Insights on the heterogeneity of thermal transfer in the chalk aquifer using fibre-optic distributed temperature sensing

The design of ground-source heat pump systems depends on reliable determination of heat transport parameters, especially the ground thermal conductivity. An effective thermal conductivity, λeff, is traditionally calculated from in-situ thermal response testing (TRT), expressed as a bulk value for the depth range being tested. However, this practice is insensitive to heterogeneous heat transport within the ground, and flowing groundwater may significantly increase λeff. To characterise high- and/or heterogeneous-flow regimes, e.g. in fractured aquifers, distributed TRT observations are advantageous. Distributed TRTs using fibre-optic distributed temperature sensing (FO-DTS) have therefore been applied at a chalk site (Berkshire, southern UK) to assess heat flow and the depth variability of effective thermal conductivity and flow conditions, within boreholes reaching up to 100 m depth. With a vertical resolution of 0.5 m, FO-DTS can isolate different thermo-hydrogeological conditions in the chalk and particularly the groundwater flow horizons. Values of λeff, measured during a 3-day TRT, vary with depth between 5 and 30 W/m K, compared to thermal conductivities of 2.3–2.9 W/m K measured on chalk core samples. Zones of enhanced λeff correlate with highly permeable flow zones in the aquifer. However, when evaluated during a 7-day cooling phase, λeff is reduced by between 10–60% compared to values measured during heating, highlighting the potential influence of complex convection effects and vertical flow within the open borehole. Overall, this study illustrates the value of distributed spatial assessments of thermo-hydrogeological conditions, and the necessity of combining different testing approaches to characterise heat flow in dual-porosity aquifers.