Evaluation of geothermal development in fractured hot dry rock based on three dimensional unified pipe-network method

Understanding the subsurface heat transmission together with thermal-hydro (T-H) coupling process in fractured rock mass is essential to the heat mining from enhanced geothermal systems (EGS). A three dimensional numerical model based on a unified pipe-network method (UPM) is developed for modeling this coupling process by explicitly introducing fracture networks embedded into porous media. Both fractures and rock matrix are discretized as connected pipe networks by a self-developed mesh generational tool in the discontinuum based method. A local thermal non-equilibrium (LTNE) model is introduced to separate the fluid and solid temperatures. Two energy balance equations are incorporated and connected by the heat transfer term. The proposed method is verified by comparing with the analytical solution and performing convergence tests. A case study is carried out by introducing randomly distributed fractures into a three dimensional rock mass. Sensitivity analyses are conduced to investigate the influence of the solid-fluid interface heat transfer coefficient, pressure differential and fracture aperture on temperature distribution in a highly fractured porous medium.