Numerical study on heat extraction performance and parameter optimization of single-well coaxial geothermal system

This study develops a 3D full-scale COMSOL Multiphysics model to optimize heat extraction in single-well coaxial geothermal systems. Simulations demonstrate that enlarged annular space (0.255 m outer/0.101 m inner diameters) significantly enhances short-term performance. Mechanistic analysis further confirms that annular expansion increases thermal contact area by 3–25% across configurations. Specifically, it achieves a peak extraction rate of 3769.33 kW, representing a 3.82% increase over the lowest-output Scheme 3 (3630.2 kW) through expanded fluid-rock contact. Nevertheless, tube geometry sensitivity diminishes over time, resulting in merely 2% performance variance after 30 years due to near-wellbore heat depletion and conductive dominance from deeper formations. Notably, turbulent water injection (40 m3/h in 0.252 m/0.101 m tubing) yields 13-fold higher cumulative heat extraction than CO2, establishing water as the optimal working fluid. Collectively, these findings indicate that transient optimization focusing on annular-turbulence synergy for immediate gains while accommodating long-term geological constraints enables sustainable geothermal exploitation. Consequently, this work provides design guidelines balancing short-term efficiency with reservoir sustainability for deep geothermal applications.