Study On The Acoustic Emission Characteristics And Cracking Mechanism Of Shale

The proven developed reserve of Chinese shale gas ranks first in the world.However,the National Shale Gas Development Plan pointed out that the forward-looking technologies for shale gas exploration should be strengthened to reduce the cost and ensure the benefit and sustainable development of shale gas.The key to increasing shale gas production is constructing fracture networks in shale reservoirs to provide shale gas channels.Given this,the shale in Shizhu county of Chongqing was investigated to study the evolution of crack initiation,propagation and coalescence behaviors affected by bedding layers and pre-existing flaw,and thus to analyze the micro-damage mechanism of shale.The main research content and results are listed as follows:（1）A mechanical-optical-acoustical comprehensive data acquisition system consisting of a rigid hydraulic machine,high-speed industrial camera and acoustic emission acquisition instrument was developed.Brazilian tests were carried out on complete shale disc specimens.Sandstone specimens without bedding layers were also tested for comparison.The influences of bedding layers angles on the strength,macroscopic fracture morphologies,acoustic emission parameters,frequency spectrum characteristics and moment tensor properties were studied.Three kinds of macroscopic fracture morphologies were found for shale discs under the effect of indirect tensile stress,i.e.,fracture in straight-line shape,fracture in arched-shape and fracture in complex shape.A quantitative evaluation method for different fracture morphologies was proposed by analyzing AE frequency spectrum characteristics.（2）Using the mechanical-optical-acoustical comprehensive data acquisition system,uniaxial compression tests of shale specimens with different bedding layers and pre-existing flaw orientations were carried out.Sandstone specimens with different pre-existing flaw angles were also tested for comparison.The effects of the orientations of bedding layers and pre-existing flaw on the uniaxial compressive strength,cracks evolution behaviors,acoustic emission frequency spectra and moment tensor inversion results were studied.The tensile and shear damage mechanism in microscopic were analyzed.The results show that there are significant differences between the cracking behaviors of shale and sandstone specimens in the uniaxial compression tests.In shale specimens,crack initiation is predominantly controlled by the pre-existing flaw and is also affected by bedding layers.Crack propagation is greatly controlled by bedding layers and stress field distribution.When the bedding layers are vertical,the cracks are most likely to propagate along the direction of the bedding layers,and tensile cracks are observed.When the bedding is 30°,the shale specimens are most likely to be controlled by the bedding layers,resulting in shear slip failure along the bedding layers.（3）Based on the built-in Parallel Bonding Model of Particle Flow Code PFC^2D,numerical models of thin-layered shale were established.The microscopic mechanisms of crack initiation and propagation evolution of sandstone and shale under tensile and compressive stress were analyzed.The results show that the arc-shaped fracture morphology of the shale disc is induced by the path selection of tensile cracks between the interlayers and innerlayers.In this process,a step-shaped evolution path is formed,and thus an arch-shape is formed in macroscope.In the uniaxial compression tests of shale,maximum principal stress mainly concentrates at the tips of the pre-existing flaw in a butterfly shape,and maximum shear stress not only distributes at the tips of the pre-existing flaw,but also concentrates in the range of 45°-60°towards loading direction.The cracks evolution process of the flawed shale specimen is mainly controlled by tensile stress,while the shear stress controls the final failure of the specimen.（4）A meso-parameters bedding layer strength weak coefficientsλwas proposed to study the influences of different bonding strength of the interlayers on the failure mechanism of shale specimens.The relative relationship of micromechanical parameters of shale bedding layers significantly controls the microscopic failure mechanism and macroscopic failure characteristics of shale specimens.When the tensile strength and cohesion strength of interlayers are similar to those of innerlayers and the bedding angles are relatively high,the straight-line shape fractures are highly possible to form,and they are mainly manifested as tensile cracks.The arched-shape fractures are highly possible to form at other bedding layers angles,and they are also mainly contributed by tensile cracks.When the tensile strength and cohesion strength of interlayers are much smaller than those of innerlayers,it is easy to form fractures in complex shape,in which the arched section is mainly contributed by tensile cracks,and the straight line section is mainly contributed by shear cracks.The bedding layer strength weak coefficientλsignificantly effects the morphology,distribution and number of cracks in flawed shale under uniaxial compression stress.The normalized maximum load is also controlled by the bedding layer strength weak coefficientλand the bedding layers angles.