Fluid flow through fractured geological formations is fundamental to successful CO₂ storage operations, as fracture networks often serve as the dominant pathways for CO₂ migration, distribution, and long-term plume containment within subsurface reservoirs. Current industry practice relies heavily on simplified steady-state flow models that assume viscous-dominated behaviour, overlooking the significant role of fluid inertia —the tendency of moving fluids to maintain their momentum and resist changes in flow direction. 

Fluid inertia, stemming from acceleration and directional changes, significantly alters flow behaviour at fracture intersections, driving non-linear partitioning, recirculation, and localised turbulence, which becomes critically important during CO₂ injection scenarios. Recent studies have shown that inertial effects can fundamentally alter flow patterns and mixing processes in fractured media, yet experimental validation of these phenomena remains limited, creating substantial uncertainty in storage predictions and injection optimisation strategies. 

This knowledge gap poses significant risks for large-scale CCS deployment, as inaccurate flow modelling can lead to poor injection efficiency, uneven CO₂ distribution, and compromised containment security. This experimental investigation addresses this critical limitation through time-resolved particle tracking velocimetry (PTV) measurements using water across a meter-scale fracture intersection, by capturing the time-dependent flow behaviour across a fracture intersection. 

Results reveal three distinct flow regimes that challenge current modelling assumptions: viscous flow at low injection rates, a critical transitional regime where turbulence maximises and intersection-driven momentum transfer peaks, and high-velocity inertia-dominated flow that bypasses secondary fractures. Importantly, the observed height-spanning vortex structures and upstream flow destabilisation effects that propagate beyond intersection boundaries, completely absent from current steady-state storage models. These findings enable the development of velocity-controlled injection protocols that either harness turbulent mixing for enhanced CO₂ distribution or avoid inertial bypassing to ensure uniform network coverage, directly supporting optimised injection strategies, improved containment modelling, and enhanced storage security for commercial CCS operations.