Numerical modeling of co-injection of N2 and O2 with CO2 into aquifers at the Tongliao CCS site

- A mutual solubility model for CO2-N2-O2-brine systems is established.
- The solubility model is incorporated into TOUGH2/EOS7C and TOUGHREACT, forming a new module EOS7Cm.
- EOS7Cm is successfully applied to simulate the reactive transport of co-injection of air with CO2 at the Tongliao field experiment.

Co-injection of impurities such as N2 and O2 with CO2 into aquifers is considered as an economical strategy for CO2 capture and storage (CCS). Compared to pure CO2 injection, impurities contained in CO2 stream will affect geophysical and geochemical properties of the systems and further affect flow and reactive transport processes. This study establishes a mutual solubility model for CO2-N2-O2-brine systems with pressure up to 600 bar, temperature up to 100 °C and salinity up to 6 mol/kg water by using the fugacity-activity method for the phase equilibrium. The model is then incorporated into TOUGH2/EOS7C and TOUGHREACT, forming a new improved module we call EOS7Cm. EOS7Cm is applied to simulate the reactive transport process of co-injection of air with CO2 into a shallow aquifer at the Tongliao field experiment using a 2-D radial homogeneous and anisotropic model. The fast reactive calcite and dolomite are considered as the only reactive mineral in the model. The results fit with the flow and reactive phenomenon observed at the site. It is shown that chromatographic partitioning processes of impurities occur in both gas and aqueous phases due to the difference in the solubility between CO2 and impurities. The front of gaseous O2 slightly lags behind that of gaseous N2, while gaseous CO2 lags behind gaseous O2. The distributions of the dissolved gases in the aqueous phase are similar to that in the gas phase. After the CO2 breakthrough the pH drops rapidly to about 5.0, which pronounces the dissolution of carbonate minerals. The ratio of N2 and O2 does not significantly impact the distribution pattern of gas species, but it affects the transport distance of the gases and the percentage of gas species at the gas front.