LONG TERM MONITORING OF BOREHOLE TEMPERATURE PROFILES USING A THERMISTOR ARRAY IN THE BALL STATE UNIVERSITY GROUND-SOURCE GEOTHERMAL SYSTEM

Ball State University (BSU) has completed the world’s largest district-scale, ground-source geothermal system that utilizes the shallow lithology and groundwater as a thermal reservoir. Since October 2014, when thermal loading started in Phase 2 of the system, we have monitored long-term temperature data from a thermistor array installed in a nested research well adjacent to the Phase 2 field of borehole heat exchangers. These data allow direct comparison of temperature collected at increments of 10 feet (3.28 m) in the well and at 10-minute intervals. The data are also part of a long-term and system-wide monitoring program of ground heating in response to thermal loading. The data from this thermistor array are therefore useful for comparisons between monitoring wells surrounding and within the geothermal system.

Data between October 2014 and April 2015 are stable and divide into four temperature regimes: 1) a zone of stable temperatures at 14° C within the glacial till, 2) an unconfined aquifer in the Silurian-age Salina Group where temperatures decrease from 14° C to 13.3° C, 3) a semi-confined aquifer in the Silurian-age Salamonie and Cataract Formations with temperatures of 13.1° C, and 4) a confined aquifer in the Ordovician-age Maquoketa Group with stable temperatures of 12.5° C. Following April 2015, temperatures have increased in response to, but delayed from, thermal loading in Phase 2. In May-October 2015, the rate of temperature change increased followed by a slow down in the rate of temperature change from October-December 2015. Maximum temperatures have increased from 14° C to 15.5° C and are most amplified in the Salina Group. In contrast, temperature increases are least pronounced in Brainard Shale where maximum temperatures remain at 14.8° C. The temperature profile is a clear representation of the concept of a ‘thermofacies’ that is related to the mineral composition, porosity structure, and rate of groundwater flow through the aquifer matrix.