Comprehensive Evaluation of Retrogression-Resistant Well Cement Systems Set at Different Temperatures and Cured at 200 °C

Traditional silica-enriched oil well cement suffers from strength retrogression at high temperatures, hindering deep geoenergy resource development. This study comprehensively evaluates novel retrogression-resistant cement systems formulated with Portland cement, silica sand, fly ash, and ground blast furnace slag. Slurries with densities of 1.9–1.6 g/cm3 were prepared by adjusting water content or adding hollow glass microspheres, and cured at 200 °C after initial setting at either 80 °C or 200 °C to simulate different field conditions. Results show that systems directly set at 200 °C maintained stable mechanical properties over 30 days, while those set at 80 °C exhibited significant strength gains. Permeability and pore structure remained stable or refined in all systems. Microscopic analyses (MIP, QXRD, TGA) reveal that low-temperature-set systems contain higher tobermorite and lower xonotlite and hydrogrossular contents, explaining their superior performance. Density reduction by water addition weakened properties, whereas hollow glass microspheres effectively preserved microstructure and strength. The C-(A)-S-H content remained stable throughout curing, and tobermorite increased over time, constituting the primary anti-retrogression mechanism. These findings provide insights into the roles of setting temperature and density regulation, supporting the design of durable cement systems for deep and ultra-deep wells.