Thermal analysis of medium-depth borehole heat exchanger coupled layered stratum thermal conductivity

Medium-depth coaxial borehole heat exchanger is a crucial option for low-carbon heating. However, the complex and variable nature of medium-deep formations poses challenges for accurate thermal conductivity determination, and its correlation with heat transfer mechanisms remains unclear. Hence, a thermal conductivity model incorporating temperature corrections and lithological variations was developed and integrated into a layered analytical heat transfer model. Model analysis shows that strata thermal conductivity and temperature both have significant effects on heat extraction. Geological tests and applications based on Shenyang case found that due to good thermal conductivity of rock skeleton (gneiss), low porosity (1.8 %) and no mud, maximum value of 3.34 W m- 1 K- 1 was observed near 1063.9-1068.8 m. The average thermal conductivity is 2.58 W m-1 K-1. Formations with low porosity, low permeability, and low mud-content are denser, which favors heat conduction, contrary to hydro-geothermal wells that rely on highly porous and fractured formations. The deviation between the calculated results and thermal response test from two regions did not exceed 5.21 %, achieving a high precision. This method enables direct determination of formation thermal properties based on well-logging, providing a convenient and engineering-friendly approach.