Evaluating anisotropy ratio of thermal dispersivity affecting geometry of plumes generated by aquifer thermal use

Abstract

Studies on shallow geothermal applications have demonstrated that mechanical thermal dispersion is an important heat transport mechanism in saturated porous media. However, most previous studies have assumed the thermal dispersivity ratio, which has not been sufficiently examined. This study investigates the validity of the general assumption that the transverse dispersivity is one-tenth of the longitudinal dispersivity. For this purpose, heat transport experiments, specifically designed at the laboratory scale for estimating dispersivity, were conducted using two different heat sources at Darcy fluxes of 4.74 to 37.47 m/d (Reynolds number Re < 0.3). The results from the analytical models showed that the dispersivity ratios differed from the thermal dispersivity assumption, indicating that the ratio can vary depending on the properties of the porous media. Additionally, heat transport modeling was performed to confirm the influence of the dispersivity ratio on the temperature plumes that develop from the thermal use of shallow aquifers, such as groundwater heat pump systems. We found that, under steady-state conditions, the decrease in the dispersivity ratio led to smaller plume dimensions in all directions, owing to the increased heat dissipation in the transverse direction. Transient simulations for flow and heat transport indicated that the effect of the thermal dispersivity ratio was time-dependent and heterogeneous and increased with the injection rate. These results suggest that the thermal dispersivity ratio is crucial for assessing the environmental impacts of aquifer thermal use, especially for long-term and large-scale applications. Therefore, the magnitude and ratio of thermal dispersivity should be evaluated at the design stage for more efficient and sustainable use of shallow geothermal resources.