Effect of impurities on the onset and growth of gravitational instabilities in a geological CO₂ storage process: Linear and nonlinear analyses

Because 75% of the total cost of carbon capture and storage (CCS) arises from the separation of CO₂ from gas streams (Nsakala et al., 2001), the co-injection of CO₂ and impurities such as H₂S, N₂, and SO₂ is considered a cost-effective alternative to pure CO₂ geological sequestration. Here, the effect of the impurities on the onset of gravitational instability has been analyzed theoretically and numerically. Linear stability equations have been derived and solved analytically and numerically. Double diffusive effects make the system stable or unstable depending on the values of the diffusivity ratio, δ_B, and buoyancy ratio, r_βr_C. In addition, using the Fourier spectral method, we have traced the temporal evolution of the gravitational fingers numerically. The shape and the growth history of the fingers are strongly dependent on the impurity content. The time-periodic oscillatory motions are not observed in the present linear and nonlinear analyses. For a given Rayleigh number, the dissolution of N₂ and H₂S impurities makes the system stable, whereas dissolved SO₂ accelerates the onset of instability.