The process of getting more oil from oil wells by injecting suitable microbes or microbial products into the oil reservoir, is called microbial enhancement of oil recovery (MEOR). It is also called tertiary oil recovery.

Aerobic bacteria yeasts and fungi are not suitable for MEOR, as most of them fail to grow in mines. Anaerobic bacteria are the right choice for oil recovery for many reasons. The microbes should have the following features:

• The microbes should be similar in size.
• They should have the ability to survive in oil reserves atleast for 3 months.
• They should have the ability to withstand the temperature between 30°C and 100°C.
• They should have the capacity to resist 50-500 atm pressure in the oil wells.
• They should have salt resistance.
• They should live without oxygen.
• They could grow on cheap carbon sources.

The mixed culture of microbes is added to dilute nutrient medium containing a cheap carbon source and nitrogen source. This is the inoculums. The inoculums are pumped into the oil reservoir. The injected microbes dislodge oil found on rock particles and increase the oil production up to 7-200%. Water is pumped into the reservoir under pressure to collect oil in the well head.

The microbes produce hydrogen, methane, carbon dioxide, organic acid, surface active products and polysaccharides. The gases maintain the gas pressure inside the reservoir and reduce the viscosity of the oil. So the oil found on rock particles tends to move.
The organic acids like acetic acid and propionic acid increase the porosity of the rock and production of carbon dioxide. The surface active products reduce the interfacial tension between oil and water while pumping water into the reservoir. The polysaccharides increase the viscosity of flooding water to allow it to go through all channels under the applied pressure. They force the released oil towards the well head. As a result more oil gets collected in the well head.

Oil recovery can also be enhanced by injecting certain products of microbial metabolism into the oil reserves. The products include surfactant polymers and non-surfactant polymers. They may be alcohols, acids and ketones.

Non-surfactant polymers are water soluble substances used to thicken water. They have high viscosity and stability over pH changes, temperature, pressure and salinity. They are resistant to microbial breakdown in the reservoir. Even they increase the viscosity at the level of 500 ppm- 1000ppm.

Xanthum gum is typical non- surfactant polymer. It is a polysaccharide produced by the bacterium Xanthomonas compestris by fermenting carbohydrates. It is a branched polymer consisting of numerous glucose units. The molecular weight ranges from 1*106 to 1* 107daltons.

Xanthum gum is mixed with water to make thickened water and then the water is pumped into the oil reserves, the thickened water, when pressure is applied, acts as a piston and pushes the oil towards the well head. So oil gets collected in the well head. Oil is taken from the well head using suction pumps.

Alginate produced from Azotobacter vinelandii and pullan produced from Aureobasidium pollutants are also used as non- surfactant polymers for enhancing the oil recovery.

Xanthan, alginate and pullan are found to be degraded, to some extent, by certain microbe in the oil reserves. This is the major limitation to this technique.
Some biopolymers do not dissolve in water, but they release oil from rock particles by their fluid dynamic properties. Such polymers are known as surfactants. Emulsan is a surfactant polymer.

Emulsan is made from Acinetobacter calcoaceticus RAGI. It is a lipopolysaccharide consisting of D-galactos-amine, aminouronic acid and fatty acids. It's molecular weight is about 106 daltons. It reduces the viscosity, and increase emulsification and rock-wetting.
When emulsan is pumped into the reservoir along with water, it releases the oil found around the rock and soil particles and moves it towards the well head. From the well head, oil is taken by using suction pumps.

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