Microbial Enhanced Oil Recovery (MEOR) is a biological-based technology involving the manipulation of functions or structures within microbial environments present in oil reservoirs. The primary objective of MEOR is to improve the extraction of oil confined within porous media, while boosting economic benefits.Lazar, I., I.G. Petrisor, and T.E. Yen, Microbial enhanced oil recovery (MEOR). Petroleum Science and Technology, 2007. 25(11–12): p. 1353-1366Ollivier, B. and M. Magot, eds. Petroleum microbiology. 1st ed. 2005, ASM Press: Washington, DC. 365Sen, R., Biotechnology in petroleum recovery: The microbial EOR. Progress in Energy and Combustion Science, 2008. 34(6): p. 714-724Van Hamme, J.D., A. Singh, and O.P. Ward, Petroleum microbiology – Part 1: Underlying biochemistry and physiology. Chimica Oggi-Chemistry Today, 2006. 24(1): p. 52Fujiwara, K., et al., Biotechnological approach for development of microbial enhanced oil recovery technique. Petroleum Biotechnology: Developments and Perspectives, 2004. 151: p. 405-445 As a tertiary oil extraction technology, MEOR enables the partial recovery of the commonly residual 2/3 of oil, effectively prolonging the operational lifespan of mature oil reservoirs.

MEOR is a multidisciplinary field incorporating, among others: geology, chemistry, microbiology, fluid mechanics, petroleum engineering, environmental engineering and chemical engineering. The microbial processes proceeding in MEOR can be classified according to the oil production problem in the field: wellbore clean up removes mud and other debris blocking the channels where oil flows through; well stimulation improves the flow of oil from the drainage area into the well bore; and enhanced water floods through stimulating microbial activity by injecting selected nutrients and sometimes indigenous microbes. From the engineering point of view, MEOR is a system integrated by the reservoir, microbes, nutrients and protocol of well injection. Enhance oil recovery of the depleting multistage fractured horizontal shale oil wells in unconventional shale oil reservoir.

Outcomes So far, the outcomes of MEOR are explained based on two predominant rationales:

Increment in oil production. This is done by modifying the interfacial properties of the system oil-water-minerals, with the aim of facilitating oil movement through porous media. In such a system, microbial activity affects fluidity (viscosity reduction, miscible flooding); displacement efficiency (decrease of interfacial tension, increase of permeability); sweep efficiency (mobility control, selective plugging) and driving force (reservoir pressure).

Reduce water cut. The indigenous microbes stimulated by the injected microbial nutrients grow fast and selectively block the "thief zones", divert the injected water to sweep the unswept oil.

The aforementioned two rationals are demonstrated in a Youtube video prepared by New Aero Technology LLC.

Relevance Several decades of research and successful applications support the claims of MEOR as a mature technology. Despite those facts, disagreement still exists.Awan, A.R., R. Teigland, and J. Kleppe, A survey of North Sea enhanced-oil-recovery projects initiated during the years 1975 to 2005. Spe Reservoir Evaluation & Engineering, 2008. 11(3): p. 497-512 Successful stories are specific for each MEOR field application, and published information regarding supportive economical advantages is however nonexistent. Despite this, there is consensus considering MEOR one of the cheapest existing EOR methods. However, obscurity exists on predicting whether or not the deployment of MEOR will be successful. 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". This is probably because MEOR is a complementary technology that may help recover the 377 billion barrels of oil that are unrecoverable by conventional technologies.

Bias Before the advent of environmental molecular microbiology, the word "bacteria" was utilised indistinctively in many fields to refer to uncharacterized microbes,Daims, H., M.W. Taylor, and M. Wagner, Wastewater treatment: a model system for microbial ecology. Trends in Biotechnology, 2006. 24(11): p. 483 and such systematic error affected several disciplines. Therefore, the word "microbe" or "microorganism" will therefore be preferred hereafter in the text.

In Microbial EOR, only the beneficial microbes are stimulated, like Nitrate Reducing Bacteria (NRB). The non-beneficial bacteria like Sulphate Reducing Bacteria (SRB) are not stimulated because the MEOR process only introduces nitrate to the reservoir, but not introduces sulphate to it. In the meantime, the growing NRB can control the activities of SRB, reduce the concentration of . To some extent, MEOR process may recover the reservoir from sour to sweet.

