Modeling flow and heat transfer of fractured reservoir: Implications for a multi-fracture enhanced geothermal system

The EGS (enhanced geothermal system), particularly the multi-fracture EGS, is expected to be a key method to effectively develop the deep geothermal resource. The accurate modeling of flow and heat transfer in an EGS reservoir, particularly the fracture, is challenging. Therefore, an improved model considering the LTNE (local thermal non-equilibrium) and non-Darcy flow in a rough fracture is proposed. Then, the flow and temperature fields in the matrix are comprehensively investigated. The relationship between matrix and fracture is analyzed. The LTNE and LTE (local thermal equilibrium) are compared to determine the specific thermal process in the fracture. The difference between non-Darcy and Darcy flow in the fracture is studied to evaluate the actual flow behavior. The influences of the rough and smooth fractures are discussed. The results show that the direct contribution of the fracture to production is close to 100%, while the matrix mainly plays an indirect role as a heat source. There is a noticeable local low-pressure region induced by fluid density change in the matrix, while the heat conduction, especially rock conduction, dominates the heat transfer of the matrix. The intense heat convection in the fracture leads to a heat transfer rate difference of up to 5000 times compared with the heat conduction of the matrix, causing a marked thermal breakthrough. The non-Darcy flow represents the actual flow in the fracture, and the velocity is significantly lower than the Darcy flow velocity when a high injection flow rate is employed in the EGS production. The total extracted heat difference between rough and smooth fractures reaches 7.51%.