Fracture Complexity and Seismicity Influenced by Mineral Heterogeneity and Fluid Viscosity in Hydraulic Fracturing of Simulated Crystalline Rock

Abstract Grain-scale heterogeneity and injection fluid viscosity significantly influence the hydro-mechanical behaviour of granites during hydraulic fracturing in Enhanced Geothermal System (EGS). However, their coupled influence on the fracture complexity and seismic response has been insufficiently explored. To investigate, a grain-based discrete element model (GB-DEM) was developed, incorporating a segmentation algorithm that integrates petrophysical imaging to accurately reproduce mineral distributions. The models were calibrated against laboratory mechanical tests and hydraulic fracturing experiments, and grain-scale heterogeneity was quantified using Voronoi-based metrics. Three synthetic granites with different heterogeneity levels were examined under different viscosities. Results reveal that fracture propagation distance from the injection point has a critical transition zone. Within this interval, fracture complexity reaches its maximum, crack energy remains elevated, and beyond it, the seismic magnitudes increase. Low-viscosity injection and higher grain-scale heterogeneity both promote shorter injection duration, greater total crack numbers, higher proportions of intergranular cracking, enhanced fracture complexity, and reduced $$b$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>b</mml:mi> </mml:math> values. Elevated crack energy and larger seismic magnitudes are predominantly associated with low-viscosity fluids. These insights provide a physics-based foundation for developing injection strategies that enhance thermal recovery whilst mitigating induced seismicity in EGS operations.