Safety-based injection strategy for carbon dioxide geological sequestration in a deep saline aquifer with complex sandstone-shale sequences: A case study from Taiwan

The purpose of this study was to decide the best injection strategy for CO₂ geo-sequestration in a deep saline aquifer with complex bedded sandstone-shale sequences. The best injection strategy is decided based on the estimates of the safety index (SFI). Numerical simulation method was used in this study. The major conclusions from this study are: (1) Safe trapping mechanisms contribute to a lower risk of CO₂ leakage by trapping CO₂ as immobile blobs or changing the phase of CO₂ from supercritical phase to aqueous, ionic, and mineral phases in the post-injection period. (2) For an aquifer with complex sandstone-shale sequences, the best injection strategy should be decided by the results of risk evaluation and the SFI estimation. (3) The well location affected the injection strategy. The risk of CO₂ leakage was lower using a down-dip injection well than an up-dip well. (4) The best strategy for this case study was to use the down-dip well to inject CO₂ into the bottom sandstone layer. The SFI for this scenario reached 0.99 at the storage time of 1000 years, which meant that the probability of CO₂ leakage occurring was nearly zero.