Quantifying Thermal Dispersivity and Darcy Fluxes with Active-DTS thermal tests

For the successful subsurface thermal energy storage, accurate characterization of a target aquifer is essential. The active-distributed temperature sensing (DTS) thermal test, which often utilizes a fiber-optic cable both to monitor temperature and to serve as a heat source, has emerged as a promising tool for high-resolution estimation of Darcy flux. For the interpretation, most studies have assumed thermal dispersion to be negligible, yet thermal dispersion is expected to become significant under high flow velocity and heterogeneous hydraulic conductivity field. Despite its importance, estimating in-situ dispersivity remains highly challenging. To evaluate the possibility and sensitivity of thermal dispersivity estimates obtained from active-DTS tests, we incorporate thermal dispersion into the moving infinite line source model and validate it through the numerical model. After that, a sensitivity analysis was performed with two-dimensional numerical simulations under various Darcy fluxes (q, 1 – 10 m/d) and thermal longitudinal dispersivity conditions (α, 0 – 0.01 m). Our results demonstrate that increasing thermal dispersivities systematically reduced the magnitude of temperature increase and delayed the time to reach the plateau, both effects intensified as q increased. Based on these results, we jointly estimate Darcy flux and thermal dispersivity from single or multiple active-DTS tests and evaluate their uncertainties quantitatively. We expect that these findings will extend the applicability of active-DTS thermal tests as a versatile tool for aquifer characterizations and provide a chance for in-situ thermal dispersivity estimation.

 

Keywords: Thermal dispersion; Active-DTS; Aquifer characterization; Moving infinite line source