Grain size effects on hydro-seismo-mechanical responses of granite during laboratory hydraulic fracturing

Geothermal energy in granite is becoming a significant renewable type, yet the hydrofracturing mechanisms within the coarse grains of granite remain largely unexplored. The triaxial hydraulic fracturing of granites with coarse grains has been simulated in laboratory settings to explore the influence of grain size on fracturing behaviour. Two types of granites with similar elastic properties but distinct grain sizes (>2.5 mm) have been examined. Source mechanism analysis indicates that in granitic coarse grains, tensile fractures are more prevalent than non-tensile fractures. The proportion of non-tensile cracks increases over time, notably after the occurrence of rock breakdown. Comparative analysis demonstrates that coarser grains are associated with reduced breakdown pressure, prolonged fracturing durations, a greater number of seismic events, lower seismic event magnitudes, and a higher proportion of tensile cracks. Interactions between hydraulic fractures and grains, such as ‘bypass’, ‘cross’, and ‘branching’, are interpreted and conceptualized based on the cohesive zone model. Our experiments also show that the fracture complexity comes from not only the fatigue hydraulic fracturing and low viscosity injection, but also continuous injection with coarse granite grains. Larger grains introduce greater fracture complexity. • Coarser grains are associated with longer failure duration and smaller breakdown pressure. • Continuous hydraulic fracturing with large grain size has favourable stimulation effects. • Grains have three interaction modes, ‘bypass, cross, and branch’, with hydraulic fractures.