| Abstract |
Self-degradable slag (S)/Class C or F fly ash blend pozzolana cements were formulated, assuming that they might serve well as alternative temporary fracture sealers in Enhanced Geothermal System (EGS) wells operating at temperatures of ? 200°C. The candidate formulas were screened based upon material criteria including an initial setting time ? 60 min at 85°C, compressive strength ? 2000 psi for a 200°C autoclaved specimen, and the extent of self-degradation of cement heated at ?200°C after it was contacted with water. After mixing with water and being autoclaved at 200°C, the calcium silicate hydrate (l) [C-S-H (l)] and aluminum-substituted 1.1 nm tobermorite crystal phases, and geopolymer as amorphous phase were identified as hydrothermal reaction products responsible for the development of a compressive strength > 2000 psi. All the S/C systems displayed the self-degradation. In the S/F systems, the 200°C-autoclaved cements had the combined phases of tobermorite as its major reaction product and amorphous geopolymer as its minor providing a compressive strength of 5271 psi. Sodium hydroxide derived from the hydrolysis of sodium silicate activator not only initiated the pozzolanic reaction of slag and fly ash, but also played an important role in generating in-situ exothermic heat that significantly contributed to promoting self-degradation of cementitious sealers. The source of this exothermic heat was the interactions between sodium hydroxide, and gaseous CO2 and CH3COOH by-products generated from thermal decomposition of sodium carboxymethyl cellulose additive at ?200°C in an aqueous medium. Thus, the magnitude of this self-degradation depended on the exothermic temperature evolved in the sealer; a higher temperature led to a severe disintegration of sealer. In contrast, the excessive formation of geopolymer phase due to more incorporation of Class F fly ash into this cementitious system affected its ability to self-degrade, reflecting that there was no self-degradation. The geopolymer was formed by hydrothermal reactions between sodium hydroxide from sodium silicate and mullite in Class F fly ash. Thus, the major reason why geopolymer-based cementitious sealers did not degrade after heated sealers came in contact with water was their lack of free sodium hydroxide. |