The role of fracture surface roughness in macroscopic fluid flow and heat transfer in fractured rocks

• JRC distributions in fracture networks were determined based on the in-situ data.
• Fracture roughness can affect the macroscopic fluid flow and heat transfer processes.
• A proper e/E model is critical in modeling fluid flow and heat transfer in DFN models.

Rock fractures are major conduits for fluid flow in fractured rocks, and the convective heat transfer between rock fracture surfaces and circulating fluid is a critical issue in heat recovery in fractured rocks. It has been demonstrated that fracture surface roughness has a significant influence on the mechanical, hydraulic, thermal and transport behavior of single fractures. This study aimed to assess the effects of local surface roughness of fractures on fluid flow and heat transfer processes at the macroscopic scale of fracture networks. Two distributions of Joint Roughness Coefficient (JRC) were determined based on the JRC data in Oskarshamn/Forsmark, Sweden and Bakhtiary, Iran. Two empirical models relating hydraulic apertures to mechanical apertures were considered. A total of ninety-one realizations that considered different JRC distributions and empirical models of mechanical-hydraulic apertures were studied. The results show that fracture surface roughness can affect the fluid flow and heat transfer processes in fracture networks to various extents, mainly depending on the empirical models of mechanical-hydraulic apertures. In other words, the role of fracture surface roughness in macroscopic fluid flow and heat transfer in fractured rocks is critical, when using a model of mechanical-hydraulic apertures that predicts significant reduced hydraulic apertures. Discrete fracture networks models with the normal distribution of JRC are less permeable than those with the lognormal distribution of JRC, using the fitting parameters of in-situ JRC data.