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Microbial biotechnology for enhancing oil recovery: Current developments and future prospects

Authors:
  • Petroleum Development Oman
  • Amity University Rajashtan
  • A'Sharqiyah University

Abstract and Figures

It is anticipated that over 2 trillion barrels of conventional oil will remain in reservoirs worldwide after conventional recovery methods have been exhausted. Other oil recovery methods depend on many economic and technological limitations. Microbial Enhanced Oil Recovery (MEOR), on the other hand, has been proposed for many years as a cheap and effective alternative to enhance oil recovery as its different processes generally do not depend on oil prices. Microbes offer useful metabolic products such as biosurfactants, biopolymers, biogas, biomass, in addition to bio-acids and bio-solvents for enhancing oil recovery. These bioproducts contribute to different microbial systems which tackle specific problems of oil recovery from a chosen target reservoir. The present review provides an overview of MEOR developments from its early stages until today. Basic aspects of petroleum engineering oil recovery stages and microbial characteristics suitable for MEOR are introduced to better link the two bioengineering technologies. The uses and types of different microbial bioproducts available in literature are reviewed and various recovery mechanisms are discussed. Successful MEOR field trials around the world are summarized which shows the potential of this technology as alternative oil recovery method. However, these processes have not been fully proven and did not receive large attention in the petroleum industry due to several reasons that are also discussed. One major reason is the absence of standardized field results and post trial analysis and the lack of structured research methodology. Also, the inconsistent technical performance and lack of understanding of the mechanism of oil recovery contributed to the fact that MEOR received little interest in the petroleum industry.
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Microbial biotechnology for enhancing oil recovery:
Current developments and future prospects
H. Al-Sulaimani1, S. Joshi2, Y. Al-Wahaibi1, S. Al-Bahry2, A. Elshafie2, A. Al-
Bemani1
1Department of Petroleum and Chemical Engineering, College of Engineering, Sultan Qaboos
University, PO Box 33, Al-Khod, 123, Sultanate of Oman; E-mail: hanaa.alsulaimani@gmail.com
2Department of Biology, College of Science, Sultan Qaboos University, PO Box 33, Al-Khod, 123,
Sultanate of Oman
ABSTRACT
It is anticipated that over 2 trillion barrels of conventional oil will remain in reservoirs worldwide after
conventional recovery methods have been exhausted. Other oil recovery methods depend on many economic
and technological limitations. Microbial Enhanced Oil Recovery (MEOR), on the other hand, has been
proposed for many years as a cheap and effective alternative to enhance oil recovery as its different processes
generally do not depend on oil prices. Microbes offer useful metabolic products such as biosurfactants,
biopolymers, biogas, biomass, in addition to bio-acids and bio-solvents for enhancing oil recovery. These
bioproducts contribute to different microbial systems which tackle specific problems of oil recovery from a
chosen target reservoir. The present review provides an overview of MEOR developments from its early stages
until today. Basic aspects of petroleum engineering oil recovery stages and microbial characteristics suitable
for MEOR are introduced to better link the two bioengineering technologies. The uses and types of different
microbial bioproducts available in literature are reviewed and various recovery mechanisms are discussed.
Successful MEOR field trials around the world are summarized which shows the potential of this technology
as alternative oil recovery method. However, these processes have not been fully proven and did not receive
large attention in the petroleum industry due to several reasons that are also discussed. One major reason is the
absence of standardized field results and post trial analysis and the lack of structured research methodology.
Also, the inconsistent technical performance and lack of understanding of the mechanism of oil recovery
contributed to the fact that MEOR received little interest in the petroleum industry.
Keywords: microbial enhanced oil recovery, microbial bioproducts, biosurfactants, biopolymers, MEOR
candidate microbes, field trials
INTRODUCTION
There are three stages of oil recovery process employing mechanical, physical and chemical
methods [1]. The first stage is the primary recovery stage where the natural energy of the reservoir,
mainly reservoir pressure, is utilized. These natural driving forces include: water drive from the
aquifer, solution gas drive that results from gas evolving from oil as reservoir pressure decreases,
gas cap drive, rock and fluid expansion and others [2,3]. The next oil recovery stage is the
secondary stage which takes place when the reservoir pressure tends to fall and becomes insufficient
to force the oil to the surface. In this stage, external fluids are injected into the reservoir either to
maintain the reservoir pressure or to displace the oil in the reservoir [4]. The usual fluid injected is
water; however, immiscible gases could also be injected in this stage. While primary recovery stage
produces generally between 5-10% of the total oil reserves, recovery efficiencies in the secondary
phase varies from 30-40% [1,5]. Based on recent world reserves statistics, nearly 2 trillion barrels of
conventional oil and 5 trillion barrels of heavy oil will remain in reservoirs worldwide after
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conventional recovery methods have been exhausted [6]. Hence, attention has been focused on the
Enhanced Oil Recovery (EOR) techniques for recovering more oil from the existing and abandoned
oil fields. The EOR methods may be divided to thermal, chemical and gas injection methods. The
thermal methods are primarily intended for heavy oils and tar sands mainly to supply heat to the
reservoir. These methods include steam or hot water injection and in situ combustion technique.
