Pore-network topology controls CO2 flow hysteresis and trapping in a heterogeneous reservoir carbonate

Reliable prediction of CO 2 storage security and injectivity in carbonate reservoirs is limited by scarce, inconsistent multiphase-flow data and by uncertainty over whether their flow behavior is governed by wettability or pore-network structure. We study a sample from a candidate carbonate storage reservoir, integrating steady-state CO 2 -brine relative permeability measurements at reservoir conditions (8 MPa, 50 ℃) with in situ X-ray microtomography and pore-network modeling to cross-validate macroscopic relative permeability, pore-scale ganglion topology, and displacement mechanisms. We observed low CO 2 relative permeability (maximum 0.30) and strong drainage–imbibition hysteresis, with high residual trapping (63% of initial saturation). In situ topological tracking links this hysteresis to an asymmetry in CO 2 connectivity: drainage coalesces dispersed ganglia into a sample-spanning backbone (normalized Euler characteristic from +5 to -16 mm − 3 ), whereas imbibition dismantles it into isolated clusters (+19 mm − 3 ). Pore-network modeling reproduces the measured relative permeabilities and identifies that snap-off—17% of throat-filling events—effectively fragments the percolating CO 2 network, serving as the controlling disconnection mechanism. Varying the modeled contact angle from water-wet to neutral-wet (θ = 0 ◦ -90 ◦ ) shifts the capillary pressure but leaves relative permeability and the invasion sequence essentially unchanged, showing that pore-network topology, rather than wettability, governs CO 2 mobility, hysteresis, and trapping in this carbonate. These results explain how heterogeneous carbonates can combine low CO 2 mobility with high storage security, and provide relative permeability functions and trapping mechanisms for use in storage forecasting.