摘要：Reservoir rocks such as sandstone and mudstone have obvious plastic characteristics.The current hydraulic fracturing initiation model cannot accurately describe the initiation characteristics of sandstone and mudstone reservoir rocks.In this paper,the nonlinear characteristics of rock deformation were characterized based on the Power-enhanced constitutive equation,and the piecewise nonlinear constitutive equation was established by using the piecewise approximation method.Based on the constitutive equation,an elastic-plastic stress field model around wells was established to analyze the stress field around wells during hydraulic fracturing.The initiation mode of elastic-plastic formation was analyzed and optimized.The results showed that the plastic yield of rock was determined by the in-situ stress field and the initial yield stress.The smaller the yield stress was,the greater the in-situ stress was,and the more easily the plastic yield occurred.Under the combined of rock mechanics parameters and in-situ stress,the tension failure occurred when the wellbore was under elastic state,and there were two failure modes of plastic tension failure and plastic shear failure after plastic state.The initiation directions of elastic tension,plastic tension and plastic shear failure were the direction of maximum horizontal principal stress,while there was a failure angle between the fracture plane and the maximum horizontal principal stress under the shear failure.When the power hardening index was lower than 0.5,only plastic shear failure occurred in this situation; when the power hardening index was higher than 0.5,the initiation mode of rock was determined by yield stress,power hardening index,internal friction angle and cohesion,and the plastic tensile failure and plastic shear failure may occur simultaneously.
