Experimental and geochemical modeling studies for the effect of CO2-water-rock interactions on the stability of CO2 geological storage: Changes in water chemistry and rock properties according to mineral compositions

Abstract

The objectives of this study are to present the alteration of geochemical properties in water and rock by the CO2-water-rock interaction and to evaluate the effect of geochemical reaction on the efficiency of storage and sealing for CO2 geological storage. This study was carried out by selecting two targets of characteristic reservoir and cap rocks which were not studied previously.

The first target is sandstone having Ca-deficient conditions and little buffering capacity, which is considered as the CO2 reservoir rock in the Pohang Basin. The SO2 were co-injected into the sandstone core with CO2 to identify the effect of SO2 on water-rock interactions. Three cases of laboratory scale experiments and geochemical reaction modeling were carried out, depending on injection gas (pure CO2, 0.1% SO2 in CO2 and 1% SO2 in CO2, respectively). The co-injection of SO2 into the sandstone core resulted in the extreme acidification of the fluid and this acidification led to significantly enhance the CO2-water-rock interaction. In the 1% SO2 case, pH remained less than 2 during the experiments due to insufficient buffering capacity. Sulfate minerals such as anhydrite, gypsum and alunite were not precipitated because of an insufficient supply of Ca2+. Because the total volume of precipitated secondary minerals was less than that of the dissolved primary minerals, total rock volume decreased by 0.63%, 1.37% and 5.56% in the pure CO2, 0.1% SO2 and 1% SO2, respectively in the modeling. Chlorite largely contributed to decrease in total rock volume although it formed only 4.8 wt.% of the rock. Results show that the co-injection of a certain amount of SO2 at CO2 storage reservoirs having low carbonate and Ca-rich minerals can significantly increase the porosity by enhancing water-rock interactions. This procedure can be beneficial to CO2 injection under some conditions.

The second target is basaltic tuff in the Janggi Basin which is considered as the cap rock when CO2 was injected into the reservoir rock. The basaltic tuff contained a high content of zeolite and smectite (23.58 wt.% and 42.14 wt.%, respectively) which can cause the cation exchange reaction. For the CO2-water-basaltic tuff interaction, the laboratory experiment and geochemical reaction modeling were carried out. We ran our geochemical reaction modeling with and without cation exchange reactions in order to identify the effect of cation exchange on cation concentration. The experimental results showed that the concentration of monovalent cations (Na+ and K+) in the fluid samples significantly increased compared to that of divalent cations (Ca2+, Mg2+ and Fe2+) at the beginning of the reaction. The Na+ and K+ concentrations increased to 250.0 mg/kg and 41.0 mg/kg after 4 days of the experiment, respectively, whereas the Ca2+, Mg2+ and Fe2+ concentrations increased 10.3 mg/kg, 1.4 mg/kg and 1.6 mg/kg after 4 days of the experiment, respectively. The Ca2+ concentrations were particularly low, even though most of the calcite (0.44 wt.%) had dissolved. This suggests that eluted divalent cations in solution were exchanged with monovalent cations in the smectite and zeolite. Similar to the experimental results, the geochemical reaction modeling results showed that the concentration of divalent cations was higher when simulated without cation exchange effects. A shift in the smectite XRD peak and increase in the specific surface area of the rock sample occurred due to the expansion of basal spacing in smectite, likely because of smaller Na ions exchanging with larger Ca2+ ions. Precipitation of carbonate minerals was not observed, due to insufficient concentrations of divalent cations for carbonate precipitation. Dissolution reaction was more dominant than precipitation reaction throughout the experiment, leading to an increase of porosity in the rock sample from 14.85% before the experiment to 18.69% after the experiment. This study indicates that cation exchange reaction is likely to reduce the mineral carbonation potential of injected CO2, even if divalent cations are eluted by mafic rocks, such as basaltic rock. However, results of the geochemical modeling showed that the effect of the cation exchange on the mineral carbonation was decreased with reaction time. Therefore, the stability of basaltic tuff as the cap rock in the Janggi Basin will not be affected in the long-term.

Results show that the alteration in the water chemistry and the rock properties by CO2-water-rock interaction depends on mineral composition (Ca-deficient conditions, poor buffering capacity and high cation exchange capacity), and can affect the efficiency and stability for CO2 geological storage. Therefore, the results also indicate that characterization of geochemical reaction depending on the mineral composition of reservoir and cap rocks is important and necessary to assess the CO2 storage reservoir. This study will be helpful in the geochemical evaluation on CO2 storage site similar to conditions of our study area.