Thermal conductivity of sands treated with microbially induced calcite precipitation (MICP) and model prediction

Thermal resistance between underground energy geo-structures and surrounding soils limits heat exchange efficiency particularly when soils are at low saturation degrees or nearly dry conditions. Microbially induced calcite precipitation (MICP) technique has great potential to enhance soil thermal properties thus increasing the possibility to expand geothermal applications to arid environments. This study investigates the thermal conductivity (k) of MICP treated sands in drying process by using a newly developed dual probe heat pulse (DPHP) sensor. The results show that thermal conductivity of sands was significantly improved after MICP treatment, and k value increased as the number of treatment cycles increased. The improvement in k is attributed to the MICP induced CaCO3 crystals functioning as "thermal bridges" among sand grains, which provides more highly effective heat transfer path and increases surface contact area in heat exchange process. Based on the normalized thermal conductivity (k(r)) concept and the experimental data obtained in this study, a modified thermal conductivity predictive model was proposed for MICP treated sands. The model prediction matched the experimental results very well, and the predicted values of k agreed with the measured values with only 10% deviation in the entire saturation range. (C) 2019 Elsevier Ltd. All rights reserved.