Effects of grain size and small-scale bedform architecture on CO(2) saturation from buoyancy-driven flow.

Small-scale (mm-dm scale) heterogeneity has been shown to significantly impact CO(2) 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 CO(2), 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 CO(2) 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 CO(2) saturation than bedforms with discontinuous or cross lamination. In addition, the "extremely well sorted" grain sorting cases tend to have lower CO(2) saturation than expected for cross-laminated domains. Finally, both a denser CO(2) phase and greater interfacial tension increase CO(2) saturation. Again, variation in fluid properties seems to have a greater effect on CO(2) saturation for cross-laminated domains. This result suggests that differences in bedform architecture can impact how CO(2) saturation values respond to other variables such as grain sorting and fluid properties.