川南龙马溪组页岩地层井壁崩塌破坏机制研究

Study on the Failure Mechanism of Borehole Collapse in Shale Strata ofLongmaxi Formation in Southern Sichuan

针对四川龙马溪组页岩井壁易坍塌问题,首先对龙马溪组地面露头开展了微细观组构分析和基础力学实验,明确了其矿物组成、裂缝分布及基础力学参数;其次利用真三轴实验设备开展了原地应力场与井筒力学环境下三轴力学加载与井壁崩塌破坏规律研究,通过改变层理缝倾角研究页岩井壁崩塌失稳方向与失稳类型,利用内窥镜记录岩石力学加载过程中裂隙动态轨迹变化,根据崩塌岩块形状研究井壁崩塌失稳类型;最后利用RFPA 2D(真实破裂过程分析)软件验证了页岩井壁崩塌破坏规律。结果表明:龙马溪组页岩矿物组成以石英、伊利石及伊蒙混层为主,水化膨胀性能较弱,页岩岩体层理较为发育,是液相侵入岩石内部的主要渗流通道;当层理缝倾角为0°时,井壁破坏为劈裂和穿越层理缝剪切破坏;随着层理缝倾角的不断增大,井壁岩石倾向沿裂缝面剪切滑移破坏;当层理缝倾角为90°时,井壁岩石倾向沿层理缝劈裂张拉破坏。研究结果揭示了页岩井壁失稳机理与主要影响因素,可为后续井壁稳定性研究提供支撑。

In order to solve the problem that the shale borehole of Longmaxi Formation in Sichuan is prone to collapse,microscopic structural analysis and basic mechanics experiments were carried out on the surface outcrops of Longmaxi Formation,and the mineral composition,fracture distribution and basic mechanical parameters were clarified;secondly,the true triaxial test equipment was used to study the in-situ stress field and the failure law of the borehole wall collapse under the mechanical environment of the wellbore,the shale borehole wall collapse instability direction and instability type were studied by changing the dip angle of the bedding fracture,the dynamic trajectory change of the fracture during the rock mechanical loading process was recorded by the endoscope,and the borehole wall collapse instability type was studied according to the shape of the collapsed rock block;finally,the RFPA2D(Real Fracture Process Analysis)software was used to verify the failure law of shale borehole collapse.The results show that the shale mineral composition of the Longmaxi Formation is dominated by quartz,illite and Yimeng mixed layers,with weak hydration and expansion performance,and the shale rock mass bedding is relatively developed,which is the main seepage channel for liquid phase intrusion into the rock;when the inclination angle of the bedding fracture is 0°,the failure of the borehole wall is splitting and shear failure through the bedding fracture;with the continuous increase of the dip angle of the bedding fracture,the rock of the borehole wall tends to fail along the shear slip of the fracture surface;when the dip angle of the bedding fracture is 90°,the borehole wall rock tends to split tensile failure along the bedding fracture.The results reveal the mechanism and main influencing factors of shale borehole instability,which can provide guidance for subsequent borehole stability research.