Heat control mechanism and productivity optimization of artificial fracture zone structure of dry hot rock in Gonghe Basin

Hydraulic fracturing is necessary to obtain geothermal energy from the hot dry rocks. The structure of the artificial fractures plays an important role in controlling the water and heat transport during the heat production. In this study, we analyzed the influence of the permeability and width of artificial fracture zone on the coupled heat and flow processes in the Gonghe Basin, China. The results showed that in the fractured reservoir with permeability lower than 5 D, the outflow temperature increases with the width of the fracture zones. This is because the injected cold water can sufficiently merge with the thermal water in the reservoir, and has a weak influence on the water temperature close to the extraction well. A special situation occurs in the horizontal reservoir with permeability higher than 10 D, where the outflow temperature decreases with the increase of fracture zone width, because the overall low temperature zone is increased due to the rapid diffusion of injected cold water. In the reservoir with horizontal fracture zone, outflow temperature decreases with the increase of the permeability, because the injected cold water easily arrives the production well. In contrast, in the vertical or tilted fracture reservoir, the outflow temperature increases with the permeability, because the free convection occurs strongly in the high-permeable reservoirs. A synthetic comparison suggested that under the same well distance, heat production is higher in the reservoirs with low-permeable horizontal fracture zone and with high-permeable vertical fracture zone, among other fracture zones.