Study on the sealing integrity of cement sheath during volume fracturing of shale gas of horizontal well

The integrity of the cement sheath is very important for reducing the band pressure of the empty cement sheath and improving the safety of the shaft (well hole). The injection of high pressure fluid may lead to the expansion of the fracture of the interface of the well cementation during the volume fracturing of shale gas of horizontal wells. The sealing property of cement sheath is affected. Therefore, the finite element calculation model of fracture expansion in the cement interface is established on the stepped finite element method, which is combined with the CZM model. This model is used to calculate the field examples. The fracture expansion of the cement interface is affected by the degree of interfacial cementation, formation and the properties of cement sheath, the influence of which is studied. The results of the study show that the initial stress of the cement sheath and the change of the formation pressure on the overlying formation do not have any significant influence on the length of the fracture expansion of the cement interface. The greater difference of the horizontal stress while, the smaller the length of the crack growth. The fracture expansion length of the cement interface decreases with the increase of the elastic modulus of the formation. The amplitude of the change is increased first and then reduced. The elastic modulus of the cement sheath and the degree of consolidation of the cement interface are beneficial to reduce the length of crack propagation of the interface. All results have certain implications for the evaluation and prediction of the integrity of the cement sheath during the volume fracturing of shale gas wells.