The transient changes of pressure and temperature in the wellbore during fracturing could cause cement sheath damage and micro-annulus. In this work, a plastic damage model that considers both damage and yielding is used to describe the full stress-strain mechanical properties of the actual cement sheath. A novel numerical model was established to evaluate the cement sheath integrity considering thermo-mechanical coupling. This model is validated by the laboratory experiment and shows high accuracy. The results indicate that tensile damage occurs inside the cement sheath during the fracturing process, and the maximum damage occurs in the direction of the maximum horizontal stress. The micro-annulus rapidly appears at the first cement sheath interface during depressurization process after injection, which can reach 39.57 μm. With the increase of shutdown time, the micro-annulus gradually decreases. This work reveals the cement sheath failure mechanism and provides a basis for wellbore integrity control.
Cement sheath integrity is critical to oil and gas development. In the process of hydraulic fracturing, thermal oil recovery, the cement sheath is usually exposed to fluctuating high-pressure and high-temperature environments, which can easily damage and destroy the cement(De Andrade & Sangesland, 2016a; Thiercelin et al., 1998). It is reported that 79.524% of the wells showed sustained casing pressure problems in the Fuling shale gas field in Sichuan, China(Guo et al., 2018; Li et al., 2020; Yan et al., 2018). Therefore, understanding the mechanisms of cement sheath damage under different operating conditions is essential to evaluate wellbore integrity better.
Several cement sheath integrity models under temperature and pressure have been developed. In previous studies, ideal elastic-plastic models have been used to describe the stress-strain characteristics of cement sheath. The Mohr-Coulomb or Drucker-Prager criterion has been used to determine whether the cement has yielded(De Andrade & Sangesland, 2016b; Feng et al., 2017; Gu et al., 2022). The actual cement stone has a large number of internal micro-cracks and micro-voids. The size and distribution of the micro-cracks change under the action of temperature and pressure, which is characterised by macroscopic degradation of the cement stiffness(Arjomand et al., 2018). Therefore, the conventional ideal elastic-plastic constitutive model can not represent the damage properties of the cement sheath effectively.
