Study On Mechanical Properties Of Shale In Triaxial Stress State

With the rapid development of the global economy,the energy needs of various countries have increased dramatically.Due to the maturity of technology,the production of conventional energy have increased a little,and the energy source faces the danger of exhaustion in the future.Countries are looking for new energy sources that can replace conventional energy sources.Shale gas has attracted much attention due to its huge reserves.With the successful development of shale gas in the United States,other countries have gradually joined the study of shale gas.The study of shale gas in China started late,but it has developed rapidly.Using foreign hydraulic fracturing technology,a large amount of shale gas resources have also been explored.However,due to the significant differences between China’s shale reservoirs and the United States,shale gas reservoirs in China are generally buried more deeply.Therefore,the use of hydraulic fracturing technology faces many new challenges:the increase in drilling depth makes the construction more difficult;the development of the natural cracks lead to a large loss of fracturing fluid;The changes of pressure,which destroy reservoirs,can produce complex cracks easily.In order to fundamentally improve the hydraulic fracturing technology,it is necessary to fully understand the mechanical properties of shale and to fundamentally understand the law of crack propagation and the form of destruction of shale.This thesis mainly studies the mechanical properties of shale from both experimental and numerical simulations.In the experiment,the influence of confining pressure on the mechanical properties of shale was studied,and the basic mechanical parameters were obtained.In the numerical simulation,the mechanical behavior of the material was simulated,and the failure form of the material was studied.The results of the numerical simulation were compared with experimental phenomena and experimental data,and the following conclusions were obtained:(1)Triaxial compressive tests on shale materials under uniaxial and different confining pressures show that shale exhibits brittle fracture characteristics under low confining pressure;shale is a reinforced material.With the increase of confining pressure,the compressive strength of the material and the slope of the stress-strain curve increase,and the deformation of the specimen increases when it fails.(2)The brittle cracking model was used to simuq2 late the mechanical behavior of shale.The calculated results show that the material undergoes elastic deformation under both uniaxial and triaxial compression.When the compressive strength is reached,brittle fracture occurs.In uniaxial direction,the failure surface is parallel to the axial direction;when the confining pressure is 15 MPa,the failure surface is at a certain angle with the axial direction.At the same time,comparing the numerical results with the experimental data,we find that the stress and strain curves of the two have the same changing trend,indicating that the model can well simulate the brittle fracture mechanics characteristics of shale under low confining pressure.(3)The Drucker-Prager model was used to simulate the mechanical behavior.The calculation results show that the material undergoes elastic deformation at the initial stage of compression;as the overall stress of the material increases,the stress near the defect increases dramatically,and materials strat to yield;then the stress further increases,and the yield area of the material increases.Shear occurs at the adjacent pores,causing greater plastic deformation.The simulated stress-strain curves are compared with the experimental data.The experimental data basically falls in the vicinity of the curves and has the same trend of change,indicating that the model can reflect the mechanical properties of such rock materials.