Effects of reservoir permeability heterogeneity on the size of solute and thermal protection zone around a geothermal reinjection well in a porous reservoir, Hungary

Sustainable groundwater management requires accurate delineation of a protection zone (PZ) around wells. While PZ has been widely studied for solute protection zone (SPZ) of drinking water wells, the growing use of geothermal applications requires also the delineation of both solute and thermal protection zone (TPZ). In this study, the effects of reservoir heterogeneity on SPZ and TPZ size were investigated using numerical simulations around a deep geothermal reinjection well, where reservoir heterogeneity was studied by stochastic permeability distributions. In the first part of the study, synthetic simulations were performed to highlight the importance of the scientific question, where the role of the permeability heterogeneity scale, and the thermal retardation were examined. The SPZ and TPZ size was characterized by monitoring the evolving position of the solute and thermal front. In the main part, the theoretical findings were applied to a real geothermal project in Hungary, proposing a novel workflow where the heterogeneity scale was derived from seismic attributes and geophysical logs. The results show that the SPZ and TPZ size increases with the heterogeneity scale, indicating that a larger protection zone is needed, although the effect of heterogeneity decreases over time. Heterogeneity has significant effect on the TPZ extension especially on small time scale, so its investigation in heat transport cannot be neglected despite greater heat diffusion. The probability maps of SPZ and TPZ areas illustrate well the uncertainty caused by reservoir heterogeneity. Finally, results highlight that TPZ cannot be directly rescaled from SPZ because of the time-varying difference between apparent and effective thermal retardation.