Managing geologic CO2 storage with pre-injection brine production: a strategy evaluated with a model of CO2 injection at Snohvit

CO2 capture and storage (CCS) in saline reservoirs can play a key role in curbing CO2 emissions. Buildup of pressure due to CO2 injection, however, can create hazards (wellbore leakage, caprock fracturing, and induced seismicity) to safe storage that must be carefully addressed. Reservoir pressure management by producing brine to minimize pressure buildup is a potential tool to manage these risks. To date, research studies on the effectiveness of brine production have largely focused on generic, hypothetical systems. In this paper, we use data from the Snohvit CCS project to perform a data-constrained analysis of its effectiveness under realistic geologic conditions. During the first phase of the Snohvit project, CO2 was injected into the compartmentalized Tubaen Fm. with lower-than-expected injectivity and capacity, which resulted in pressure buildup sooner than was expected. Using a reservoir model calibrated to this observed behavior, we analyze an alternative scenario in which brine is produced from the storage unit prior to injection. The results suggest that pre-injection brine production in this particular formation would be 94% efficient on a volume-per-volume basis - i.e. for each cubic meter of brine removed, an additional 0.94 cubic meters of CO2 could have been injected while maintaining the same peak reservoir pressure. Further, pressure drawdown observed during brine production is a mirror image of pressure buildup during CO2 injection, providing necessary data to estimate reservoir capacity before CO2 is injected. These observations suggest that this approach can be valuable for site selection and characterization, risk management, and increasing public acceptance.