基于含可信度地层压力剖面的精细井身结构设计方法

Methods for precise casing design based on formation pressure profiles with different credibility

对现有深井井身结构设计中安全密度窗口约束条件进行分析,发现井涌允量、破裂压力安全系数、抽吸压力系数等经验性安全系数的取值较为保守,且没有考虑循环钻进时防漏的约束条件,这对深井尤其是下部安全密度窗口窄的井钻井不利。通过引入含可信度的地层压力剖面以及附加钻井液密度和抽吸压力的计算分析,使安全密度窗口的上下限得以精确确定,在此基础上加入考虑环空压耗当量密度的约束条件,并给出迭代计算方法,解决了环空压耗计算中需要井身结构参数的难题,避免了深井循环钻进过程中的井漏问题,进而形成了一套基于含可信度地层压力剖面的、适用于深井的精细井身结构设计方法,可以有效避免深井中由于钻井液密度设计不合理导致的井下复杂事故的发生,且其设计结果具备一定的风险控制功能,有利于结合现场实际对设计进行风险控制。

The constraints of the safe density window in the existing casing design program for deep well drilling were ana-lyzed. It is found that the values taken for the empirical safety factors, such as kick tolerance, fracture pressure coefficient and swabbing pressure factor, are relatively conservative, and there is no constraint considering the loss during well circula-tion operations, which is a weak point for deep well drilling, especially for the wells with narrow safe density window in the lower stratum. A method to accurately determine the upper and lower limits of the safe density window was proposed consider-ing the formation pressure profiles with different credibility, additional drilling fluid density and swabbing pressure, and a constraint considering the loss during well circulation operations was also added in the calculation program. Thus, a new method for precise casing design for deep wells was established, which can be based on formation pressure profiles with dif-ferent credibility, and simplify the calculation of the annulus pressure loss and can avoid the loss problems during well circu-lation operations. The new method can effectively control and reduce the risks caused by irrational fluid density during casing design of deep wells.