CO2 convective dissolution in water-saturated unconsolidated porous media at reservoir conditions

Abstract

During CO2 storage into saline aquifers, CO2 plume mixes with the fluid phases present in the reservoir mainly due to diffusion. The mixing process creates a density gradient (e.g. increase the density of water/brine). This phenomenon leads to a convective flow which will accelerate the CO2 mixing and mass transfer which will significantly enhance the underground CO2 storage rate. It is important to know the transport rate of CO2 through a porous media pack during this convective mixing process to get a better understanding of the CO2 mixing process. Hence CO2 convective dissolution experiments were conducted in vertically packed unconsolidated porous media at 100 bar and 50 °C. Porous media with the permeability of 0.5 D, 4 D, 16 D, and 76 D were prepared using glass beads of different sizes. A high-pressure 3D steel column cell was used, with a Bromothymol blue water solution-filled sapphire cell connected at the bottom. Colour change inside the sapphire cell was used as an indication of the CO2-breakthrough. Pressure decay of the system as a result of CO2 dissolution and convection was monitored along with CO2 breakthrough through the porous media. It has been found that CO2 transport rate in bulk fluid (i.e. without porous media) is 4.3 mm/min, with repeatable consistency. With increasing Rayleigh number, the CO2 transport rate was found to increase as a function of power in the form of V=a2*Ra^(b2) in 3D experiments. Comparing the results with 2D experiments (i.e. conducted in Hele-Shaw cells) few differences were identified. It has identified that the differences between 2D and 3D-experiments were due to geometry, boundary effects, slight heterogeneities within the porous pack, and the contact area between CO2 and porous media. It has also been found out that Ra number and dimensionless time have a relationship in the form of power (Ra=c*τ^(-n)) in 3D space.

Keywords: Convection, Porous media, Reservoir conditions, CO2 dissolution, 3D steel column