R. Farajzadeh, SPE and Shell International Exploration and Production, P.
Ranganathan, SPE, P.L.J. Zitha, SPE, and J. Bruining, SPE, Delft University of
Technology, The Netherlands
Canadian Unconventional Resources and International Petroleum Conference,
19-21 October 2010,
Calgary, Alberta, Canada
The efficiency of mixing in density-driven natural-convection is largely
governed by the aquifer permeability, which is heterogeneous in practice. The
character (fingering, stable mixing or channeling) of flow-driven mixing
processes depends primarily on the permeability heterogeneity character of the
aquifer, i.e., on its degree of permeability variance (Dykstra-Parsons
coefficient) and the correlation length. Here we follow the ideas of Waggoner
et al. (1992) to identify different flow regimes of a density-driven natural
convection flow by numerical simulation. Heterogeneous fields are generated
with the spectral method of Shinozuka and Jan (1972), because the method allows
the use of power-law variograms. We observe from our simulations that the rate
of mass transfer of CO2 into water is higher for heterogeneous media.
Efficient storage of carbon dioxide (CO2) in aquifers requires dissolution in
the aqueous phase. Indeed the volume available for gaseous CO2 is less than for
dissolved CO2. The inverse partial molar volume (virtual density) of dissolved
CO2 is around 1300 kg/m3 (Gmelin, 1973) leading to more efficient storage than
CO2 remaining in the supercritical state (< 600 kg/m3) at relevant storage
temperatures. Moreover, dissolution of CO2 in water decreases the risk of CO2
leakage. The mass transfer between CO2 and underlying brine in aquifers causes
a local density increase (Gmelin, 1973), which induces convection currents
accelerating the rate of CO2 dissolution (Yang and Gu, 2006; Farajzadeh et al,
2006, 2009). This system is gravitationally unstable and leads to unstable
mixing enhancement in the aquifer (Riaz et al, 2006; Meulenbroek et al, 2010;
Hassanzadeh et al, 2007; Farajzadeh et al, 2007).
The effect of natural convection increases with increasing Rayleigh number,
which, for a constant-pressure CO2-injection scheme, mainly depends on the
permeability. This means that the efficiency of the mixing (caused by natural
convection) is largely governed by the aquifer permeability (Green et al, 2009;
Farajzadeh et al, 2007a), which is subject to spatial and directional
variations in practice. Previous studies on this subject are mostly concerned
with homogeneous porous media and despite attention of a few papers (e.g.
Farajzadeh et al., 2008; Green et al, 2009; Nield and Simmons, 2007;
Ranganathan et al, 2010) the effect of heterogeneity on the CO2 mass transfer
in aquifers is not fully understood.