Study On Influence Of Cement Ring Defect On Casing Carrying Capacity

The cement sheath defect will directly affect the force condition of casing and often result in stress concentration phenomenon even lead to casing damage. However, there is only a little research about the influence of cement sheath defects on casing strength at home and abroad and the cement sheath defect model is also too idealized to match the fact. Therefore, based on the elastic-plastic mechanics, this Paper deduces the stress model of the maximum equivalent effect of casing while the cement ring subjected to uniform in-situ stress condition. Then establishes models of different types of cement sheath defect according to the field data and laboratory test data. Finally, simulations were carried out to study the influence of the cement sheath defect on the casing strength by finite element software and get the logic of the effect of cement sheath defects on bearing capacity of casing. The following results can be achieved through this study:(1)Eccentric annulus cement loss often appears in the narrow annulus and the capacity will rise rapidly with the increase of the missing area decreased again and recovery rate decreased gradually, and eventually close to but lower than that of casing cement sheath intact bearing capacity.(2)The casing bearing capacity will decrease with the increase of gas channel defect area and increases with the increase of the number of defects, and distance from the casing wall distance increases, and the gas channel defects more dispersed, from the narrow annulus is more close, the bearing capacity of the lower casing.(3)The casing bearing capacity will increase with the increase of micro annulus width, but when the micro annulus is not complete the casing bearing capacity will decrease and then gradually increase with the micro ring gap defect ring opening increases, and the maximum equivalent stress of the casing even much higher than when the cement sheath is good and according to the micro annulus position and width of different casing, reduce the carrying capacity and the increase will be different.(4)Half moon shaped defects on casing veering capacity were studied and effects of gas channel defects is similar, but under the same conditions, the casing crescent shaped defects lead to bearing capacity was more.(5)The casing bearing capacity decreased with the increase of cement material defect depth and Poisson’s ratio, with the increase of defects to the ring opening and elastic modulus and decreased first and then increased gradually.This study has laied a good foundation for further in-depth understanding of the influence of the cement sheath on the casing bearing capacity.