深水钻井导管下深区间确定及风险评价方法

深水钻井导管的下入深度直接关系到海底井口的力学稳定性及后续钻井作业的安全。通过分析,确定导管存在最小下深和极限下深,并给出极限下深计算模型。基于概率统计和地质统计学方法,将目标点邻井的承载力增长系数进行概率统计,并通过差值算法对目标点进行移植预测,得到该系数在目标点处含有概率信息的预测值,将导管最小下深由单值变为含概率信息的分布带,同时结合导管极限下深的概率化分布,对目标井的导管设计下深进行定量风险评价。实例分析表明:承载力增长系数这一区域经验变量对深水导管下深影响很大,尤其当取值较小时,导管下深随着该系数的减小呈指数增长;该方法在给定可信度条件下可计算导管绝对安全下深区间,有效避免了因承载力增长系数不确定等因素导致的导管下深风险。

Determination and risk assessment methods for setting depth of conductor during deepwater drilling

The penetration or setting depth of conductor pipe can directly affect the mechanical stability of subsea wellhead and the safety of subsequent drilling operations. There should be a minimal setting depth and a limit setting depth, and a mathematical model for calculating the limit setting depth can be established. In this paper, probability statistics and geo-statistics methods were used for the assessment of the bearing capacity of the target point based on the data from adjacent wells. The bearing capacity growth factor of the target point can be predicted using a differential arithmetic method, in which the statistics information was included. Therefore, a distribution range for the minimal setting depth of the conductor pipe can be determined using the statistics data, and by combining its limit setting depth data, a quantitative risk assessment can be carried out for the conductor ''s setting depth. Case studies show that the bearing capacity growth factor is a regional and empirical variable that has a great impact on the setting depth of deepwater conductor, and the setting depth of conductor grows exponentially with the decrease of the growth factor, especially when its value is small. This method can calculate an absolute safe range for conductor ''s setting depth under the conditions of a given confidence, which can effectively avoid the risks caused by uncertainties of the bearing capacity growth factor and other factors.