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

Abstract

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.

Failure Analysis of Cement Sheath Mechanical Integrity Based on the Statistical Damage Variable

“…Portland cement stone is a brittle material with congenital defects. During the drilling and production of oil and gas wells, under the action of the surrounding rock stress and wellbore stress, , the cement sheath is easily broken to form macroscopic cracks, which become gas channeling paths and cause sustained casing pressure. − Maintaining the sealing integrity of the cement sheath is the key to ensuring the long-term safe production of oil and gas wells, and how effectively evaluating the sealing performance of the cement sheath is a basic problem that needs to be solved urgently . At present, the direct way is to evaluate the damage and failure mode of the cement sheath through annulus isolation simulation experiments.…”

Effects of the porous structure of cement sheaths on the deformation failure mechanism of wellbore cement sheaths

“…To explore the failure mechanism of cement sheaths, a set of integrity test experiment device for cement sheaths was developed, and comparative experimental research on cement sheath integrity was conducted under concentric and eccentric conditions. Existing research indicates that reducing the elastic modulus of a cement sheath can decrease stress and prevent its failure 33 . Some scholars added rock asphalt particles modified by plasma technology into cement sheaths and tested their mechanical parameters.…”

Effects of porous structure on the deformation failure mechanism of cement sheaths for wellbores

“…To explore the failure mechanism of cement sheaths and establish preventive measures, a set of integrity test devices for cement sheaths was developed, and experimental research on cement sheath integrity was conducted under concentric and eccentric conditions. Available research indicates that reducing the elastic modulus of a cement sheath can decrease stress and prevent its failure 33 . Some scholars added rock asphalt particles modified by plasma technology into cement sheaths and tested their mechanical parameters.…”