The primary objective of MEOR is to improve the extraction of oil confined within porous media, while boosting economic benefits.
[7] Successful stories are specific for each MEOR field application, and published information regarding supportive economical advantages is however nonexistent.
MEOR is, therefore, one of the future research areas with great priority as identified by the "Oil and Gas in the 21st Century Task Force".
[3] Before the advent of environmental molecular microbiology, the word "bacteria" was utilised indistinctively in many fields to refer to uncharacterized microbes,[8] and such systematic error affected several disciplines.
[2][3][4][7] Microbes are living machines whose metabolites, excretion products and new cells may interact with each other or with the environment, positively or negatively, depending on the global desirable purpose, e.g. the enhancement of oil recovery.
Such complexity is increased by the interplay with the environment, the later playing a crucial role by affecting cellular function, i.e. genetic expression and protein production.
Despite this fundamental knowledge on cell physiology, a solid understanding on function and structure of microbial communities in oil reservoirs, i.e. ecophysiology, remains inexistent.
The aim of MEOR is to enhancing oil recovery continuously by making use of the metabolic process of the indigenous beneficial microbes.
From a molecular point of view, the review of Daniel[14] shows that at high pressures the DNA double helix becomes denser, and therefore both gene expression and protein synthesis are affected.
[16][17][18] The oxidation potential (Eh, measured in volts) is, as in any reaction system, the thermodynamic driving force of anaerobic respiration, which takes place in oxygen depleted environments.
In this process, the effects of nitrate reduction on wettability, interfacial tension, viscosity, permeability, biomass and biopolymer production remain unknown.
Besides, electrolytes promote an ionic strength gradient across cellular membrane and therefore provides a powerful driving force allowing the diffusion of water into or out to cells.
However, some microbes from hypersaline environment such as Pseudomonas species and Halococcus thrive at lower aw, and are therefore interesting for MEOR research.
Organic matter and metabolic products between geological formations can diffuse and support microbial growth in distant environments.
The mechanism can be explained from the client-operator viewpoint which considers a series of concomitant positive or negative effects that will result in a global benefit: Changing oil reservoir ecophysiology to favour MEOR can be achieved by complementing different strategies.
In situ microbial stimulation can be chemically promoted by injecting electron acceptors such as nitrate; easy fermentable molasses, vitamins or surfactants.
This knowledge has been obtained from experiments with pure cultures and some times with complex microbial communities but the experimental conditions are far from mimicking those ones prevailing in oil reservoirs.
Biopolymer production and the resulting biofilm formation (less 27% cells, 73–98% EPS and void space) are affected by water chemistry, pH, surface charge, microbial physiology, nutrients and fluid flow.
[12][13] Microbial produced surfactants, i.e. biosurfactants reduce the interfacial tension between water and oil, and therefore a lower hydrostatic pressure is required to move the liquid entrapped in the pores to overcome the capillary effect.
Secondly, biosurfactants contribute to the formation of micelles providing a physical mechanism to mobilise oil in a moving aqueous phase.
Immiscible CO2 helps to saturate oil, resulting in swelling and reduction of viscosity of the liquid phase and consequently improving mobilization by extra driving pressure.
Successful MEOR field trials have been conducted in the U.S., Russia, China, Australia, Argentina, Bulgaria, former Czechoslovakia, former East Germany, Hungary, India, Malaysia, Peru, Poland, and Romania.
[1][3][7] Lazar et al.[1] suggested China is leading in the area, and also found that the most successful study was carried out in Alton field, Australia (40% increase of oil production in 12 months).
[7] As reviewed by Lazar et al.,[1] field application followed different approaches such as injection of exogenous microorganisms (microbial flooding); control of paraffin deposition; stimulation of indigenous microbes; injection of ex situ produced biopolymers; starved selected ultramicrobes (selected plugging); selected plugging by sand consolidation due to biomineralization and fracture clogging in carbonate formations; nutrient manipulation of indigenous reservoir microbes to produce ultramicrobes; and adapted mixed enrichment cultures.
Published MEOR models are composed of transport properties, conservation laws, local equilibrium, breakdown of filtration theory and physical straining.
(B) Filtration model to express bacterial transport as a function of pore size; and relate permeability with the rate of microbial penetration by applying Darcy's law.
Monod equation is commonly used in modelling software but it has a limited behaviour for being inconsistent with the law of mass action that form the basis of kinetic characterization of microbial growth.
This makes things difficult for in situ biosurfactant production because controlled experimentation is required to determine specific growth rate and Michaelis–Menten parameters of rate-limiting enzyme reaction.
Modelling of bioclogging is complicated because the production of clogging metabolite is coupled nonlinearly to the growth of microbes and flux of nutrients transported in the fluid.
Thus, a field strategy needs a simulator capable of predicting bacterial growth and transport through porous network and in situ production of MEOR agents.