Inertia-driven flow dynamics across rock fracture intersection: Experimental insights into subsurface reservoir flows

Fluid inertia critically governs transport phenomena in subsurface reservoirs, impacting a range of engineering applications, including hydrocarbon and geothermal recovery, carbon dioxide and hydrogen storage and proppant injection during hydraulic fracturing. Despite its importance, the mechanisms by which fluid inertia modulates flow redistribution and energy dissipation at fracture intersections remain inadequately understood, especially for time-dependent, spatially heterogeneous flow regimes that characterise realistic subsurface conditions. This study employs time-resolved, two-dimensional Particle Tracking Velocimetry (PTV) to investigate inertia-driven flow dynamics across a metre-scale fracture intersection under systematically varied Reynolds numbers (Re = 800–10000), capturing instantaneous velocity fields and vorticity structures with unprecedented temporal and spatial resolution. Results reveal that intersection-driven turbulence maximises within a critical transitional regime (Re = 4000–6000), where burst intermittencies and velocity fluctuations peak at the downstream of the intersection. Power spectral density analysis demonstrates the emergence of Kolmogorov inertial subranges at Re ≥ 2000, with spectral energy reaching a maximum in the transitional regime, providing the first experimental validation of energy cascade mechanisms across a fracture intersection. Coherent vortex structures extend across the entire fracture height and propagate into both downstream and secondary fractures, challenging the prevailing view that eddies remain confined to the intersection or planform region. Upstream flow destabilisation manifests through premature spectral cascade development during transitional regimes, evidencing non-local hydrodynamic coupling that extends beyond the intersection region. These findings underscore the necessity of incorporating inertial effects to accurately predict rates and distribution of flows within discretely fractured systems.