Effects of bacterial cell density and alternating microbial- and enzymolysis-enhanced oil recovery on oil displacement efficiency

- Low cell density had higher oil displacement efficiency than high cell density.
- Microbial-enzymolysis enhanced oil recovery had higher efficiency than water flooding.
- Vaporizable fractions decreased (increased) in microbial (enzymolysis) displaced oil.
- Residual oil contained fewer saturates for the treatments compared to the control.
- Mainly injected and indigenous bacterial species harbored the displaced fluid.

In the present study, the effects of bacterial cell density on microbial enhanced oil recovery (MEOR) and enzymolysis enhanced oil recovery (EEOR) were investigated. A simulated oil displacement test was conducted to determine the displacement efficiency of bacterial (Pseudomonas aeruginosa) fermentation broth with different cell densities (high, HCF; low, LCF), crude fungal (Aspergillus oryzae) enzyme solution (CES), and MEOR alternating EEOR (HCF-CES and LCF-CES). The displacement efficiency for LCF was 1.7-fold that of HCF, and the displacement efficiency for CES was 4.6-6.0-fold that of water (control). The total displacement efficiencies of LCF-CES and HCF-CES for five displacements were 9.4-fold and 6.2-fold higher than controls, respectively. The relative quantity of vaporizable fractions in the displaced oil decreased after MEOR and increased after EEOR. The residual oil in the upper portion of the displacement tube contained fewer saturates (half or less) and increased resins (2-fold or higher) for LCF-CES and HCF-CES, compared with controls. Compared with the injected fluid, the displaced fluid had a lower pH, oil-spreading diameter, and surface tension, with substantial gas production and no dehydrogenase activity. The displaced fluid contained numerous culturable bacteria dominated by injected (P. aeruginosa) and indigenous (Bacillus atrophaeus, B. cereus) species. The results demonstrate a significant effect for bacterial cell density on oil displacement efficiency, and increased oil displacement following EEOR. Alternating MEOR and EEOR combines oil displacement by bacterial surfactants and oil degradation by fungal enzymes and this novel technical approach appears to substantially improve crude oil recovery.