In vertical cased well, cement sheath is mainly used to support casing, seal off annulus, isolate porous zones, and protect the casing from corrosive formation fluids. During the production, the integrity of casing-cement-formation system is generally damaged by perforating and fracturing processes, leading to failure of hydraulic fracturing operation. Previous studies on cement sheath rarely involve coupled numerical simulation applied to hydraulic fracturing, gaining scarce industrial experience. In this paper, a coupled numerical model for fracturing operation in vertical cased well has been set up by FLAC3D(Fast Lagrangian Analysis of Continuain in Three Dimensions), to analyze the effect of formation and cement sheath on the integrity of casing-cement-formation system. The results show that cement with low Young's modulus and high Poisson's ratio is crucial to retain the integrity of the casing-cement-formation system. Fracturing operation should be initiated in sections of cement with lower Young's modulus and higher Poisson's ratio, and formation with higher Young's modulus and lower Poisson's ratio. This numerical coupled model not only can gain insight on interaction of casing, cement and formation, but also optimize the design of perforating operation for reservoir stimulation in vertical cased well.
During well cementing operation, cement hardens to form cement sheath, and then form the casing-cement-formation system. Cementing is an important procedure for well completion. The ultimate purpose is to provide effective zonal isolation of the formation which have been penetrated by the wellbore. Once a well is put into product, the wellbore conditions will change under various processes, such as the perforation of the casing, fracturing stimulation, acid treatment, etc. Pressure increases when perforation accompanied with the firing of the guns can lead to cement damage. If fracturing stimulation or acid treatment is adopted, the increase of the wellbore pressure is more harmful to the casing-cement-formation system because the fluid pressure is high and the fluid injection lasts from minutes to hours. Moreover, the change of outer loading can be applied to the cement sheath. For example, the pressure on the external surface of the cement sheath will increase because of formation creep. In this case, it may lead to fracture propagation or annulus cracks around the cement sheath, eventually lead to the failure of fracturing operation. Scholars have done a lot of experiments to study the properties of cement sheath[1,2]. Many scholars also studied the effect of wellbore pressure on the cement sheath, used the analytical model to predict well life and used the finite element model to simulate the actual working conditions.