Study On Mechanical Property Finite Element Modelling Of Cement For Shale Gas Formation

Shale gas has rich and widely distributed reserves, and has been getting into commercial exploitation phase. But its low porosity and permeability demands the need for reservoir fracturing to increase production. Currently, most domestic shale gas well completion chooses perforated casing method, high fracturing pressure will cause damage to cement, thus bringing down cementing quality. Their fore, in this paper, a "casing- cement- formation" threedimensional finite element model of the perforation was established to solve this problem, and then the impact of elastic modulus, Poisson’s ratio, density, cement thickness, perforation parameters on the mechanical properties of the cement under conditions of production and fracturing were analyzed. Finally, the preferring properties of the cement stone were proposed. The results were as follows:Under production: the perforation model has similar stress distribution with no-perforating model except stress concentration at the hole; the second interface pressure of cement sheath is higher than that of the first interface, acting as the dangerous interface; the shape of stress concentration region at the hole resembles a spiral spring, which is distributed forward along the perforations. One effective way to improve the quality of cement is improving cement modulus of elasticity; cement stress has positive linear correlation with and cement thickness, while cement density and Poisson’s ratio have little impact on cement stress; cement contact pressure increases with cement modulus of elasticity, Poisson’s ratio and thickness, while the density has little effect on the contact pressure.Under fracturing: cement displacement, stress and strain have greater stress concentration at perforations, stress peak can reach more than 2 times the normal stress, and the shape of stress concentrated area resembles a rounded rectangle or ellipse, with the first interface acting as the dangerous interface. Perforation is the main cause of casing and cement damage; cement sheath stress increases with the increase of elastic modulus and decrease of Poisson’s ratio. The contact pressure increases with the increase of Poisson’s ratio and elastic modulus. When the elastic modulus is certain, cement sheath stress reaches the minimum with Poisson’s ratio of 0.25, and cement sheath strain reaches the minimum with Poisson’s ratio of 0.2.