Stratum thermal conductivity and thermal analysis of medium-deep borehole based on field test: From a geological view

Medium-depth ground source heat pump is a crucial option for low-carbon heating. However, research from a geological perspective is limited, and there is a lack of calculation methods for the thermal conductivity of medium-deep complex strata. Hence, we conducted geological and thermal tests on a 1700 m coaxial borehole at Shenyang. A hybrid thermal conductivity model considering rock types was established. CSAMT results show that the strata present a trend of being loose in the shallow part and dense in the deep part, with local faults. The sedimentation of the stratum is incomplete, with the Archaean (>20 m) being dominant. The rock skeleton is thermally conductive and locally fragmented. The average thermal conductivity of 1700 m stratum is 2.58 W·m⁻¹·K⁻¹. Due to good thermal conductivity of rock skeleton (gneiss), low porosity (1.8 %) and no argillaceous, maximum value of 3.34 W·m⁻¹·K⁻¹ was observed near 1063.9 – 1068.8 m; due to the fractured layers leading to higher porosity (44.28 %), minimum value of 0.85 W·m⁻¹·K⁻¹ occurred in 1149.9 – 1153.9 m. Field tests showed that stable heat extraction reached 163.5 kW under a flow rate of 27.0 m³·h⁻¹, maintaining a stable heating of 213.7 kW under a supply temperature of 60 ℃.