Study on the Failure Mechanism of Borehole Collapse in Shale Strata of Longmaxi Formation in Southern Sichuan
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 in-stability 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 col-lapse.The results show that the shale mineral composition of the Longmaxi Formation is dominated by quartz,il-lite 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 inclina-tion 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.
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