Heat extraction mechanism in a geothermal reservoir with rough-walled fracture networks

Abstract

This study aims at understanding the mechanism of heat extraction from a geothermal reservoir characterized by rough-walled fracture networks. A unified pipe-network method (UPM) which simplifies both fractures and the rock matrix as pipes is developed considering the local thermal non-equilibrium (LTNE) theory, and it is verified against an analytical solution. Three-dimensional simulations of macroscopic fluid flow and heat transfer in a fractured geothermal reservoir are conducted to take account of fracture roughness. The channeling effect and the heterogeneous distribution of fluid temperature in a core-scale model with a rough-walled fracture surface are simulated. An equivalent heat transfer coefficient (EHTC) is obtained from numerical experiments with respect to the flow rate, mechanical aperture and the equivalent hydraulic aperture. A representative element volume is then used to investigate the flow and heat transfer process in a geothermal reservoir with rough-walled fracture networks by applying the obtained EHTC. Results demonstrate that it is essential to use the proposed EHTC since the constant heat transfer coefficient (HTC) recommended in previous studies underestimates the final outlet fluid temperature in cases with rough-walled fractures.