CO2 Channelling in porous media: A three-phase displacement study using the lab-on-a-chip for CO2 sequestration in depleted oil reservoirs

Abstract. Carbon dioxide (CO2) sequestration in depleted hydrocarbon reservoirs emerged as a promising strategy for mitigating global climate change. Along with large and long-term CO2 storage opportunities, the pre-existing installations of hydrocarbon fields can be used for CO2 injection and monitoring processes, making depleted oil and gas reservoirs an encouraging carbon sink with minimal environmental influences. However, the complex multiphase flow observed in underground geo-formations needs further investigation through the flow dynamics of CO2 injection and displacement of native fluids, which are crucial for efficient and safe CO2 storage. This study investigates CO2/brine/oil three-phase displacement dynamics in a porous media following the lab-on-a-chip approach. A highly heterogeneous porous microchannel is used to mimic the underground porous media, which is placed under a high-resolution digital microscope for visualization and quantification of CO2 trapping. It was observed that initially, CO2 prefers high permeable flow paths, which results in channelling due to high viscosity contrast (viscosity ratio, M = 0.103). Subsequently, CO2 invades oil from dead-end portions of the microfluidic device by elongating the oil droplets, owing to a reduction in surrounding phase viscosity and interfacial tension (IFT). The elongation of oil increases the snap-off of oil at the small throat radius, which increases oil recovery. CO2 snap-offs were also observed due to high-pressure drop during the invasion of flow paths with smaller throat radii. Unlike viscous fingering, snap-off events increase the trapping of CO2 in the porous media. Surfactant additives form emulsions and amplify the immiscible multiphase interaction, optimizing CO2 trapping through structural and capillary trapping mechanisms. The findings can be valuable while strategizing efficient, enhanced, and secured subsurface CO2 sequestration.