Use of Calcium Aluminate to Enhance the High-Temperature Resistance of Hydroceramic Systems for Well Cementing

Summary Recently, the calcium oxide (CaO)-aluminum oxide (Al2O3)-silicon dioxide (SiO2)-water (H2O) hydroceramic system has demonstrated strong potential for use as a cementing material for deep wells due to its stability at high temperatures. However, the typically used aluminum source α-alumina (α-Al) has extremely low hydration reactivity. With this study, we aim to improve the participation of Al2O3 in the hydroceramic reaction by replacing α-Al with calcium aluminate (CA). CaO-Al2O3-SiO2-H2O hydroceramic systems with various Ca/Si/Al molar ratios were prepared to have a constant density of 1.65 g/cm3 and sufficiently retarded to meet the placement requirements in deep wells. The hydroceramic systems were then cured at 240°C for 2 days, 30 days, and 90 days, respectively, to study their long-term stability by various evaluation methods, including compressive strength test, water permeability test, mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) analysis. Test results indicated that CA had significantly higher reactivity than α-Al, and the total hydration heat of the hydroceramic system showed a strong correlation with its compressive strength at 2 days. Overall, the high calcium systems had markedly superior performances compared with the low calcium systems, primarily due to increased silica reactivity. For the low calcium case, hydroceramic systems with a Ca/Si/Al ratio of 1:2:1 exhibited inferior performance than those with a Ca/Si/Al ratio of 2:4:1, primarily due to alumina’s competition with silica for calcium sources, which led to the formation of the wairakite (CaAl2Si4O12·2H2O) phase. For the high calcium case, hydroceramic systems with a Ca/Si/Al ratio of 2:2:1 exhibited similar performance as those with a Ca/Si/Al ratio of 4:4:1, suggesting that the content of reactive alumina had a relatively small effect in these systems. Although various systems showed complex strength retrogression behavior, the use of CA to replace α-Al improved the compressive strength by 88.4% on average in the high-calcium hydroceramic systems.