Evaluation of heat extraction performance for vertical multi-fractures in enhanced geothermal system

Abstract

ABSTRACT Enhanced geothermal system (EGS) is an important heat extraction method for geothermal energy of hot dry rocks (HDRs). With the development of directional well and multilateral well technologies, EGS with vertical multi-fractures may be a greatly potential heat extraction method. However, the previous studies mainly focused on the analytical model for single fracture or numerical simulation for complex fractures, while the analytical models for the fluid flow and heat transfer in the EGS with vertical multi-fractures are seldom investigated. Therefore, a fluid flow-heat transfer analytical model was first proposed for vertical multi-fractures in the EGS. Then, the analytical solution of the multi-fractures system was obtained. The model was verified with both analytical and numerical models. In addition, the influence of parameters on the instantaneous heat extraction efficiency was analyzed. The results indicated that the fracture aperture, fluid initial temperature, fluid density, and rock initial temperature have a significant impact on the instantaneous heat extraction efficiency. In the research scope of this paper, when the development time exceeds 5000 days, the instantaneous heat extraction efficiency difference generally exceeds 10.0%, and some parameters can even reach 25.0%. The vertical multi-fracture instantaneous heat extraction efficiency is more than 5.0% higher than the horizontal multi-fracture under the same conditions. The present model can be used to guide the evaluation of instantaneous heat extraction efficiency for EGS with vertical multi-fractures.