History It was in 1926 when Beckam proposed the utilisation of microorganisms as agents for recovering the remnant oil entrapped in porous media. Since that time numerous investigations have been developed, and are extensively reviewed. In 1947, ZoBell and colleagues set the basis of petroleum microbiology applied to oil recovery, whose contribution would be useful for the first MEOR patent granted to Updegraff and colleagues in 1957 concerning the in situ production of oil recovery agents such as gases, acids, solvents and biosurfactants from microbial degradation of molasses. In 1954, the first field test was carried out in the Lisbon field in Arkansas, USA. During that time, Kuznetsov discovered the microbial gas production from oil. From this year and until the 1970s there was intensive research in USA, USSR, Czechoslovakia, Hungary and Poland. The main type of field experiments developed in those countries consisted in injecting exogenous microbes. In 1958, selective plugging with microbial produced biomass was proposed by Heinningen and colleagues. The oil crisis of 1970 triggered a great interest in active MEOR research in more than 15 countries. From 1970 to 2000, basic MEOR research focused on microbial ecology and characterization of oil reservoirs. In 1983, Ivanov and colleagues developed the strata microbial activation technology. By 1990, MEOR achieved an interdisciplinary technology status. In 1995, a survey of MEOR projects (322) in the USA showed that 81% of the projects successfully increased oil production, and there was not a single case of reduced oil production. Today, MEOR is gaining attention owing to its low cost (less than $10 per incremental bbl) and low CAPEX requirement (the operator does not need to invest in surface facilities as traditional chemical or EOR, and can reduce the number of infill drilling wells). Several countries indicated they might be willing to use MEOR in one third of their oil recovery programs by 2010. In addition, as the Wall Street, shale oil operators, and the US DOE realize the extreme recovery factor of the US shale oil wells (lower than 10%), the US SBIR sponsored the first MEOR pilot of multistage fractured shale oil well in the world in 2018, "Field pilot test of Novel Biological EOR Process for Extracting Trapped Oil from Unconventional Reservoirs", conducted by New Aero Technology LLC.

Advantages There is a plethora of reviewed claims regarding the advantages of MEOR.Singh, A., J.D. van Hamme, and O.P. Ward, Petroleum microbiology – Part 2 – Recovery, biorefining and biodegradation processes. Chimica Oggi-Chemistry Today, 2006. 24(2): p. 65-67 There are lots of publications in the website www.onepetro.com maintained by the Society of Petroleum Engineering and other websites or databases. Some field applications are also shared by petroleum microbiology companies.

Advantages can be summarised as follows: Injected microbes and nutrients are cheap; (the injection of microbes is out of date. The new microbial EOR technology does not need to inject microbes to the reservoir, but only inject nutrients to stimulate the indigenous microbes) easy to handle in the field and independent of oil prices. Economically attractive for mature oil fields before abandonment. Increases oil production. Existing facilities require slight modifications. Easy application. Less expensive set up. Low energy input requirement for microbes to produce MEOR agents. More efficient than other EOR methods when applied to carbonate oil reservoirs. Microbial activity increases with microbial growth. This is opposite to the case of other EOR additives in time and distance. Microbial nutrients are biodegradable and therefore can be considered environmentally friendly.

Disadvantages Disadvantages of MEOR:

Microbial growth is favoured when: layer permeability is greater than 20 md; reservoir temperature is inferior to 85 0C, salinity is below 100,000 ppm and reservoir depth is less than 3,500m. The recent cases proved that there is no corrosion during MEOR based on continuous field monitoring results. In addition, the stimulated indigenous microbes do not affect crude oil qualities, and there is no sign of increasing microbes in the produced liquid.

Environment of an oil reservoir Oil reservoirs are complex environments containing living (microorganisms) and non living factors (minerals) which interact with each other in a complicated dynamic network of nutrients and energy fluxes. Since the reservoir is heterogeneous, so do the variety of ecosystems containing diverse microbial communities, which in turn are able to affect reservoir behaviour and oil mobilization.

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. All these entities, i.e. enzymes, extracellular polymeric substances (EPS)Flemming, H.C. and J. Wingender, Relevance of microbial extracellular polymeric substances (EPSs) – Part II: Technical aspects. Water Science and Technology, 2001. 43(6): p. 9-16Flemming, H.C. and J. Wingender, Relevance of microbial extracellular polymeric substances (EPSs) – Part I: Structural and ecological aspects. Water Science and Technology, 2001. 43(6): p. 1-8 and the cells themselves, may participate as catalyst or reactants. 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.

Environmental constraints

Several factors concomitantly affect microbial growth and activity. In oil reservoirs, such environmental constraints permit the establishment of criteria to assess and compare the suitability of various microorganisms. Those constraints may not be as harsh as other environments on Earth. For example, with connate brines the salinity is higher than that of sea water but lower than that of salt lakes. In addition, pressures up to 20 MPa and temperatures up to 85 °C, in oil reservoirs, are within the limits for the survival of other microorganisms.

Some environmental constraints creating selective pressures on cellular systems that may also affect microbial communities in oil reservoirs are:

Temperature