Abby L. Harvey
GHG Monitor
1/23/2015
Carbon dioxide injected into saline aquifers for storage may not react with brine within the aquifers as previously thought, raising efficiency questions, according to a study published this week by researchers at the Massachusetts Institute of Technology. The study sought to discover what chemical interactions take place within the aquifers when injected CO2 reacts with brine over a long period of time. Ideally, the injected carbon will eventually solidify in the aquifer. The new study however, suggests that only a small portion of injected carbon solidifies. “As CO2 is injected into a brine–rock environment, a carbonate-rich region is created amid brine. Within the carbonate-rich region minerals dissolve and migrate from regions of high-to-low concentration, along with other dissolved carbonate species. This causes mineral precipitation at the interface between the two regions. We argue that precipitation in a small layer reduces diffusivity, and eventually causes mechanical trapping of the CO2. Consequently, only a small fraction of the CO2 is converted to solid mineral; the remainder either dissolves in water or is trapped in its original form,” the study says.
According to an MIT press release, the researchers, Yossi Cohen and David Rothman, discovered that CO2 only mineralized at the point where the CO2 and brine met, creating a solid layer which then separated the remaining CO2 from the brine, leaving it in its original form at the top of the aquifer. “This can basically close the channel, and no more material can move farther into the brine, because as soon as it touches the brine, it will become solid,” Cohen said in the release. “The expectation was that most of the carbon dioxide would become solid mineral. Our work suggests that significantly less will precipitate.”
Because the CO2 does not turn into a solid, some response to the study has indicated that there is a greater risk for the gas to leak back into the atmosphere. However, the study does not directly address this issue and in fact, the halted mineralizing is of little concern, Bruce Hill, Chief Geologist with the Clean Air Task Force, told GHG Monitor this week. “Storage geologists don’t rely on mineral formation as a primary means for trapping CO2,” Hill said. “This is because minerals are slow to form. In fact, the timeframe at which mineralization may occur may be well beyond the timeframe that we need to capture and store CO2. Instead, the security of CO2 is provided by vertical separation of the injection zone from the surface through a thick section of impermeable rock.”
