Simulation investigation of vertical heterogeneity on co2 trapping in deep saline aquifers

Abstract

Geological storage of CO2 in deep saline aquifers has been widely proposed as a long-term mitigation option. However, the influence of vertical reservoir heterogeneity on CO2 trapping mechanisms remains insufficiently studied, representing a critical gap. This study uses high-resolution reservoir simulations to systematically quantify how vertical permeability trends influence CO2 trapping in deep saline aquifers. A vertical cross-section model with a 5-year CO2 injection followed by 195 years of monitoring, with heterogeneity levels from homogeneous to highly heterogeneous, explicitly comparing upward-decreasing vs upward-increasing permeability profiles, was developed using CMG software. The results show that permeability architecture ultimately affects trapping outcomes. Upward-decrease profiles create partial barriers that slow vertical migration, compartmentalize the CO2 plume, and increase residual trapping and dissolution, especially at high heterogeneity. In contrast, models with upward-increase permeability preserve vertical connectivity, which leads to a continuous plume with greater structural trapping but less long-term immobilization of CO2. Under high heterogeneity (VDP=0.93), the upward-decreased case immobilizes a large fraction of the injected CO2 (as residual and dissolved), whereas the upward-increased case retains a large connected gas cap. Permeability trends also affect injectivity: low-permeability layers near the well (coarsening-upward) raise the injection pressure, whereas a high-permeability base (fining-upward) maintains lower pressure. These findings provide practical guidance for site selection: in moderately heterogeneous sandstones (VDP ≈ 0.3-0.5), upward-increase trends favor efficient injection and enhanced CO2 dissolution within ~50 years; in highly heterogeneous reservoirs (VDP>0.5), upward-decrease trends support long-term (century-scale) CO2 trapping, though potentially at the cost of higher injection pressure.