High-Temperature Mechanical and Microstructural Properties of Well Cement Modified with Ethylene-Vinyl Acetate Polymer and Polypropylene Fibers for Geothermal Well Applications

The high-temperature performance of well cement is critical for the construction of deep geothermal wells and high-temperature energy storage wells, where mechanical integrity and pore structure stability govern wellbore reliability. To address the strength degradation and structural deterioration of conventional cements under high temperature, the G-class cement was modified by ethylene-vinyl acetate (EVA) polymer and polypropylene fibers (PF), and their impact under various temperatures was explored. Results show that at 600 °C, the compressive strength of modified cement remains above 30 MPa. While the cumulative pore area decreases at 500 °C, a significant increase in larger pores and a major restructuring of the pore network occurs at 600 °C, reflecting the dual effects of high temperature on the pore structure. The modified cement retains structural integrity and excellent mechanical performance up to 400 °C with minimal strength loss, uniform strain distribution, and stable pore structure. At 500 °C and above, it still maintains load-bearing capacity and deformation adaptability, meeting the service requirements for geothermal wells and high-temperature energy storage wells. Even at 600 °C, the reinforcing effect of EVA and PF degradation products slow down crack propagation, ensuring durability in extreme conditions. The research findings lay the foundation for the development of well cement for high-temperature service environments.