Research On Constitutive Model Of Brittle Damage Of Shale

Shale gas is currently the focus of exploration and development.Most shale reservoirs are typical tight reservoirs and need to be fractured to effectively developed.Therefore,researching the mechanical properties of shale represented by the constitutive relation to provide theoretical basis of damage evolution is very important.Taking the shale developed in chang7 reservoir of Ordos Basin as the research object.X-ray diffraction and argon ion polishing-electron microscopy scan experiments are used to comprehensively analyze the mineral composition and pore fracture structure of shale,with paper research and Comsol finite element software to study the damage mechanism of shale under external loads.The mechanical response characteristics of shale specimens under static and dynamic loads were investigated using axial and SHPB impact tests to get threshold stress;Based on the elastic constitutive model of Zhu-Wang-Tang,combined with the damage variable following the Weibull distribution,the equation was modified properly,and the constitutive equation that can describe the mechanical properties of shale under dynamic load was finally obtained.The results show that the stress concentration phenomenon of the micro-fractures and micro-pores in shale during loading is the most vulnerable structure in shale;the brittle minerals in shale are more easily damaged than ductile minerals when loaded;Brass piece shaper and determine the appropriate specimen size,so that the impact test has achieved constant strain rate loading;Through the dynamic performance analysis of the shale specimen in the test,it is found that the peak stress and strain rate are linear relationship,the dynamics of shale The strength increases with the increase of impact velocity,which indicates that shale is a strain rate sensitive material;by comparing with the static compressive strength of shale,the strength enhancement coefficient of shale in this impact test is between 2.456 and 2.975.The damage freezing test was used to measure the damage value of shale.The relationship between shale damage and peak stress was fitted.The quantitative expression between damage and peak stress was established,and the threshold stress at damage was found to be 28.65 MPa.The corresponding threshold strain rate is 32.19s-1.According to the shale stress response and SHPB strain rate range,the Zhu-Wang-Tang equation is simplified.The fragmented shale constitutive equation was established by measuring the damage threshold and compared withthe one-dimensional brittleness model.The results show that the modified Zhu-Wang-Tang model can better reflect the stress-strain relationship in shale damage and can accurately describe the peak front dynamic mechanical properties.