Time-Resolved Synchrotron-Based Investigation of Reactive Pore Evolution in Sandstone for CO 2 Storage

Abstract Understanding mineral dissolution in heterogeneous porous media is critical for predicting the evolution of subsurface systems during geological CO2 storage. Here, we integrate time-resolved synchrotron techniques, 4D X-ray microtomography (μ-CT), X-ray fluorescence (TR-XRF), and X-ray diffraction (TR-XRD) to investigate acid-induced dissolution in cemented sandstone from the Rio Bonito Formation in the Paraná Basin (South America) across multiple spatial scales. The results reveal that dissolution is strongly controlled by mineralogical heterogeneity and fluid accessibility, resulting in spatially localized, temporally evolving reaction fronts. Carbonate cement (calcite) dissolves rapidly under acidic conditions, as evidenced by a sharp depletion of the Ca signal (TR-XRF) and a continuous reduction in diffraction intensity (TR-XRD), driving early-stage porosity enhancement observed by μ-CT. In contrast, silicate phases (microcline) exhibit slower, surface-limited dissolution accompanied by partial structural modification, while quartz remains comparatively unreactive under the investigated conditions and experimental time frame. Dissolution rates show an exponential dependence on pH, confirming the dominant role of proton activity in controlling reaction kinetics. This behavior has direct implications for predicting injectivity evolution and geochemical buffering during the early stages of CO2 injection. Integrating structural, chemical, and mineralogical observations establishes a mechanistic link between phase-specific reactivity and pore-scale evolution. These findings highlight the importance of reservoir mineralogy, petrophysical properties, and diagenetic history in governing reactive transport processes. The results provide experimental constraints for reactive transport models addressing permeability evolution and early-stage geochemical alteration in CO2 storage systems.