GEOCHEMICAL AND MICROBIOLOGICAL INFLUENCES ON SEAL INTEGRITY DURING SC-CO2 EXPOSURE, ARBUCKLE AQUIFER, SE KANSAS

The emission of CO₂ and other greenhouse gases into the atmosphere has been linked to global climate change. In order to mitigate these effects, CO₂ can be captured and injected into deep saline aquifers as a form of Geologic Carbon Storage (GCS). One such aquifer in Southeast Kansas, the Arbuckle dolomite, is being evaluated as a potential GCS site. The Arbuckle Group consists of cherty dolomite and is overlain by the Simpson Group and the Chattanooga Shale, which is considered to be the main aquifer seal. Above the Chattanooga Shale lies the Middle Pennsylvanian Cherokee Shale, considered a secondary seal. Reservoir brine and rock samples were collected via two wells drilled in Wellington, KS.

Under reservoir pressures and temperatures, the injected CO₂ will be in the supercritical state, and will behave similarly to a buoyant liquid. This buoyancy will allow the supercritical CO₂ (CO_{2 (sc)}) to migrate upward through the aquifer and eventually come into contact with the caprock (Chattanooga Shale). In order to understand the potential geochemical and biological influences on reservoir materials and seal integrity upon exposure to CO_{2 (SC)}, a series of batch experiments was conducted. Powdered seal material, pyrite, and dolomite and collected brine with and without native microbial consortia were exposed to 100% pCO_{2 (SC)} at reservoir temperatures and pressures (2500 psi and 50°C) for a duration of 5 to 45 days.

Aqueous geochemistry, XRD, and SEM reveal changes in mineralogy after CO₂ exposure in experiments containing Chattanooga Shale and pyrite. While no mineralogical changes were indicated for experiments containing dolomite or Cherokee Shale, aqueous geochemistry reveals considerable changes were still occurring in these experiments. There were no significant differences between biotic and abiotic experiments, which may be a function of low reservoir biomass. The results indicate pyrite-bearing phases may precipitate secondary gypsum in the presence of CO₂ and oxygenated reservoir brine. The precipitation of gypsum under these conditions could beneficially fill cracks and microfractures in the reservoir seal. Conversely, gypsum precipitation could fill valuable pore space near the injection site, resulting in a decrease in injection capability.