(4c) Role of Geochemical Interactions in Assuring Permanence of Storage of Co2 in Geologic Environments

When CO₂ is injected into deep (> 800 m) subsurface environments for long term storage it interacts with the pre-existing rock-water system. These interactions must be critically assessed to assure that the net effect is acceptable in terms of favoring long term storage and isolation of resulting CO₂ -brine fluid systems from groundwater resources and from the atmosphere. Major rock-water- CO₂ interactions include dissolution of CO₂ into brine, dissolution and sorption of CO₂ into any organics such as coal or oil that are present, dissolution of existing minerals especially calcite, and precipitation of new minerals. Most of these processes strongly favor trapping the CO₂ to retain it in the subsurface. However, mineral dissolution caused by acidic pH could, under certain conditions, lead to scenarios that might increase leakage risk. US regulations for underground injection control (UIC) require characterization of the rock-brine system coupled with geochemical modeling to assure that interactions are understood and that adequate assurance can be provided that injectate will be retained within the injection zone. Dissolution of CO₂ into brine is the limiting step that controls how strongly geochemical interactions favor trapping. Solubility of CO₂ into brine at subsurface temperatures and pressures is well known. However, the contact area between the CO₂ and brine that controls the amount of dissolution is poorly quantified in rock pore systems. The contact area evolves though three phases of development of the plume: a highly dynamic phase during injection, a moderately dynamic phase as the plume migrates to a stable geometry, and a sluggish phase of long duration when the plume is stable except for dissolution into water and mineral precipitation. Rock pore systems, formation geometry, basin hydrology, and injection strategies can alter the contact area and must be considered in assessing the role of geochemistry. Results of recent field tests at the Frio brine pilot provide a first look at the rates and processes by which these phases and reservoir properties interact.