LONG-TERM MONITORING OF A GEOTHERMAL EXCHANGE BOREFIELD WITH FIBER OPTIC DISTRIBUTED TEMPERATURE SENSING

Fiber Optic Distributed Temperature Sensing (FO-DTS) is a technique to collect temperature measurements along continuous profiles in space and time. FO-DTS is a suitable tool to monitor the performance of ground heat exchange systems. We are using over 2.4 km of sensing FO-DTS to monitor the response of a district-scale borefield (that includes over 2500 borefield heat exchangers) and assess the long-term performance of a geothermal exchange field. This assessment includes quantifying heterogeneous thermal properties and groundwater flow and their effect on subsurface heat transfer and storage. The aim of the study is to evaluate the potential for borefield overheating caused by a yearly imbalance in heating and cooling loads. This imbalance in heating and cooling loads increases the temperature of the field and leads to diminished borefield performance. Over a two-year period, we observed seasonal heating and cooling loads with an overall increase of the temperature in the field. Within these overall observations, we found accelerated cooling associated with layers of highly voided rock, saturated rocks below the water table heat and cool quicker than dry rocks, and shale formations tend to have poor heat transfer relative to siliciclastic and carbonate formations. Given these observations, an optimal borefield design to offset overheating should increase contact with a saturated, voided or fractured, clay-constituent-free sedimentary bedrock.