Development of geothermal energy sources is an important contribution to ensure the “green shift” from fossil fuels to more sustainable sources of energy. Currently, most geothermal wells operate at temperature range of 150–300° C, but development of super-high temperature geothermal reservoirs may be needed to increase power production. However, the high temperature conditions to be found in such wells, up to 400–500 °C, are very challenging and may be detrimental for the integrity of well cement.
In this paper, several cement integrity challenges for high temperature geothermal wells are reviewed, such as mechanical failures during well start-up and potential shut-in periods, and long-term issues caused by chemical alterations of the cement during high temperature exposure. Experimental tests have been performed with two different, potential geothermal well cement systems: a Portland-based system with silica flour and MicroSilica as additives, and a non-Portland, calcium aluminate cement system. For both cement systems, high temperature ageing tests have been performed at 500 °C for 8 weeks, where unconfined mechanical properties were determined before and after exposure. Furthermore, down-scaled tests of radial crack formation in casing-cement-rock samples have been performed, as simplified functions tests of cement sheath integrity during well start-up.