| In this paper,the macroscopic and microscopic failure behaviors of longmaxi shale with different stratification were studied by experiment,simulation and theory.The variation rules of compressive strength,elastic modulus,poisson's ratio,fracture energy and other parameters were obtained.The influence of precast gap and bedding angle on the initiation,propagation and fracture of shale cracks was studied.While,the relationship between the elastic modulus of shale and the inclination of bedding faces and the strength formula of shale splitting failure are established.The conclusions obtained can provide a theoretical basis and practical guidance for the safe and efficient mining of shale gas.The main conclusions are as follows:?1?Based on the macroscopic failure behavior of longmaxi shale under different static strains and different strain rates,the evolution of compressive strength,elastic modulus and poisson's ratio of shale with the change of bedding angle is obtained.Revealing the transformation mechanism of failure mode caused by layering angle.Discussing the variation of strength and failure mode of layered shale under low strain rate.?Under quasi-static load and different strain rates,the stress-strain curve of layered shale will go through four stages:compaction,elasticity,yield and failure.As the angle between the bedding faces and the prefabricated notch increases,the compressive strength and elastic modulus of the shale have changed in a "U" shape.While,the poisson's ratio decreases first and then increases,and both the compressive strength and modulus of elasticity are lowest at an angle of about 45°.?The failure mode of shale is obviously affected by the dip angle of the bedding faces.When the angle is 0°?15°,the shale is mainly caused by splitting failure.When it is 15°?30°,the shale is mainly also caused by splitting failure,but the shear failure is gradually exerting its effect;at 45-60°.The shale exhibits shear failure obviously,and the shear failure effect at this time reaches the maximum.And 60°?90°,the effect of shear failure weakened,while the effect of splitting damage strengthened.?Strain rate effect has a significant impact on shale damage.When the strain rate is increased from 2.5 X 10-6/s to 2.5 X 10-4/s,the compressive strength of shale with the bedding angle of 0°,30° and 90° increases first and then decreases and the change is larger.However,the compressive strength of shale shows a different trend with the increase of strain rate when the bedding angle is 60°.?Under different loading conditions,the shale is dominated by splitting failure.When 2.5 10-5/s,there is shear failure inside the shale.With the further decrease of strain rate,the shear mechanism becomes more and more obvious.When the normal stress and the bedding faces are at an angle of 60°,the shear failure of the shale is the main failure mechanism.Combined with the maximum shear stress occurring on the weak surface of 45°,we believe that when the angle between the bedding faces and the normal stress is 45-60°,the bedding of shale has a great influence on the failure mechanism in the shale.So that this major influencing factor needs to be considered during the development of hydraulic fracturing.?2?Based on the static three-point bending failure experiment of shale with different bedding angles,the influence law of precast notch and bedding angle on the initiation,expansion and fracture of shale cracks was obtained.?With the increase of the Angle between the bedding surface and the precast notch,the peak failure load,elastic modulus,fracture energy and other mechanical parameters of shale increase gradually.?The failure of shale is usually controlled by the main crack,which usually has branch cracks.Due to the shale mineral particles are small,the main crack propagation path is generally smooth.?The three-point bending load-displacement curve shows that the failure of shale shows obvious brittleness.The propagation path of the main crack is usually random.When the main crack propagation encounters the bedding crack,the crack will expressed as stop cracking,turning,expanding along the crack and continuing to expand through the crack which are determined by the stress received,the shale matrix material and the crack body.?3?Fracture toughness of shales with different stratification angles was calculated based on experiments,fracture morphology,failure mechanism of shales were analyzed,and three-point bending failure under different conditions was simulated.?In the three-point bending experiment of the microscopic shale the fracture toughness of the shale will increase gradually with the increase of the bedding angle.The fracture toughness of different shale specimens under the same bedding angle has discreted.?The observation of shale fracture morphology shows that after three-point bending failure,the main micro-fracture types of shale are:undulating fracture,flake fracture,bedding faces fracture,tearing fracture along the layer structure surface,wear layers and tearing fracture plane,the micro cracks crack fracture,etc.,which depend on shale material and loading stress displacement boundary conditions.?The shale microscopic failure mode is mainly based on particle destruction,particle wear failure and composite failure.The failure mechanism is mainly tensile failure and shear failure.?During the shale rupture process,the crack propagation is dominated by a main crack,and the main crack has branch cracks.While,the path of the main crack is usually smooth,and the bifurcation crack generally appears at the weak structural plane of the original crack at the edge of the main crack.The extension path of the main crack of shale has some randomness.The influence of original bedding crack on the main crack propagation of sample is local and mainly competes with bending stress.The competition mechanism of the two determines the final crack propagation path.The higher the loading rate,the faster the shale crack growth rate and the corresponding increase of branch cracks.In the hydraulic fracturing process of shale development,the greater the impact rate of hydraulic fracturing is,the more branch fractures will be generated,which is more conducive to the formation of migration channels of shale gas.?The KIC of the type I shale fracture toughness is calculated to be 0.736 MPa.M0.5.As the notch angle increases,K? rises firstly and then decreases.As the notch angle increases,the type ? stress intensity factor K? increases gradually from the notch angle.When the notch angle reaches 45°,K? tends to be stable;T stress is a stress at the tip of the crack parallel to the crack direction.During the transition from ?/? composite fracture to type ? fracture,the T stress is gradually increasing.?4?Establish the relationship between the elastic modulus of shale and the inclination of bedding surface and the strength formla of shale splitting failure. |
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