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The CO2 flow in deep saline reservoirs is controlled by different forces depending on the distance from the well. Far from the well, where capillary forces dominate, it is important to understand at which saturations the CO2 connects through the porous medium in order to better predict its migration within the reservoir.We investigate CO2 breakthrough behaviour in Boise sandstone, a well-characterized porous reservoir analogue, using controlled laboratory-scale experiments on cylindrical core samples. A low uniaxial load (~5% UCS) was applied to the fully brine-saturated samples to maintain mechanical stability and acoustic sensor coupling. CO2 was injected at flow rates spanning the transition from capillary-controlled to Darcy-dominated flow regimes, ranging from 0.04–0.1 mln/min, and 5–80 mln/min, respectively. The CO2 breakthrough on the top of the setup was detected by using a portable and autonomous mass spectrometric system for on-site environmental gas quantification (“miniRuedi”) together with helium, nitrogen and water as background gases. The use of mass spectrometric detection allows for highly sensitive, real-time identification of CO2 breakthrough at very low concentrations, providing precise constraints on breakthrough timing and flow connectivity that cannot be resolved from pressure data alone. In addition, micro-CT scans before and after the experiments were made, showing the formation of microfractures.The early experimental results show a clear correlation between the injection rate and breakthrough time, and the intensity of CO2, detected by the miniRuedi (Fig. 1). The observations highlight the role of pore structure in controlling CO2 migration pathways under capillary-dominated conditions, displaying the unsteady-state effects on the inlet pressure. The experimental setup proved to be highly responsive enabling the detection of small deviations within the system. These findings could contribute to improved understanding of pore-scale flow mechanisms relevant for safe and efficient geological CO2 storage and offer experimental constraints for numerical and upscaled flow models.