Influence of Casing Eccentricity on the Mechanical Integrity of Cement Sheaths in Fractured Wells

During the fracturing process of oil and gas wells, casing eccentricity directly affects the mechanical integrity of cement sheaths, but the law and degree of influence are not clear at present, and there are no measures to address the influence of casing eccentricity on the mechanical integrity of cement sheaths. This paper took the lead in developing a set of experimental devices for cement sheath integrity. A comparative experimental study on the mechanical integrity of cement sheaths was carried out for the first time under casing concentric and eccentric conditions, and the influence of casing eccentricity on the mechanical integrity of cement sheaths in fractured wells of the Wushi 17-2 oilfield was also investigated. The numerical simulation method was used to perform stress analysis on cement sheaths of fractured wells under casing concentric and eccentric conditions. The influences of casing wall thickness, cement sheath thickness, the elastic modulus of cement sheath, and formation on radial stress of the cement sheath were analyzed, and the mechanical integrity safeguard or remedy measures of the cement sheath under the casing eccentric condition were proposed for the first time. The results show that casing eccentricity can easily lead to stress concentration at the narrow edge of the cement sheath. Under the condition of the same strength of the cement sheath, the integrity of the cement sheath is more likely to fail under the casing eccentric condition. The tensile failure cracks are concentrated at the narrow edge of the cement sheath. With the increase in casing eccentricity, the stress at the narrow edge of the cement sheath increased, the critical failure pressure of the cement sheath decreased, and the failure pressure of cement sheath integrity decreased by 17.72% at a casing eccentricity of 33% compared within a casing eccentricity of 0%. Improving the casing center degree, increasing the casing wall thickness and cement sheath thickness, and reducing the elastic modulus of the cement sheath can minimize the stress of the cement sheath and prevent mechanical integrity failure. This study is helpful to evaluate the mechanical integrity of the cement sheath in fractured wells accurately and can provide a technical reference for optimizing the casing center degree.