Visualization experiments on the swelling and mixing behavior of CO₂–oil systems in realistic rock microstructures are essential for underground energy applications involving compressed fluids. In such systems, fluid properties and transport processes strongly depend on pressure, temperature, and pore structure. Most micromodel studies, however, are conducted under ambient conditions or rely on simplified geometries, neglecting the stochastic nature of real rock formations. This has led to the common assumption that swelling is governed by slow, unremarkable flow processes.

In contrast, our results demonstrate that realistic 2D microstructures exhibit complex flow dynamics and rapid CO₂ absorption. We systematically investigate these phenomena across microstructures of increasing geometric complexity and pore connectivity using both experiments and quantitative modeling. Starting with simplified four-channel systems including stochastic dead-end pores, we identify fundamental flow mechanisms. We then extend the analysis to 2D glass beads, sand, and sandstone microstructures.

In more complex systems, such as glass beads and sand structures, increasing oil volume leads to pronounced structural reorganization, including the formation of spanning decane clusters. Sandstone microstructures, however, display distinctly different swelling and mixing behavior, primarily due to their unique pore connectivity. These findings provide new insights into CO₂–oil interactions in realistic porous media.