The purpose of this chapter is to lay out potential geochemical impacts of geologic sequestration. Injection of supercritical carbon dioxide into a brine formation shifts rock-dominated reaction systems to fluid-dominated systems controlled by acid-generating reactions and mixed-fluid equilibria. Increased carbonic acid content in the brine reduces the pH of in situ brine by approximately 1.5―4 pH units, depending on brine chemistry, formation lithology, and temperature, to a pH value between 3.5 and 4. Alkalinity is also produced by reaction of carbonic acid with reservoir minerals, but alkalinity of in situ brine cannot overcome the acidity produced by dissolution of supercritical carbon dioxide fluid. Analysis suggests that displacement of brine as injection proceeds will lead to separation from supercritical carbon dioxide fluid and loss of saturated carbon dioxide, wherein alkalinity can neutralize the acidity, yielding near-neutral to alkaline pH. Silica concentrations and dissolution rates will become enhanced, whereas silica precipitation is inhibited by acidic brine. Acidified brine will also react with both reservoir rock and caprock, enriching the brine in metal cations and creating alkalinity. As silica-supersaturated, metal-laden brine migrates into areas without carbon dioxide, in situ monitoring can be used to indicate repository performance. Return of silica-supersaturated brine to a rock-dominated reaction system buffered to neutral pH conditions may enhance precipitation of quartz, chalcedony, or amorphous silica. Reaction kinetics among supercritical carbon dioxide, brine, and rock are comparable to rates in systems containing gaseous carbon dioxide.
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