Strength Retrogression Mechanisms of Silica-Enriched Oil Well Cement under 240°C Curing Conditions

The long-term strength retrogression of silica-enriched oil well cement poses a significant threat to wellbore integrity in deep and ultra-deep wells. This study tested the performance evolution of silica-rich cement under two setting temperatures (80°C and 240°C) followed by 240°C/20 MPa curing. Results indicated that 80°C-set cement exhibited strength growth during curing, whereas 240°C-set cement suffered strength decline over 28 days; increasing the dosage of silica sand and adding coal gangue powder both cannot prevent this decline trend. SEM combined with XRD quantitative analysis revealed that, the content of C-(A)-S-H of set cement at low-temperature setting increased cured from 3d to 28d and it directly generated more xonotlite at 3d early stage, although this mineral phase was unfavorable relative to the compressive strength of set cement, it was structurally stable at this temperature, which was the reason for its stable strength, in contrast, the C-(A)-S-H of set cement at high-temperature setting decreased cured from 3d to 28d and the content of xonotlite continued to increase during the long-term curing process, resulting in the coarsening of the microstructure and retrogression in strength; this difference may be due to the different structure of C-(A)-S-H under the two setting temperatures; in addition, increasing the dosage of silica sand and adding coal gangue powder both cannot prevent the above-mentioned structural and compositional changes of set cement. The results of above microanalysis can explain all systems compressive strength.