Thermal depletion dynamics and heat recovery mechanisms in the medium-deep geothermal system

Medium-deep geothermal systems are a critical focus in geothermal energy research, yet their large-scale commercialization is hindered by periodic thermal depletion during prolonged heat extraction. This study systematically investigates thermal depletion and heat recovery mechanisms in a medium-deep coaxial borehole heat exchanger (CBHE) over a 20-year heat extraction period, emphasizing geothermal gradients (0.025-0.045 K/m), rock thermal conductivities (0.5-3.5 W/(m·K)), and lithological variations (basalt, sandstone, limestone). Based on validated field data from heating system, numerical results reveal that prolonged operation induces cumulative cold accumulation, reducing adjacent formation temperatures by up to 20.5 K over two decades. Steeper geothermal gradients amplify long-term thermal depletion while marginally improving short-term recovery. High rock thermal conductivity accelerates near-field heat transfer but propagates cold plumes, causing distal thermal inversion (e.g., at y=1m, 3.5 W/(m·K) yields 4.6 K higher temperatures than 0.5 W/(m·K), while at y=20 m, it results in 1.1 K lower temperatures). Lithology critically governs system inertia: high-diffusivity basalt exhibits rapid thermal responses but broader thermal influence zones due to accumulated losses compared to low-diffusivity limestone. Optimal CBHE performance requires that operational strategies be tailored to the site-specific formation thermal properties to mitigate long-term thermal depletion. These findings provide actionable guidelines for enhancing geothermal sustainability across diverse geological settings.