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<p>The long-term strength retrogression of silica-enriched oil well cement seriously threatens wellbore integrity in deep/ultra-deep wells. This study tested the performance evolution of silica-enriched oil well cement, initially set at 80&#xa0;°C or 240&#xa0;°C and then cured at 240&#xa0;°C/20&#xa0;MPa. Results showed: cement set at 80&#xa0;°C showed strength growth during curing, while that set at 240&#xa0;°C underwent strength loss within 28&#xa0;days; neither increased silica sand dosage nor coal gangue powder addition inhibited this decline. SEM-XRD quantitative analysis revealed: for low-temperature-set cement, C-(A)-S-H content increased from 3 to 28d, and more xonotlite formed at 3d; though xonotlite impaired compressive strength, its structural stability ensured cement strength stability. In contrast, high-temperature-set cement had decreased C-(A)-S-H and continuously increased xonotlite during curing, causing microstructural coarsening and strength retrogression—this difference may stem from distinct C-(A)-S-H structures under the two setting temperatures. Notably, increasing silica sand dosage or adding coal gangue powder also failed to prevent such structural/compositional changes, and these microanalytical results explain the compressive strength of all systems.</p>

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Strength retrogression mechanisms of silica-enriched oil well cement under 240 °C curing conditions

  • 期刊论文

  • 发表日期：

    2025年11月19日

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  • Guodong Cheng,
  • He Li,
  • Haoya Liu,
  • Haoguang Wei,
  • Shiming Zhou,
  • Kuizhen Fang

摘要

The long-term strength retrogression of silica-enriched oil well cement seriously threatens wellbore integrity in deep/ultra-deep wells. This study tested the performance evolution of silica-enriched oil well cement, initially set at 80 °C or 240 °C and then cured at 240 °C/20 MPa. Results showed: cement set at 80 °C showed strength growth during curing, while that set at 240 °C underwent strength loss within 28 days; neither increased silica sand dosage nor coal gangue powder addition inhibited this decline. SEM-XRD quantitative analysis revealed: for low-temperature-set cement, C-(A)-S-H content increased from 3 to 28d, and more xonotlite formed at 3d; though xonotlite impaired compressive strength, its structural stability ensured cement strength stability. In contrast, high-temperature-set cement had decreased C-(A)-S-H and continuously increased xonotlite during curing, causing microstructural coarsening and strength retrogression—this difference may stem from distinct C-(A)-S-H structures under the two setting temperatures. Notably, increasing silica sand dosage or adding coal gangue powder also failed to prevent such structural/compositional changes, and these microanalytical results explain the compressive strength of all systems.