Sodium-silicate-activated slag for acid-resistant geothermal well cements

This paper evaluates the usefulness of sodium-silicate-activated slag (SSAS) cement for completing geothermal wells containing highly concentrated H₂SO₄ and some CO₂· Using a 20wt% sodium silicate solution (SiO₂/Na₂O mol ratio of 3·22) as the alkali activator, the SSAS cements autoclaved at temperatures up to 200°C displayed an outstanding compressive strength of more than 80 MPa, and a minimum water permeability of less than 3·0 × 10⁻⁵ darcy. The combination of calcium–silicate–hydrate (C–S–H) and tobermorite phases was responsible for strengthening and densifying the autoclaved cement. At 300°C, an excessive growth of wellformed tobermorite and xonotlite crystals generated an undesirable porous microstructure, causing the retrogression of strength and enhancing water permeability. Although all the phases formed in the autoclaved cements were vulnerable to reactions with H₂SO₄, so depositing bassanite scales as corrosion product over the cement's surfaces, the C–S–H phase played an important role in retarding the rate of acid erosion. Thus, after the uptake of Ca by the H₂SO₄, Ca-destitute C–S–H preferentially reacted with the Mg from the slag to form the lizardite phase that not only retarded the rate of acid erosion, but also retained the integrity of the cementitious structure. Therefore, SSAS cement has good potential as an acid-resistant geothermal well cement at temperatures up to 200°C.