Optimization of microbial enhanced oil recovery using Bacillus subtilis: Theoretical modeling and experimental validation

A reaction–diffusion model coupling bacterial growth, substrate consumption, and biosurfactant production is presented and validated experimentally for microbial enhanced oil recovery. The physical problem addressed is the mobilization of residual oil through the biosurfactant-induced interfacial tension (IFT) reduction. The model employs Monod-type kinetics for biomass growth and a linear production term for biosurfactant formation, coupled to diffusion in a 2D porous domain. Dimensional consistency is verified, and all parameter units are provided. Numerical simulations were conducted using an explicit finite-difference scheme (100 × 100 grid, Δx = Δy = 0.1 cm, Δt = 0.01 h) with Neumann (zero-flux) boundary conditions. A sensitivity analysis with respect to μmax, biosurfactant yield α, and substrate yield YBS identifies these parameters as primary controls of additional oil recovery. Laboratory micromodel experiments using Bacillus subtilis (surfactin producer) and glucose medium (10 g l−1) demonstrate an IFT reduction from 28 mN m−1 to 2.5 mN m−1 over 72 h and an average additional recovery of 18% of the original oil in place (OOIP), in agreement with model predictions 17.5% OOIP, relative error <3%, R2 = 0.96). The novelty of this work lies in the combined analysis of reaction–diffusion induced spatial instabilities (Turing-type patterns) with experimentally measured IFT and quantitative oil recovery within a unified predictive framework. Limitations, including the exclusion of microbial gas production and acidification, are discussed, and directions for extension to heterogeneous reservoir scales are proposed.