Abstract: During staged fracturing of shale oil horizontal wells in the Longdong area of the Ordos Basin,the second cementing interface is prone to cracking because of its low bonding strength,which may further induce interlayer channeling.To solve this problem,the mechanical response of the cement sheath system under alternating loads and the evolution of interface failure were investigated.Based on the principles of elasticity and fracture mechanics,a mechanical model covering the entire loading and unloading process during fracturing was established.Triaxial mechanical experiments revealed a stepwise decrease in the elastic modulus of cement stone with increasing cycle number,and field data from production wells were used to validate the accuracy of the mechanical model.The results show that the weakening effect of alternating stress on the contact pressure of the second cementing interface is significantly stronger than that on the first cementing interface.The energy dissipation mechanism of crack propagation at the second cementing interface was clarified,and it was demonstrated that a physical equilibrium boundary exists for crack growth under specific operating conditions.The maximum propagation length of the second cementing interface under typical conditions was calculated to be 12.7 m.Further analysis indicates that the failure length of the second cementing interface increases with increasing wellhead pump pressure and in-situ stress.The greater the difference in elastic modulus between the formation and the cement sheath,the more slowly the interfacial energy release rate decays,the more significantly the risk of interlayer sealing failure increases.These findings can provide scientific and effective technical guidance for hydraulic fracturing of horizontal wells and field cementing operations in this area,and are of important academic value and engineering significance for optimizing fracturing parameters and determining their limits.

Key words: hydraulic fracturing of horizontal wells, second cementing interface, alternating stress, interface failure, shale oil