Study On The Macro And Mesoscopic Mechanism Of Crack Propagation In Fractured Rock Mass During Hydraulic Fracturing

The damage and destruction of fractured rock mass in the process of hydraulic fracturing is co mmon in energy exploitation projects,water conservancy and hydropower projects and water rich tunnel projects.The research on this problem is closely related to the efficiency of energy exploitation,the safety of construction personnel and the long-term stability of rock mass engineering.The mechanism of hydraulic crack propagation is closely related to the physical and mechanical properties,local heterogeneity,in-situ stress state,fracturing fluid characteristics of fractured rock mass,involving rock mechanics,fracture mechanics,damage mechanics,fluid mechanics and many other disciplines.A large number of scholars have carried out extensive and in-depth research on this issue,but it is still difficult to accurately and intuitively observe and quantitatively analyze the whole process of hydraulic fracturing of real rock samples.In this paper,fracture mechanics method,hydraulic fracturing visual test method and discrete element numerical simulation method are comprehensively applied to study the hydraulic crack propagation law in fractured rock mass under the influence of multiple factors,so as to provide theoretical basis and experimental support for the improvement of hydraulic fracturing technology.Firstly,this paper studies the theory of hydraulic crack initiation in fractured rock mass,and studies the calculation method of hydraulic crack initiation angle and hydraulic pressure based on the maximum circumferential tensile strain criterion;By combining the self-developed visual true triaxial hydraulic fracturing test device with DIC analysis software,a visual test method for the whole process of hydraulic crack propagation in fractured rock mass under complex mechanical environment is proposed;Then the visual hydraulic fracturing test of natural rock samples with prefabricated cracks is carried out by using this method.Considering the factors such as liquid injection flow,stress state,crack inclination,rock bridge angle and rock characteristics,the propagation law of hydraulic cracks is studied according to the change of water pressure,the distribution and evolution of displacement field and strain field;Finally,the numerical simulation of hydraulic fracturing is carried out in the discrete element simulation software PFC2 D based on the improved flat joint contact model(FJM).The main research results and conclusions are as follows:(1)The maximum circumferential tensile strain criterion is used to analyze the initiation characteristics of hydraulic cracks in fractured rock mass.Considering the influence of nonsingular stress term(T stress)in Williams expansion on fracture characteristics,the maximum circumferential tensile strain criterion is combined with DIC analysis of hydraulic fracturing test of brittle rock.When the maximum circumferential tensile strain criterion is used to judge the initiation of hydraulic cracks,the maximum circumferential tensile strain obtained by DIC analysis is introduced as one of the criteria for the initiation of hydraulic cracks,and the formulas for the initiation angle of hydraulic cracks and the initiation water pressure are derived.(2)The self-developed and reformed real triaxial hydraulic fracturing visualization test device maintains good sealing effect and water pressure bearing capacity while ensuring the visualization effect,and realizes the whole process visualization of the deformation,damage and crack propagation of hydraulic fracturing samples in a fully sealed environment.Improve the DIC calculation method based on Ncorr program,realize the efficient and accurate analysis of hydraulic fracturing test images in complex environments,and creatively complete the combination of hydraulic fracturing visualization test under true triaxial compression and DIC method,and put forward a complete set of hydraulic fracturing visualization test and analysis method of fractured rock mass.(3)The maximum circumferential tensile strain fracture criterion based on DIC analysis is applied to the analysis of hydraulic crack initiation characteristics,and its accuracy is verified by co MParing the initiation angle and the initiation water pressure in the visual hydraulic fracturing test.The location of hydraulic crack initiation corresponds to the maximum tensile stress concentration area at the tip of the prefabricated crack,and is affected by the mineral distribution near the tip of the prefabricated crack and the original damage.The crack initiation angle is random under low injection flow,and the injection flow has little effect on the crack initiation pressure;Under uniaxial compression,with the increase of vertical pressure,the crack initiation angle gradually increases,and the crack initiation water pressure first increases and then decreases.Under biaxial compression,with the increase of lateral pressure coefficient,the crack initiation angle decreases from about 45° to about 0°,and the crack initiation water pressure gradually increases;Under triaxial compression,with the increase of crack inclination angle,the crack initiation angle and crack initiation water pressure decrease first and then increase,and the minimum value occurs along the prefabricated crack direction;the fracturing water pressure of sandstone is significantly lower than that of granite and shale due to its low tensile strength.When the bedding direction is parallel to the prefabricated crack,the fracturing angle of shale is 0°.(4)The hydraulic crack of single crack rock sample basically extends along the crack initiation direction of the prefabricated crack tip,and will turn at the location where the mineral grain distribution is uneven or there is original damage.The propagation process is mainly dominated by tensile action.At low liquid injection flow rate,the propagation path of hydraulic crack turns more and is easy to form secondary cracks.With the increase of liquid injection flow rate,the propagation speed of hydraulic crack accelerates and the proportion of shear action decreases;the hydraulic crack propagation direction is most affected by the stress state.The hydraulic crack opening and propagation speed increase with the increase of uniaxial vertical pressure and decrease with the increase of biaxial lateral pressure coefficient;The crack inclination has little influence on the propagation direction of hydraulic crack.With the increase of crack inclination,the failure mode of hydraulic crack propagation presents tension shear mixed failure and gradually transits to pure tensile failure;The growth rate of hydraulic crack sandstone < granite < shale,the distribution of granite biotite and the weak bedding of shale will lead to the diversion of hydraulic crack and bear a certain amount of shear action.(5)With the increase of liquid injection flow,the increase of uniaxial vertical pressure and the decrease of biaxial lateral pressure coefficient,the adjacent hydraulic cracks of double cracked rock samples are more likely to interfere with each other,turn and connect.The rock bridge angle has a significant i MPact on the propagation direction and penetration mode of hydraulic cracks in the rock bridge area.The penetration mode in the rock bridge area changes from tensile penetration to tensile shear mixing penetration and then to non penetration as the rock bridge angle increases from 0° to 180°.There is a specific rock bridge angle in the hydraulic fracturing process,which minimizes the mutual interference and inhibition between adjacent hydraulic cracks.Due to the complex mineral composition and uneven distribution of granite,the hydraulic crack propagation path in Yanqiao area is complex.The hydraulic crack propagation in Yanqiao area of sandstone and shale is greatly affected by the original damage and weak bedding.(6)PFC discrete element simulation of hydraulic crack propagation is carried out based on the improved flat joint contact model(FJM).The results show that the closer the angle between the natural crack and the direction of large principal stress is to 90 °,the easier the hydraulic crack is to deflect,and the closer it is to 0 °,the easier it is to fracture;When the rock bridge angle is smaller(larger)and the lateral pressure coefficient is closer to 0(1),the hydraulic crack in the rock bridge area is more difficult to penetrate.Under the condition of high ground stress,the shear dislocation between particles is more likely to occur in hydraulic fracturing,and the hydraulic crack growth is mainly affected by the difference between the size of the principal stress;Under the condition of low ground stress,the mutual interference between the propagation direction of hydraulic cracks and adjacent hydraulic cracks in the rock bridge area is not affected by the lateral pressure coefficient.