Mineralogy and Geochemistry of the Arbuckle Aquifer: Insights into Characterization for CO2 Sequestration

Barker, Robinson L.; Watney, Willard L.; Rush, Jason; Strazisar, Brain; Scheffer, Aimee; Datta, Saugata

In response to increasing concerns over release of anthropogenic greenhouse gases the deep saline aquifer in south-central Kansas has been proposed as a potential site for geologic storage for CO2. Two wells (KGS 1-32 and 1-28) have been drilled to provide data for site specific determination of the storage potential of the Arbuckle. Cores from specific depths within Arbuckle (4164`-5130`) were utilized for study and flow-through experiments. Examination of formation rocks by thin section studies, SEM, XRD and CT scans were carried out to characterize the mineralogy of the cores.

Dominant mineralogy throughout the formation is dolomite with sporadic large chert nodules and occasional zones of argillaceous and pyritic minerals. Silica occurs as authigenic megaquartz, radial chalcedony, microcrystalline quartz and porous silica. Carbonate-silica contacts contain extensive heterogeneity with sulfide minerals and argillaceous material in between. Extensive vugs and microfractures are common. This study focuses on three zones of interest: Mississippian pay zone (3670`-3700`), potential baffle in Arbuckle (4400`-4550`) and proposed CO2 injection zone (4900`-5050`).

Drill stem tests and swabbed brine samples collected from 13 depths throughout the aquifer reveal a saline brine (~50,000-190,000 TDS) dominated by Na⁺, Ca²⁺ and Cl^-. Elemental ratios of major cations with Cl^- demonstrate a typical saline aquifer system. Ca/Cl and Mg/Cl ratios suggest effect of dolomitization within the brines. Cl/Br ratios reveal hydrochemical separation of the upper and lower Arbuckle and δ¹⁸O and δ²H isotopes and Li/Cl ratios support the separation of upper and lower Arbuckle by a baffle zone. Swabbed waters provide Fe speciation data and reveal the importance of it in the system.

Laboratory experiments carried out at 40°C and 2100 psi using formation core plug and collected brine identify reaction pathways to be anticipated when supercritical CO2 is injected. Results showed varying concentrations of elements with Ca²⁺, Mg²⁺, Na⁺ and Cl^- increasing during the first 15 hours, while Fe, S, and SO4^2- decrease. For the next 15 hours a reverse trend of the same elements was observed. Alkalinity and pH show inverse relationship throughout the experiment. We conclude that dominant reactions will occur between brine, CO2 and dolomite, calcite, chert, pyrite and argillaceous minerals. There is no perceived threat to freshwater resources in Kansas due to CO2 injection.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013