Chemical flooding involves injection of certain chemicals that might change either the
characteristics of the reservoir fluids or improve the recovery mechanisms. These include polymer,
surfactants and alkaline flooding. Miscible flooding (either first- or multi- contact miscible) includes
CO2 miscible gas injection, N2 miscible injection and others. Now, more advanced technologies are
being implemented in the oil industry to recover the trapped oil. These include seismic/sonic
stimulations and electromagnetic methods [1]. However, economics are the major deterrent in the
commercialization of the above mentioned EOR methods [6].
Microbial Enhanced Oil Recovery (MEOR) is one of the technologies that can be potentially
implemented with an exceptionally low operating cost. It has several advantages compared to
conventional EOR processes where it does not consume large amounts of energy as do thermal
processes, nor does it depend on the oil price as do many chemical processes [7]. MEOR is simply
the process of utilizing microorganisms and their bio-products to enhance the oil recovery. Bacteria
are the only microorganisms used for MEOR by researchers due to their small size, their production
of useful metabolic compounds such as gases, acids, solvents, biosurfactants, biopolymers as well as
their biomass [8]. Also, their ability to tolerate harsh environments similar to those in the subsurface
reservoirs in terms of pressure, temperature, pH and salinity increased their attraction to be used for
EOR purposes. Bacteria’s average cell size ranges between 0.5-5.0 m which makes it easier for
them to penetrate through the reservoir’s porous media [9]. For MEOR processes which involve the
injection of bacteria into the reservoir, it was calculated that the microbes have to be small,
spherical and less than 20% of the size of the pore throat in the formation [10,11]. Most of the oil
reservoirs are sedimentary basins, reservoir lithology is usually sandstone or carbonates, mostly
fractured limestone for the carbonates reservoirs, with pore size being greater than 30 m for
productive reservoirs and pore throat size not less than 10 m [12]. It was reported that for
reservoirs having permeability higher than 0.6 Darcy (D), an area of 60,000m2 was affected by
microbial treatment [13]. It is also believed that sandstone reservoirs need to have permeability
greater than 0.1 D for the microbes to be able to pass through them [9]. However, for reservoirs with
tight formations having permeability around 0.1 D, the effect was limited to the wellbore region.
Jang et al. [13] conducted a bench scale study on the transport of three bacterial species (Bacillus
subtilis, Pseudomonas putida and Clostridium acetobutylicum) in highly permeable and porous
rock. They have also provided a quantitative screening criterion for selecting proper potential
bacterial strains for in situ MEOR applications. Jansheka [14] compiled a partial list of some
bacteria that have been used in the MEOR experiments and field studies. It was found that Bacillus
and Clostridium species are the most common species used for MEOR purposes since they can form
dormant, resistant endospores that can survive under stressful environmental conditions and they
can produce the useful bioproducts for MEOR [14-16].
The idea of using bacteria for the production of oil was first suggested by Beckman back in
1926 [17]. However in 1946, Zobell and his co-workers were the first to perform actual
experimental work to confirm Beckman’s theory. Their work continued till 1955 and they patented
a process for secondary recovery of petroleum using anaerobic bacteria, hydrocarbon utilizing, and
sulfate reducing bacteria [18]. Later, extensive experimental work on the potential of microbes for
MEOR purposes was conducted [19-22]. Although the results were promising, the research in this
area lost its interest in the 1970s due to economic reasons [23]. However, in the 1980s and 1990s,
the global decline in oil prices raised the need for a cost effective process that is both technically
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and economically feasible. Thus, considerable research in the area of MEOR was performed during
that period [3,9,14,24-36].
