Exploring supercritical fluids and investigations of their characteristics for geothermal use requires risk minimization similar to those in oil and gas applications. The geothermal operating company leading the DESCRAMBLE (DESCRAMBLE 2018) project consortium within the European Union's (EU) research and innovation programme Horizon 2020, requested a tailored cement slurry suitable as a dependable long-term barrier and appropriate cementing practices which suit the targeted K-horizon's expected supercritical conditions.

A scouting study was performed testing two different Portland-cement silica flour blends under targeted conditions. Additionally, a blend comprising calcium aluminate phosphate cement was selected for this test series. Non-Portland slurry designs have been used in high-temperature geothermal wells (Berard, 2009) as well as in steam injection applications (Li, 2016).

Because of testing equipment limitations, samples were cured at 200°C/206 bar and then heated to 450°C at ambient pressure during the scouting phase. The non-Portland design proved to be the best candidate, although all three cement designs showed cracking. As a result, it was decided to further improve the non-Portland cement's mechanical properties by adding modifiers that would not affect the high-temperature rating of the cement. During the second phase, it was also necessary to consider the operational side of slurry design, as placement temperature was 200 °C with a requirement of more than 5 hours thickening time, which is near the upper limit for a non-Portland cement blend. Additionally, testing conditions were ramped up close to supercritical conditions during initial curing, and samples then underwent further curing at 550 °C and ambient pressure. Tri-axial mechanical properties were evaluated after cooldown.

From an operational aspect, it became apparent during lab testing that mixing order was important as well as extended temperature simulation to help ensure the well was properly cooled down before cement placement. Once all laboratory testing, simulations, and operational preparations were complete the actual 7-inch liner cementing operation was performed with the desired results. Use of a specially modified calcium aluminate phosphate cement blend was successful and initial results from the well showed that this cement system was properly tailored to the geothermal well conditions.

Only a few geothermal wells have been attempted previously under such conditions, hence it was necessary to create and deliver a modified cement slurry design for both geothermal and oil and gas applications. Further monitoring of the cement sheath will be conducted to evaluate long-term properties.

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