Advanced 3D thermo-hydro-mechanical modelling of thermal aperture evolution in enhanced geothermal systems
Musa D. Aliyu
Abstract
• A novel 3D THM model for thermal aperture evolution in EGS. • First study to quantify 3D thermal aperture effects on reservoir performance. • Colder injection (40 °C) enhances dilation but accelerates thermal decline. • Higher injection (70 °C) delays thermal breakthrough and sustains permeability. • Optimal injection (55 °C–65 °C) improves heat recovery and long-term efficiency. Fracture aperture evolution under coupled thermal–hydro–mechanical (THM) interactions governs permeability and long-term performance in enhanced geothermal systems, yet its three-dimensional behaviour remains poorly understood. This study presents and validates a fully coupled THM model that captures thermal aperture dynamics under varying injection temperatures. Simulations with 40 °C and 70 °C injection at 150 kg/s show that colder fluid causes greater thermal contraction, widening the fracture aperture to 160 mm and enhancing early heat extraction. However, this also accelerates stress redistribution, leading to faster thermal breakthrough and a drop in production temperature from 175 °C to 150 °C within 30 years. In contrast, 70 °C injection yields a smaller aperture of 137 mm but maintains a more stable thermal gradient, sustaining production at 155 °C and enthalpy above 650 kJ/kg for over 22 years. Injection pressure reaches 120 MPa in the colder case due to contraction-driven permeability loss but remains below 95 MPa with warmer injection. The results highlight that thermal aperture evolution not only controls flow but also feeds back into stress fields affecting fracture stability. An injection range of 55–65 °C is identified as optimal, balancing heat extraction, pressure demand, and mechanical integrity for efficient enhanced geothermal system operation.