| Rock fracture is the basis of hydraulic fracturing in petroleum engineering.In order to reveal the fracture characteristics of rock under confining pressure,this thesis focuses on the type I fracture softening model of sandstone under confining pressure.The main research contents include sandstone fracture characteristics under confining pressure,fracture softening model with confining pressure,and the relationship between softening model and deformation response.Based on the three-point bending experimental technology,a test method of rock fracture characteristics under confining pressure is proposed,and the displacement field of sandstone specimen surface is obtained based on digital image correlation technology.The fracture characteristics of sandstone under confining pressure are characterized by displacement nephogram,displacement gradient nephogram and crack opening displacement(COD)distribution curve.The length of the fracture process zone and the critical crack opening displacement increase with the increase of confining pressure.Based on the inversion method,the tensile softening curve of sandstone is obtained,and the bilinear softening model and linear softening model with the influence of confining pressure are established according to the tensile softening curve.The variation characteristics of the characteristic parameters of bilinear softening model and linear softening model with confining pressure are determined.For example,the tensile strength,cohesion at turning point and the opening displacement at the end of cohesive crack in bilinear softening model increase with the increase of confining pressure,and the opening displacement of crack at turning point does not change with confining pressure.The fracture initiation and instability propagation characteristics of sandstone are described by double- fracture model.The double- fracture toughness increases with the increase of confining pressure.The cohesion toughness calculated by bilinear softening model is in good agreement with the difference of double- fracture toughness,which verifies the effectiveness of bilinear softening model.By extracting the stress distribution curve of the fracture propagation path of the numerical model in the inversion method,the sensitivity of the overall response characteristics of sandstone to the parameters of the fracture softening model is determined,and the influence of confining pressure on the overall response behavior of sandstone in the process of fracture is revealed.In the overall response relationship,the peak value is controlled by tensile strength and initial fracture energy,and the post peak value is controlled by total fracture energy.The contribution of sandstone physical properties and confining pressure boundary conditions to the maximum bearing capacity of sandstone is quantified.The contribution of physical properties to the maximum bearing capacity is greater than that of boundary conditions. |
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