Effect of fracture geometry characteristics on thermal convection of basin-scale groundwater

Abstract

This study investigates fracture geometry impacts on basin-scale groundwater thermal convection, focusing on aperture, length, inclination, and density. A two-dimensional numerical model evaluates interactions between forced and free thermal convection using temperature fields and Rayleigh numbers. In our scenario, larger fracture apertures tend to enhance forced thermal convection, while fractures whose apertures larger than 0.08 m will promote free thermal convection to some extent. Increased length and density expand heat transfer areas, yet rapid fluid flow through preferential pathways may reduce efficiency. Equilibrium occurs at 300 m length and 50 fractures, beyond which forced convection raises the critical Rayleigh number threshold by 2–4 times. Fractures at 45° inhibit free convection by forming "V"-shaped channels that enhance forced convection, increasing thresholds by 1.5–3.2 times. Through sensitivity analysis, fracture length and density were identified as the most influential factors among geometric characteristics. Additionally, we defined the heat extraction rate of geothermal systems and found that optimal geothermal targets in fractured karst reservoirs require larger apertures, moderate lengths (300 m), 45° intersections, and moderate densities (50 fractures). These findings provide valuable insights into optimizing geothermal resource exploitation, highlighting the role of fracture networks in shaping groundwater heat transfer dynamics.