Comparative study on effects of macroscopic and microscopic fracture structures on the performance of enhanced geothermal systems

The spatial configuration of fracture structures controls the efficiency of heat production in enhanced geothermal systems. However, it is still impossible to fully characterize complex fracture structures, because in practice limited information is available in deep geothermal reservoirs. A simplified representation of fracture structures is necessary, but requires insight into the major parameter(s) controlling the flow and heat transport. In this study, we examine fracture structures at macroscopic and microscopic scales; the fracture orientation is used to characterize the macroscopic fracture structure, and the mean, standard deviation and correlation length of apertures are used to represent the microscopic fracture structure. Effects of these parameters on flow and heat transport and subsequently on heat production are systematically analyzed and compared. It is found that the effect of macroscopic structure on heat production is dependent on the density-driven flow induced by tem-perature contrast in geothermal systems, and the effect of microscopic structure is related to perturbations in flow paths, cross-sectional flow area and density-driven flow. A sensitivity analysis clarifies that in enhanced geothermal systems, priority should be given to characterize the macroscopic structure of the fracture network, followed by the estimation of the mean and correlation length of fracture apertures.

Automated summary

The spatial arrangement of fractures in geothermal systems plays a crucial role in determining the efficiency of heat production. However, characterizing complex fracture structures is challenging due to limited information in deep geothermal reservoirs. This study explores the macroscopic and microscopic characteristics of fractures and evaluates their impact on flow, heat transport, and heat production. The findings suggest that the macroscopic structure influences heat production through density-driven flow, while the microscopic structure affects flow paths, cross-sectional flow area, and density-driven flow. A sensitivity analysis highlights the importance of characterizing the macroscopic structure followed by estimating the mean and correlation length of fracture apertures in enhanced geothermal systems.