Distributed thermal response testing in a fractured chalk aquifer with high groundwater flow

In situ thermal response testing (TRT) is routinely undertaken to determine the ground’s effective thermal conductivity around closed loop borehole heat exchangers. In recent years, development of state of the art in TRT has included the addition of distributed temperature sensing (DTS) to allow insights about the relative thermal properties of specific geological horizons around the ground heat exchanger. In this context, the SmartRes project implemented a novel combined application of fibre optic DTS to track thermal plume development around an open borehole in the Chalk aquifer that had been equipped with a closed loop ground heat exchanger and subject to a heat injection TRT. The test site, Trumplett’s Farm in Berkshire (England), is known to have very high rates of groundwater movement through a dual porosity material, with particular flow concentrations in specific horizons of the highly fractured Chalk aquifer.

The fibre-optic DTS system allowed for measurement of temperature change every 0.5m down the 100m ground heat exchanger, on a 4-minute cycle, and clearly illustrated the different flow horizons in higher detail than with a standard TRT. The high resolution of measurement in space and time permitted even relatively thin geological units with differential ground water flow to be identified. Meanwhile, thermistors at 5m spacing were also installed in adjacent boreholes to monitor temperature changes and showed good agreement with the average temperatures recorded DTS in the same holes.

Based on DTS data and temperatures of the heated circulating fluid, the ground’s thermal conductivity was calculated during the 72h heating phase of the TRT and the recovery phase when heat injection was stopped. The results clearly illustrated the difference in overall thermal behaviour captured by the fluid, and the different stratigraphic units through which the borehole was constructed. The temperature changes in the high ground water flow zones were so subdued due to advection effects that it makes interpretation of the traditional TRT difficult and of limited use in the context of classic closed loop thermal design.

While the techniques illustrated in this field experiment are unlikely to be commercially viable for closed loop geothermal system deployment, they are potentially significant for the development and subsequent monitoring of open loop systems and/or aquifer thermal energy storage. In these scenarios it is much more important to understand the nuances of the in situ hydrogeological regime which may impact plume development and long-term system sustainability.