Estimation of seismic velocity changes in a HT-ATES system using THM modelling

In Central Europe, a substantial emitter of CO2 in the energy sector corresponds to the thermal energy required for heating and cooling. Seasonal underground heat storage presents a viable option for storing excess heat generated during the summer months for usage in winter, reducing the need for conventional sources of energy. Today, high-temperature aquifer thermal energy storage (HT-ATES) systems are attracting large interest as they represent a sustainable means of meeting heat demand.

In HT-ATES systems, hot water is injected into a reservoir during summer, while exchanged cold water is injected over the winter season. These fluctuations in temperature and pressure have an impact on the geomechanical and thermo-hydraulic properties of both the reservoir and the surrounding layers. Monitoring the changes in the reservoir properties is a critical aspect of running a heat storage system safely and efficiently. We try to determine whether active seismic imaging could be a suitable method to characterize the temporal and spatial evolution of the reservoir.

With view on designing future geophysical assessment and monitoring systems, we first perform thermo-hydro-mechanical (THM) modelling to estimate the variations in the poroelastic properties due to the geothermal processes. Our modeling is based on the characteristics of the DeepStor demonstrator, currently under development in the north of Karlsruhe (Germany), at the Karlsruhe Institute of Technology (KIT). The three layers model includes different mechanical properties with one borehole. The simulation of cyclic hot water injection and production over time allows to quantify its effect on the underground material properties. In addition to assessing the expected operational parameters of the DeepStor demonstrator, we test additional injection schemes with varying underground properties to simulate the different ranges of porosity changes and look at their effects on the elastic properties.

Linking the THM model parameters to seismic sensitive variables such as velocities and impedances, through empirical equations, allow us to determine the conditions under which active seismic surveys could effectively detect these changes. This approach provides a valuable tool for evaluating the potential of active seismic imaging for monitoring HT-ATES systems.