Impact of the vertical resolution of corner-point grids on CO₂ plume migration predictions for layered aquifers

Design of subsurface CO₂ storage sites largely relies on numeric simulation-based predictions of plume extent and progressive immobilization. In most cases, sensitivity analyses are performed with corner-point grid representations of the geo-model and first-order IFD methods using two-point flux approximation (TPFA). Here, we have conducted a comprehensive analysis of the impact of the vertical resolution of such grids on the predicted plume extent and capacity in a simplified layered aquifer system. Four different CO₂ mobility and buoyancy scenarios were analyzed. To minimize grid-orientation effects, the analysis was performed for predominantly grid-axes parallel flow through regularly gridded cross-sectional models with uniform cell size and variable cell width-over-height ratios. The analysis of the role of vertical grid resolution indicates a first-order correlation between plume extent and this parameter. The results also reveal that capillary forces reduce plume extent and enhance aquifer storage for low permeability contrasts between layers. Furthermore, model sensitivity to grid resolution scales with the magnitude of the permeability contrast between layers. Inspection of these results reveals that an underestimation of CO₂ mobility at the top of the plume is the root cause of the observed plume retardation. A comparison with two alternative simulators that discretize mobility as piecewise linear within cells as opposed to piecewise constant and are less resolution sensitive confirms this interpretation.