Mehran Sohrabi1; Masoud Riazi, Christian Bernstone2, Mahmoud Jamiolahmady1
and Niels Peter Christensen2
1 Centre for Enhanced Oil recovery and CO2 Solutions, Heriot-Watt University,
2 Vattenfall Research and Development
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.