Numerical simulation of geothermal energy productivity considering the evolution of permeability in various fractures

The optimization of geothermal exploitation model and simulation of productivity is a hot spot in the thermal industry recently. Previously, the fracture permeability was considered as a constant during the thermal exploitation. However, the fracture permeability varies constantly during the thermal exploitation, and this causes an error in the numerical simulation of geothermal productivity. Here, the experiment in evolution of the fracture permeability in granite was performed, and the equations for evolution of the permeability in major fractures, branch fractures and natural fractures were established. On this basis, the thermal-hydro-mechanical coupling geothermal model was established to investigate the effect of well profiles, conditions for evolution of fracture permeability and natural fracture development degree on geothermal exploitation capacity. The results show that when considering the evolution of fracture permeability, the vertical well model is affected after 20 years of exploitation, and the produced thermal energy in the vertical well with developed fractures is about 4% (0.4 MW) lower than in the conventional vertical well model. Under the same conditions, the difference between horizontal wells is within 1%, which can be ignored. More heat is outputted in reservoirs with developed fractures than in those without fractures.