(724h) Numerical Modeling of CO2 Sequestration in Deep, Saline, Dolomitic-Limestone Aquifers

Geological sequestration of CO₂ is one of the most promising technologies for large-scale CO₂ mitigation. As per U. S. Department of Energy [1] the United States has an estimated potential of 1.8â??20 trillion metric tons of underground CO₂ storage. To assess the suitability of a potential CO₂ storage site, we must understand the thermodynamics [2] and kinetics of the CO₂-brine system under the relevant conditions. Using TOUGHREACT [3], we have estimated the changes in physical and chemical properties of limestone-dolomitic aquifers over a simulated period of 100 years after injecting high pressure (160-200 bars) CO₂ for 10 years under isothermal conditions. The objectives of the numerical model are to 1) estimate the amount of CO₂ sequestered by solubility trapping, mineral trapping, and residual trapping; 2) estimate the dissolution and precipitation of different minerals in the system; 3) estimate the concentration of the major chemical species and pH change in the aquifer brine; and 4) estimate the changes in porosity and permeability of the aquifer. Also, effects of other parameters like temperature, pressure, and salinity on CO₂ sequestration have been studied for long-term feasibility of the process.

Results from the study suggest that solubility trapping accounts for the highest amount of CO₂ sequestered as compared to mineral and residual trapping. Due to formation of carbonic acid, the pH of the brine dropped to below 5. This drop in pH led to dissolution of minerals like calcite and dolomite. Also, there was precipitation of minerals like gypsum. Changes in porosity and permeability due to dissolution and precipitation of minerals have been estimated. Although the study was conducted primarily for calcite, dolomite, and gypsum, it may be applicable to other formations as well. The study gives an overall estimate of the amount of CO₂ sequestered under different conditions (salinity, pressure, temperature, and mineralogy). It also predicts the changes in chemistry and permeability of the system under different conditions. The study thereby helps to choose the feasibility of sites for CO₂ sequestration more efficiently.

References

1. NACAP (2012) The North American Carbon Storage Atlas, The U.S. Department of Energy (DOE), Natural Resources Canada (NRCan), and the Mexican Ministry of Energy (SENER).
2. Thomas, M. W., Stewart, M., Trotz, M., and Cunningham, J. A. (2012). Geochemical modeling of CO₂ sequestration in deep, saline, dolomitic-limestone aquifers: Critical evaluation of thermodynamic sub-models. Chemical Geology, 306, 29-39.
3. Xu, T., E.L. Sonnenthal, N. Spycher, and K. Pruess, (2006) TOUGHREACT - A simulation program for non-isothermal multiphase reactive geochemical transport in variably saturated geologic media: Applications to geothermal injectivity and CO2 geological sequestration, Computers & Geosciences, v.32, p.145-165.