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Description
This study focuses on the monitoring of high-temperature aquifer thermal energy storage (HT-ATES) systems, where geothermal fluids undergo cyclic injection and production. It evaluates the effectiveness and sensitivity of the Full-Waveform Inversion (FWI) technique in mapping the anticipated changes within the reservoir using forward modeling. The feasibility study is applied to the DeepStor HT-ATES project in Karlsruhe, Germany, which aims at storing high-temperature fluids at circa 1.3 km depth across multiple layers between 7 and 10 meters thick. A baseline elastic model of the site was created, followed by a second model that includes the geomechanical changes expected after five years of fluid injection and extraction. Due to the depth and the narrow thickness of the layers, surface seismic methods have not been considered; instead, four different cross-well seismic configurations are examined. Distributed Acoustic Sensing (DAS) on fiber optic cable is used as receiver line. Applying acoustic FWI in such a cross-well configuration would not be appropriate nor provide any reliable image. Hence, an elastic rheology in the FWI modeling and inversion processes was used, which leveraged the information contained in shear-wave data (including P-to-S wave conversions) leading to increased spatial resolution and sensitivity in detecting the changes within the target layers. The findings indicate that placing receivers across the reservoir greatly enhances the ability to image thin layers and improves the detection of anomalies. In configurations where the receiver line do not intersect the reservoir layers, the best results are obtained when the FWI is applied with tighter constraints and more precise initial models. Independently of the seismic survey geometries, the single-component-receiver-like optical fiber makes difficult to capture the full lateral extent and magnitude of the changes. However, all geometries consistently detect variations at the reservoir depth. These results emphasize the importance of optimizing the design of the monitoring survey and demonstrate the potential of elastic FWI methodologies to enhance the detection, mapping and, under favorable conditions, quantification of subtle geomechanical changes expected in HT-ATES systems.
