Debonding at casing-cement interfaces poses a leakage pathway risk that may compromise well integrity in CO2 storage systems. The present study addresses the effects of long-range, CO2-induced, reactive transport on the conductance of such interfacial pathways. This is done by means of reactive flow-through experiments, performed on simulated wellbore systems consisting of cement-filled steel tubes, measuring 1.2–6.0 m in length. These were prepared by casting Class G HSR Portland cement into steel tubes (inner diameter 6–8 mm), followed by curing for 6–12 months. The tubes were subsequently pressurized to permanently inflate them off the cement, creating debonded cement-steel interfaces. Four experiments were performed, at temperatures of 60–80 °C, employing flow-through of CO2-bearing fluid at mean pressures of 10–15 MPa, controlling the pressure difference at 0.12–4.8 MPa, while measuring flow-rate. The results show decreases in sample permeability of 2–4 orders, which microstructural observations reveal to be associated with downstream precipitation of calcium carbonates, possibly aided by migration of fines. This demonstrates that reactive-flow on the metre-scale significantly enhances the self-sealing potential of cement-casing interfaces relative to near-static reaction experiments. The results and method presented can be used not only to understand the long-range behaviour of annuli in wells qualitatively, but also to test reactive transport models which can then be applied at the field scale.