Numerical simulation of cement-to-formation interface debonding during hydraulic fracturing of shale gas wells

The hydraulic fracturing process involves high pump pressure and large displacement, which increase the risk of debonding on the interface of the cement sheath and rock formation. Therefore, in this study, a three-dimensional numerical calculation model of casing-cement sheath-formation assembly was established, and the failure mechanism of the bonding surface caused by fracturing due to fluid migration was investigated. The effects of the cement sheath's Young's modulus and Poisson's ratio, bonding strength, perforation azimuth angle, and non-uniform in-situ stress on the anti-debonding ability of the cement-to-formation interface were analyzed. Finally, the following conclusions were drawn: The calculation results show that the failure of the bonding surface is greatly affected by the bonding strength, and its anti-debonding ability significantly increases linearly with increasing bonding strength. Furthermore, while increasing the Young's modulus improves the anti-debonding ability, the Poisson's ratio of the cement sheath and the perforation azimuth angle have little effect. Under non-uniform in-situ stress, the anti-debonding ability decreases with the increase of the difference between the horizontal and vertical in-situ stress. Thus, the non-uniform in-situ stress accelerates the failure of the bonding surface.