Experiment study on the evolution of permeability and heat recovery efficiency in fractured granite with proppants

Hydraulic fractures in enhanced geothermal system (EGS) are main channels for geothermal exploitation from hot dry rock. However, due to the in-situ stress and chemical effects, the hydraulic fractures will close gradually, which will cause rapid reduction of permeability and heat recovery efficiency in EGS. Hydraulic fracturing proppant treatment is effective to decrease fracture closing and maintain fluid circulation in geothermal exploitation. In this paper, a self-developed triaxial seepage test device was used to accommodate granite samples with single artificial fracture and study the effects of proppants on rock permeability and heat recovery efficiency during geothermal energy extraction. In the experiment, the rock sample temperature was between 100 and 280 °C, and the confining pressure was between 26 and 35 MPa. The experimental results showed that both permeability and heat recovery efficiency of samples decreased with the increase of rock temperature and confining pressure. The addition of proppants was unable to prevent the reduction of permeability and heat recovery efficiency in the samples with the increase of confining pressure and temperature, but could effectively reduce the negative influence. The further analysis reveals the pressure dissolution and water weakening effects on fracture were the main reason for the decrease of permeability and net heat extraction rate in EGS, which could give a better understanding on geothermal reservoir reconstruction.