Structural evolution in micro-calcite bearing Ca-montmorillonite reinforced oilwell cement during CO2 invasion

• Ca-montmorillonite, modified by ScCO 2 , is used as an anti-CO 2 corrosion additive. • Micro-calcite (MC) bearing Ca-montmorillonite reduces degradation of oilwell cement. • MC induces formation of a dense carbonate layer, which enhances CO 2 resistance. High concentration CO 2 invades and forms carbonate layers in oilwell cement under geological CO 2 sequestration conditions. Long-term invasion of CO 2 makes the carbonation structure unstable and prone to damage. For the reinforcement of oilwell cement, Ca-montmorillonite (Ca-MMT) was modified using supercritical CO 2 (ScCO 2 ) as the solvent and intercalator. The micro-calcite bearing Ca-montmorillonite (MC Ca-MMT) was added to the cement slurry to reinforce the oilwell cement after the modification. Microcrystal calcite was identified in the MC Ca-MMT through transmission electron microscopy(TEM) and selected-area electron diffraction(SAED) analyses. The mechanical properties and microstructural evolution of the MC Ca-MMT reinforced oilwell cement were investigated, revealing that the carbonation was inhibited by MC Ca-MMT. Through the analysis of microcrystal formation and densified carbonation area, the CO 2 -resisting mechanisms of MC Ca-MMT reinforced oilwell cement were revealed in this study.