Numerical simulation of thermo–hydraulic coupling processes in fractured karst geothermal reservoirs

Fractured karst geothermal reservoir is a kind of typical geothermal reservoirs with the advantages of abundant storage water and easy reinjection of tail water during the period of geothermal utilization. Such geothermal system is also one of the geothermal reservoirs with the greatest potential for the development and utilization of geothermal energy in China. However, its geological structures are diverse (e.g. pore, fracture and vug), exhibiting complex characteristics of multiple scales, strong heterogeneity and various flow regimes. Therefore, the fluid-heat transfer processes and geothermal production performance of fractured karst geothermal reservoirs are not clarified. In this paper, a numerical model considering thermo–hydraulic coupling processes based on the discrete fracture–vug network approach is put forward, according to the characteristics of fractured–vuggy geothermal reservoirs. In addition, the accuracy of the numerical model is verified. The results obtained from this research are as follows. First, the numerical model considering the thermo–hydraulic coupling process is put forward, in which the Darcy's law is used to describe the flow zone of porous medium, the Navier–Stokes equation is used to illustrate the free flow zone of vugs, and the Beavers–Joseph–Saffman boundary condition is used to couple the fluid flow between these two zones. Second, the connectivity of fracture network is the key parameter to control and evaluate the flow and heat transfer effects in fractured vuggy geothermal reservoirs. The existence of vugs plays an important role in the fluid flow and heat transfer in geothermal reservoirs. Third, the thermo–hydraulic coupling model based on the discrete fracture–vug network can effectively describe the fluid flow and heat transfer processes in fractured vuggy geothermal reservoirs. The connectivity of fracture networks controls the thermo–hydraulic coupling processes in fractured vuggy geothermal reservoirs. Fourth, the existence of vugs seriously impacts the thermo–hydraulic coupling processes in geothermal reservoirs. For instance, on the one hand, it increases the number of high-speed flow channels spanning across the system and even makes the system get connected. On the other hand, it increases the speed of local flow channels inside the system. In conclusion, this proposed method is of great significance for studying the development characteristics and optimizing their geothermal production performance of fractured vuggy geothermal reservoirs.