Study On Failure Characteristics And Numerical Simulation Of Fissured Shale

Shale gas is widely distributed in China,with abundant reserves and great development potential.In recent years,the exploration and development of shale gas is a key link to ensure the security of the national energy supply system,and reservoir reconstruction has become an important means of development.The structure of shale is complex,and besides bedding,it contains natural or artificial defects such as fissures（joints）with different numbers and shapes.These defects have an important i MPact on the crack propagation of shale reservoirs,and then affect the reservoir reconstruction.Therefore,it is necessary to explore the crack propagation and evolution law of shale with fissures under stress.In this paper,in order to study the shale with multi-fissures,the shale with single and double fissure（s）is taken as the research object.Acoustic emission（AE）and digital image correlation（DIC）are used for monitoring,and uniaxial compression test and numerical simulation are carried out to study the mechanical characteristics,failure mode and crack propagation law of the shale with fissure（s）.The main results are as follows:（1）In the uniaxial compression test,the peak strength,peak strain and elastic modulus of shale with single and double fissure（s）are positively correlated with fissure angle,and the peak strength,peak strain and elastic modulus of shale with single fissure are all higher than those with double fissures.（2）AE count can reflect the damage accumulation of the specimen,and DIC can effectively monitor the cracking process of the specimen and predict the initiation position of crack.Based on the characteristics of RA-AF,the tensile crack is dominant,but more shear cracks are produced when it finally fails.（3）The initiation and distribution of the initial macroscopic crack of the fissured specimens are similar,basically starting from tensile crack,starting from the middle of the fissure at low fissure angle,and then starting from the fissure tip.With the increase of fissure angle,the crack initiation stressσ_imcand the crack initiation stress levelβincrease,and the crack initiation angleθ_imcdecreases,all of which show a strong linear relationship.It is more difficult for the specimen to initiate the initial macroscopic crack and lose stability.（4）With fissure angle increasing,the failure mode of the fissured specimen changes from tensile to tensile-shear mixed failure.In the process of instability,the specimen with double fissures is easily affected by tension,and the tensile through crack is formed along the initial tensile crack,while the specimen with single fissure is prone to the activated crack along the horizontal direction.The fissure angle has an important influence on the characteristic parameters,distribution and reservoir volume of cracks in the failure mode.Specimens with single fissure（α=30°-45°）and specimens with double fissures（α=45°-60°）form the failure mode which includes more and longer macroscopic cracks with larger volume and more complex distribution.（5）Combined with the characteristics（number,distribution）of micro-cracks and contact force chain,with the fissure angle increasing,the micro-crack initiation and stress concentration change from the middle of the fissure to the fissure tip to the whole specimen.The influence of the fissure on the initiation and failure mode of the specimens is weakened.Cumulative micro-crack count also reflects the internal damage of fissured shale.（6）According to the velocity field,the macroscopic cracks of fissured shale are discriminated,and the classification methods of tensile cracks,shear cracks and tensile-shear mixed cracks are put forward.The results show that with the fissure angle increasing,the tensile cracks of the fissured specimens gradually decrease,while the shear cracks and tensile-shear mixed cracks gradually increase.This is basically consistent with the laboratory test result,which is verified and further explained from the perspective of micro-crack evolution.