The Research About Damage Mechanism Of Wellbore Rock During Excavation Process In Tight Sandstone Formation

During the actual drilling process,the wellbore rock will experience the stress redistribution due to the excavation process,which will cause damage to the wellbore rock.In this paper,the damage mechanism of wellbore rock is studied during excavation process in tight sandstone formation.Based on the first and second laws of thermodynamics,a macroscopic phenomenological damage constitutive model is established step by step from the point of view of mesomechanics.Then the damage variables,the release rate of damage energy and stress,strain tensor are solved by the finite element numerical method.Finally,according to the physical and mechanical parameters of the tight sandstone,the damage evolution of the wellbore rock in the process of wellbore excavation in tight sandstone formation is calculated.The main research work includes the following points:(1)The damage model of tight sandstone under external load is constructed: the damage variable is described in tensor form at mesoscale level,and the concept and method of continuum mechanics are introduced to construct the constitutive relation of tight sandstone in irreversible thermodynamic process.Then the potential function of damage evolution and the evolution rule of damage are defined by analogy with the plastic potential function and plastic evolution rule.The theoretical framework of damage constitutive relation of tight sandstone is established.(2)The governing equation and numerical algorithm of damage model of wellbore in tight sandstone formation during excavation process are established.The mechanical process of wellbore excavation in tight sandstone formation is described.The governing equation in excavation process is established by using the damage model constructed before.Then the governing equation is discretized by finite element method,and the rock is judged whether the rock is in elastic state or in damage evolution stage by iteration calculation method.The damage variable,damage energy release rate and stress,strain tensor in the final stage of the rock can be obtained by cyclic iteration calculation.(3)The damage evolution of the wellbore rock in the process of wellbore excavation in tight sandstone formation is calculated.According to the damage model of tight sandstone and the numerical algorithm of damage model under external load,the damage of wellbore rock during excavation is calculated.The stress and damage state around the wellbore under different confine pressure and wellbore pressure in the uniform hydrostatic in-situ stresses are analyzed,and compared with the stress value under elastic state.The damage mechanism around the wellbore is analyzed according to the distribution graphs of stress components and damage components,and the evolution rule of tensile damage,compressive damage and shear damage around the wellbore is revealed.In addition,the influence of different stress difference ratios and different wellbore pressures on the damage of wellbore rock in non-hydrostatic in-situ stresses is analyzed.By enumerating the distribution graphs of damage components and stress components under different stress difference ratios and different wellbore pressures,the principle of the change of evolution direction of damage components is analyzed.