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Abstract
Worldwide, significant efforts and resources are being directed at evaluating potentials of CCS (carbon capture and storage) for the long term storage of large quantities of CO2 that would otherwise be released in the atmosphere. Despite many years of experience with CO2 injection in oil reservoirs, our current understanding of brine/CO2 interactions that occur during CO2 injection in aquifers (brine-bearing rocks) remains very limited. This is a source of uncertainty and concern not just for the governments and companies interested in investment in CCS but also for the public in relation to the safety of long term injection and storage of CO2 in geologic formations.

In this paper we report new insights into the pore-scale interactions between super-critical CO2 and brine obtained from the results of a series of CO2 injection visualisation experiments carried out in novel high-pressure transparent porous media. In these experiments, we have physically simulated and visually investigated the micro-scale behaviour of CO2 in brine-bearing porous media. In particular, through vivid images of fluids distribution taken during the experiments, we highlight a new mechanism in which CO2 evolution follows CO2 dissolution in brine. In parts of the porous medium in which CO2 injection was taking place, it was observed that a free CO2 phase nucleated and came out of solution and gradually expanded. The phenomenon accelerated when the brine salinity increased or when the CO2 injection rate increased.

The observed mechanism is expected to affect many important aspects of CO2 flow and retention in porous media. It may increase CO2 storage capacity by displacing more brine. On the other hand, it can adversely affect the ability of rock to safely contain the stored CO2.