Comparison of different methods for ground thermal properties determination in a clastic sedimentary environment

Energy Strategy of the Republic of Croatia by 2020 relies on renewable energy resources as one of the main priorities. The use of geothermal energy sources is specifically encouraged. Within the research project “Research and the Promotion of the Use of Shallow Geothermal Potential in Croatia”, an improved method, the so called distributed thermal response test (DTRT) has been applied on a 100 m deep borehole heat exchanger (double U pipe) within a borehole of 152 mm outer diameter. The fundamental difference from the thermal response test (TRT) is the measurement of the carrier fluid temperature along the borehole heat exchanger (BHE) using an optic fiber cable placed inside the BHE pipes. Hence, in DTRT vertical distribution of the ground thermal conductivity and borehole thermal resistance are determined along the borehole heat exchanger. The undisturbed ground temperature profile along the BHE was also determined using this method. The work presented in this paper shows measurements from the borehole heat exchanger installation located in the city of Osijek, and determination of the ground thermal conductivity for specific geological settings using three different approaches: DTRT, TRT and direct measurement of sediment thermal properties. In a fluvial sedimentary sequence found in Osijek the discrepancy of DTRT and TRT results are 3.65% for thermal conductivity and 2.25% for borehole thermal resistance, while comparison of weighted averaged ground thermal conductivity results of DTRT and the direct thermal measurement yielded difference of 8.74%.