Determining fracture properties by tracer and thermal testing to assess thermal breakthrough risks for ground source heating and cooling in the Chalk

Numerous open loop ground energy systems are under construction or have been constructed in central London. The majority of these systems use the Chalk aquifer as a water source. A significant proportion of the abstracted water must be returned to the aquifer to maintain groundwater levels. If the ground system is to function correctly, the temperature of the abstracted groundwater must not be altered significantly by early thermal breakthrough of the returned water. Groundwater flow within the Chalk is predominantly through fractures and these provide the primary route by which thermal breakthrough might take place. The nature of the fracturing and its impact on the thermal transport beneath a proposed site must be understood to provide confidence that the ground energy system will function correctly. Two tracer and thermal test methods to determine fracture properties are examined. The results suggest that the flow in the Chalk is carried by a small number of fractures and that careful modelling and interpretation can provide valuable constraints on the frequency of fractures, their spatial pattern and their hydraulic aperture. The results highlight difficulties for designing within borehole temperature monitoring systems for fractured aquifer thermal experiments.