MEOR is also considered as an inexpensive process since it can be implemented with minor
modifications to existing field facilities [9]. However, despite the positive and promising
experimental and field tests results, it did not receive wide spread attention in the oil industry due to
several factors suggested by many researchers. Some of these reasons were the negative perception
on the use of bacteria and handling them in the field for MEOR processes although it was verified
by tests conducted by public health laboratories which reported that the mixed cultures of bacteria
are safe to handle and pose no threat to the environment, plants, animals or human beings [15,39].
Besides, the reservoir’s environment is not favorable for the pathogenic organisms to grow. That’s
why, it is recommended to perform a toxicity test for any organism to be used in the field for
MEOR to assure the safety of involved parties. Another factor was the inconsistent technical
performance and lack of understanding of the mechanism of oil recovery [38]. It is difficult to
extrapolate the results from one microbial field trial to other reservoirs as each reservoir has its
unique properties and microbial population for indigenous MEOR cases [39]. One of the major
reasons for MEOR not receiving wide popularity was the absence of standardized field results and
post trial analysis [16]. Most field trials were not followed for enough amount of time to determine
the long term effect [2]. In addition, another reason might be that extensive laboratory tests are
needed to determine the microbe to be used, its survival and competitiveness in the reservoir,
feeding regime strategy and to evaluate the effectiveness of the process.
MICROBIAL CANDIDATES FOR MEOR
Microbes can be classified in terms of their oxygen intake into three main classifications; aerobes
where the growth depends on a plentiful supply of oxygen to make cellular energy. Strictly
anaerobes, by contrast, which are sensitive to even low concentration of oxygen and are found in
deep oil reservoirs. These anaerobes do not contain the appropriate complement of enzymes that are
necessary for growth in an aerobic environment [40]. Lazar [41] found predominantly spore-
forming bacilli and cocci in deep reservoir and non spore-forming bacilli in shallow ones [42]. The
third group of bacteria is facultative microbes, which can grow either in the presence or reduced
concentration of oxygen [40]. Successful field experiments mostly used the anaerobic bacteria [16].
There are many sources from which bacterial species that are MEOR candidates can be isolated.
Lazar [39] suggested four main sources that are suitable for bacterial isolation. These are formation
waters, sediments from formation water purification plants (gathering stations), sludge from biogas
operations and effluents from sugar refineries. Oil contaminated soil could be used as a good source
of microbes isolation for MEOR [43]. Isolation from hot water streams was also reported [15].
Nutrients are the largest expense in the MEOR processes where fermentation medium can
represent almost 30% of the cost for a microbial fermentation [44]. The microbes require mainly
three components for growth and metabolic productions: carbon, nitrogen and phosphorous sources,
generally in the ratio of C, 100: N, 10: P, 1. Media optimization is very important since the types of
bioproducts that are produced by different types of bacteria are highly dependent on the types,
concentrations and components of the nutrients provided. Sometimes, cheap raw materials are also
used as nutrients such as molasses, cheese whey, beef extract and others that contain all the
necessary nutritional components. Huge varieties of raw materials are currently used for industrial
fermentations which are important to the overall economy as they accommodate high percentage of
the final production cost [45,46]. Joshi et al. [46] reported the possibility of using cheese whey for
biosurfactant production. Cheese whey is a liquid byproduct of cheese production. It is composed of
75% of lactose and 12-14% protein in addition to organic acids, vitamins and minerals. Molasses is
a byproduct of sugar production and its low price and presence of vitamins which are valuable for
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fermentation made it an attractive carbon source used by many researches. It contains several other
compounds besides sucrose which include minerals, organic compounds and vitamins. Some
microbes utilize oil as the carbon source, which is excellent for heavy oil production, since it will
reduce the carbon chain of heavy oil and thus increase its quality. Cooper and his co-workers [47]
tested microbes using kerosene as the carbon source to release bitumen from tar sands. Moses [2]
showed that the presence of crude oil in the media can significantly increase the production of
methane and carbon dioxide while the growth rate was reduced [42]. Thus, it is important to
carefully test the nutritional preferences of the studied microbes that would maximize the
production of desired metabolites provided that cost effective supplies are assured.
MICROBIAL BIOPRODUCTS
Microorganisms produce a variety of metabolites that are potentially useful for oil recovery [48].
There are six main bioproducts or metabolites produced by microbes. Table 1 shows a summary of
these bioproducts and their application in oil recovery.
Table 1. Microbial bioproducts and their applications in oil recovery [48].
Product Microorganism Application in oil recovery
Biomass Bacillus licheniformis, Leuconostoc
mesenteroides, Xanthomonas
campestris
MPPM, selective plugging, viscosity
reduction, oil degradation, wettability
alteration
Biosurfactants Acinetobacter calcoaceticus,
Arthrobacter paraffineus, Bacillus
sp., Clostridium sp., Pseudomonas
sp.
