气体钻井井斜有限元岩石应力分析

气体钻井易井斜，钻井施工主要通过调整钻具组合和钻井参数进行轨迹控制，然而在高陡构造地区，气体钻井井斜变化严重超出井身质量控制要求，为后期作业带来了严重影响。因此，气体钻井井斜变化必须考虑井筒中不同循环介质的影响和井壁稳定程度的影响，而现有的数学模型复杂，计算量大，不能真实地模拟井下钻井的力学变化。为此，利用有限元理论，建立了实体三维力学模型，可以充分考虑井筒中不同循环介质的影响和井壁稳定性的影响进行Von Mises应力计算，根据地层应力值的变化，可以直观地分析出气体钻井井斜规律。通过对该模型力学计算结果的分析表明：在相同地层倾角和岩石强度下，气体钻井井斜幅度大于钻井液钻井的井斜幅度，井壁更不稳定，钻井时更易出现井眼扩大现象；当地层倾角小于45°时，井斜具有上倾趋势；地层倾角等于45°时，井斜趋势不明显；地层倾角大于45°时，井斜具有下倾趋势。

Finite element analysis on rock stress status during gas drilling deviation

Well deviation is common in air drilling, and the profile is often controlled by adopting anti deflection BHA or selecting proper drilling parameters. However, the gas drilling deviation is so serious in high dip structure, it is not possible to maintain a good well profile by those control measures. The effects of different circulation media and wellbore stability must be considered when studying gas drilling deviation. The mathematic models now available are complicated, lots of calculation required and the drilling dynamics variation unable to be simulated, so it cannot reflect the true well deviation variation. Here in this paper the finite element theory is used to set up a 3D mechanical model, and the effects of different medium and wellbore instability are considered when doing VON MISS stress calculation. Therefore the gas drilling deviation rule can be directly summed up based on the formation stress variation. The calculation results using this model show that in comparison with conventional mud drilling, when drilling in conditions with same formation dip and same rock strength, the deviation in air drilling is larger than that in conventional mud drilling, the wellbore is more unstable in air drilling, and it is easier to have well enlargement in air drilling. it is also found that than when formation dip is less than 45 °, the hole deviation tends to go updip; when formation dip is equal to 45 °,  the hole deviation trends is not certain; when the formation dip is greater than 45 °, the well deviation tends to go downdip.