CO2 sequestration in brines: what actually happens?

“I am flying home from Europe in late August with nothing but a notebook and the 2011 Goldschmidt conference Geology giveaway issue to keep me occupied. Using the old-fashioned method of reading and writing on paper, I will blog my way through the compilation of highlighted geochemistry papers as time allows. These will then be posted via time delay to keep the blog moving while preventing paper burnout.”

We humans are perplexing beasts. In order to power the computers and airplanes and steel mills and blogs of our increasingly technological society, we are digging up and burning every source of fossilized plant and algae matte rthan we can find. In the process, we are dumping CO₂ into the atmosphere at the fastest rate since at least the Paleocene / Eocene thermal maximum 55 million years ago.

The accumulation of this gas in the atmosphere has been identified as a potential problem, so society is looking for alternative dump sites. Although some agriculturalists think that trees or soil or other surface effects can securely hold this excess carbon, geologists tend to concentrate on shoving it where the sun doesn't shine. In science talk, we replace shove with ‘sequester’ since scientists like elongated words.

The theoriticians like to daydream about ‘sequestering’ their carbon in al sorts of fanciful places, but a perennial favorite is the deep, dark, hot, salty brines of saline aquifers. Often, these aquifers underlie current (or former) oil and gas reservoirs, in which case a fair amount is learned about them in the petroleum extraction process. However, carbon dioxide is supercritical at the pressures and temperatures of these deep reservoirs, and becomes highly reactive as a result. Brine can also be quite chemically reactive. In a nutshell, the supercritical CO₂ dissolves into the brine to form carbonic acid. But predicting the details is tricky.

In order to stop the theoriticians from talking smack about CO₂-brine reactions, 1600 tons of CO₂ was injected into a brine in an abandoned oil well. Carefully monitored aqueous geochemical hijinks ensued.

By measuring the composition of the reservoir fluids both before and after CO₂ injection, Kharaka et al. are able to quantify these hijinks.

In short, the pH plummets as the HCO₃- skyrockets, and dissolved alkali earths, transition metals, and base metals increase as a result. This is predicted to be a result of dissolution of carbonate and iron hydroxide cement. Obviously, dissolving the intragranular cement should increase the porosity and permeability, making it easier for the fluids to migrate. Despite this, no leakage was observed into the overlying sandstone unit.
Another disturbing observation was the increase in dissolved organic molecules, some of which are quite toxic. This observation was unexpected, and not fully understood.

The last experiment was to use the d₁₈O values of the brine and CO₂, which were initially quite different, to calculate mixing and residual supercritical CO₂ which had not dissolved into the brine.

My only complaint is that they did not look at the behavior of sulfur. Sulfur can be present in brines and co-existing residual hydrocarbons in either oxidized or reduced forms, and can also form a variety of minerals. Sulfur oxidation is what generates acid mine drainage, and it is an important constraint on both the acidity of the fluids present and on the solubility of various metals.
Kharaka, Y., Cole, D., Hovorka, S., Gunter, W., Knauss, K., & Freifeld, B. (2006). Gas-water-rock interactions in Frio Formation following CO2 injection: Implications for the storage of greenhouse gases in sedimentary basins Geology, 34 (7) DOI: 10.1130/G22357.1