Experimental investigation on the influencing factors of the thermal response test using actively heated fiber optics

Shallow geothermal energy has received extensive attention as renewable energy for sustainable development in the last few decades. Thermal conductivity, usually estimated in-situ by thermal response test (TRT), is a key parameter for evaluating shallow geothermal energy resources and designing boreholes heat exchanges. However, it is still challenging to collect accurate data about strata thermal conductivity. With the application of copper mesh heated optical cable, actively heated fiber (CMCH), optics based thermal response test (ATRT) offers the possibility of fast and accurate in-situ estimation of thermal conductivity. However, the heat transfer process of ATRT is different from the traditional TRT and there is no standardized ATRT method yet. In this study, the influences of heating duration, heating power and recover time were studied using a physical model box. The results indicate that: (1) A more accurate calculated thermal conductivity requires a longer heating duration, considering the costs and errors, the heating duration is recommended to be in the range of 90∼400 minutes. (2) The heating power has no significant effect on the calculated thermal conductivity but 15∼40 W·m⁻¹ is recommended considering the measurement accuracy of DTS equipment. (3) If ATRT was interrupted, there should be a recover time longer than 24 hours before the following ATRT to avoid the interference of the previous test.