Emulsification, interfacial tension reduction,
viscosity reduction
Biopolymers Bacillus polymyxa, Brevibacterium
viscogenes, Leuconostoc
mesenteroides, Xanthomonas
campestris
,
sp.
MPPM-Injectivity profile modification,
mobility control, viscosity modification
Bio-solvents Clostridium acetobutylicum,
Clostridium pasteurianum,
Zymomonas mobilis
Emulsification, viscosity reduction
Bio-acids Clostridium sp., Enterobacter
aerogenes
Permeability increase, emulsification
Biogases Clostridium sp., Enterobacter
aerogenes, Methanobacterium sp.
Increased pressure, oil swelling, interfacial
tension reduction, viscosity reduction,
permeability increase
Biosurfactants
They are amphipatic molecules with both hydrophilic and hydrophobic parts which are produced by
variety of microorganisms. They have the ability to reduce the surface and interfacial tension by
accumulating at the interface of immiscible fluids and increase the solubility and mobility of
hydrophobic or insoluble organic compounds [49]. There are five major types of biosurfactants,
namely lipopeptides, phospholipids, glycolipids (which include rhamnolipids, trehalose lipids and
sophorolipids), fatty acids and neutral lipids [42,47]. Table 2 shows the details of some of the
biosurfactants along with their producing organisms [50].
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Table 2. Types of biosurfactants and their producing organisms [50].
Biosurfactant type Producing organism References
Lipopeptides
Surfactin B. subtilis [38,93]
Lychenysin B. licheniformis [7,46,55,94]
Glycolipids
Rhamnolipids P. aeruginosa [98,99]
Trehalose Lipids Pseudomonas sp.,
R. erythropolis
[96,100]
Sophorolipids Arthrobacter sp.,
Mycobacterium
s
p.
[47,55,95]
Phospholipids Acinetobacter sp.,
T. thiooxidans
[97]
Polymeric
biosurfactants
Emulsan
Acinetobacter sp. [101]
Surfactants are known to reduce the interfacial forces between oil and water and thus improve
the mobilization of oil [51]. It was reported that some microbes can produce biosurfactants that
reduced IFT between oil and water from typical values of 10mN/m to as low as 0.005mN/m [52,53].
In the past few years, biosurfactants have gained attention because of their biodegradability, low
toxicity, and its cost effectiveness [7,45,46,54-57]. Since biosurfactants can be produced from
carbohydrates by fermentation process, it is possible to produce huge amount more cheaply than the
synthetic surfactants, for which they are also developed for use in the oil industry [42]. There is a
wide variety of microorganisms that are reported to produce different types of biosurfactants. For
instance, many Bacillus species were found to produce lipopeptides; Pseudomonas and Candida
species are known to produce glycolipids while Thiobacillus thiooxidans produce phospholipids
[58]. Al Araji et al. [59] reported that the type, quality and quantity of biosurfactants produced are
influenced by the nature of the carbon substrate and the concentration of nitrogen, phosphorous,
magnesium, ferric and manganese ions in the medium. The culture conditions such as pH,
temperature, agitation and dilution rate in continuous culture are also additional factors [59].
Jenneman et al. [60] isolated a Bacillus licheniformis strain, JF-2, from oil field injection water that
is reported to produce biosurfactants that are potentially useful for in situ MEOR processes.
McInerney et al. [61] and Youssef et al. [58] found that the lipopeptide biosurfactant produced by
JF-2 mobilized large amounts of residual hydrocarbon from sand-packed columns and it generated a
low interfacial tension needed for substantial oil recovery. This strain grew and produced
lipopeptide anaerobically at salinity up to 8% and temperatures up to 45C. Furthermore, the growth
of JF-2 was not inhibited by the presence of crude oil [55]. There are several other potential
applications of biosurfactants such as detergents, cosmetics, pharmaceutical, sewage sludge
treatments for oily wastes, pipeline transportations and many others [46]. However, the largest
market for biosurfactants is the oil industry for petroleum production enhancement.
Biopolymers
These are polysaccharides which are secreted by many strains of bacteria mainly to protect them
against temporary desiccation and predation as well as to assist in adhesion to surfaces [1,62]. The
proposed processes of biopolymers are mainly selective plugging of high-permeability zones and
thus permeability modification of the reservoir to redirect the waterflood to oil rich channels [1].
