Evaluation of ground thermal properties and specification of the geological structure using thermal response test, natural gamma, and resistivity data

Abstract

The primary objective of this study is to identify the ground thermal properties of the local geological structure and to investigate the geological structure. The knowledge of underground thermal properties is necessary to design borehole heat exchangers (BHE) for Ground Source Heat Pumps (GSHP). Particularly for commercial GSHP, main thermal parameters should be measured on site. A thermal response test (TRT) is performed on one or more borehole heat exchangers in a pilot borehole. This borehole is a part of the borehole field. A TRT enables to collect the data to be used for ground thermal parameters calculation, i. e. the thermal conductivity and resistivity. The heat load to be injected into the BHE is predefined in advance; the resulting temperature changes of the circulating fluid and flow rate are measured. The paper presents a short description of the basic concept and theory of TRT, shortly reviews the experience of this technology. The main aim of the presented study and calculations is to start creating efficient methodology to estimate main thermal parameters (thermal conductivity and thermal resistance) in Lithuania. The most used line-source methods and the results will be presented and compared against the numerical method. Circulating fluid and borehole temperatures would be analytically calculated in order to estimate the impact on the underground temperature field.