ISOTOPIC RESULTS FOR MONITORING CO2 GAS INJECTION AT AN ENHANCED OIL RECOVERY HUFF-N-PUFF SITE IN SOUTH CENTRAL ILLINOIS

The Huff-n-Puff site in south central Illinois was the first CO₂-Sequestration test for the Midwest Geological Sequestration Consortium in the Illinois Basin. This was a small-scale CO₂ injection pilot program to test equipment, monitoring techniques, logistics and enhanced oil production. Approximately 43 tons of CO₂ were injected into the Cypress sandstone formation (≈1550 ft deep) over a period of 5 days and shut in for two weeks before oil extraction. The isotopic composition of gases and aqueous samples from the site were determined to help detect migration of CO₂ from the injection well to surrounding production wells and to shallower freshwater aquifers and the vadose zone. The δ¹³C of the injected CO₂ was -35 ‰. The d¹³C of the CO₂ associated with the formation brines averaged around -23 ‰. A major increase in the percentage of CO₂ was observed at one of the production wells 182 days after the first day of injection as indicated by a large CO₂ concentration (94 %) in the gas sample collected. The δ¹³C of this CO₂ was very negative, -32.5 ‰, representative of the injection gas. Prior to this, small increases in CO₂ concentrations were observed at the same production well that also showed -33 ‰ δ¹³C values, suggesting a small amount of injected CO₂ may have bled to this production well even before 182 days. An alternative explanation for these early events may be oxidation of hydrocarbons to produce such negative δ¹³C values but %O₂ remained very low. The δ¹³C of the dissolved inorganic carbon (DIC) for the aqueous samples collected indicated isotopic equilibration began to be established in the injection well between the CO₂ and DIC but not in the production wells. This data suggests the small amount of gas that bled or was pulled through the formation brines to the production wells was out of geochemical equilibrium with the brines. There was no indication of CO₂ migration to the shallow monitoring wells or vadose zone probes adjacent to the injection and production wells.