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Another important process of biopolymers is their potential as mobility control agents by increasing
the viscosity of the displacing water hence improving the mobility ratio and sweep efficiency [63].
There are different types of biopolymers produced by different bacteria such as Xanthan gum
produced by the Xanthomonas sp. [64], Levan produced by the Bacillus species [63], Scleroglucan
produced by the Sclerotium sp. [65] and many others [1]. There are many applications where these
biopolymers can be used in addition to enhancement of oil recovery. These include their use in
medical field in drug delivery systems, wound closure, healing products and others. In addition to
their use in food containers, waste bags, agriculture and protective clothing [66]. One of the
biopolymers that is currently in commercial product and have been subjected to extensive studies is
the Xanthan gum. It is produced by fermentation of carbohydrates and it is well known as a
thermally stable heteropolysaccharide. In addition, its physical properties of viscosity, shear
resistance, temperature and salt tolerance made it almost an ideal polymer for use in EOR [64,67]. A
very successful MEOR field trial using the biopolymer process of selectively plugging the high
permeable zones was reported in Fuyu oilfield, China [68]. These MEOR tests started since 1996 by
using the strain CJF-002 which was identified to be an Enterobacter sp. This strain was able to
produce insoluble biopolymers that formed a jelly-like substance at high molasses concentrations.
Oil production was increased more than twice by regulating the water flow and reducing the
channeling effects [68].
Biogases
Bacteria can ferment carbohydrates to produce gases such as carbon dioxide, hydrogen and methane
gas. These gases can be used for enhancing oil recovery by exploiting the mechanisms of reservoir
re-pressurization and heavy oil viscosity reduction. These gases can contribute to the pressure build-
up in pressure depleted reservoirs [62]. They may also dissolve in crude oil and reduce its viscosity
[42,69]. Some of the reported gas-producing genera are Clostridium, Desulfovibrio, Pseudomonas
and certain methanogens [70]. Methanogens produce about 60% methane and 40% carbon dioxide
where the methane will partition between oil and gas phase while carbon dioxide will partition to
the water phase as well and hence improve the mobility of oil [71].
Bio-Acids
Some bacteria when given certain nutrients can produce acids such as lactic acid, acetic acid and
butyric acid [61]. These acids can be useful in carbonate reservoirs or sandstone formations
cemented by carbonates, since it can cause dissolution of the carbonate rock and hence improve its
porosity and permeability [69]. Production of organic acids by bacteria is a normal phase of
anaerobic fermentation of sugars. Clostridium sp., for example, can produce 0.0034 moles of acid
per kilogram of molasses [71].
Bio-Solvents
Sometimes solvents can also be produced as one of the metabolites of the microbes. These include
ethanol, acetone and butanol. They may also help in reduction of oil viscosity and can also
contribute as a co-surfactant in reducing the interfacial tension between oil and water [58].
Microbial biomass
Bacteria are known to grow very fast as some are reported to multiply every 20 minutes under
aerobic conditions [40]. The mechanism of the microbial biomass in MEOR involves selective
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plugging of high permeability zones where the microbial cells will grow at the larger pore throats
restricting the undesirable water flow through them [72]. This will force the displacing water to
divert its path to the smaller pores and hence displacing the un-swept oil and increasing the oil
recovery. Polymers, biofilms and slimes may also contribute to the selective plugging process.
Several laboratory and field tests were conducted to test the feasibility of this mechanism. Jenneman
[60] showed that the addition of nutrients (carbon, nitrogen and phosphate) in sufficient
concentrations into Berea sandstone cores resulted in permeability reduction of 60-80%. The
advantage of Microbial Permeability Profile Modification (MPPM) process is that it does not
interfere with the normal waterflood operation. It is also eco-friendly and is considered as the
cheapest MEOR mechanism [73].
