Objective With the growth in the demand for natural oil and gas and the technological progress of their production, the oil and gas well depths have been constantly setting new records. The increase in the depth corresponds to sharply increasing in situ stress, leading to increasingly severe damage to the casing of ultradeep oil and gas wells. To enhance the bearing capacity of the casing under high in-situ stress, high-strength casing with a large wall thickness is typically employed, a measure which has been proved, however, less effective.

Methods This study established a finite element model of the casing-cement sheath-formation system to corroborate the presence of arching effects in the casing system and the protective effect of flexible cement sheath on the casing. Based on this, the impacts of different formations were investigated, along with the material parameters and thickness of cement sheath on the mechanisms behind the stress transfer in the system through a series of parametric analysis.

Results and Conclusions  The findings indicate that flexible cement for well cementing enabled the formation near the casing to bear in-situ stress by allowing limited deformations of the formation, thereby reducing the load on the casing. In contrast, rigid cement for well cementing bore in situ stress by restricting the deformations of formations near the casing, thereby sharing the in situ stress with the casing. In the force system applied to the flexible cement for well cementing, a smaller elastic modulus of the flexible cement corresponded to a lower casing stress, suggesting an active arching effect. In contrast, in the force system applied to the rigid cement for well cementing, a higher elastic modulus of the rigid cement was associated with a lower casing stress, hinting at a passive arching effect. However, active arching effects exhibited a much higher decreasing amplitude of stress than passive arching effects. Therefore, active arching effects produced more effective protection for the casing, especially in hard formations. Additionally, the Poisson's ratio and thickness of cement sheath also affected the casing stress, while their impacts were smaller than those of the elastic modulus of the cement sheath. Therefore, this study proposes application of the flexible cement for well cementing depends on the presence of the arching effects in formations, that is, the presence of shear strength in formations rather than their hardness.