Acid-resistant cements for geothermal wells: sodium silicate activated slag/fly ash blends

Sodium silicate activated slag/class F fly ash (SSASF) blend cements were prepared by varying two parameters, the SiO2/Na2O mol. ratio in the sodium silicate activator, and the slag/fly ash weight ratio, and then autoclaving them at 100, 200 and 300°C. The usefulness of the autoclaved SSASF cements as acid-resistant geothermal well cements was evaluated by exposing them for 15 days to 90°C CO2-laden H2SO4 (pH 1·1). Among the combination of these two parameters, the most effective one in inhibiting acid erosion consisted of 50/50 slag/fly ash ratio cement and 2·50 SiO2/Na2O ratio activator. The weight loss by acid erosion of this cement was less than 7%. The major contributor to such minimum acid erosion was the zeolite phase formed by interactions between the mullite in fly ash and the Na ion liberated from the activator. Further characteristics of this cement included a long setting time for its slurry of approximately 1770 min at room temperature, and a water permeability ranging from 2·4 × 10−4 to 1·9 × 10−5 Darcy, and a compressive strength from 4·0 to 53·4 MPa after autoclaving at 100–300°C for 24 h.