Thermal Effect on Cement State of Stress in a Cement-Casing Structure

ABSTRACT Temperature fluctuations within a wellbore are common occurrences in various applications, including carbon sequestration, hydrogen storage, and thermal storage. To ensure long-term well integrity, it is necessary to investigate the mechanical response of the cement annulus under thermal loading. In this study, we designed a two-layer cement casing structure to evaluate the stress state at the cement-casing interface under high pressure and high temperature conditions. We cured Class-G cement with a water-cement ratio of 0.4 and poured it inside the casing, subjecting it to 40 MPa and room temperature for seven days before elevating the temperature to 35, 45, 55, and 65°C. After correcting for casing thermal expansion, we found that the cement-casing interface stresses were 9.2, 11.1, 16.4, and 22.7 MPa under these temperatures, respectively. The corresponding coefficient of thermal expansion of the cement was 1.6 × 10−5 /°C. Throughout the experiment, there was no water exchange between the curing cement and the external reservoir. The cement pore pressure dropped to nearly zero after seven days of hydration, but it increased significantly with the elevated temperature and dissipated slowly. Additional tests are needed to correct for the pore pressure effect on the cement-casing interface state stress. Overall, this work contributes to a better understanding of cement's pore-mechanical behavior with thermal cycling under subsurface conditions. INTRODUCTION The global energy sector is shifting from fossil based systems to clean energy resources in response to the growing concern about climate change. The topics of carbon sequestration, hydrogen storage, and thermal storage become the target of the whole society. Despite this shift, wellbore remain the primary effective channel connecting subsurface space utilization and ground activities, making long-term well integrity crucial to ensure sustainable, reliable, and efficient energy storage and sequestration. In a typical wellbore, cement annulus is usually the most vulnerable part, therefore, we care about cement's overall performance in maintaining zonal isolation, including rheological properties, chemical properties, and mechanical properties [1]. Usually, researchers propose new formulations with specific types of additives to improve displacement efficiency, reduce chemical shrinkage, and promote proper mechanical strength. Of all the aspects of performance, cement mechanical behavior directly determine the quality of zonal isolation throughout its functional life.