Microbial Enhanced Oil Recovery (MEOR) is a tertiary oil recovery process where microorganisms and their metabolites are used to retrieve unrecoverable oil from mature reservoirs. Stimulation of biosurfactant production by indigenous microorganisms can reduce the capillary forces that retain the oil into the reservoir. The studied reservoir is characterized by alternated oil and water sand layers, with an average porosity of 25% and a permeability of 50 mD. It's a flat structure at 450 m depth, with an initial pressure of 32.4 bars and a temperature of 42.5°C. The oil is paraffinic, with low viscosity, high pour point and a gravity of 25° API, with no gas dissolved. Due to these properties, the wells can't flow naturally and the production has to be lifted, making this reservoir a good candidate for MEOR application. This work addresses the isolation and identification of microorganisms capable of producing biosurfactants and degrading heavy oil fractions at the oil reservoir conditions. Five Bacillus subtilis strains isolated from oil samples were able to grow and produce extracellular biosurfactants at 40°C under anaerobic conditions in medium supplemented with hydrocarbons. In addition, some of the isolates displayed a capacity to degrade, both in aerobic and anaerobic conditions, the large alkyl chains, and reduce the viscosity of hydrocarbon mixtures. A sand-pack column model was designed to simulate the oil recovery operations and evaluate the mobilization of residual oil by microorganisms. Additional oil recovery using B. subtilis isolates ranged from 19.8 to 35.0%, suggesting that stimulation of biosurfactant production by these strains in situ can contribute to mobilize entrapped oil. The novelty of this technique, compared to the conventional EOR methods, is the application of indigenous microorganisms to increase the oil recovery.

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