Factors And Laws On Cement Sheath Structure Integrity Under Non-uniform Crustal Stress

When the oil gas wells and gas storage wells at the productive operations of casing pressure test, fracturing and repeated gas injection, the cement and the interface of cement sheath will be deformed and even damaged by the stresses, which affect the cement sheath.Especially when the cement in the formation of a plastic non-uniform, the non-uniform formation will cause stress and strain distributions nonlinear changes, causing compression,tension and shear cutting and other complex stress state of cement and interface of the cement ring exits at the same time at the process of load or unload at loading, which increase the difficulty of analysis of the structural integrity of cement. So, this paper based on the conditions of the horizontal wellbore, establish a horizontal stress calculation model; using finite element and elasticity theory, analyze the mechanics effect relationship of the casing–cement-formation combination under the station of the non-uniform formation, establish contact finite element model, establish evaluation criteria of the structural integrity of the cement sheath; analyze the laws and effects of the unevenness of the formation stress, the sleeve eccentric stress, inclination and mechanical parameters of the cement to the interface of cement and the structural integrity of the cement at the process of load and unload at the state of loading, evaluate adaptive questions of the mechanical parameters of the cement and structural integrity, for providing the basis of the reasonable choice of the mechanical parameters of the cement and design of the slurry system.The results shows that, with the uneven stress increases, the uneven distribution of all the stresses of the cement suffered increase; and the size of the sleeve’s eccentricity, to some extent, exacerbated uneven distribution of all the stresses of the cement; at the same time,with changes of the deviation angle and depth of the well, make uneven distribution of the crustal stress around the well increase, which make the stresses of the cement appear significant changes; changes of the mechanical parameters of the cement make the different responses of the cement to the non-uniform crustal stress, increase the changes of the stresses which the cement suffered.