IN SITU AND EX SITU MEOR
There are two processes for MEOR depending on the site of the bioproducts production. These are
namely in situ and ex situ processes. The in situ process involves producing the bacterial
bioproducts inside the reservoir. This can be done either by stimulating the indigenous reservoir
microbes or injecting specially selected consortia of bacteria (exogenous microbes) that will
produce specific metabolic products in the reservoir which will lead to enhancement in oil recovery
[72]. The ex situ process, in turn, involves the production of the bioproducts at the surface outside
the reservoir then injecting them separately either with or without the separation of the bacterial
cells. In this case, commercial size bio-reactors are needed to scale-up the production of the desired
metabolite for field applications. For the in situ process where the exogenous microbes are
introduced into the reservoir, it is important to conduct compatibility studies to determine the
interaction of the injected microbes with indigenous microbes, nutrients, oil and rock [9]. As
described by Jang et al. [13], the success of an in situ MEOR process depends on the selection of the
candidate reservoir, the proper choice of potential bacterial species, the viability of bacteria under
reservoir conditions, the amount of metabolites generated and their effects on releasing residual oil
and other economic factors. However, care must be taken when nutrients or sulfate-containing
waters are injected to ensure that indigenous sulfate-reducing bacteria (SRB) are not stimulated or
overgrown by the injected microbes. These SRBs play a very negative role in MEOR due to the
production of hydrogen sulfide [74]. The major concerns of the global oil industry with SRBs
include oil souring, corrosion caused by H2S production, plugging by iron sulfide, the related
financial burden and the threat to health and safety of the operators [11]. Hitzman [75] patented the
concept of adding a biocide to the water in a waterflood to kill or inhibit SRB (US patent 2917428).
In the laboratory scale, promising microorganisms are isolated from different sources such as water,
oil and soil samples. Then they are screened for desirable metabolites for oil release, followed by
bench-scale experiments showing release of oil-saturated sand packs, cores or even micro models
[76].
MEOR FIELD TRIALS
The first MEOR field test was carried out in Lisbon field Union County, Arkansas, in 1954 [77,78].
Since then, several field trials were performed and by 2003, more than 400 MEOR field tests have
been conducted in the US alone [11], in addition to numerous other field tests carried out in the rest
of the world. There are two main purposes to go for MEOR field applications as single well
treatment and full field treatment.
Single well treatment includes well stimulation, well bore clean-up and others. In this treatment
process, improvement in oil production can result from removal of paraffinic or asphaltic deposits
from the near well bore region or from mobilization of residual oil in the limited volume of the
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reservoir that is treated [9]. This is similar to huff and puff process where the microbial effect is
utilized in this case. In this process, the well is initially inoculated by the desired microbes and
nutrients are injected to stimulate the indigenous microbes. Then, the well is shut-in for a while to
allow microbial growth and to produce the desired metabolites around the well bore. Finally, the
well is opened for production and operation [79]. Most of the reported successful field trials were
single well treatments in the US, China, Romania, India, Russia and Argentina where incremental
oil varied from no impact to 204% [16].
Full field treatment includes microbial enhanced water flooding, MPPM, and other processes
that involve both injection and production wells. In this case, the microbes and nutrients are injected
through an injector well where the metabolites will be produced in situ such as biopolymers that
will help in mobility control of the water flooding. Biomass can also develop from growth of
bacterial cells and block the high permeable zones which will divert the flow of displacing fluids
and allow the displacement of the un-swept oil. Incremental oil is produced from the production
wells in this case.
The success of the full field applications has been mixed and the data from MEOR field tests is
limited [42]. The main problem is that the design of the microbial system and oil production
response are not well documented. Several field trials were summarized and reviewed in literature
[2,3,16,17,22,39] where it was remarked that many of the field tests lacked explanation on the
mechanisms of the oil recovery. Post-treatment analyses were also missing in most of the cases
which might be the major reason why MEOR technology has not gained credibility in the oil
industry [11]. Moses [2] stated that most field trials were not followed for a long enough time to
determine the long term effects. Monitoring and follow-up of results are important factors for
successful MEOR tests. Furthermore, previous work on MEOR has not been informed by a
reservoir engineering perspective such as placement and propagation of biochemicals, effects of
reservoir heterogeneity, mobility control and others [80]. It is important to characterize the target
reservoir prior to designing the MEOR treatment. This includes structural, geological and reservoir
engineering analysis of the target reservoir to better diagnose the problem that would help in
selecting the appropriate microbial mechanism or process for enhancing the oil recovery for this
particular reservoir. Bryant and Burchfield [9] set a minimum criteria requirement that reservoirs
targeted for microbial treatment have to meet. This include reservoir’s permeability to be greater
than 0.1D, temperature to be less than 160 ºF and preferably total dissolved solids in brine not
exceed 100,000 ppm.
On the other hand, Portwood [81] obtained a database of information collected from 322
projects all treated with the MEOR process in the US. One of the objectives was to determine if any
reservoir characteristics is a dominating factor in determining the applicability of the MEOR
process. He concluded that reservoir lithology neither enhances nor impedes the effectiveness of
MEOR as 73% of the projects were conducted in sandstone reservoirs and the rest were in carbonate
reservoirs. He also concluded that MEOR process is applicable in wide range of reservoir
temperature as microorganisms can survive the temperatures present in most of the oil reservoirs.
