Micro-organisms are the creatures that in the prepared condition for their growth, they can be beneficial for so many fields. One of these fields is Enhanced Oil Recovery. Microbial enhanced oil recovery method (MEOR) includes the usage of Micro-organisms of their productions.

One of the most important productions of micro-organisms is surfactants. Surfactants include cationic surfactants, anionic surfactants, and neutral surfactants. These conditions have two parts which are water-wet and oil-wet, and by putting their selves in the surface between oil and water, they decrease the interfacial tension, and in this way they help oil flow going towards wells. For instance, one of the problems in the movement of oil inside an environment of porosity is a phenomena which is called pinching off, in which the surfactants decrease the trapped oil and make it continuous, and this makes the diameter of oil molecules decrease to the size smaller than gate diameter, so that the oil molecules start bypassing.

The advantages of these biological combinations compared with their chemical types are as follows: more surface activity, better performance in various temperature conditions, PH, salinity, and being less poisonous. Nc is related to the residual oil saturation through the capillary desaturation curve. The critical capillary number and the shape of the capillary desaturation curve depend on the rock properties

Such as pore size distribution and wettability. The more the Nc, the less the IFT, so the oil recovery gets more.

The high potential of EOR is illustrated in the fact that over %65 of the original oil-in-place still remains in many worldwide reservoirs after primary and secondary oil recovery. Micro-organisms are used in this method to reduce permeability and homogenization of reservoir by sealing the areas with high permeability. Their productions are being used in other fields too; for example biopolymers are used to increase the viscosity of oil, biosurfactants are used to decrease the interfacial tension of oil-water, biogases and solvents are used to reduce the viscosity and organic acids of oil to increase the porosity by solving the carbonate rocks.

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