Application of Bacillus subtilis strain for microbial-enhanced oil recovery

In order to improve the oil recovery, injection of exogenous bacteria into the oil reservoir is one of the most widely used microbial flooding methods. In this study, a screened strain of Bacillus subtilis (B. subtilis) was introduced to perform the microbial flooding. The biosurfactants produced by B. subtilis was one kind of cyclic lipopeptides, which could reduce the surface tension of the culture solution from 68 mN/m to 25 mN/m and also decrease the interfacial tension of water/oil from 25.6 to 4.6 mN/m. Emulsification tests indicated that the strain and the biosurfactants could degrade and emulsify the crude oil. In the oil displacement experiments, oil recovery was increased by 32.4% by injecting fermentation broth into the simulated formation. By respectively performing the emulsification and oil displacement tests, it was demonstrated that the biosurfactants and degradation of the microbes in the heavy components of the crude oil are the main factors to enhance the oil recovery. Besides, the optimal cultural temperature for strain of B. subtilis was set as 40 degrees C. Nevertheless, the strain was inappropriate for the oil displacement under acidic conditions. In addition, the hydrophilic sands and an optimal culture solution volume of 0.7 pore volume (PV) would be in favor of the oil recovery. It was further confirmed that the efficiency of microbial flooding was much higher than that of the chemical oil displacement.