Due to the extremely low porosity and permeability of shale, fracturing is often used to develop shale gas reservoirs. During shale fracturing, extremely high fracturing pressure may invalidate the integrity of the cement sheath and bring hidden dangers to the safe development of shale gas. This paper compares the stress state of the three-dimensional unperforated and perforated finite element models of casing-cement sheath-formation to obtain the influence of perforation on the stress concentration and failure area of the cement sheath. The stress state comparison incorporates three sets of perforation models with different hole densities and diameter verifies the stress concentration law of perforation on the cement sheath. By studying the effects of the elastic modulus and Poisson’s ratio of the cement sheath on the maximum tensile and compressive stresses of the cement sheath under fracturing pressure, the integrity failure form of the cement sheath and the measures for integrity failure are obtained. The results show that the peak stress concentration of the perforated cement sheath is about twice the normal value of the stress. The failure area is the two ends of the perforation, the shape is similar to the bottom surface of the elliptical cone, and the thickness is up to half of the wall thickness of the cement sheath. The line length of the largest failure area is 4.5 times the diameter of the hole. Under fracturing conditions, the cement sheath generally undergoes tensile failure. The lower the elastic modulus and Poisson’s ratio of the cement sheath, the smaller the tensile stress of the cement sheath, and the less likely it is to cause tensile failure.

Perforation and fracturing are typically associated with the development of coalbed methane wells. As the cement sheath is prone to failure during this process, in this work, the effects of the casing pressure, elastic modulus of the cement, elastic modulus of the formation, and casing eccentricity on the resulting stresses are analyzed in the frame of a finite element method. Subsequently, sensitivity response curves of the cement sheath stress are plotted by normalizing all factors. The results show that the maximum circumferential stress and Mises stress of the cement sheath increase with the casing internal pressure, elastic modulus of the cement and casing eccentricity. As the elastic modulus of the formation increases, the maximum circumferential stress of the cement sheath decreases, and its maximum Mises stress increases slightly. The cement sheath undergoes tensile failure during coalbed methane fracturing. The stress sensitivity of the cement sheath to the influential parameters is in the following order: casing internal pressure > elastic modulus of cement sheath > casing eccentricity > elastic modulus of formation. KEYWORDSCoalbed methane fracturing; finite element method; cement sheath integrity; sensitivity analysis; failure analysis Nomenclature a casing eccentricity rate Δl casing eccentricity distance, mm r b borehole radius, mm r c outer radius of casing, mm Abbreviations CCFC casing-cement sheath-formation combination CIP casing internal pressure CS circumferential stress This work is licensed under a Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.