This paper develops an approach to understanding the feasible space of Enhanced Geothermal Systems (EGS), based on recognition of the key role of the geometry and properties of natural, induced, and reactivated fractures in controlling both the rate and quality of heat delivered from the EGS reservoir. EGS systems rely on a combination of conductive heat transport from the rock matrix to flowing fractures, and convective heat transport through these fracture networks to producing wells. Gringarten et al. (1975) developed dimensionless analytical solutions which help illustrate the relative roles of convective and conductive heat transport processes using idealized systems of uniformly spaced parallel fracture networks. This paper uses the Discrete Fracture Network (DFN) approach to extend the Gringarten solutions to more realistic fracture networks, including variable fracture aperture, transmissivity, orientation, size, and spatial structure. For the simple fracture geometries that Gringarten considered, dimensionless time shows that heat delivery has a second power dependency on flow rate and fracture area. As a result, Gringarten concluded that distributing flow rate over multiple fractures with uniform spacing and properties greatly improved EGS performance. The analytical solutions developed in this paper demonstrate that even simple networks of parallel fractures with variable spacing and flow properties largely negates the improvement that Gringarten found from multiple fractures. The paper verifies and extends the analytical solutions presented here using a series of DFN heat and mass flow simulations. These simulations demonstrate that realistic fracture networks produce thermal decline trends similar to the analytic solutions with variations that account for early thermal depletion of intensely fractured rock within the stimulated volume, and late time single-fracture like behavior as the entire stimulated volume acts as a single heat sink. The feasible space for EGS systems therefore depends on the ability of fracture stimulation to produce rock masses providing a large amount of surface area for thermal conduction, and a relatively slow rate of convective heat transport in each fracture, while providing sufficient fracture pathways to deliver the required total heat flux.

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