In this work, we combine measurements and modeling to investigate the application of pressure-decay experiments towards delineation and interpretation of CO₂ solubility, uptake and mass transfer in water/brine systems at elevated pressures of relevance to CO₂ storage operations in saline aquifers. Accurate measurements and modeling of mass transfer in this context are crucial to an improved understanding of the longer-term fate of CO₂ that is injected into the subsurface for storage purposes. Pressure-decay experiments are presented for CO₂/water and CO₂/brine systems with and without the presence of unconsolidated porous media. We demonstrate, via high-resolution numerical calculations in 2-D, that natural convection will complicate the interpretation of the experimental observations if the particle size is not sufficiently small. In such settings, we demonstrate that simple 1-D interpretations can result in an overestimation of the uptake (diffusivity) by two orders of magnitude. Furthermore, we demonstrate that high-resolution numerical calculations agree well with the experimental observations for settings where natural convection contributes substantially to the overall mass transfer process.