Properties of hydraulic fractures influenced by fluid viscosity: insights from grain-based CFD–DEM modelling on tight granites

Abstract Grain-scale heterogeneity and injection fluid viscosity significantly influences hydro-mechanical behaviour of granites during hydraulic fracturing in enhanced geothermal system (EGS). However, their coupled influence on the fracture complexity and hydraulic properties has been insufficiently explored. The hydro-mechanical behaviour of crystalline rocks under fluid injection is investigated using a grain-based discrete element method model (GB-DEM). The mineral distribution, mechanical and hydro-mechanical properties of the modelled granites have been accurately calibrated. Models with different heterogeneity levels are examined under fluid injection of two viscosities. Then, a computational fluid dynamics (CFD) based flow channel system is reconstructed to evaluate the post fracturing performance of the induced hydraulic fractures. The results show that injection fluid viscosity and grain scale heterogeneity jointly control hydraulic fracture morphology and fluid conductivity by modifying the fracture geometry. Low-viscosity injection generates more complex fractures with increased intergranular cracking but results in lower fluid flow velocities and permeabilities. Injection efficiency analyses further reveal that fracture complexity raises the critical viscosity threshold for non-linear saturation behaviour, with complex fractures imposing a disproportionately larger time penalty on high-viscosity fluids.