Transversely Isotropic Shale With Prefabricated Cracks Under Brazilian Tests

On the edge of the Sichuan Basin,southeastern of Chongqing,the cap rock of the oil and gas reservoirs is mainly shaly.As one of the key shale gas exploration and development areas in China,its cap rock’s continuity is good and the development of the cap rock is also high.Once the mining technology becomes mature,it can efficiently output shale gas,which will provides a powerful guarantee for alleviating energy shortage.However,there still exists poor understanding of the mechanical properties of shale in the shale gas exploration and mining process,especially its transversely isotropic mechanical properties.Which will cause many problems,such as hydraulic fracturing optimization and wellbore instability in the process of shale gas extraction,resulting in shale gas can’t be produced on a large scale efficiently.Therefore,the study of the mechanical properties of shale in the southeastern of Chongqing can provide necessary theoretical supports for the efficient extraction of shale gas.To study the transversely isotropic mechanical properties of shale,the indoor mechanical tests,theoretical analysis,and numerical methods of finite element method were all used in the Brazilian split test of pre-cracked shale specimens under different bedding angle.The main research contents are as follows:(1)The Brazilian split test of transversely isotropic shale specimens at different bedding angles was performed with the using of high-speed camera and acoustic emission monitoring.By analyzing the experimental results,the effects of bedding angle on the mechanical properties of transversely isotropic shale are obtained.Combined with the transverse isotropy constitutive theory,the elastic parameters E 、v 、 E?、v? and G? of transversely isotropic shale are calculated.(2)According to the load-displacement characteristics,acoustic emission monitoring and high-speed camera monitoring results obtained from the test,the mechanical properties,crack propagation patterns,and failure modes of the shale under the joint action of the bedding plane and the pre-crack were analyzed.It is found that when the bedding angle α and the pre-crack angle β are equal or perpendicular,the peak load drop of the test piece is very obvious,which is basically the lowest level among the peak load results of all the test pieces.(3)According to the experimental results of intact shale`s Brazilian split test and the Brazilian split test of pre-cracked shale specimens,the effects of pre-crack on the mechanical properties of shale were compared and analyzed.It was concluded that the pre-crack severely weakens the strength of shale,and the pre-crack also affects the crack propagation morphology and the ultimate failure mode of these specimens.Specifically,when the angle of pre-crack is β=30°,the bedding angle satisfies 0°≤α<60°,and when the crack angle satisfies 30°<β≤90°,compared with the bedding plane,pre-crack is the main influencing factor of shale fracture mechanics properties in brazilian split test.When the pre-crack angle β= 0 °,pre-crack mainly affects the strength of the specimen,and have no obvious effect on the specimen’s failure mode.(4)By using ABAQUS(a finite element software),the elastic parameters obtained in the experiment were substituted into the established numerical model,and numerical calculations were performed on specimens with pre-crack at different angles.Comparing the numerical results with their experimental results,the results show that the numerical results were highly similar to the real test results.At the same time,according to the numerical calculation of the stress distribution nephogram and the maximum principal stress vector distribution map,the above analysis of its experimental results were verified.