He also found that as the porosity increased, the incremental oil production decreased. However,
even at highest porosity ranges (26-30%) the incremental production was nearly 20%. Thus,
porosity was not considered as a limiting factor of the MEOR process. Another conclusion was that
reservoirs with low oil gravity (30 °API or less which indicates heavier oil) are found suitable for
MEOR applications. It is important to note that microbial EOR is not a single technology based on a
common approach; rather, it is the adaptation of microbial systems to specific problems of oil
recovery from a chosen target reservoir [3]. These microbial systems include wettability alteration,
viscosity reduction of oil, selective plugging, scale and corrosion control and many others [82].
The technology of MEOR has advanced from a laboratory-based evaluation of microbial
processes to field application internationally [53]. Many of the early field trials were conducted in
155
the US. Johnson and his co-workers [83] injected 150 stripper wells (production less than
10bbl/day) with mixed cultures of Bacillus and Clostridium species using crude molasses. Most of
these wells produced, on average, 2 bbl/day of oil and reservoirs depths varied between 200-1000 ft.
In successful cases, Johnson [83] reported that 20-30% of additional oil in place was recovered.
Hitzman [17] reported on some preliminary field testing with 24 wells during 1977-1982. The
depths varied from 300-4600 ft and he reported that 75% of the wells showed a pressure increase of
10-200 psi. Most of the wells doubled the production for a period between three to six months [8].
Hitzman patented in 1962 a process for the injection of bacterial spores along with the nutrients into
a reservoir (US patent 3032472).
A very successful, well documented and characterized field trial was conducted and supervised
by Lewis Brown since 1994 [73]. The field was the North Blowhorn Greek Unit in Lamar,
Alabama, USA. It had 20 injectors and 32 production wells. The treatment process was MPPM by
adding KNO3 and NaH2PO4 to the waterflood to stimulate the indigenous microbes. Brown [11]
reported in his review that the production decline rate decreased from 18.9% per year to 7-12% per
year and that the field is still producing till today, although it was scheduled to be abandoned in
1998. Several field tests were conducted in other countries which include Romania [39], Argentina
[84], Russia [47,85] and others. Lazar [39] reported an extensive review on MEOR field
applications that was conducted in Romania during the period from 1971-1991. He emphasized on
three main areas of research namely examination of the bacterial populations present in the
formation water of the reservoir, adaptation of the microorganisms to field conditions prior to
injection and finally, field testing of the adapted microorganisms. He concluded that the successful
trials resulted in a two-fold increase in the oil production for one to five years.
In Eastern Asia, some MEOR experimental and field trials were reported in China, Malaysia,
India and Indonesia. Several large-scale field tests were carried out in China including Jilin,
Xinjiang, Daqing, Fuyu and many others [86]. One of the successfully reported field trials was in
Daqing oilfield which is the largest oilfield in China with an average effective thickness of 30ft
[87]. In this application, Pseudomonas aeruginosa (P-1) and its metabolic products were used which
reduced the oil viscosity by 38.5%. It was reported that 80% of the wells showed a significant
increase in oil production and total enhancement of oil recovery of 11% was observed [87]. Another
MEOR application in the same field was by using Brevibacillus brevis and Bacillus cereus. The
mechanism suggested was petroleum hydrocarbon degradation of heavy compounds by the stated
microbes and bio-oxidation process [88]. In this field trial, the microbes were injected between 2002
and 2004 in a huff and puff manner as well as microbial enhanced water flooding method which
was carried out in 2004. Corresponding oil production increased from 20.8 tons per day before
bacteria injection to 36.9 tons per day was reported [88].
In the Arab world, some MEOR laboratory experimental tests were conducted by Sayyouh and
his co-workers since 1992 in Cairo University and in King Saud University [89]. They isolated their
bacteria from the Egyptian and Saudi crude oils and brine. They tested experimentally the effects of
nutrient types and its concentrations, bacterial type, salinity and permeability on oil recovery. Some
other experimental work was conducted by Zekri et al. [15] in United Arab Emirates University
where they studied the possibility of increasing oil recovery from UAE reservoirs using bacterial
flooding. They also investigated the parameters which affected the optimization of microbial
flooding in carbonate reservoirs [15,90,91]. A study was presented by Sayyouh [98] on the
applicability of MEOR for recovering more oil under the Arab reservoir conditions where data was
obtained from more the 300 formations from seven Arab countries (Saudi Arabia, Egypt, Kuwait,
Qatar, UAE, Iraq and Syria). He anticipated that MEOR technology may recover up to 30% of the
residual oil under the Arab reservoir conditions [92]. Some initiatives were taken in the Sultanate of
Oman at Sultan Qaboos University to experimentally investigate the potential of MEOR in Omani
oil fields [122]. Biosurfactant producing strains were isolated from oil contaminated soil samples.
