No abstract

Microbialenhanced oil recovery has gained more attention in recent years due to its low cost and eco-friendliness. However, studies on the application of this technique in the low-permeability reservoirs are few. In this study, eight strains from the Ordos Basin low-permeability reservoir that produce biosurfactant and reduce viscosity were identified. Strains could produce biosurfactants (lipopeptide and glycolipids) to emulsify the oil and had good tolerance on temperature (25°C-50°C), salinity (1 g/L-50g/L), and pH (5-10). After the actions of A-3, SC4534(2), SC4561, and JSC4535, the content of long-chain n-alkanes were decreased by 51.2%, 28.3%, 35.2%, and 28.9%, respectively. The naphthalene in aromatic hydrocarbons was also effectively degraded by the strains that were screened, and the degradation rate was higher than 84%. Additionally, all strains were able to reduce oil viscosity, which was reduced by 27-51% (in seven days). 16S rRNA gene sequencing analysis indicated that seven strains belong to the genus Bacillus and one belongs to the genus Rhodospirillaceae. The results revealed that the existence of high-efficiency strains that can significantly improve the properties of crude oil in low-permeability oil fields though biodegradation and use of biosurfactants, which is of great significance for the application of MEOR in low-permeability oil fields.

<p>Many bacteria have been proved to change physical (viscosity, wettability, and tension), and compositions of crude oil, which can make it easier for oil to be released from rock pores and achieve the purpose of improving recovery, which is called Microbial Enhanced Oil Recovery (MEOR). Our team has previously isolated six emulsified and viscosity-reducing bacteria (<em>Bacillus. sp.</em>) in low permeability layers (Chang 4+5 and Chang 6) of Ordos Basin. However, environmental tolerance of the strains is limited, and the components of crude oil used by the strains were also different. The combination of strains of different species and genera may enhance the effects of single bacteria, surpass the tolerance upper limit, and optimize the viscosity reduction and degradation. Therefore, in this study, it is extremely necessary to study the bacterial consortium. Two consortia were obtained and each consortium consisted of three bacterial strains and was designated as Consortium A (51+61, 61+H-1, 51+H-1; A-ALL) and Consortium B (34(2)+42, 34(2)+A-3; 42+A-3. B-BLL). The performance of the mixed strains was evaluated by the analysis of change in emulsification rate, crude oil composition, viscosity, and the tolerance (temperature, salinity, and pH) though GC-MS, rotational rheometer, and other methods. The results showed that bacterial consortiums had higher alkali resistance and could survive temperatures of 55 °C and salinity of 50 g/L in comparison to single bacterium. The emulsification rate was 22%-48%. Consortium B has better effects than Consortium A. The viscosity reduction rate of the Consortium A after 7 days was exceeded 30% as a whole, and the rate of Consortium B was more than 35%. The crude oil of Consortium B is basically non-stick to flask. Compared with single bacteria, the utilization components of crude oil to bacteria are still different, including both long chain hydrocarbons and short chain hydrocarbons. However, the proportion of long chain n-alkanes is further reduced compared with that of single bacteria, and the highest ratio is reduced by 23.81% (B-ALL). Overall, the bacterial consortium outperforms the single strain in terms of tolerance, viscosity reduction, and degradation, which further optimizes the application of MEOR.</p>