Influence of local thermal non-equilibrium on enhanced geothermal system

Abstract

Effective development of hot dry rock depends on complex fracture network formed by the enhanced geothermal system. Different scales of mass and heat transfer environment are formed between tight matrix and different fractures, which leads to the complexity of mass and heat transfer process of porous media. In order to analyze the influence of local thermal non-equilibrium on the mass and heat transfer process of complex fracture system, a micro pore scale mass and heat transfer model considering local non-thermal equilibrium is established. The microscopic mechanism of the effect of local non-thermal equilibrium assumption on mass and heat transfer process in matrix-fracture system is revealed. An embedded discrete fracture model considering local thermal non-equilibrium assumption and the solution method are also established. The necessity of considering local thermal non-equilibrium is analyzed by studying the sensitivity of formation physical parameters. The results show that ignoring the local thermal non-equilibrium assumption will underestimate the rock temperature near the fracture zone and overestimate the fluid temperature in the heat transfer front zone. The effect of local thermal non-equilibrium on the enhanced geothermal system is mainly reflected in the early stage of injection. The higher the injection intensity, the lower the matrix fracture permeability, the greater the matrix fracture porosity difference, the greater the rock thermal diffusion coefficient, the smaller the convective heat transfer coefficient, the more it is necessary to consider the local thermal non-equilibrium assumption. The research results can provide reference for the collection and collation of field data.