Carbon Storage in the Arbuckle Aquifer, Wellington Kansas: An experimental Investigation of the Geochemical and Microbiological Effects of Supercritical CO₂ Exposure and Implications for Seal Integrity, Reservoir Storage Capacity, and Injectivity

Abstract

Captured CO2 emissions can be injected and stored in geologic formations as a way to mitigate the effects of rising atmospheric CO2 levels on global climate. Mineral weathering reactions in the presence of CO2 can affect reservoir injectivity, seal (caprock) integrity, and ultimately, the fate of CO2 sequestration over time. The effects of supercritical CO2 exposure on the deep saline Arbuckle Group reservoir and “Cowley Facies” seal of South-central Kansas were investigated through a series of 13 controlled batch experiments under injection conditions (50 °C and 172 bar). Powdered rock and synthetic brine were reacted with CO2 and peptidoglycan, a microbial biomass proxy, for 32 to 76 days. Control experiments, pressurized with N2, were conducted in parallel to the CO2 experiments. Exposure to supercritical CO2 (CO2(SC)) caused a decrease in brine pH and an increase in alkalinity, Ca2+, and Mg2+ concentrations in Arbuckle and “Cowley Facies” experiments. “Cowley Facies” experiment brine also had higher SO42- concentrations after exposure to CO2(SC). Arbuckle and “Cowley Facies” rock are rich in carbonate minerals, which are sensitive to changes in solution pH. The decrease in solution pH during CO2(SC) exposure promoted dissolution of dolomite and ankerite, which contributed to the rise in Ca2+, Mg2+, and HCO3- concentrations and mineral surfaces exhibited dissolution features (i.e. pitting, rounding, and etching). Dissolution of ankerite in “Cowley Facies” experiments mobilized Fe2+, which was oxidized subsequently and precipitated in the presence of minor O2, a common impurity associated with captured CO2 gas. Nano-scale iron oxide crystals were observed on mineral surfaces in CO2(SC)-reacted “Cowley Facies” experiments. Increased SO42- concentrations after exposure to CO2(SC) in “Cowley Facies” experiments is likely a result of anhydrite dissolution and/or oxidative dissolution of pyrite in the presence of minor O2. The added peptidoglycan suppressed dissolution of carbonates in Arbuckle and “Cowley Facies” CO2(SC)-reacted experiments, but enhanced carbonate dissolution in N2 controls. Peptidoglycan enhanced release of Fe2+ in both experiments and controls. Peptidoglycan suppressed the release of SiO2(aq) in CO2(SC)-reacted “Cowley Facies” experiments, but had no effect on silica release in N2 controls. Exposure to CO2(SC) caused a 6-12% decrease in rock mass, which translates to a 6-11% increase in porosity. Areas of high subsurface biomass concentration could see inhibition of carbonate and phyllosilicate mineral dissolution rates, and enhanced Fe release rates, which may impact total rock mass lost. Porosity enhancement near the injection well may ease CO2 injectivity, while an increase in porosity may be detrimental to caprock sealing efficiency. Minor precipitation of iron oxides in the caprock may be sufficient enough to affect pore connectivity by clogging pore throats. A reduction in pore connectivity (permeability) would reinforce seal integrity, effectively trapping CO2 for decades or longer. As pCO2 decreases in areas of the reservoir over time through injection cessation and plume migration, precipitation of carbonate minerals will occur, trapping some of the CO2 in mineral form indefinitely.