Numerical Study on the Effects of pH Buffering Minerals for CO2 Sequestration in Geology by CMS Technology

Among the technical alternatives of CCS, Koide and Xue (2009) proposed the technology, CO₂ Microbubble Storage (CMS). In CMS, CO₂ is injected into shallow geology as microbubbles in water. Suzuki et al. (2012) studied on the legal, economic, and effective operation of CMS and showed an appropriate one as follows. CO₂ microbubbles are injected in a vertical injection well with water withdrawn from the surrounding wells. When the microbubbles are mixed with water, the water is quickly saturated with CO₂ because of the large surface area of the bubbles. As a result, CO₂ is injected into geology as the solute, mostly bicarbonate ion. The cost efficiency of this operation is reasonbale.   

There is an issue that should be assessed when CO₂ dissolved water is injected in shallow geology. That is the migration of low pH groundwater. The low pH pore solution might have impacts on the underground environment. Here the preliminary numerical study on the pH buffering capacity for CMS was done.  

In the numerical study, groundwater migration dominated by Darcy law, the advective and dispersive transport of the ions, and the reaction between pore water and minerals were considered. One dimensional geology model was used. There are various mechanisms that bring pH buffering effect in an underground circumstance. Here calcite dissolution by low pH groundwater was considered because it is one of the most casual phenomena that can bring pH buffering effect.   The results of the case studies where the different mass fractions of calcite in the model layer were supposed show that the pore solution in the case with calcite was neutralized and that in the case without calcite was not. They also showed that the mass fraction of calcite does not change much the pH buffering effect. Those results were brought by the principle that the pH of the pore solution is controlled by the equilibrium constant of the reaction between minerals and the pore solution. That is, the excess portion of calcite is not related with the pH increase at the moment. However, the results that the mass fraction of calcite at the most up flow grid gradually decreased indicate that the initial mass fraction of calcite affects the longevity of pH buffering capacity.