3. Effect of Salinity and Pressure on the Rate of Mass Transfer in Aquifer Storage of Carbon Dioxide

The growing concern about global warming has increased interest in improving the technology for the geological storage of carbon dioxide (CO₂) in aquifers. One issue is the limited storage space for carbon dioxide. Part of the storage space is in a gas tongue overlaying the aquifer. However, more storage space is available in aquifer. Storage in the aquifer has the advantage that the partial molar volume of dissolved carbon dioxide is about twice as small as the partial molar volume of the gas phase under optimal conditions. One important aspect for aquifer storage is the rate of transfer between the overlying gas layer and the aquifer below. It is generally accepted that density driven natural convection is an important mechanism that enhances the mass transfer rate. The density effects occur because water with dissolved carbon dioxide has a higher density than fresh water or brine. There is a lack of experimental work that study the transfer rate into water saturated porous medium at in situ conditions, i.e., above critical temperatures and at pressures above 60 bar. Representative natural convection experiments require relatively large volumes (e.g., a diameter 8.5 cm and a length of 23 cm). We studied the transfer rate experimentally for both fresh water and brine (2.5, 5 and 10 w/w %). The experiment uses a high pressure ISCO pump to keep the pressure constant and allows determining the corresponding injection rate and cumulative injected volume. To our knowledge this is the first transfer experiments at constant pressure. A log-log plot reveals that the mass transfer rate is proportional to t^0.8, and thus much faster than the predicted by Fick’s law in the absence of natural convection currents. Moreover, the experiments show that natural convection currents are weakest in highly concentrated brine and strongest in pure water.