|M.Sc Student||Michel-Meyer Itamar|
|Subject||The Role of Water Flow and Dispersion on the Dissolution|
of CO2 in Deep Saline Aquifers
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Uri Shavit|
|Dr. Ravid Rosenzweig|
|Full Thesis text|
CO2 geological sequestration is a promising technology for reducing greenhouse gas emissions. This process is based on the compression of CO2 to a supercritical phase and injecting it to deep saline aquifers. As the density of the supercritical CO2 is lower than that of the ambient groundwater, it flows as a gravity current to the top of the aquifer and accumulates under the sealing layer, where it holds a risk that it may leak back up to the atmosphere through cracks and faults in the sealing layer. However, when CO2 is dissolved in the brine it becomes heavier than the CO2-free brine. The dissolution of CO2 into the brine leads to a hydrodynamic instability in which fingers of dense CO2-rich brine are formed and propagate downwards, causing CO2-unsaturated brine to move upwards. This convection process is desirable as it accelerates the dissolution rate of CO2 into the brine. Most studies have neglected the presence of groundwater flow in the aquifer, though it was found there is a small, yet non-zero water flow in aquifers suitable for CO2 sequestration. The only studies that considered the effect of groundwater flow on the dissolution process are pure numerical studies. These studies have shown a decrease in the formation of the convective fingers.
In this research, the effect of groundwater flow was studied by performing laboratory experiments in a vertical Hele-Shaw cell using a mixture of methanol and ethylene-glycol (MEG) as a CO2 analog. The experiments were conducted in two different porous media and applying a range of horizontal water flow rates including experiments with no flow. MEG was colored with dye and was injected into the top of the cell, and the experiments were recorded by a camera. The images were translated to concentration maps which were used to calculate the dissolution rate, the number of fingers and the morphological characteristics of the fingers.
The different flow rates in the two porous media were characterized by using the ratio of the dimensionless numbers Peclet and Rayleigh, Pe/Ra, which describes the ratio of the horizontal water velocity and the characteristic gravitational velocity. It was found that the wave number of the fingers decreased exponentially with the increase in Pe/Ra, indicating that generation of fingers is suppressed by the flow rate. Additionally, the fingers' downward propagation rate decreased with an increase in Pe/Ra. In the no-flow experiments, the convective flux followed the same power law of Ra set by previous studies. However, our experimental results show that the convective dissolution was hardly affected by the increase in the horizontal flow. Our assumption is that on one hand the aquifer flow and associated dispersion suppresses the generation of fingers and the convective dissolution while on the other hand they enhances the diffusive dissolution. This research provides a better understanding of the hydrodynamic instability and the dissolution mechanism in flowing aquifers. This is needed to better estimate the efficiency and the risks associated with CO2 geological sequestration.