Experimental Observation of CO2 Breakthrough in Boise Sandstone

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.