Sodium-polyphosphate-modified fly ash/calcium aluminate blend cement: durability in wet, harsh geothermal environments

摘要

To assess the durability of sodium-polyphosphate-modified fly ash/calcium aluminate blend (SFCB) cement, which was used to complete geothermal wells in Indonesia, the cement specimens were exposed for up to 7 months in autoclave containing a simulated geothermal brine with 20,000 ppm CO_2 and 400 ppm H_2S at 280degC. Hydrothermal reactions between sodium polyphosphate (NaP) and fly ash or calcium aluminate cement (CAC) generated two major crystalline phases, hydroxyapatite (HOAp) and analcime (AN), which were responsible for developing a densified microstructure in cement bondies, offering their ultimate compressive strength. The AN phase was susceptible to a reaction with CO_2, transformating it into the cancrinite (CAN) phase. The AN→CAN phase transition continuously took place during exposure between 7 days and 4 months. This phase transition caused the alteration of a dense microtexture into a porous one, thereby resulting in some loss of strength. However, once the conversion of AN into CAN neared completion, there was no further significant change in compressive strength. After 7 months of exposure, the phase composition of cement consisted of HOAp and CAN as the major phases, and AN as the minor one. No decomposition of the cement was observed, nor was there any carbonation-caused erosion, suggesting that the SFCB cement has an excellent durability in a hostile geothermal environment.