MICROBIAL-WATER-ROCK-CO2 INTERACTIONS AFTER A CO2 EOR INJECTION

Coalbeds are one of the most promising reservoirs for geologic CO₂ sequestration, as CO₂ can strongly adsorb onto organic matter; however, little is known about the long-term fate of CO₂ sequestered in coal. The “2800-foot sand” of the Olla oil field is an oil and gas-producing reservoir of the Paleocene–Eocene Wilcox Group of north-central Louisiana. In the 1980s, this field was flooded with CO₂ in an enhanced oil recovery (EOR) project, and 9.0×10⁷ m³ of CO₂ were not recovered during this process. The goal of this study is to determine if the CO₂ EOR injection led to increased microbial activity and generation of biogenic gas in the Olla field; specifically, it examines CO₂ sequestration in a sandstone interval interbedded with coal located approximately 854 m (~2800 ft.) below the surface. Previous studies suggest that Wilcox Group coals host microorganisms that convert CO₂ and H₂ into CH₄. The Olla oil field has also shown indicators of enhanced microbial methanogenesis compared to surrounding fields that also produce biogenic gas from the Wilcox Group. Isotopic and geochemical data from formation waters and gases collected from the Olla field, and sands from other fields (to establish pre-injection conditions for the 2800-foot sand) were examined along a 90-km transect, to determine the ultimate fate(s) of the residual injected CO₂ (mineral trapping, dissolution, methanogenesis, sorption, and/or migration). Using field data and published isotopic fractionation factors, biogeochemical models were created to predict changes in the isotopic values of carbon reservoirs [dissolved inorganic carbon (DIC), CO₂, CH₄] in the presence of injected CO₂. These models show that up to 37% of the injected CO₂ remains in the “2800-foot sand” as CO₂ gas and DIC in equilibrium. Models for CO₂ and CH₄ show opposing results (CO₂ models convey migration while CH₄ models convey methanogenesis), suggesting multiple CO₂ storage mechanisms or extensive mixing of CO₂ sources in the formation.