Development of Reactive Transport Models for Very High Temperature Heat Aquifer Storage (VESTA) at a Pilot Site in Germany

Thermal energy storage at large scale has significant potential for large scale clean energy deployment. However, it is necessary to understand and address the challenges (Dobson et al., 2023) associated with high temperature reservoir thermal energy storage (HT-RTES). Lessons learned from the previous demonstrations identify insufficient site characterization, thermal short-circuiting, lack of available heat, scaling, corrosion, and biofouling as key factors affecting the performance of HT-RTES. The objective of this paper is to develop reactive transport modeling strategies to understand the reactive-transport processes associated with HT-RTES in high-saline reservoirs by evaluating the significance of changes in temperature, pressure, mineralogy, and porosity of the formation during the HT-RTES operation. The results from the model will also evaluate the retrograde solubility of minerals, changes in permeability, and changes in redox conditions during the HT-RTES operation.

For this study, an isolated injection-production well doublet is used for injecting hot and cold fluids during the seasonal cycle. During summer, brine at 75 °C is surface heated to 140 °C and injected into the reservoir with 15% porosity. Produced brine from the heat-exchanger at 60 °C is injected back into the cold well during winter. Reactive transport simulations are carried out using TOUGHREACT-EOS7(Dobson et al., 2004; Sonnenthal et al., 2021) for 5 years of cyclic RTES operation. Representative geochemical data were obtained from the depleted Leopoldshafen oil field of Leopoldshafen around the DeepStor site (Banks et al., 2021).

In agreement with findings of Banks et al., we observe an increase in the porosity, in our case of 1.5%, near the hot wells after 5 years of operation. After five years, there has been a marginal decrease in porosity, approximately 1%, near the cold well. This suggests that mineral dissolution is more prevalent in the vicinity of the hot wells, likely a result of the injection of hot, somewhat acidic brine, whereas mineral precipitation is predicted around the cold well as a result of the temperature decrease. Iron minerals such as goethite show dissolution near the hot wells and precipitation in the relatively colder brine slightly away from the hot well. Also, changes in permeability have been evaluated using a cubic law of porosity-permeability correlation. There is no significant interference of hot and cold plumes, which indicates that thermal short-circuiting has not occurred under the simulated operating conditions. Future work will include a modeling scenario under strong oxidizing conditions such as the presence of dissolved oxygen in the injection brine, which can better quantify the possibility of corrosion and scaling due to air intrusion.