| Microbial enhanced oil recovery (MEOR) techniques, especially indigenous microbial enhanced oil recovery technique, are gaining increased attention due to their economic, environmentally-friendly and simpler application advantages. Investigating microbial resources and their potential applications on MEOR in oil reservoirs are crucial and essential.In this study, microbial communities in typical oil reservoirs with different temperatures and geological conditions were analyzed; changes on indigenous microbial communities during microbial flooding and plugging were monitored; microbial effects on MEOR were investigated; crude oil biodegraded fractions saturates aromatics resins and asphaltene were investigated by Gas chromatography (GC) and mass spectrometry (MS) as well as electrospray ionization (ESI) coupled with high-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) techniques; the mechanism of transformation of heteroatoms containing nitrogen and oxygen were investigated in the range of molecule; and effects of emulsification and lowing interface tension by microbial cultures on crude oils are assessed. The main items and results of this investigation were as follows:(1) Microbial communities of a production well and a water injection well in an oil reservoir with low in situ temperature in Xinjiang were analyzed and compared. Results showed that dominant bacteria in the production well were uncultured Desulfobacterales bacterium (GQ354918) covering 66.4% of the total microbial population while the dominant bacteria in the water injection well were uncultured bacterium clone (AY327241) covering 47% of the total microbial population. Overlapping microbial communities were Uncultured Desulfuromusa sp. (EU283459),Denitrovibrio acetiphilus (NR027535), Syntrophus sp. (AJ133795), Uncultured Syntrophus sp. (GU112190), and Uncultured Desulfobacterales bacterium (GQ354918).(2) Microbial communities of DaQing mesothermic oil reservoir after polymer flooding were dominant byα-proteobacteria (20.47%),γ-proteobacteria (23.81%),δ-proteobacteria (32.21%) and uncultured bacteria (21.16%). Pseudomonas and Acinetobacter related to biodegradation of hydrocarbons were 17.79% and 6.02%, respectively. Pseudomonas and Acinetobacter are main contributing bacteria to MEOR because of their abilities to carry out degradation of hydrocarbons, desulfidation, denitrification, production of biosurfacant and oil emulsification. In addition, Pseudomonas can degrade polyacrylamide.(3) Microbial communities of the production well in Liaohe thermal oil reservoir were dominant by Petrobacter sp. NFC7-F8 (34.8%), Alishewanella sp.620 (23.6%), and Pseudomonas aeruginosa(18%). Microbial communities of another production well were predominant by Alishewanella jeotgali (53.3%). In addition, the overlapping microbial communities in the both wells were mainly Pseudomonas, Acinetobacter, Alishewanella, and Rhizobium, all of which have the ability to produce biosurfacant and the degradation of long-chain hydrocarbons and heavy oils.(4) Microbial communities of hyperthermal condensate gas reservoir showed that microbial populations in water samples were dominant by Pseudomonas sp. (AY486375) of 41%, Shewanella sp. (FM887036) of 22%, and Enterobacter sp. (GU086162) of 14%. The microbial populations in two oil samples were dominant by Pseudomonas aeruginosa (HM582426) of 67.82% and Vibrio sp. (FJ457366) of 56.32%. Detected sequences had high similarity (more than 99%) with cultured Acinetobacter, Pseudomonas and Sphingobacterium which are related to production of biosurfactant, which indicated that the microbial communities in the oil reservoir could be used in MEOR.(5) The microbial communities in the oil reservoirs with different temperatures (20-110℃) showed different microbial populations in the four oil reservoirs. The temperature of oil reservoirs had greatest effect on microbial communities. In addition, the characteristics of crude oils and mineral compositions of the oil reservoirs also had a major contributions to the variations of microbial communities. Results of microbial communities also showed that bacteria having potential application on MEOR inhabited all oil reservoirs with different temperatures, which indicated MEOR could be applied in these oil reservoirs with wider temperature range compared to traditional temperature range (20-80℃)(6) After microbial plugging dominant microbial communities in the NO.2 northern block of Daqing Oil Field changed obviously. Three months after microbial plugging cultured bacteria occupied more than 80% of the total microbial populations. Six months after microbial plugging, the existing microbial communities having the ability to produce biosurfacant and degrade polycyclic aromatic hydrocarbons covered more than 80% of the total microbial populations.(7) According to analysis of microbial communities after microbial plugging in the NO.2 northern block of Daqing Oil Field, microbial communities of Brevibacillus agri, Bacillus licheniformis, and Pseudomonas aeruginosa appeared in enrichment cultures of bio-polymer producing bacteria in the laboratory. Compared with the microbial communities activated in the laboratory, the bacteria activated in the studied oil reservoir were different; they included Pseudomonas, Sphingomonas, and Acinetobacter, but no Bacillus licheniformis appeared, which indicated that the complex environmental factors of oil reservoirs might make greatest contributions to the discrepancies appeared in the study, in addition, Enterobacter cloacae XW (EF592491) another injected bacterium could not survive in this oil reservoir.(8) After biodegradation by indigenous bacteria, the compositions of the four fractions in crude oil appeared changes. Relative abundances of saturated hydrocarbons and asphaltene increased, and relative abundances of aromatic hydrocarbons and resins decreased. Normal alkanes with carbon number≤18 were biodegraded, especially those with carbon≤12 were degraded completely, while relative abundances of normal alkanes with more carbon number (≥18) increased. Side chains of aromatic hydrocarbons with two aromatic cycles (naphthalene) were susceptible to microbial metabolism, causing increase of their relative abundances. Relative abundances of basic compounds containing nitrogen (carbazoles) increased too. Phenanthrene and its homologues appeared more reluctant to metabolism of bacteria.(9) After biodegradation by indigenous and recombined microbial communities, heteroatoms in heavy crude oil from Xinjiang changed. Generally, relative abundances of N-species with higher molecular weight decreased, while those with lower molecular weights increased. In addition, the distribution of double-bond equivalent (DBE) and carbon number showed few changes, which indicated that side chains of N-species were metabolized preferentially, and cyclic core structure containing nitrogen were more reluctant to microbial metabolism. Molecular weight of crude oil decreased from 403Da to 382Da. Relative abundance of heteroatoms increased, which provide more data to support the conclusion that oxidation of hydrocarbons produced carboxylic acids. Similarity to N-species, side chains of O-species were metabolized preferentially. In addition, relative abundance of O2-species with carbon number of 16 showed an obvious increase.(10) Four strains producing biosurfactant were screened from Daqing and Xinjiang Oil Fields. The viscosity of crude oil was reduced by 20-30%, oil-water interface tension dropped to 30 mN/m and all pH values decreased in microbial cultures after cultivation with the four strains. Biosurfacant and organic acids produced by the four strains caused crude oil emulsification and dispersion, which changed the interface natures of crude oils, adjusted the interacting force of crude oils and stratum minerals, lowered the capillary force and improved the mobility of crude oils. All theses changes would have positive potential applications in MEOR. |
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