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This study investigates how four different injection schemes, (i. constant rate, ii. stepwise increasing rate, iii. stepwise decreasing rate, and iv. cyclic rate), constrained by the cumulative amount of CO2 injected, affect the likely extent of pressure buildup and CO2 plume, which play a role in the appraisal of environmental risk performance at CO2 storage sites. This objective is achieved using a representative model of a realistic site consisting of multi-layer sandstone that is extremely permeable (between 1 to 2 Darcy) and separated by thin layers of shale extending laterally with sporadic discontinuities in form of perforations. Results show that cyclic injection tends to keep the pore pressure lower than the other three injection schemes, while the highest pressure increase over the entire injection period (50 years) is observed with stepwise decreasing rate. The compressibility of CO2 plays a role in attenuating the impact of fluctuating cyclic injection signals on pressure after 30 years of injection, where this time decreases by 5 years in case of heterogeneous scenario. Except for the cyclic injection scheme, all other three injection schemes lead to almost the same magnitude and areal extent of CO2 saturation, while it shows a cyclic behavior in the case of the cyclic injection. Major observations are similar in both homogeneous and heterogeneous scenarios, although layered heterogeneity in the representative site introduces small differences in results. The results imply that it would be preferable to store CO2 using a cyclic injection scheme in storage reservoirs that may be prone to high pressure buildup during injection because of their geology (e.g. fractured shale reservoirs). These results also carry important implications for enhanced oil recovery (EOR) using CO2 where the primary goal is to drive the oil out by increasing pore pressure; for EOR by CO2 , stepwise decreasing rate would be most preferable as it leads to highest increase in pore pressure.

Impact of four different CO2 injection schemes on extent of reservoir pressure and saturation