Investigating the potential of microbial enhanced oil recovery in carbonate reservoirs using Bacillus persicus

•The potential of the bacterial strain Bacillus persicus for EOR was proved through imbibition and core flooding tests.•The mechanisms of the oil recovery enhancement were studied and identified in several experiments.•Bacillus persicus provides an economical and eco-friendly EOR option for fractured/non-fractured carbonate reservoirs. In this study, several laboratory experiments were conducted to evaluate the performance of Bacillus persicus as a microbial enhanced oil recovery (MEOR) agent in conventional and fractured carbonate reservoirs. In these experiments, it was observed that using the microbial solution as an imbibing fluid can produce 12.5% of the original oil in place. Complementary imbibition experiments demonstrated that lower recoveries are achievable using Sodium Dodecyl Sulfate (SDS) surfactant and supernatant solutions. In order to investigate effective mechanisms, various experiments, including wettability alteration measurements, biosurfactant production tests, viscosity, and pH measurements, were performed. The contact angle tests exhibited that the selected microorganism can alter the wettability of carbonate surfaces by reducing the water/oil contact angle from 132° to 54°, representing a change from an oil-wet to water-wet state. Bacillus persicus also showed favorable results in biosurfactant production tests, including interfacial tension, emulsion index E24%, and oil displacement experiments. The viscosity measurements showed that the bacterial solution could decrease oil viscosity by about 27%. Moreover, no significant changes were found in the pH of the microbial solution during incubation and after treatment. Finally, core flooding experiments exhibited that microbial solution injection as a tertiary oil recovery method could recover 37.52% of the original oil in place. The results obtained in this study demonstrated the potential of MEOR plans targeting oil-wet/mixed-wet fractured and non-fractured carbonate reservoirs.