Mechanical behavior and cement sheath damage characteristics of casing pullout based on peridynamics

Abstract

In petroleum exploration and production, the recovery and replacement of casing are crucial. However, difficulties often arise during casing pullout operations due to the strong bonding strength between the casing and the cement sheath. Currently, there are few researches on casing pullout process, which leads to the fact that casing pullout operations often relies on experience and lacks theoretical support. To comprehend the mechanical behavior, interface shear failure mechanisms of casing pullout process, and to propose rational solutions for practical engineering applications, this study establishes a Peridynamics (PD) numerical model for casing pullout to conduct research and analysis on its process. Based on the characteristics of the cement sheath, a damage model for the cement sheath is proposed in this paper. Additionally, a kernel function is introduced to reflect the variation of long-range forces with the distance between material points, aiming to characterize the changes in the stiffness of bond constant within the horizon of material points. The mechanical responses of the cement sheath and casing at different phases of the casing pullout process are analyzed and characterized. The research results indicate that an increase in the casing-cement sheath thickness ratio promotes the damage at the casing-cement sheath interface, reducing the time required for casing pullout but inhibiting the expansion of cement sheath cracks. As the thickness ratio increases from 0.1 to 0.18, the degree of cement sheath damage decreases from 6.904% to 6.453%. The increase in Young's modulus of the cement sheath is negatively correlated with the time required for casing pullout, positively correlated with the interface failure rate, and it does not exhibit a linear relationship with the degree of cement sheath damage. Within the range of Young's modulus from 4 to 20 GPa, the maximum difference in cement sheath damage degree is 3.715%. In engineering practice, the Young's modulus of well cement and the thickness ratio of casing-cement sheath can be appropriately increased to enhance pullout efficiency.