Analytical-experimental based inversion algorithm of stratum thermal properties for medium-depth coaxial borehole

Energy saving and decarbonization become consensus of the world. Medium-depth ground source heat pumps employing coaxial borehole heat exchangers have emerged as a pivotal choice for low-carbon heating. Heat transfer performance of coaxial borehole is vital for design, whose fundamental determinant is stratum thermal properties. However, direct measurement of these properties proves challenging for the complex stratum. Inversion methods, though avoiding this limitation, lack applicability to medium-depth boreholes. Therefore, a stratum thermophysical inversion algorithm was established based on an analytical model coupling the spatial-temporal dynamics of in-borehole fluid and stratum. Notable for its independence from initial values and the absence of parameter fitting, this algorithm offers a user-friendly solution. To validate the algorithm's accuracy and investigate heat extraction characteristics in Songliao Basin, an inaugural thermal response test at depth of 2500m was conducted in Shenyang, Liaoning. The maximum mean relative error is 9.4 % or the maximum absolute temperature error is ±0.94 °C. Correlation analysis suggests using the initial 0–13 h of data for volume heat capacity inversion and subsequent data for effective thermal conductivity inversion to enhance precision. Furthermore, a recommended minimum test duration of 80 h ensures robust results.