Pore-scale dynamics of multiphase reactive transport in water-wet carbonates under co <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si7.svg" display="inline" id="d1e1164"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:

Depleted carbonate reservoirs are promising sites for geological CO 2 storage, yet the presence of residual hydrocarbon introduces complex pore-scale interactions that influence the dynamics of solid dissolution. We combined time-resolved X-ray microtomography (micro-CT), core-flooding experiments, and pore-scale modeling to investigate how residual hydrocarbon affects dissolution patterns and effective reaction rates during CO 2 -acidified brine injection into Ketton limestone under reservoir conditions. We find that the pore structure and fluid distribution control flow heterogeneity, reactive surface accessibility, dissolution patterns and the reaction rates. At low injection rate, two distinct dissolution patterns were observed: 1) a positive feedback loop of channel widening that efficiently enhanced transport properties; and 2) a suppressed regime in which heterogeneity and hydrocarbon blockage resulted in only a modest increase in permeability. At high injection rates, a more uniform dissolution occurred caused by re-mobilization of hydrocarbon that initially blocked the flow of brine. Effective reaction rates in two-phase flow were lower than in the equivalent single-phase case and up to two orders of magnitude lower than the batch rates due to persistent transport limitations. These findings provide mechanistic insights into multiphase reactive transport in carbonates and highlight the importance of accurately understanding the impact of the residual phase on reactions to improve predictions of CO 2 storage efficiency. • Pore-Scale imaging and modeling of CO 2 reactive flow in a sample containing residual oil was performed. • We captured micron-resolution displacement dynamics, leading to channel formation that increased porosity and permeability. • Heterogeneity of pore structure, hydrocarbon distribution, and re-mobilization controlled the dissolution patterns and effective reaction rates. • Two-phase flow reaction rates are transport-limited and lower than single-phase or batch rates.