LABORATORY INVESTIGATION OF CO2 – ROCK – BRINE INTERACTIONS USING NATURAL SANDSTONE AND BRINE SAMPLES FROM THE SECARB TUSCALOOSA INJECTION ZONE

The Southeast Regional Carbon Sequestration Partnership (SECARB) launched their Phase III CO₂ injection into the Tuscaloosa sandstone formation at Cranfield, Mississippi. Changes in formation brine chemistry during CO₂ injection were monitored in field-collected brine samples (see complementary abstract by Thordsen et al.). Our investigation focuses on laboratory-scale experiments using natural samples from the Tuscaloosa formation. Results from our work will be compared with results from the field study to determine whether laboratory-scale experiments may be used to predict changes in CO₂ storage formation fluid chemistry due to CO₂-water-rock interactions.

Core samples from the Tuscaloosa loose sediment injection zone were collected and preserved, and natural brines were collected via U-tube, prior to CO₂ injection. XRD analysis of the sandstone showed mainly quartz, chert and feldspar with some volcanic rock and metamorphic rock fragments. Iron-chlorite (chamosite) was present at about 14 % wt. of the core sample, and only a small fraction can be attributed to carbonate minerals. The brine was a natural Na-Ca-Cl brine with 152,000 mg/L TDS. The experiment was conducted using a rocking autoclave at 350 bar pressure and 125 °C constant temperature for a reaction period of 120 days, which included 40 days of brine-rock mixing in the absence of CO₂ to ensure steady-state conditions prior to CO₂ injection.

After CO₂ injection, solution pH decreased, and we observed increases in major cations (Ca, Mg, K). Fe(II) and total Fe increased immediately after injection; Fe(II) declined to pre-CO₂ levels within 48 hours, and total Fe rose towards the end of the experiment. These data suggest a rapid reaction of Fe(II)-bearing carbonate minerals and/or iron-chlorite phases upon CO₂ injection, with subsequent geochemical reactions involving Fe-bearing solid phases. Additionally, iron-chlorite dissolution in the presence of CO₂ is possible since Al, Si, and Mg concentration changed over time. These results suggest a rapid initial reaction of Lower Tuscaloosa natural brine and sandstone upon CO₂ injection, with subsequent precipitation and dissolution reactions occurring in the presence of CO₂-charged brine.