Experimental assessment of sealing integrity of grouts and clay pellets under thermal cycles representative of HT-ATES operation

High-temperature aquifer thermal energy storage (HT-ATES) can play a key role in the energy transition. For well completion of conventional low-temperature ATES and groundwater wells, grout and/or clay pellets are typically utilised as annular materials to ensure the long-term well integrity. It is not yet known if such materials can also be used in HT-ATES working conditions. In this work, a novel approach to evaluate the sealing performance for such completion materials is proposed and tested over multiple thermal heating and cooling cycles representative of the conditions of HT-ATES operation. The experimental framework utilises a novel experimental design to test the apparent transmissivity of the annular material, followed by micro-CT scanning. During each test, up to 11 thermal cycles are applied, with temperature variations between 22oC and 90oC. For grouts after 7 days of curing, micro-CT scans reveal debonding and the occurrence of micro-annuli with an equivalent diameter of approximately 26% of the original cross-section. After 28 days of curing, the thermal cycles had a much reduced impact on micro-annulus formation. The corresponding apparent transmissivity decreased up to 80% for samples containing a high percentage of cementitious minerals and a low water-to-grout ratio. The clay pellets, saturated with fresh water, demonstrated effective sealing capacity and an impermeable behaviour. However, clay pellets saturated with 0.25 mol/L NaCl, showed up to an 85% decrease in swelling capacity yet still exhibited impermeable behaviour. The results indicated that thermal cycles affect the integrity of grouts, while clay pellets show resilience to them. Furthermore, longer curing periods and specific chemical compositions improve sealing performance and provide resilience to thermal cycles.