Geological CO₂ storage is a promising strategy for reducing greenhouse gas emissions; however, its underlying multiphase reactive flow mechanisms remain poorly understood. We conducted steady-state imbibition relative permeability experiments on sandstone from a proposed storage site, complemented by in situ X-ray imaging and ex situ analyses using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Despite our use of a brine that was pre-equilibrated with CO₂, there was a significant reduction in both CO₂ relative permeability and absolute permeability during multiphase flow due to chemical reactions. This reduction was driven by decreased pore and throat sizes, diminished connectivity, and increased irregularity of pore and throat shapes, as revealed by in situ pore-scale imaging. Mineral dissolution, primarily of feldspar, albite, and calcite, along with precipitation resulting from feldspar-to-kaolinite transformation and fines migration, were identified as contributing factors through SEM–EDS analysis. This work provides a benchmark for storage in mineralogically complex sandstones, for which the impact of chemical reactions on multiphase flow properties has been measured.