深水钻井钻遇水合物地层井壁稳定性研究

The Wellbore Stability During Deep-water Drilling Operations in Hydrate Formation

深水浅部地层钻井过程中易钻遇天然气水合物,由于其易分解的特性,使得水合物层在钻井过程中极易出现井壁失稳。本文通过Abaqus有限元软件,建立了考虑水合物分解的多场耦合井壁稳定模型,分析了钻井时间、钻井液温度、海水深度对井壁稳定的影响。结果表明:钻井工程的扰动会导致水合物的分解,造成地层强度降低,增加井周的塑性屈服区域,引发井壁失稳;随着井眼钻开时间的增加,这种不稳定性会进一步增大;井壁的不稳定程度随钻井液温度增加非线性增加。井底压差一定时,海水深度越小,井眼周围地层的井壁不稳定性越大。因此,在实际的深水钻井过程中,选择水深更大方位进行钻探更利于维持水合物层井壁的稳定。

Natural gas hydrates are easily encountered during deep-water drilling operation at shallow formations.Because hydrate is easily decomposed,the wellbore will be easily at risk of instability during drilling.In this paper,a thermal-fluid-solid coupling wellbore stability model with hydrate decomposition was established based on Abaqus finite element software,and the effects of drilling time,drilling fluid temperature and seawater depth on wellbore stability were analyzed.The results show that the disturbance of the drilling engineering may cause the decomposition of hydrates,reduce the strength of the formation,increase the plastic yield area around the wellbore,and then cause the wellbore instability.As the borehole drilling time increases,this risk of instability will further increase.The instability of the wellbore increases non-linearly with the increase of drilling fluid temperature.When the bottom hole pressure difference is constant,the lower the seawater depth,the greater the instability of the formation around the wellbore.Therefore,choosing a larger water depth is more conducive to hydrate formation stability when the underbalanced drilling method is used in deep water.