Effects of grain size and small-scale bedform architecture on CO₂ saturation from buoyancy-driven flow

Small-scale (mm-dm scale) heterogeneity has been shown to significantly impact CO₂ migration and trapping. To investigate how and why different aspects of small-scale heterogeneity affect the amount of capillary trapping during buoyancy-driven upward migration of CO₂, we conducted modified invasion percolation simulations on heterogeneous domains. Realistic simulation domains are constructed by varying two important aspects of small-scale geologic heterogeneity: sedimentary bedform architecture and grain size contrast between the matrix and the laminae facies. Buoyancy-driven flow simulation runs cover 59 bedform architecture and 40 grain size contrast cases. Simulation results show that the domain effective CO₂ saturation is strongly affected by both grain size and bedform architecture. At high grain size contrasts, bedforms with continuous ripple lamination at the cm scale tend to retain higher CO₂ saturation than bedforms with discontinuous or cross lamination. In addition, the "extremely well sorted" grain sorting cases tend to have lower CO₂ saturation than expected for cross-laminated domains. Finally, both a denser CO₂ phase and greater interfacial tension increase CO₂ saturation. Again, variation in fluid properties seems to have a greater effect on CO₂ saturation for cross-laminated domains. This result suggests that differences in bedform architecture can impact how CO₂ saturation values respond to other variables such as grain sorting and fluid properties.