Heterogeneity effects on possible salinity-driven free convection in low-permeability strata

Although studies of free convection commonly focus on highly permeable strata, but numerical analyses indicate that density-driven free convection may also occur in heterogeneous low-permeability strata. Traditional Rayleigh number criteria are overly conservative in predicting thermohaline convection in these systems; so, numerical models are used to make inferences on the process. Simulations with stochastic realizations of permeability fields show that dense plumes can take preferential pathways to sink through generally low-permeability strata; patch analysis using percolation theory shows that the threshold permeability for the onset of free convection can be as low as 10⁻¹⁶ m² even with a mean permeability of 10⁻¹⁸ m². Threshold permeability for the percolation pathways decreases with increasing concentration gradient, vertical correlation length and the mean and variance of the permeability. The connectedness of relatively high-permeability zones is important in initiating and controlling plume fingers of free convection in both single-layer and sand-shale sequence models. Permeable units above and below are conducive to free convection through intervening low-permeability strata if buoyancy gradients exist. This heterogeneity is on scales that are difficult to sample by drilling and too localized to be simulated in regional models but may be significant in solute transport in these systems.