Role of Nanofluids in Heat Extraction for Mid-Deep Geothermal Wells: Numerical Study on Thermofluidic Characteristics

Global climate change has intensified the need for clean and stable energy sources. Geothermal energy, with its consistent availability, is crucial for the transition to renewable energy systems. This study aims to numerically evaluate the enhancement of heat extraction in a mid-deep coaxial geothermal heat exchanger (GHE) when using water-based Al2O3 and SiO2 nanofluids. A comprehensive 1D pipe flow- and 3D subsurface heat transfer-coupled model was developed and validated against field experimental data. The results demonstrate that the nanofluids significantly enhanced heat extraction. The water–SiO2 nanofluid achieved the highest outlet temperature, exceeding pure water by approximately 0.2 °C after 2000 h. A lower inlet temperature of 5 °C increased heat extraction by 88.57% compared to 25 °C, despite a lower outlet temperature. The thermal influence radius expanded from <2 m at 300 h to ~6 m at 1800 h. This study provides quantitative insights and a validated framework for optimizing GHE performance through nanofluid selection and operational control.