Alternating mechanics of fracturing and two-interface failure characteristics in Changqing tight gas horizontal wells

Abstract

The cementing interface quality in tight reservoir horizontal wells significantly influences fracturing design and effectiveness. Weak cementation strength at the second interface often leads to cracking and interlayer fluid flow during fracturing, hindering efficient oil and gas extraction. Studying the pressure-bearing capacity and failure extension of the second interface during fracturing is critical for optimizing reservoir development. In this paper, considering the fracturing characteristics of horizontal wells in Changqing tight reservoir, the casing-cement ring-formation is regarded as a unified cemented body, and a stress mechanics model of cemented interface is established by considering the alternating stress during fracturing loading and unloading process, and the mechanical change rule of the cemented interface of the cemented body is calculated and analysed. This study, based on actual data from a single well in the Changqing tight gas reservoir, quantified the effects of various construction parameters on interface contact pressure and failure length. The results indicate that the contact pressure of the first and second interfaces gradually decreases with an increasing number of alternating load cycles, with the changes in the second interface being more pronounced. Under loading pressure, the contact pressure of the second interface decreased by an average of 1.57%. The failure length of the interface is significantly correlated with the differences in properties between the formation and the cement sheath; higher elastic modulus results in shorter failure lengths. Higher in situ stress exerts a suppressive effect on interface failure, while reducing the viscosity of the fracturing fluid significantly increases the failure length of the interface. This study provides a scientific basis for optimizing fracturing construction parameters and boundary condition selection for horizontal wells in the Changqing tight gas reservoir.