Investigation on Radial Crack of Cement Sheath During CO2 Injection Using a TM-XFEM Coupled Model

ABSTRACT Cement sheath integrity is critical to the safety and efficiency of CO2 capture and storage technology. Radial cracking of cement sheath is one of the pathways leading to CO2 leakage. Cement sheath radial cracking has been little studied, and the models usually ignore the effect of fluid temperature, while it is challenging to achieve fracture tortuosity. In this paper, a fully coupled thermo-mechanical (TM) model is developed to simulate cement sheath radial fracture using the XFEM method. The influences of various parameters on the fracture trajectory of cement sheath are studied, including in-situ stress, casing pressure, and fluid temperature. The results indicate that tensile stresses may be generated in the circumferential direction of the cement sheath during supercritical CO2 fluid injection. When the tensile stress exceeds the tensile strength of the cement sheath, radial crack of the cement sheath will occur. The fracture size increases with casing pressure and decreases with fluid temperature. This paper is of great significance in studying the failure mechanism of cement sheath integrity, and has guidance for optimizing cement sheath design to ensure the long-term wellbore integrity. INTRODUCTION CO2 geological storage (CGS) is the main approach to solving the greenhouse effect. The major challenge in the operation of CGS project is CO2 leakage (Zhang & Bachu, 2011). The Kansas region of the USA has been severely polluted by the wellbore sealing failure, which led to 3,000 tons of gas leaking into the atmosphere (Metz et al., 2005). Wellbore integrity is critical to maintaining operation safety, operation efficiency, and environmental sustainability for CO2 sequestration. The major functions of the cement are to seal the annular between the casing and formation, prevent the inter-zonal fluid migration, and provide additional support and protection for the casing. As one of the essential physical barrier units in a wellbore barrier system, the sealing ability of the cement sheath is critical for the safe and effective long-term storage of CO2 (Bai et al., 2016; Choi et al., 2013). The sealing integrity of cement sheath consists of two main components: the cement stone sealing ability and the interface sealing ability. Most previous studies focused on the failure analysis of the cement sheath interface (Feng et al., 2017; Gu et al., 2022; Yin et al., 2019). However, during CO2 injection, the cement sheath could develop radial cracks under high pressure and low temperature, seriously affecting the wellbore sealing ability (Hassani et al., 2014). Therefore understanding the development of cement radial cracks under different operating conditions is essential to evaluate wellbore integrity.