Establishment Of Fracture Initiation Criterion For Hydraulic Fracturing Based On Softening Cohesion Resistance

With the turbulent international situation,the cost of importing crude oil in China is becoming increasingly high.In order to ensure China’s energy security,it is necessary to develop unconventional petroleum resources such as shale oil with huge underground reserves.In recent years,the development trend of shale oil in China has been strong.However,due to the late start of unconventional oil and gas resources such as shale oil,there is a lack of understanding of the mechanism of hydraulic fracturing crack initiation.The crack initiation is influenced by multiple factors,and common fracture initiation criteria only consider the maximum tensile stress situation,and cannot take into account factors such as cohesion and softening at the crack tip.This project takes into account the relationship between cohesion,cohesion resistance,fracture toughness,and rock and soil softening at the crack tip,establishes a hydraulic fracturing crack initiation criterion,and explores the mechanical mechanism of hydraulic fracturing shale crack initiation.Using finite element software to simulate the initiation of shale fractures,a damage model is established using the fracture zone method and implicit gradient method.Based on the Mohr Coulomb criterion and Drucker Plague criterion,parameters such as cohesion and internal friction angle are introduced to conduct finite element simulation of crack propagation,and stress,damage,deformation cloud maps,and displacement load curves are obtained.By analyzing the simulation results,the plastic deformation and softening laws in the crack tip area were obtained,and it was found that cohesion and softening in the shale crack tip area have a significant impact on the initiation of shale cracking.The similarity theorem is used to make similar shale test pieces,and the crack propagation test pieces with simple materials and reasonable size are made through the reasonable proportion of P.F325 fly ash Portland cement,80-120 mesh quartz sand and water.Load the specimen using the QBG-300 multifunctional testing machine,and record the initiation and evolution process of the specimen using an in-situ micro camera system to obtain the displacement load curve.Use DIC(Digital Image Correlation)technology to obtain the crack opening displacement and elongation displacement.Analyze the displacement load curve to obtain the deformation law of the plastic zone at the crack tip,and verify the effectiveness of the finite element simulation results of crack initiation.Further verify the effects of softening and cohesion on shale fracture initiation,and obtain the relationship between cohesion resistance and stress intensity factor through experiments.From the perspective of fracture mechanics,this paper studies the blocking effect of the internal cohesion of rock mass on the destruction of shale under load and its impact on the cracking of shale under load,introduces the fracture toughness with the same effect,studies the relationship between cohesion and fracture toughness on the basis of experiments,and obtains the change law of the fracture toughness under the action of cohesive resistance and the cracking of shale cracks,Further considering the influence of plastic deformation at the crack tip on cohesive resistance,based on the nonlinear softening constitutive relationship,the fracture toughness considering cohesive resistance under plastic deformation conditions is obtained.A hydraulic fracturing initiation criterion model for shale is established,and the effectiveness of the initiation criterion is verified through experiments.