Cement–rock interaction in geothermal wells at high temperatures

The lifetime of a geothermal well is strongly dependent on the long-term performance of the cement used for its construction. The critical zones in well cements are the outermost layers, where the cement encounters either the steel casing or the rock formation. The current paper presents a study of the changes in the interfacial transition zone that forms between geothermal cements and volcanic rocks, after hydrothermal treatment at 290°C. Cement slurries were poured into cavities drilled into ignimbrite blocks and allowed to set at 90°C. The assemblages were autoclaved under typical geothermal conditions, both with and without a carbon dioxide overpressure at 290°C. The addition of silica to the cement dictates the phases that form in cement. When exposed to carbon dioxide, these calcium silicate hydrate phases carbonate, at distinct carbonation rates. The rate at which these phases form and/or carbonate affects the reaction extent of the cement with the rock and, eventually, the cement durability.