Study On Crack Propagation Law And Failure Mechanism Of Layered Shale Under Uniaxial Compression

As traditional fossil energy produces a large amount of harmful gases when burned,it further intensifies global environmental pollution and the greenhouse effect.In order to protect the earth's ecological environment,it is urgent to develop alternative clean energy.In the national"13th Five-Year Plan",my country will increase the optimization and improvement strategy of energy composition,among which the development of shale gas is one of the research hotspots.Shale gas is stored in the dense shale,and it is generally necessary to create artificial cracks during mining to form a channel for oil and gas flow.Based on the consideration of multiple factors such as improving the mining efficiency of shale gas,avoiding hidden accidents caused by rock instability,and ensuring the safety of engineering construction,it is necessary to fully grasp the crack propagation law and failure mechanism of shale.Up to now,there have been a large number of research results on the crack propagation law and failure mechanism of fractured rock masses.Previous studies mainly focused on the influence of factors such as hydraulic coupling and bedding angle on the mechanical behavior of shale,and less attention was paid to the study of mechanical behavior of shale with holes.However,in the actual mining process of shale gas,the holes and openings left after the excavation of horizontal wells have a greater impact on the mechanical properties of shale.In response to this problem,this paper conducts uniaxial compression tests on layered shale with different apertures.Acoustic emission is used to collect and analyze the acoustic emission signals of the sample during the entire compression process,and DIC technology is used to analyze the tensile strength of the sample at the moment of failure.Shear strain field evolution,and based on RFPA^2Dnumerical simulation software,the crack propagation law and failure mechanism of shale with different bedding angles are studied.The main conclusions of the paper are as follows:(1)Obtain the basic mechanical parameters of shale through uniaxial compression and triaxial compression tests.Based on the Poisson's ratio and elastic modulus evolution of porous layered shale samples under different confining pressures,the anisotropy of shale is revealed.The longitudinal wave velocity of the shale sample decreases significantly with the increase of the bedding angle.(2)When the bedding angle is less than 45°,the specimen will produce a large number of tensile cracks during the uniaxial compression process,which manifests as tensile failure;when the bedding angle is 45°,the specimen will be in the uniaxial compression process Shear cracks appear,and their failure forms are tensile-shear mixed failure;when the bedding angles are 60°and 75°,large-aperture specimens appear as tensile failure,and small-aperture specimens appear as tensile-shear mixed failure;When the bedding angle is 90°,perforation cracks appear in the sample during uniaxial compression,and tensile cracks are the main ones.The results show that the larger the hole diameter,the more obvious the tensile effect of the sample,the easier it is to produce perforation cracks;the smaller the hole diameter,the more shear-type cracks are likely to appear in the sample.(3)Based on the Digital Image Correlation Technology(DIC),when the bedding angle is 0°-75°,the final failure mode of the sample with an aperture of 2 mm is mainly divided into shear failure along the bedding angle and non-bedding failure.Tensile-shear mixed failure;tensile high-strain areas mostly appear on the left in low-angle bedding,and tensile high-strain areas mostly appear on the right in high-angle bedding;when the bedding angle is 90°,the samples are all A strong tensile strain zone appears.When the hole diameter is 2 mm,the sample appears to be shear strain concentrated and tensile-shear mixed failure;when the hole diameter is 6mm,the sample appears to be tensile failure;when the bedding is lower at 0°,the sample appears to be tensile-shear mixed Failure:In addition to the bedding angles of0°and 75°,the specimens with a diameter of 10 mm have high tensile strain areas at the round holes,which are tensile failures.(4)The acoustic emission amplitude of the shale sample is basically between60-80 d B;the maximum ringing counts all appear in the sample with an aperture of 2mm,followed by the sample with an aperture of 10 mm,and finally The sample with an aperture of 6 mm;when the bedding angle is 0°,15°and 90°,the sample with an aperture of 2 mm has the largest acoustic emission cumulative ringing count;when the bedding angle is 30°-60°,the aperture The accumulative ringing count for the acoustic emission of the 10 mm sample is the largest.According to the difference of acoustic emission response and stress state evolution,the failure of shale is divided into two modes:brittle failure and weak brittle failure.Brittle failure is divided into four stages,namely compaction stage,crack initiation stage,crack propagation stage and failure stage;weak brittle failure is divided into two stages:compaction stage and failure stage.(5)Combining the results of acoustic emission and DIC analysis,when the bedding angle is 0°and 15°,the cracks produced by the sample are mainly shear cracks.When the aperture is 6 mm,it appears as a tensile crack;the bedding angle At30°and 45°,when the aperture is 2 mm,the cracks produced by the sample are tensile-shear mixed cracks,and the samples with other apertures are all tensile cracks;when the bedding angle is 60°and 75°,the aperture When the diameter is 2 mm,the cracks produced by the specimens are tensile cracks,and the specimens with other apertures are all tensile-shear mixed cracks.When the bedding angle is 90°,the aperture has little effect on the failure mode of the sample,and the samples all have mixed tensile-shear cracks,and the cracks are mainly tensile cracks.(6)Based on the results of CT scans,an RFPA^2Dmodel of porous layered shale was established.The crack propagation and failure modes of shale are studied.The simulation results are in good agreement with the experimental results,which verifies the reliability of the modeling method in this paper.