The mechanisms of Microbial Enhanced Oil Recovery (MEOR) are generally thought to be through oil biodegradation, production of biosurfactants, biogases, low molecular acids and biopolymers by microbes. The first four processes contribute to Interfacial Tension (IFT) reduction while the final one to plugging high permeability zones. Microbes can produce diverse metabolites thereby modifying the oil-water IFT in an oil reservoir. This study attempts to investigate the effect of microbial metabolites on the oil viscosity and IFT under reservoir P/T conditions.

A reservoir fluid with 20% crude oil and 80% formation water was treated with indigenous microbes for 21 days under anaerobic condition using molasses (5%) as nutrients. The microbially treated and the un-treated oils were analysed geochemically by using Gas Chromatography-Mass Spectrometer (GC-MS). The oil density, viscosity and the oil-water IFT of both the un-treated and bio-treated oils were measured under reservoir P/T conditions.

The analytical results indicated that there were no significant difference in chemical compositions and densities among different oil samples, suggesting little or none microbial biodegradation present. The effect of biogases and low molecular acids was excluded because of the measurement methodology used.

In contrast to the un-treated oil, the IFT of the bio-treated crude oil was found to be reduced from the un-treated 7.2 to 3.7×10-3 mN/m, a 48% reduction. The oil viscosity of the bio-treated crude oil was decreased by 50%. The viscosity and IFT reduction might be primarily due to the microbial metabolites (biosurfactants) produced, which may play a key role in altering the oil-water interface. Macro-micro emulsification was also observed at the oil-water interface. It is concluded that microbial metabolites could potentially reduce the oil viscosity (via emulsification) and improve the crude oil-water IFT under reservoir P/T conditions.

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