ALTERATION OF CLAY MINERALS IN THE MT. SIMON SANDSTONE DURING LABORATORY-SCALE CARBON SEQUESTRATION

During geological sequestration, CO₂ will react with formation fluids and reservoir rocks in the injection zone. The resulting acidification of the fluids may result in the dissolution of solid phases and formation of the new solid phases which may cause changes in rock composition and overall fabric. We are conducting laboratory-scale geochemical and mineralogical studies on CO₂ sequestration in the Illinois Basin that complement the on-going Illinois Basin - Decatur Project (IBDP), a field-scale demonstration of CO₂ sequestration in the Mt Simon Sandstone. Samples from the IBDP injection well (CCS - #1) have been selected for study with synthetic brine and CO₂ in Parr pressure reactors at 323 K and 20.7 mPa pressure for 6 months. Using petrographic techniques, samples are analyzed before and after reactor experiments to report mineralogical and textural changes. Chemical changes are also being examined in the brine compositions. PHREEQC and React^® modeling programs in addition to Ucode optimization software are being used to model changes in mineral mass and brine chemistry. The Mt. Simon Sandstone at the injection zone at IBDP is a medium to coarse grained, mostly arkosic sandstone to conglomerate (porosity maximum at 26% and permeability maximum up to 400 mD). Porosity is predominantly framework with some secondary porosity resulting from k-feldspar dissolution. Diagenetic minerals such as mixed-layered illite, chlorite, and authigenic quartz occlude pore space and commonly clog pore throats. Post-reaction samples show increased porosity as a result of the expulsion and likely dissolution of diagenetic clays. Thus far, X-ray diffraction (XRD) analyses indicated little change in bulk mineral composition. Although geochemical modeling suggested that illite formation was unlikely, results from XRD and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) suggested that some type of illitization of the clay minerals occurred. These results suggest that the capacity of the Mt. Simon Sandstone to accept CO₂ may be enhanced by clay-mineral dissolution.