Equivalent simulation method of three-dimensional seepage and heat transfer coupling in fractured rock mass of geothermal-borehole system

The study on the seepage and heat transfer coupling in fractured rock mass of a geothermal doublet system is of great importance to the exploitation of deep geothermal energy and clean energy utilization technology. Based on the coupling theory of fluid flow and heat transfer as well as a discrete fracture network model, the 3D equivalent numerical method is proposed to model a geothermal doublet system. Presumed that the natural fractured reservoir consists of a block matrix and discrete fracture network, the numerical simulation for thermal extraction is implemented where the zero-thickness elements and line elements are used to model complex fracture networks and inlet/outlet wells, respectively. Seepage flow and heat transfer in fractures, wells and matrix are calculated, along with their flux and heat exchange. The proposed method is validated against results from the analytical models and refined modeling approach, and further employed for modeling the thermal recovery process in fractured rock mass containing large-scale fracture network and for assessing the effects of fracture apertures on average temperature and heat extraction ratio. It shows that the proposed method is capable to precisely simulate the hydraulic-thermal behaviors in discrete fractures and wells, which would bring down the computational cost on the premise of ensuring calculation accuracy. The temperature field in fractured rock mass is nonuniformly distributed due to the spatial inhomogeneity and anisotropy of fracture network. And the characteristics of flow and heat transfer could also be captured. The cold front moves along the percolated fracture network, and the convective heat transfer of fluid is obviously observed. Fracture aperture is an essential factor affecting the heat transfer.