Characterization of an Aquifer Thermal Energy Storage system with an Active Distributive Temperature Sensing Experiment

Aquifer Thermal Energy Storage systems (ATES) are reversible open loop geothermal systems involving a minimum of two reversible boreholes. It gains more and more popularity with a great potential to reduce greenhouse gas emission of the building sector.

During operation, ATES storage efficiency and energy recovery rate depend on the cold and warm thermal plumes extension. The thermal plumes shaped are particularly impacted by the aquifer vertical and horizontal heterogeneity especially regarding the flux and thermal properties distribution. In addition, ATES performance also depends of the sustainability of the wells and the major issues of their aging. Among the major causes, biofouling, chemical and physical clogging are well documented for open loop system.

However, field methods existing to quantify aquifer properties are not well suited to estimate both thermal and hydrodynamic aquifer heterogeneity with a high spatial resolution. Recent developments in Fiber-Optic Distributed Temperature Sensing (FO-DTS) have solved this issue. In particularly, active -DTS experiments (ADTS) were shown to be a promising avenue for imaging the spatial distribution of subsurface heterogeneities. It consists on monitoring the thermal response along the FO cable induced by a heat source placed in the borehole which allow to estimate thermal and hydrodynamic properties distributed along boreholes.

In that context, we performed a series of ADTS experiments on a ATES site. The field experiments were run under cross-borehole configuration and replicate in two different piezometers to check the reciprocity of the results. Our work demonstrates the potential of ADTS to estimate both thermal conductivity and groundwater flux along the two boreholes. At the same time, it was shown to be a good tool to detect clogging locations along the boreholes. The proposed experimental design is simple and the tests can be run without opening boreholes and stop the pump. First, this method enables the characterization of thermal and hydrodynamic heterogeneities to develop more advanced numerical models. Secondly, it can be used as strategy surveillance to monitor clogging evolution into geothermal boreholes and to plan maintenance works before major deterioration.