Injection of free-phase supercritical into deep geological reservoirs is associated with risk of considerable return flows towards the land surface due to the buoyancy of , which is lighter than the resident brine in the reservoir. Such upward movements can be avoided if is injected in the dissolved phase (). In this work, injection of -saturated brine in a subsurface carbonate reservoir was modelled. Physical and geochemical interactions of injected low-pH -saturated brine with the carbonate minerals (calcite, dolomite and siderite) were investigated in the reactive transport modelling. -saturated brine, being low in pH, showed high reactivity with the reservoir minerals, resulting in a significant mineral dissolution and conversion in reactions. Over the injection period of 10 yr, up to 16% of the injected was found consumed in geochemical reactions. Sorption included in the transport analysis resulted in additional quantities of mass stored. However, for the considered carbonate minerals, the consumption of injected was found mainly in the form of ionic trapping.