Microbial-water-rock-CO2 interactions after a CO2 enhanced oil recovery injection
University of Arizona, Department of Hydrology and Water Resources Tucson, Arizona, USA
Coalbeds are one of the most promising reservoirs for CO2 sequestration, as CO2can 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, the 2800-foot sand was flooded with CO2 in an enhanced oil recovery project, and 9.0×107 m³ of CO2 were not recovered. Previous studies suggest that Wilcox Group coals, which interbed the 2800-foot sand, host microorganisms that convert CO2 and H2 into CH4, and that enhanced microbial methanogenesis occurred in the Olla field. The goal of this study is to determine if the CO2 injection led this microbial “hot-spot.” Isotopic and geochemical data from formation waters and gases collected from the Olla and surrounding fields were examined 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, 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 dissolved inorganic carbon 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.
AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects