Fate of injected CO_2 in the Wilcox Group, Louisiana, Gulf Coast Basin: Chemical and isotopic tracers of microbial-brine-rock-CO_2 interactions

摘要

The ''2800' sandstone'' of the Olla oil field is an oil and gas-producing reservoir in a coal-bearing interval of the Paleocene-Eocene Wilcox Group in north-central Louisiana, USA. In the 1980s, this producing unit was flooded with CO_2 in an enhanced oil recovery (EOR) project, leaving ~30% of the injected CO_2 in the2800' sandstone post-injection. This study utilizes isotopic and geochemical tracers from co-produced natural gas, oil and brine to determine the fate of the injected CO_2, including the possibility of enhanced microbial conversion of CO_2 to CH_4 via methanogenesis. Stable carbon isotopes of CO_2, CH_4 and DIC, together with mol% CO_2 show that 4 out of 17 wells sampled in the 2800' sandstone are still producing injected CO_2. The dominant fate of the injected CO_2 appears to be dissolution in formation fluids and gasphase trapping. There is some isotopic and geochemical evidence for enhanced microbial methanogenesis in 2 samples; however, the CO_2 spread unevenly throughout the reservoir, and thus cannot explain the elevated indicators for methanogenesis observed across the entire field. Vertical migration out of the target2800' sandstone reservoir is also apparent in 3 samples located stratigraphically above the target sand. Reservoirs comparable to the 2800' sandstone, located along a 90-km transect, were also sampled to investigate regional trends in gas composition, brine chemistry and microbial activity. Microbial methane, likely sourced from biodegradation of organic substrates within the formation, was found in all oil fields sampled, while indicators of methanogenesis (e.g. high alkalinity, d13C-CO_2 and d13C-DIC values) and oxidation of propane were greatest in the Olla Field, likely due to its more ideal environmental conditions(i.e. suitable range of pH, temperature, salinity, sulfate and iron concentrations).