Geochemical responses of a saline aquifer to CO2 injection: experimental study on Guantao formation of Bohai Bay Basin, East China

Abstract Geochemical responses of saline aquifers toward large quantities of CO2 injection are important to the monitoring and safety assessment of CO2 storage. Detailed studies on water‐rock‐CO2 interactions of non‐marine saline aquifers through three batch‐reaction experiments were carried out under simulated reservoir conditions (T = 100°C, P = 10 MPa). pH decreased by 1∼2 and TDS increased by 268 mg/L ∼561 mg/L. Water type evolved from original Cl·SO4‐Na to HCO3Cl·SO4‐Na with a water/rock ratio of 3:1. Chemical components of HCO3, Ca, Mg and K and trace elements of Al, Cr, Mn, Ni and Zn showed a significant increase post CO2 injection. Heavy metals including Cr, Mn, Ni, and Zn in the post‐reaction water exceeded drinking water standards, indicating potential pollution to shallow fresh aquifers by brine/CO2 leakage. K‐feldspar and albite were variably dissolved and kaolinite, chlorite, and Fe‐bearing minerals of pyrite and hematite were newly formed, confirmed by suspended materials in the post‐reaction water, red sediments on the bottom of the reactor, and mineral saturation indexes. δ2HH2O shift was observed and discussed, which was not reported in previous water‐rock‐CO2 interactions studies. One possible reason is geochemical processes of new clay minerals formation under low temperature causing water consumption and δ2HH2O enriched in the residual water. The CO2 saturation and fraction of DIC from CO2 dissolution were calculated to be 23%–36% and 33%–49% under experimental conditions and it is suggested that they can be used for CO2 storage capacity assessment.