Design And Experimental Research Of Casing Anti Extrusion Mechanical Experiment Device

Conventional collapsing strength design of casing does not consider the influence of cement sheath and uneven distribution of in-situ stress.In actual wellbore environment,casing is sometimes subjected to uneven in-situ stress in the presence of cement sheath.The eccentricity of cement sheath directly affects the stress distribution of casing,and the uneven distribution of in-situ stress also has great influence on casing strength.With the continuous improvement of deep tight sandstone or shale gas reservoir reconstruction,the heterogeneity of borehole stress is more significant.The design method of casing collapsing strength using hydrostatic pressure as effective external extrusion pressure is not accurate for complex stress wellbore.In order to study the effect of eccentricity of cement sheath and inhomogeneous stress on the stress of casing in formation environment,this paper independently developed a casing stress test device which is subjected to eccentric and inhomogeneous stress of cement sheath under the formation environment.The purpose of the experiment is to test the stress distribution law of casing under the condition of cement sheath and uniform external extrusion force and different uneven coefficient external extrusion load.The bearing capacity of different eccentric cement sheath to casing under uniform external extrusion pressure is evaluated by experimental data.At the same time,the stress variation law of casing under eccentric cement ring and non-uniform load coupling is studied.The experimental study on the self designed casing resistance to the collapsing force on the casing with different center degree shows that the complete cement sheath has a certain unloading capacity for anti extrusion of the casing under the uniform external extrusion load.But the more serious the eccentricity of the cement sheath is,the more obvious the external unloading capacity of the casing is.Through theory and experiment,it is proved that the unloading capacity of cement sheath to casing is related to the modulus of elasticity of cement sheath,Poisson’s ratio,ratio of inside and outside diameter of cement sheath and middle degree of casing.When the degree of inhomogeneous stress increases,the stress values of the inner wall of the casing wall vary in different directions:the circumferential stress in the direction of the maximum principal stress is gradually changed from negative to positive,and the circumferential pressure of casing gradually becomes circumferential,and the circumferential stress state in the direction of the minimum principal stress is always negative and pressured.And the amplitude of stress variation is larger than that of the circumferential stress in the direction of maximum principal stress.Through this study,we have a good foundation for further understanding the influence of the eccentricity of the cement sheath and the uneven stress of the stratum on the casing’s anti extrusion capacity.