Study On Failure Characteristics Of Cement Annulus Under Non-Uniform Stress

After cementing,the change of the pressure inside the wellbore may cause the sealing failure of cement annulus.So the process of pressure change inside the wellbore can be divided into two stages,pressure increase stage and pressure reduction stage.In the stage of pressure increase,under the effect of high pressure and geostress,the plastic deformation of the cement occurs,and there may appears microcracks on the cement when the stress distribution of the cement reaches the shear strength and the tensile strength.In the pressure reduction stage,because of the previous plastic deformation and due to the different deformation capacities of casing,cement and formation,there may generate micro annulus gap at the first interface or the second interface of cement.This may result in connection between layers and pressure inside annulus and reduce oil production and a series of consequences.On the basis of elastic mechanics theory,the Mohr-Coulomb criterion and the maximum tensile stress criterion,considering casing and formation as elastomers and cement as elastoplastic body,this paper emphatically researches sealing integrity of cement and its first interface.This paper judges that whether the area of the cement may occur shear failure and tensile damage during the pressure increase stage.Then,by the analysis of generative mechanism of micro annulus gap,this paper establishes the calculation model of micro annulus gap.Based on the results of Jackson test,the finite element simulation software Abaqus and the actual production example,the correctness of the theoretical model is verified.The influence of the cement elastic modulus and Poisson’s ratio of the area of the rupture zone in cement under the actual production are analyzed by using the theoretical model.The results show the failure of the cement during the pressure increase stage(shear failure and tension fracture).The area of the shear failure and tensile failure expands with the increase of the elastic modulus,and reduces with the increase of the Poisson’s ratio.The research results of this paper can be used to calculate the damage range of cement ring under actual conditions and the width of the micro annulus gap of the first cement interface,and also provide the theoretical basis for the design of cement system.