Numerical modeling of groundwater flow in an abandoned mine for geothermal use - Development of a digital twin for the mine Neuhoffnung in Bad Ems using a multidomain approach

After the cessation of active operations and the discontinuation of pumping, many mines are flooded with groundwater. These mines often reach depths of several hundred meters, resulting in water temperatures higher than those typically found in shallow groundwater and soil. In recent years, there has been growing interest in utilizing the geothermal energy stored in these relatively easily accessible water bodies. This energy potential is seen as a promising source of renewable geothermal energy. To evaluate the potential and assess different configurations of extraction and injection locations, detailed numerical models are required.

Such a model must account for the two distinct flow regimes present in the mine and its vicinity: porous medium flow in the soil and free flow in the mine workings. The challenge lies in capturing the relevant hydrological and thermal processes while keeping the computational costs at an accept­able level.

Therefore, a process-based, coupled two-domain model using dimensional reduction has been de­veloped. In the first domain, a single-phase, non-isothermal Darcy model is solved to simulate groundwater flow in the porous medium. This domain is then coupled with an embedded 1D net­work representing the mine workings. Beyond the coupling of the two domains, a significant chal­lenge is the accurate representation of flow processes within the mine workings using a 1D model. Flow processes in mine workings are more complex than those in simple pipe flow. The temperature field in the subsurface, along with the intricate geometries, requires an extension of the classical 1D pipe flow description.