Casing damage mechanism and failure segments determination of in situ combustion for heavy oil

To thoroughly investigate the impact of high temperatures at the bottom of in-situ combustion wells on casing integrity, a finite element model (FEM) was developed to simulate the casing-cement sheath-formation system. The model was constructed based on the principles of heat transfer and material mechanics, enabling an in-depth analysis of the temperature distribution and equivalent stress distribution under combustion temperatures ranging from 500 ℃ to 650 ℃. This approach aimed to elucidate the thermal and mechanical behaviors of casing materials under extreme high bottomhole temperature conditions, thereby providing a reliable basis for casing risk assessment and design optimization.

High-temperature tensile tests were performed on casing materials specifically designed for in-situ combustion wells. The tests assessed the mechanical properties of these materials, including yield strength and tensile strength, under elevated temperatures typically encountered in in-situ combustion operations. Allowable stress for different temperature ranges was determined using the safety factor method, which is widely used in engineering field to ensure the reliability of the casing structure. Additionally, a microscopic analysis of tensile fracture surfaces at various temperatures was conducted using scanning electron microscopy to investigate the temperature-induced casing failure mechanisms.

The results reveal significant changes in the fracture morphology of casing materials, highlighting a progressive degradation in the material properties as temperature increases. The Von Mises yield criterion is used as the failure assessment standard, allowing for a detailed comparison between equivalent stresses obtained from numerical simulations and experimental allowable stresses. This comparison identifies casing segments at a high risk of deformation and failure. Numerical simulations demonstrate that the casing within oil-bearing formations experiences substantial thermal expansion. Meanwhile, due to axial constraints at the top and bottom of a wellbore, the casing undergoes significant axial compression, resulting in compressive deformation. Furthermore, in ordinary rock formations, the casing expands freely but is subjected to formation stresses, leading to pronounced necking deformation. Experimental results show that once the temperature exceeds 500 ℃, the yield strength and tensile strength of casing materials decrease dramatically. The tensile fracture surfaces exhibit typical ductile fracture characteristics, accompanied by severe oxidation, confirming the adverse effects of high-temperature exposure on the mechanical performance and structural integrity of the casing. This mechanical performance degradation poses a considerable risk of failure, especially in high-temperature zones near the ignition segment.

This study further reveals that the equivalent stress distribution within the casing is positively correlated with the temperature field. High bottomhole temperatures significantly expand the risk zones for casing damage, though the failure locations remain confined to the casing segment near the igniter. The primary failure mechanism is identified as compressive deformation caused by thermal stress, with oil-bearing formation segments being particularly vulnerable to failure due to restricted casing expansion caused by surrounding constraints. These findings provide valuable insights into the failure mechanisms of the casing under high-temperature conditions and offer practical guidance for improving the casing design and reliability in in-situ combustion for heavy oil. By integrating numerical simulations with experimental tests, a more comprehensive understanding of the casing behavior under extreme thermal and mechanical conditions achieves. This approach not only enhances risk assessment strategies but also supports the development of more robust casing solutions capable of withstanding harsh environments encountered in in-situ combustion wells. Ultimately, this research contributes to the sustainable development and improves safety of heavy-oil extraction projects, reducing operational risks and ensuring long-term profitability.