Geologic and thermal conductivity analysis based on geophysical test and combined modeling

Strata thermal conductivity (λ_e) influences efficiency of medium-depth ground heat exchangers. However, a systematic approach to the acquisition of λ_e is still lacking. Hence, stratigraphic heat conduction was analyzed using geophysical test in Shenyang, combined with composite modeling. Cenozoic strata (0–730m) exhibited an average λ_e of 1.32 W·m⁻¹·K⁻¹ due to higher porosity and mud content; Archaean (730–2500m) strata displayed a higher λ_e of 2.80 W·m⁻¹·K⁻¹, due to dense quartz sandstone with lower porosity and mud content. The CBHE in Shenyang can achieve excellent heat extraction relying on good thermal conduction. Spearman correlation revealed strong negative correlations between λ_e and acoustic time difference, permeability, and porosity, while positive correlations were observed with depth, resistivity, wave speed, and rock skeleton. Higher rock skeleton thermal conductivity intensified the decrement effect of mud content on λ_e, while higher mud content diminished the enhancement effect of rock skeleton thermal conductivity. As porosity increased, the decline in λ_e slowed, particularly pronounced at higher saturation levels. λ_e exhibited a gradual decrease with temperature, with a more notable change rate observed at lower porosity levels. Principles for enhancing thermal conduction were elucidated through a three-phase analysis. These findings provide foundation for the study and design of medium-depth boreholes.