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Carbonate formations are critical for gigatonne-scale geological carbon storage; however, the multiphase flow mechanisms governing CO2 transport and trapping within their heterogeneous pore networks remain unclear. We conducted steady-state relative permeability experiments on a reservoir carbonate sample, integrated with in-situ X-ray microtomography imaging under capillary-dominated conditions. We observed low CO2 relative permeability with a maximum value of 0.3 and significant hysteresis between drainage and imbibition, accompanied by a high residual CO2 saturation of 0.27 from a maximum initial saturation of 0.43. Pore-scale imaging captured the dynamic evolution of CO2 ganglia: during initial drainage, CO2 occupied large pores with a normalized Euler characteristic of 5 mm−3; as drainage progressed, CO2 connectivity increased, yielding a Euler characteristic of -16 mm−3 at the end. In contrast, imbibition induced fragmentation of CO2 clusters, disrupting connectivity with a normalized Euler characteristic of 19 mm−3 at the end point. Pore occupancy analysis showed that CO2 initially displaced brine from larger pores during drainage, then increasingly from smaller ones as saturation increased; during imbibition, swelling water layers in small throats triggered snap-off events. These behaviors arose from pronounced structural heterogeneity (variable pore-throat sizes and poor connectivity) combined with strong water-wet properties, as evidenced by contact angles of ∼36.0° to 42.0° and supporting curvature measurements. This work provides direct insights into CO2 flow dynamics in porous media, advancing the optimization of CO2 storage practices.

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