[justify]It is well known from several core scale experiments that microbial activity inside a core may lead to enhanced oil production. In this work we argue that the only realistic microbial mechanism that contributes to oil production is that of the biofilm type, simply because of the low concentration of microbes inside the porous media. Microbial activity can lead to formation of a biofilm on the rock surface and the oil water interface. By modelling the microbes as immiscible drops we show that they can change the wetting properties of the rock. The model used is a Lattice Boltzmann algorithm for solving the multiphase Navier- Stokes equations. Experiments with two strains of microbes from oil fields have been performed. The experiments are focused on studying the ability of microbes to attach to interfaces and surfaces and thereby change the wetting properties of oil, brine and rock. The first type is a microbial capillary tube experiment where microbes grown inside capillary tubes may change the interfacial or wetting properties of the tubes. A change in interfacial tension or wetting characteristic can be observed as a change in height of the oil water interface. The second type is a sessile drop experiment, where the contact angle of an oil drop has been observed over time, while subjected to microbial activity.
Microbial enhanced oil recovery (MEOR) is motivated by the fact that numerous core scale experiments have shown an increased oil production due to microbial activity. In some cases the increased oil production has been extremely high while in some cases very low (see Bryant and Lockhart (2000)). The experimental evidences are convincing that something is going on inside the core which increases the oil production. The interpretation of core scale experiments is complicated due to one simple reason that when oil is released, one never really knows what kind of mechanism is responsible for the increased oil production. Even if the microbes or product produced by the microbes was responsible for the extra oil produced, one does not know precisely what they did. The core acts as a black box. In order to do a field trial or pilot one needs to understand in detail what the microbes are doing. As an example core scale experiments are often performed on water wet cores. If the mechanism for extra oil production is wettability change towards more oil wet behaviour, then one needs to take into account that the reservoir is probably not water wet, but mixed wet.
Biofilms can grow on the surface of the porous rock, which may lead to a change of surface properties and/or a decrease in permeability (Gandler et al. (2006)). Permeability reduction can not explain increased oil production from water wet cores. The properties of the biofilm will be different from the rock properties. The change in surface properties inside the porous rock can thus lead to a change in the wetting properties. If the microbes locally change the wettability close to a trapped oil cluster, this oil cluster can be mobilized when the receeding contact angle is reduced sufficiently. In addition microbes attached to the oil water interface will not detach easily (we will return to this point). Microbes would then be transported with the oil cluster to a new location and may induce new oil mobilization.[/justify]