SACROC Research Project: Water Quality Issues

A primary objective of carbon capture and storage (CCS) environmental regulations will be to protect drinking water resources. Currently, the underground injection control program of the EPA under the Clean Water Act regulates injection of fluids in the deep subsurface. Two water quality issues related to CCS are (1) geographic and stratigraphic (i.e. depth) locations of CO₂ injection, and (2) impacts to water quality from introduction of large volumes of CO₂.

Many proponents of geologic sequestration think that CO₂ should only be injected into zones well below the deepest underwater source of drinking water (USDW). However, this will restrict CO₂ injection from taking place in large parts of the U.S. where suitable sediments for CO₂ injection are either absent from or only exist at shallow depths. As a result there would be little to no vertical separation between zones that could serve as CO₂ sinks and USDWs. Because geological sequestration may not be viable below many of the large volume CO₂ emitters in the U.S., some power companies are requesting that the restriction to only inject CO₂ below the deepest USDW be lifted.

A second potential water issue is the question of how USDWs will be affected if CO₂ leaks from the deep storage reservoirs. GCCC researchers have conducted preliminary laboratory rock-water-CO₂ reactions and are in the process of repeating more sophisticated version of such experiments. When carbon dioxide is injected into a water-bearing geological formation it will partially dissolve and reduce the pH of the solution, making it more acidic. The chemical reactions at different pH ranges are as follows:

CO₂(g) + H₂O ↔ H₂CO₃(aq)
H₂CO₃(aq) ↔ HCO₃(aq)^– + H⁺(aq)
HCO₃(aq)^– ↔ CO₃(aq)^2– + H⁺(aq)

Based on preliminary GCCC laboratory experiments, the hydrogen ions that result from these reactions have been shown to chemically interact with the rocks that make up the geological formation. These reactions take two different forms:

• For the following dissolved cations, the concentrations increase: B, Ba, Ca, Co, K, Mg, Mn, Sr, and Zn
• In the following dissolved cations, the concentration increases and is then followed by concentration decline: Al, As, Cs, Cu, Fe, Mo, Ni, Rb, U, and V

Measurable differences in such ion concentrations inside and outside the boundaries of SACROC oilfield would be an indication that the injection of carbon dioxide had an impact on water quality in the vicinity of the SACROC oil field. Our results show no such differences.

In summary, the SACROC oilfield groundwater study is of crucial importance to water quality issues related to CCS. We find no degradation of shallow drinking water resources water as a result of more than thirty-five years of carbon dioxide injection into deep geological formations.