Corrosion-Resistant Foamed Cements for Carbon Steels in Enhanced Geothermal Systems

摘要

The cementitious material consisting of Secar #80, Class F fly ash, and sodium silicate designed as an alternative thermal-shock resistant cement for the Enhanced Geothermal System (EGS) wells was treated with cocamidopropyl dimethylamine oxide-based compound as foaming agent (FA) to prepare air bubble-foamed low density cement slurries of ≤1.3 g/cm~3. These foamed slurries were modified with acrylic polymer (AP) emulsion as corrosion inhibitor. We evaluated the hydrothermal stability of the AP in 200°C-autoclaved cements and its effect on the composition of crystalline phases. The polymer positively affected the microstructure developed in 200°C-autoclaved cements, cements' mechanical behavior and the corrosion mitigation of carbon steel (CS) by the polymer-modified cement coatings. Ca-or Na-carboxylates formed from the polymer in cement pore solution improved its thermal stability. Addition of AP did not alter any of the crystalline phases responsible for the strength of the cement. Furthermore, the AP-modified cement developed the porous microstructure with numerous defect-free disconnected voids. These effects together contributed to the improvement of compressive-strength and -toughness of the cured cement. There were four major factors governing the corrosion protection by AP: 1) reduction of the infiltration and transportation of corrosive electrolytes through the cement layer deposited on the underlying CS surfaces; 2) inhibition of the cathodic reactions at the corrosion sites of CS; 3) extended CS coverage by modified cement and, 4) improved adherence of the cement to CS surfaces. The CS's corrosion rate of 176 milli inch/per year (mpy) for 1 wt% FA-foamed cement without AP was considerably reduced to 69 mpy by adding 2 wt% AP and to less than 10 mpy with 10 wt% AP.