Establishment Of Damage Constitutive Model Establishment Of Cement Sheath Under Triaxial Stress

Cement sheath plays an important role in protecting and fixing casing and sealing oil,gas and water layers.During the operation of oil and gas wells,the integrity of cement sheath will be destroyed and the sealing will be lost,which is directly related to the production safety and service life in the oil and gas wells.Accurately describing the constitutive relationship of cement sheath is the material basis for realizing mechanical analysis and structural integrity analysis of cement sheath.Based on the stress-strain test of cement paste,this paper studies the establishment of the constitutive model of cement paste mechanics.The mechanical properties of cement paste are tested from different angles through three test modes: uniaxial stress,triaxial stress and cyclic loading.The effects of confining pressure,curing time,temperature and cyclic loading on the mechanical properties of cement paste are analyzed.According to the deformation characteristics of cement paste under triaxial stress,a mathematical model of statistical damage constitutive relationship of cement paste is established by using damage mechanics and statistical theory,and verified by experimental data.On this basis,using ANSYS numerical simulation software and control variable method,the effects of different parameters on the stress of cement sheath in creep,elastic and rigid strata are analyzed.The experimental results show that the mechanical deformation behavior of cement paste under triaxial stress is obviously different from that under uniaxial stress,which is mainly manifested in the increase of strength and plastic deformation.With the increase of temperature and curing time,the compressive strength and elastic modulus of cement paste also increase,and the strength of cement paste is proportional to elastic modulus;the toughening materials such as latex can be improved.The mechanical deformation capacity of cement paste has not changed,but the mechanical behavior of cement paste has not changed.The stress-strain curve of cement paste is similar to that of original cement paste.Based on Weibull probability density function and D-P failure criterion,the statistical damage constitutive relation of cement paste is established by using damage mechanics theory.The model parameters and their physical significance are determined.The experimental data are used to verify the model.The results show that the model fits well with the experimental results.The smaller the elastic modulus of cement sheath is,the smaller the stress of cement sheath is,and the more uniform the stress of cement sheath is,but the strength of cement sheath is proportional to the elastic modulus,so the elastic modulus of cement sheath should not be too small,choose between 5-7GPa;the larger the Poisson's ratio of cement stone,the stronger the lateral deformation capacity and pressure transmission capacity of cement sheath under stress,the stronger the unloading capacity of cement sheath.The effective stress decreases gradually with the increase of thickness,but the decrease is not significant.Under different strata conditions,the influence of cement sheath parameters on interface stress is the same,but the influence level is different.The stress parameters of cement sheath in elastic and rigid strata are not affected by crustal stress,while the creep strata are greatly affected by crustal stress.With the increase of elastic modulus,the equivalent stress of cement sheath increases.Compared with elastic strata and rigid strata,the stress of cement sheath in creep strata is more obvious.In rigid strata,the stress of cement sheath varies linearly,and decreases with the increase of Poisson's ratio.In creep strata,the stress of cement sheath decreases with the increase of Poisson's ratio,but the decrease is not significant.