Experimental and numerical study of CO2 sequestration in a heterogeneous porous medium

We investigate both experimentally and numerically the gravitational instability due to the dissolution of carbon dioxide into brine in heterogeneous porous media. To do so, we consider a two dimensional Hele-Shaw cell of 0.073 m x 0.49 m, in which a log-normally distributed permeability field has been engraved. Permeability fields with a mean gap of 370 µm and 500 µm, a correlation length  λ_x = 0.032 m, λ_z = 0.016 m and a variance of 0.137 are considered in order to see the effect of heterogeneity on the convective instability. Experiments in cells with a constant gap are also performed. The CO₂ partial pressure is varied between 12% and 85%. The convective patterns are visualized using a pH sensitive dye (Bromocresol green).

Experimental results show that fingers tend to merge faster in the heterogeneous cases than in the homogeneous ones and tend to look more distorted. The number of fingers at late times is smaller in the heterogeneous cases than in the homogeneous ones. The gap thickness has little effect in the heterogenous cells but a small increase of fingers with the gap width is observed in the absence of heterogeneity. Moreover, the amplitude of the instability is higher in the case of the heterogeneous experiments whereas the growth rate at early times is a bit smaller compared to the homogeneous ones. The amplitude of the instability at late times is higher for the cases with bigger gap thickness for homogeneous and heterogeneous cases. Increasing the partial pressure of CO₂ intensifies the amplitude of the instability as well as the number of fingers at a given time. As for the Numerical simulations, they reproduce well the evolution of the number of fingers and the amplitude of the instability. However, the numerical time for the onset of convection is longer.

Key words: CO₂ sequestration, Rayleigh-Taylor instability, heterogeneity, fingering patterns, amplitude, growth rate.