Lamina Influences on Tensile Strength of Shallow Marine Shales from Upper Ordovician, Western Ordos Basin

To investigate the influence of the lamina effect onthe tensilestrength of shallow marine shales and improve the shortcomings ofthe existing Brazilian standard disc splitting method, the Laminashale of the Upper Ordovician Wulalik Fm in the western margin ofthe Ordos Basin was selected as the experimental object. Based onthe radial wave velocity anisotropy test, the direction of crustalstress was determined, and standard cores were drilled. The Brazilianstandard disc splitting experiment on Lamina shale with differentloading angles was designed and carried out. The influence of laminaon the tensile strength of shale was summarized, and an improved calculationmethod of tensile strength was proposed. The experimental resultsindicate that the presence of lamina makes the tensile strength ofshallow marine shale exhibit significant anisotropy, and the fracturesurface morphology of standard discs under different loading anglesvaries greatly. The overall failure characteristics can be classifiedinto two types: linear and curved. When the loading angle is 0 & DEG;or 90 & DEG;, the fracture surface of the disc belongs to tensile failure(linear type), and the traditional splitting method has good applicability.When the loading angle is greater than 0 & DEG; and less than 90 & DEG;,the fracture surface of the disc belongs to tensile shear failure(curve type), and traditional splitting methods are not applicable.There is a difference in tensile strength between vertical and horizontalwells, and vertical wells should consider the comprehensive tensilestrength of the rock matrix and lamina at a 90 & DEG; loading angle.Horizontal wells should consider the tensile strength of the weaklamina plane with a loading angle of 0 & DEG;. The improved Braziliansplitting method solves the problem of the traditional method, calculatinglower tensile strength values when the loading angle is greater than0 & DEG; and less than 90 & DEG;. This provides important basic datasupport for wellbore stability evaluation and reservoir stimulationtransformation.

Automated summary

This study investigates the influence of the lamina effect on the tensile strength of shallow marine shales and improves the existing Brazilian standard disc splitting method by selecting the Laminashale of the Upper Ordovician Wulalik Fm as the experimental object. The study determines the direction of crustal stress through the radial wave velocity anisotropy test and drills standard cores. The Brazilian standard disc splitting experiment was designed and carried out on Lamina shale with different loading angles. The results show that the presence of lamina makes the tensile strength of shallow marine shale exhibit significant anisotropy and the fracture surface morphology of standard discs varies greatly under different loading angles.