156
One Bacillus subtilis strain was found to be able to produce biosurfactant that reduced the interfacial
testion (IFT) from 47 mN/m to 3.28 mN/m and yielded a total of 23% of additional oil recovery in
coreflooding experiment [122]. However, to our knowledge, no field trials were reported so far for
testing the applicability of MEOR in the Arab region.
CONCLUSION
In conclusion, MEOR is well-proven technology to enhance oil recovery from oil wells with high
water cuts and also to improve it in mature oil wells, but still in order for MEOR processes to be
well accepted and successful, extensive laboratory tests are required prior to field implementation to
select the suitable microbes, to understand their growth requirements and production conditions.
Also, optimization of nutrients and testing the microbes and their bioproducts compatible with
reservoir conditions are required. During field tests, design of the microbial system and oil
production response has to be well documented and results have to be monitored and followed up.
Finally, it is recommended to conduct toxicity tests on the microbes that are to be used in the field
to assure that it is safe to handle and pose no threat to human or to the environment.
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Microbial enhanced oil recovery (MEOR) technology has advanced, since 1980, from a laboratory-based evaluation of microbial processes, to field applications internationally. In order to adequately support the decline in oil production in certain areas, research on cost-effective technologies such as microbial enhanced oil recovery processes must be focused on both near-term and long-term applications, Many marginal wells are desperately in need of an inexpensive improved oil recovery technology today that can assist producers in order to prevent their abandonment. Microbial enhanced water flooding technology has also been shown to be an economically feasible technology in the United States. Complementary environmental research and development will also be required to address any potential environmental impacts of microbial process. The feasibility of microbial EOR processes for reservoirs in the North Sea is discussed.
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A biosurfactant produced by Tsukamurella sp. 26A was purified by procedures including acid precipitation, ethylacetate extraction, and adsorption chromatography. The purified biosurfactant reduced the surface tension of water from 72 mN/m to 30 mN/m at a concentration of 250 mg/l, whereas the minimum interfacial tension against n-hexadecane was lowered to 1.5 mN/m at a concentration of 40 mg/l. The compound stabilized oil-in-water emulsions with a variety of commercial oils and had strong emulsification and stabilization activities when compared to those of commercial emulsifiers and stabilizers. Surface tension was stable over a broad range of pH (2~12) and temperature (100°C, 3 h). The biosurfactant was identified as glycolipid having a hydrophilic moiety of trehalose.
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A biotechnology for the enhancement of oil recovery, based on the activation of the geochemical activity of stratal microflora, was subjected to a pilot trial at the Romashkinskoe oil field. In the main technological variant, activation of microbial activity was achieved by cyclic introduction of aerated water supplemented with mineral salts of nitrogen and phosphorus through injection wells. In an additional variant, crude oil was also introduced through the injection well. The use of this biotechnology led to the activation of microbial processes, first in the near-bottom zones of injection wells and then along the stratum in accordance with hydrodynamic conditions. At the first stage, the activity of aerobic oil-oxidizing microorganisms increased, resulting in an increase in the bicarbonate and acetate content of the stratal water. The second stage was characterized by an enhancement of microbial methanogenesis in the anoxic zone of the stratum and by a consequent increase in the portion of biogenic isotopically light methane in the total methane pool in stratal fluids. The dynamics of microbial metabolites in stratal waters correlated with an increase in the production of the producing wells. The pilot trials of the MEOR biotechnology at four sites of the Romashkinskoe oil field yielded 41560 tons of additionally recovered oil (29% of the oil extracted at these sites during the trials).
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A field test to evaluate the gaseous bacterial fermentation of sugar as a means of enhancing oil recovery is described. The test was conducted on the Lisbon Unit, Union County, Arkansas, using a two-spot pattern with 400 feet between wells. Laboratory experiments, which preceded the field test, showed that bacterial gases produced within the reservoir rock prior to waterflooding led to better oil recoveries than could be obtained by either waterflooding or gas flooding alone.