Wonjun, thank you very much for reply. Indeed, Horner analysis is the idea. Since I am petroleum engineer originally I have done a lot of well testing on oil wells (mainly pressure buildup tests). Since line source is also applicable and frequently used in petroleum engineering, Horner method and derivative curves could give more precise results in thermal conductivity determination. It is quite the same procedure to get permeability of reservoir.

Since I am focused on geothermal applications for awhile now, I would like to connect TRT and well testing from petroleum research in a science paper. Also to introduce skin factor in thermogeology terms, as skin is degree of formation damage around the wellbore (lower permeability) due to mud infiltration in pores. In thermogeology, as you know it is the same thing with thermal resistance, if grout is lower conductivity than the ground (plus thermal resistance from pipes etc.).

I have done in Croatia more than 20 TRT's on different geologic environment for commercial projects (1U, 2U and coaxial) but in practice is very hard to fully test recovery time due to timeframe of the project.

Therefore, we have drilled borehole at University, 2*50m of inclined boreholes (110mm) and installed coaxial heat exchangers 63/32. Heat exchangers are connected in series to get 100m in length. TRT was carried out  at the borehole proximity as much as possible with complete insulation (to avoid thermal disturbance) and with 4 different heat steps. The general idea is since TRT most often use electricity from construction site there is power fluctuations during day/night, to analyze recovery period as a more precise method. I have made all the calculations and results are very interesting.