Interaction of CO 2 Storage and Hydrocarbon Production in Vertically Communicating Subsurface Systems

The security of CO₂ storage requires all the injected CO₂ to be contained entirely within the storage site. CO₂ is not allowed to leave the subsurface system during and after cessation of injection. In stacked subsurface systems, the pore volume overlain or underlain by the CO₂ storage site might be used by another subsurface user, potentially with a different philosophy. For example, it might be used by a hydrocarbon producer where the philosophy is to deplete the site and reduce its pressure for maximum hydrocarbon exploitation. If the two systems become hydraulically connected, CO₂ may migrate from the storage site to the production site where it makes the security of storage more complex. In this study, we use numerical simulation to understand and address the characteristics of this phenomenon. We simulate CO₂ injection and hydrocarbon production in two vertically separated stacked systems. The modelling study represents two separate formations, one overlying the other, but separated by an impermeable shale layer. CO₂ injection is carried out in the one formation, whereas hydrocarbon is produced from the other formation. The two separated formations become connected via a well which penetrates through both formations and provides hydraulic communication between them. Different configurations of overlying and underlying CO₂ storage site relative to the hydrocarbon production site have been investigated in this study. Both pre- and post-storage hydrocarbon production have also been included in the analysis. Results show that in all the investigated scenarios, leakage of CO₂ is expected, though with different characteristics. CO₂ always migrates to the overlying production site due to combination of gravity and sufficient pressure gradient. Interestingly a reverse migration of CO₂ from the overlying storage to the underlying production site was also observed should a sufficient pressure difference between the two formation allows brine to migrate from the overlying CO₂ storage site to the underlying production site. Such brine migration dissolves CO₂ on its migration path and brings it to the production site. Results of this study illustrate the significance of correctly understanding this leakage phenomenon and including it in the overall long-term assessment of storage security in target storage sites.