Study on the Retrogression Behavior and Mechanism of Oil Well Cement Under Ultra-High Temperature Conditions

To address the unclear mechanism governing strength retrogression in oil well cement under ultra-high temperature conditions, this study investigates the evolution of strength and hydration products in silica-sand-enriched (45%–50%) oil well cement systems at temperatures exceeding 200 °C. The results demonstrate that at 200 °C, cement strength follows a “V”-shaped trend over time, with negligible changes in permeability and microstructure. Hydration products evolve from initial C-S-H gel to tobermorite and xonotlite in the intermediate stage, and finally to densely structured xonotlite. At 260 °C, strength development exhibits an inverted “V”-shaped pattern with significant late-stage retrogression. Early-stage hydration products primarily comprise afwillite and xonotlite, with afwillite content gradually increasing during the intermediate stage. The terminal stage is dominated by acicular and reticular xonotlite, accompanied by progressive crystalline evolution and microstructural coarsening. Incorporation of 35%–45% high-quality silica sand effectively mitigates strength loss at 200 °C. However, at temperatures ≥260 °C, even 50% silica sand addition fails to prevent strength degradation, underscoring the imperative to develop effective mitigation strategies for high-temperature cement strength retrogression.