One of the most attractive topics in geothermal development is the realization of enhanced geothermal systems in ductile zones. Thermal energy extraction from hot dry rock may require hydraulic fracturing through rocks in ductile zones, where rocks are supposed to deform in a ductile way due to the high pressure and high temperature (HPHT) conditions. One of the key factors in its realization is, therefore, to understand mechanical behavior of ductile rocks and hydraulically created fractures under the HPHT conditions, which have not been well investigated in the past. Numerical simulations are widely accepted as the effective approach to understand the mechanism of hydraulic fracturing. Numerical studies taking cooling effects from injected fluid on hydraulic fracturing into consideration have been conducted. Though distinct element methods (DEM) are frequently used to understand brittle failure or ductile deformation mechanism of rock, hydraulic fracturing simulations including both ductile behaviors and cooling effects due to injected fluid in the HPHT environment has not been fully investigated yet. Since the mechanical response to the fluid injection shows drastic changes at the brittle-ductile transitional condition, incorporation of the transitional behavior of granite into DEM is an essential step. In this study, we demonstrated hydraulic fracturing simulations with degradation approaches for the bond properties in DEM, i.e. degradation model and bi-linear approximation model, to replicate semi-brittle or ductile behavior at the HPHT condition. The numerical simulation results showed that results from bi-linear approximation model, which is suitable for replicating ductile behavior of granite, were consistent with laboratory experiment results under the HPHT condition. We can observe the cooling of rock mass due to injected fluid, while a considerable interaction between solid and fluid cannot be observed due to the shortness of injection time.


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