Investigation into the Mechanism of Microbial Thermal Enhanced Oil Recovery

Abstract As a widely-distributed microorganism in oil reservoirs, bacillus is the primary bacterial species utilized in microbial enhanced oil recovery (MEOR). However, the ambiguous oil recovery mechanisms have been the principal obstacle hindering the widespread adoption of MEOR. Laboratory experiments and physical simulations were conducted using two strains of Bacillus to elucidate the primary mechanisms of the microbial oil recovery process. These studies aimed to develop specific methodologies to spread MEOR in extra-heavy oil recovery. First, laboratory evaluation experiments were conducted. Sucrose, NaCl, urea, K2HPO4, and yeast extract were added to formation water, mixed with bacterial suspension (9:1). The bacteria were cultured in anaerobic bottle under mild-thermal conditions (55°C) with oil-to-water ratio 1:9, changes in bacterial concentration, oil status, were recorded regularly. Crude oil was saturated in physical models at 85°C, aged, then water flooding at 90°C to simulate post-thermal recovery. Bacillus was injected, and the model was braised under mild-thermal-conditions (55°C) for 14 days. Secondary water flooding was then performed with 50°C water, changes in pressure and water content were monitored. After water flooding, Bacillus, blank nutrient, sophorolipid, surfactant, and Bacillus + sophorolipid were injected sequentially for comparative experiments with parallel samples. Results showed that live bacteria effectively blocked channeling pathways in sand-filled pipes. The maximum pressure during secondary water flooding increased by 100% with mixed Bacillus injection. Specifically, mixed Bacillus + sophorollipid increased pressure by 125.6%, while Bacillus + surfactant by 57.0%. Other slugs showed no significant pressure rise. Pressure increases strongly correlated with enhanced recovery efficiency: 1.6% for surfactant, 2.25% for sophorolipid, 2.5% for blank nutrient, 9.5% for mixed Bacillus, 9% for mixed Bacillus + surfactant, and 13% for mixed Bacillus + sophorolipid. Observations of the produced liquid revealed a significant correlation between microbial flocs and pressure changes. During the first 0.5 pore volumes (PV) of secondary flooding, no flocs appeared, and displacement pressure rose. After 0.5 PV, flocs output, causing pressure to decrease. Laboratory experiments showed that live bacteria movement in the reservoir played a crucial role in dispersing crude oil, with high concentrations achieving excellent dispersion. As bacterial levels decreased, the oil-water interface became clearer, and crude oil could no longer disperse into the water. Minimal changes in viscosity, interfacial tension, and surface tension post-treatment suggest that MEOR mechanisms fundamentally differ from chemical flooding. Oil recovery using weak thermal compound microorganisms has been tested in Chunfeng oilfield (China). Unlike traditional MEOR studies, this study examines the impact of microbial life activities (aggregation, movement, cell rupture) on reservoir development due to the extremely high viscosity of oil (50°C, 8000 mPa·s) and difficulty in emulsification. The synergistic effects of bacteria with surfactants and heat were also investigated. This research provides valuable insights for selecting bacterial species and technologies in similar reservoirs.