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Microbial Resetting After a CO2-EOR Flood May Allow Stimulation of Microbial Methanogenesis to Recover Residual Oil

Abstract

Injecting carbon dioxide (CO2) into depleted hydrocarbon reservoirs to recover additional crude oil is a common enhanced oil recovery (EOR) technique. However, little is known about how microbial communities residing in these reservoirs may be modified by a CO2-EOR flood. In this study, formation water (n=2) was collected for biomass from an oil field that was intermittently flooded with CO2 from 1983 to 1986. As samples could not be collected prior to the commencement of the CO2-EOR project, formation water (n=7) was also collected from different sands in the same oil field that were not impacted by the CO2-EOR flood to serve as analogues for pre-injection conditions. Biomass was collected from the water via filtration, the DNA therein was extracted and amplified, and 16S rRNA gene sequencing was performed using the Illumina MiSeq platform. Data from both sample groups – CO2-exposed versus CO2-unexposed - were compared to determine if any major and minor microbial community differences occurred due to the CO2 flood.

The microbial community composition of both sample groups was very similar. The dominant taxa of the microbial communities observed in both areas of the oil field were Archaea, specifically methane-generating organisms. Supporting geochemical data suggest that methane production is abundant and has occurred since the CO2-EOR flood. Statistical analyses were performed to determine if there was a significant difference between the CO2-exposed and CO2-unexposed samples. An analysis of similarity (ANOSIM) test revealed that the CO2-exposed samples were not significantly different from the CO2-unexposed samples (statistic R = -0.2597, significance = 0.769), and principal coordinates analysis did not reveal any clustering of the two groups. However, several species were found to be significantly associated with the CO2-exposed group (p values between 0.024 and 0.046), very few of which are known to metabolize CO2 or are reported to be associated with CO2-rich habitats. These results suggest that reservoirs impacted by a CO2 injection may have the ability to revert back to their original microbial composition post-injection, or that the microbial communities simply were not significantly altered by CO2 injection. Furthermore, if pre-EOR methane production exists, it may be possible to target those methanogens post-EOR for enhanced microbial methanogenesis.