Numerical simulation of geothermal energy production from hot dry rocks under the interplay between the heterogeneous fracture and stimulated reservoir volume

Efficient development of hot dry rock (HDR) relies on a thorough assessment of the extraction potential. Stim-ulated reservoir volumes (SRV) and hydraulic fractures are the primary sites for water circulation and heat exchange in the heat mining process, and thus are crucial to HDR development. The objective of this study is to investigate the heat extraction performance of HDR reservoir under the interplay between the heterogeneous fracture and SRV, which can enhance the thorough understanding of the heat extraction process of stochastic HDR reservoirs and contribute to optimizing the design of the HDR development. A series of 3D thermal -hydraulic-mechanical coupling models with different permeability distributions were established innovatively. Results indicated that the homogeneous reservoir with a 35-year operation time yields an outlet temperature ranging from 161 to 131.8 degrees C, a thermal breakthrough time of 18 years, a production thermal power between 14.5 and 10.5 MW, and a maximum heat extraction ratio of 32%. Spatial heterogeneity in the fracture perme-ability significantly impacted the reservoir productivity by creating preferential flow channels. Introducing heterogeneity with a variance of 1 and correlation length of 60 m resulted in a reduction of 4.4 degrees C in the outlet temperature, 1 year in thermal breakthrough time, 0.6 MW in production thermal power, and 1% in heat extraction ratio compared to homogeneous reservoirs. Moreover, the heterogeneous permeability in SRV further decreased heat mining capability, with a reduction of approximately 14.8 degrees C, 10 years, 1.9 MW, and 5.17% observed when incorporating SRV heterogeneity (variance of 2 and correlation length of 120 m) into the reservoir already subjected to heterogeneous fracture. This study also provided valuable insights into how stress conditions affect the performance of a reservoir with the heterogeneous fracture and SRV. The results pointed out that applying in situ stress lowered reservoir attenuation rate and allowed for greater heat extraction compared to the reservoir without in situ stress. This can be attributed to the less dispersed fracture permeability occurring under in-situ stress, which caused by the reduced sensitivity of permeability to stress under high-stress condi-tions. Results also demonstrated that the thermal breakthrough time is the most sensitive performance metric to the variations of the fracture and SRV permeability, followed by the output thermal power, heat extraction ratio, and finally the produced temperature.

Automated summary

This study investigates the heat extraction performance of HDR reservoirs with heterogeneous fractures and stimulated reservoir volumes (SRV), indicating that the heterogeneity significantly impacts the heat extraction capability and that in situ stress plays a role in enhancing heat extraction.