Analysis of heat transfer based on complex Embedded Discrete Fracture Network (EDFN) for field-scale EGS

Abstract

This study proposed a doublet horizontal well injection enhanced geothermal systems(EGS) model for heat extraction via the embedded discrete fracture network. Thermal-hydraulic(TH) coupling was established and applied in this model. The performance of thermal exploitation by the comparison of four specific models (reservoir without fractures, reservoir with hydraulic fractures, reservoir with discrete fractures, and reservoir with hydraulic and discrete fractures) to optimize the base model were investigated. Meanwhile, the sensitivity analysis of reservoir parameters and injection parameters were conducted. Four specific indexes including fractures permeability, injection temperature, injection mass flow rate and well layout schemes were selected for evaluating the performance of heat extraction. The results shown that increasing the fracture number and improving the connectivity can obtain an excellent heat extraction effect. Firstly, there is a threshold for fracture permeability. Only the adequate stimulation measures were applied to increase the fracture permeability, did the stimulation of thermal power output can be reached. Secondly, increasing injection temperature will induce the enhancement of viscosity and pressure loss, and thus decrease the velocity of fluid and the temperature difference. Which is reflected in the descending of thermal power output finally. Furthermore, increasing the injection mass flow rate will give rise to the thermal power output greatly. However, it will also require greater injection pressure and energy input. Therefore, it is necessary to strike a balance between the injected mass flow and the thermal output power. Finally, the results of simulation in various well layouts illuminate that the area of the fluid flowing through the reservoir determines the temperature of the production fluid. Overall, this study provides reasonable suggestions and guidance for field-well layouts design and analysis, and heat extraction sensitivity parameters optimization under the condition of complex embedded discrete fracture networks.