Numerical study of the effect of geomorphic units on the heat transfer performance of medium-depth buried pipes

Based on the geological and geothermal information of 20 wells in the deep layer of five geomorphic units of Xi’an China, the effect of local geomorphic units on the heat transfer performance of buried pipes was analyzed. Three-dimensional full-scale numerical models were established for the inner–outer heat transfer behaviors of two coaxial casing-type coupled pipes with different depths (2000 and 2500 m), and the reliability of the models was validated with the measured data of a project in operation. Results revealed that the outlet water temperature and heat transfer intensity were the highest for the well in the tertiary alluvial–proluvial fan (30.8°C and 768 kW, respectively), while they were lowest for the well in the primary terrace landform unit (20°C and 459 kW). Furthermore, the effect became more obvious with increase in burial pipe depth. By comparing the increase rate of heat transfer of the burial pipe at depths of 2500 m and 2000 m in the same geomorphic unit, it was found that the tertiary alluvial–proluvial fan geomorphic unit and the primary terrace geomorphic unit showed the maximum and minimum increase rates (107.80% and 28.32%, respectively). This study revealed the key influence of geological factors on geothermal development efficiency, which is of great significance to improving large-scale utilization of deep geothermal energy.