Hydraulic Tomography and Heat Transport Simulation as a Combined Method for the Highly Resolved Characterization of Geothermal Systems

Recently, more and more concepts for the combination of solar thermal systems and ground-coupled heat pump systems are being proposed by companies and system manufacturers. In this study, this thermal process is simulated, for which the spatial distribution of the subsurface hydraulic parameters is required. However, it is difficult to reconstruct a natural heterogeneous aquifer with high spatial resolution. Hence, an investigation approach is utilized, which is based on the combination of hydraulic tomography and numerical simulation. This approach allows us to simulate the heat transfer in the subsurface based on a highly resolved aquifer reconstruction. Traditional hydraulic tests, e.g. pumping tests, can provide averaged hydraulic parameters only for relatively large volumes, thus the transport-relevant small-scale variability of hydraulic parameters can not be determined. In this study, two inversion methods for the characterization of the aquifer are used to determine the spatial distribution of hydraulic parameters: (1) hydraulic travel time inversion, which determines diffusivity (D) and (2) hydraulic attenuation inversion, which determines specific storage (Ss). Both inversions are based on the transformation of the groundwater flow equation into a form of the Eikonal equation that can be computationally solved efficiently with particle-tracking or ray-tracing techniques. Subsequently, the spatial distribution of hydraulic conductivity (K) can be determined indirectly, based on the ratio of K = D × Ss. The data set used for the inversions consists of 16 short-term pumping tests (lasting approximately 5 min each) with a tomographical configuration in an approximately 40m thick marl-/clay-stone aquifer between two 3" wells. The distance between the two wells is 2.9 m. Subsequently different independent hydraulic tests, i.e. the flow meter test and fluid injection logging are also carried out for the validation of the combined inversions. At the test site, one well is surrounded by three Borehole Heat Exchangers (BHEs). They are connected to a heat pump and a system which is able to emulate different scenarios of heat demand (for example heating a house) or heat abundance from a solar thermal system. The resulting subsurface heat transfer in- and outside of the BHEs is simulated using a COMSOL Multiphysics 3D model. It consists of the BHE system and the surrounding subsurface considering all necessary parameters for the calculation of heat transport between the BHEs. Since this transport proceeds diffusively and convectively, it is necessary to know the subsurface flow parameters for accurate calculations and predictions. The distributions of the hydrogeological parameters with a resolution of approximately 2.5m × 0.4m between two wells, which are achieved through hydraulic tomography are incorporated as input parameters into the model. This combined approach enables a detailed investigation of the thermal interaction between BHEs and thus can optimize the use of geothermal reservoirs for thermal energy storage.