MONITORING INJECTION GAS WITH ISOTOPE ANALYSES AT A CO2-SEQUESTRATION PILOT STUDY IN A COAL SEAM IN ILLINOIS

Carbon dioxide was injected in the Springfield Coal at a depth of about 274m in Wabash County, southeastern Illinois. A total of 101 tons of CO₂ gas was injected in pulses over 7 months beginning June, 2008. Gas chromatography and isotopic analyses of gases collected from 3 observations wells in the coal seam were completed to monitor the migration of the CO₂ and coalbed methane. Two observation wells were aligned with the butt cleat orientation about 30m and 15m from the injection well and the third well was aligned with the face cleats about 30m from the injection well.

The injected CO₂ was a corn-based byproduct of an ethanol processing plant with a δ¹³C of -10.8 ‰ and a carbon-14 (¹⁴C) activity of 104 pMC. Values close to these would be expected for breakthrough of the injected CO₂. Prior to breakthrough the concentration of CO₂ in gas from the observation wells was very low, from <0.01 to ~ 0.2 % by volume with δ¹³C values from -4 to -32 ‰. Breakthrough occurred at different times in each observation well. It was nearly one month before the closest butt cleat well showed evidence of CO₂ breakthrough with an abrupt increase of CO₂ to about 12%. After nearly 4 months the concentration of CO₂ in the second butt cleat well rose to 3.1% and by 6 months increased to 45% CO₂. Breakthrough at the face cleat well (88% CO₂) occurred nearly 6 months after the initial injection.

The isotopic results confirmed the increase in CO₂ concentrations at the observation wells originated from the injection. An interesting pattern was observed for the isotopic composition of the CO₂ collected at the observation wells. The δ¹³C of the CO₂ collected for the initial breakthrough was virtually the same as that injected; however, isotopically lighter values were observed as the concentration of CO₂ leveled off. As the concentration of CO₂ eventually increased isotopically heavier values were observed. This was also true for the ¹⁴C results. The trend of negative to more positive isotopic values can be explained by the tendency of coal to retain isotopically heavier molecules (¹³CO₂) relative to the isotopically lighter CO₂ (¹²CO₂), as observed in the literature for laboratory canister desorption studies of coal. A somewhat similar pattern of isotopic results were also observed for the evolution of CH₄ at the observation wells.