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中国页岩储层钻探方面处于起步阶段,钻井过程中复杂情况频发,井壁坍塌问题严重。借鉴北美地区页岩气钻井经验,对比中国四川盆地与北美Haynesville的页岩气水平井井身结构,发现随井眼尺寸减小,钻井复杂情况有所降低。研究过程中,结合岩石强度的尺寸效应理论,比较不同强度尺寸模型的优缺点,优选Vutukuri模型分析微小井眼井壁围岩的强度变化规律,并根据McLamore各向异性破坏准则确定岩石单轴抗压强度随主应力与层理面法向之间夹角的关系,以此作为页岩储层强度准则;根据定向井坍塌压力计算方法分析井周应力状态,结合尺寸效应与强度准则,分别得出标准井眼和微小井眼在层理性地层中的坍塌压力随井眼轨迹的变化规律,同时探究了固定井眼方位下的坍塌压力随井斜角变化规律。对比不同井斜角下两者的差距,结果表明,不论直井或定向井,采用微小井眼均能显著降低钻井坍塌压力,降低幅度约为10%,研究结论揭示了小井眼减少钻井复杂的内在原因。
The shale reservoir drilling in China is in its infancy. The complicated situation in the drilling process is frequent and the wellbore collapse problem is serious. Drawing on the experience of shale gas drilling in North America, comparing the wellbore structure of shale gas horizontal wells in China’s Sichuan Basin and North America with Haynesville, it is found that the drilling complexity decreases as borehole size decreases. In the course of the study, the strength-dependent size effect theory was used to compare the strengths and weaknesses of different strength and dimensional models. The Vutukuri model was used to analyze the strength variation of the surrounding rock of the micro-well borehole. The uniaxial resistance of the rock was determined according to McLamore’s anisotropic failure criterion The relation between the compressive strength and the included angle between the principal stress and bedding plane normal is taken as the shale reservoir strength criterion. According to the calculation method of well collapse pressure in directional well, the circumferential well stress state is analyzed. Combined with the size effect and strength criterion, The collapsing pressure of standard boreholes and micro-boreholes in the stratigraphic strata is changed with the trajectory of wellbore. At the same time, the variation law of collapse pressure with wellbore angle under fixed wellbore orientation is investigated. Comparing the differences between the two wells, the results show that the use of micro-boreholes can significantly reduce the collapse pressure of wells, both in vertical wells and directional wells, with a reduction of about 10%. The conclusions of the study reveal that the slim borehole can reduce the complexity of drilling the reason.