the safe production test time of hydrate formation was determined according to the Mohr-Coulomb failure criterion. Finally, a case study was carried out on a certain jet drilling operation in the natural gas hydrate formation of the South China Sea. In view of the engineering and geological characteristics of shallow soil and the decomposition of hydrate formation caused by the invasion of drilling fluid, the safe setting depth of conductor is recommended to be 98 m. And in order to ensure the safe bearing load of hydrate formation, it is suggested to set the safe production test time less than 60 d. The research results provide the technical reference for the field operation of drilling conductor in deepwater natural gas hydrate formations.