Impact of Tight Horizontal Layers on Dissolution Trapping in Geological Carbon Storage

As a method to reduce atmospheric emissions of carbon dioxide (CO2), this greenhouse gas can be separated at large point-sources, compressed and injected into geological formations such as saline aquifers. Several physical features of the fluids and the rock sequester the CO2 in the pores of the rock for geologic time-scales. One such feature is dissolution into the brine, which increases the density of the brine and thereby reduces the risk of upward migration. This is referred to as dissolution trapping, and it is often an important trapping mechanism. The efficiency of dissolution trapping is largely due to density-driven convective mixing. Convective mixing of dissolved CO2 has been studied in homogeneous systems (e.g. ). All real storage formations are heterogeneous in permeability and porosity and this may affect the efficiency. Farjazadeh et al. studied the effect of random heterogeneity fields. In their simulations they found that heterogeneity implied enhanced dissolution rates compared with the homogeneous case. Green et al. showed that the dissolution rate in an aquifer with horizontal low-permeability layers of small lateral extent can be modelled approximately based on an anisotropic permeability that uses the effective vertical permeability. Post and Simmons studied the effect of low-permeability layers of larger extent on sequestration of a solute (salt in their case) in the low-permeability strata, but they did not focus on the overall dissolution. The efficiency of CO2 dissolution trapping in systems with large low-permeability structures is therefore still largely unknown. In this paper, we perform numerical simulations to investigate the efficiency of CO2dissolution in aquifers with tight horizontal layers of extent larger than the intrinsic wave-