Mechanistic Insights into Biosurfactant-Mediated Microbial Enhanced Oil Recovery under Reservoir Conditions: Ex Situ and In Situ Core Flooding Using Bacillus subtilis SN1

Abstract Microbial enhanced oil recovery (MEOR) has emerged as a sustainable alternative to conventional recovery methods for mobilizing trapped oil through biosurfactant-mediated interfacial interactions. Herein, a facultative anaerobic strain, Bacillus subtilis SN1 was isolated from refinery sludge, which produced diverse lipopeptide biosurfactant mixture comprising surfactin, iturin, and fengycin. The SN1 strain reduced the surface tension (ST) and interfacial tension (IFT) to 28.8 and 0.71 mN/m, respectively, with a crude biosurfactant yield of 6.15 g/L and critical micelle concentration (CMC) of 320 mg/L. The strain remained metabolically active under elevated salinity (10%), temperature (70 °C), and crude oil concentration (10%). It also produced ∼1.81 g/L of crude biosurfactant under anaerobic reservoir-simulated conditions at 60 °C. Additionally, biosurfactant retained structural stability and surface activity at high temperature (90 °C), salinity (10%), and pressure (3000 psi). The produced biosurfactant exhibited shear thickening behavior (n = 1.17) with a viscosity of 1.20 mPa.s. Core flooding experiments under reservoir-simulated conditions demonstrated additional oil recovery (AOR) of ∼15.6% through ex situ biosurfactant (1 CMC) flooding, while in situ flooding using SN1 culture with 5-day shut-in period showed ∼9.6% AOR. A cyclic huff-and-puff strategy further improved the AOR to ∼15.0% due to prolonged metabolite production during repeated microbial stimulation. During biosurfactant flooding, the capillary number increased by ∼74 times with a favorable apparent mobility ratio of 0.33–0.41. Overall, the enhanced AOR was governed by IFT reduction, biosurfactant-mediated wettability alteration (reduced contact angle from ∼93° to ∼48°), and fragmentation of adhered oil layers, as confirmed by field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) analyses, thereby facilitating the mobilization of capillary-trapped residual oil.