ARTICLE IN PRESSProgress in Energy and Combustion Science 34 (2008) 714– 724 Contents lists available at ScienceDirect Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs Biotechnology in petroleum recovery: The microbial EOR Ramkrishna Sen à Department of Biotechnology, Indian Institute of Technology (IIT), Kharagpur, West Bengal 721302, India a r t i c l e in f o Article history: Received 17 January 2006 Accepted 5 May 2008 Available online 20 June 2008 Keywords: Microbial enhanced oil recovery Mechanisms Solvents Gases Biosurfactants Biopolymers Biofilms Selective plugging SRB Modeling Field trials a b s t r a c t Biotechnology has played a significant role in enhancing crude oil recovery from the depleted oil reservoirs to solve stagnant petroleum production, after a three-stage recovery process employing mechanical, physical and chemical methods. Biotechnologically enhanced oil recovery processes, known as microbial enhanced oil recovery (MEOR), involve stimulating indigenous reservoir microbes or injecting specially selected consortia of natural bacteria into the reservoir to produce specific metabolic events that lead to improved oil recovery. This also involves flooding with oil recovery agents produced ex situ by industrial or pilot scale fermentation. This paper essentially reviews the operating mechanisms and the progress made in enhanced oil recovery through the use of microbes and their metabolic products. Improvement in oil recovery by injecting solvents and gases or by energizing the reservoir microflora to produce them in situ for carbonate rock dissolution and reservoir repressurization has been enunciated. The role of biosurfactants in oil mobilization through emulsification and that of biopolymers for selective plugging of oil-depleted zones and for biofilm formation have been delineated. The spoil sport played by sulfate-reducing bacteria (SRB) in MEOR has also been briefly reviewed. The importance of mathematical models used in predicting the applicability of an MEOR strategy and the microbial growth and transport has been qualitatively discussed. The results of some laboratory studies and worldwide field trials applying ex situ and in situ MEOR technologies were compiled and interpreted. However, the potential of the MEOR technologies has not been fully realized due to poor yield of the useful microbial metabolic products, growth inhibition by accumulated toxic metabolites and longer time of incubation. A complete evaluation and assessment of MEOR from an engineering standpoint based on economics, applicability and performance is required to further improve the process efficiency for writing more success stories. Thus, this review attempts to address almost all the issues concerning the MEOR, its past and recent trends and its future prospect and directions. & 2008 Elsevier Ltd. All rights reserved. Contents 1. 2. 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715 Enhanced oil recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715 Microbial enhanced oil recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715 3.1. MEOR mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716 3.1.1. Role of biosurfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 717 3.1.2. Role of biopolymers and biofilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 718 3.1.3. Use of gases and solvents as MEOR agents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719 Sulfate-reducing bacteria—Notorious villains in MEOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719 Importance of mathematical modeling in MEOR—a qualitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 719 Field trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 720 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723 4. 5. 6. 7. à Tel.: +91 3222283752; fax: +91 3222278707. E-mail address: [email protected] 0360-1285/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pecs.2008.05.001 which have become increasingly important in the post-Iraq war scenario. gases and also enzymes to increase recovery of oil from depleted and marginal reservoirs. which fracture the hydrocarbon-bearing formation to improve the flow of oil and gas to the wellhead.10]: NCap ¼ Viscous forces vm ¼ .doe.htmlS. oil and rock interfaces. obtained from Improved Oil Recovery Scheme Thermal (i) Steam flood (ii) Combustion (iii) Hot water Chemical (i) Polymers (ii) Surfactants (iii) Alkali Gas Injection (i) CO2 (ii) N2 (iii) Flue gas Biotechnological Novel (i) Seismic / Sonic stimulations (ii) Electromagnetic MEOR/MIOR Fig.14–16]. Other techniques involving mechanical and physical means such as pumping and gas lift help in oil production when the reservoir pressure dissipates. Hence. While primary recovery produces 5–10% of the total reserve. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 715 1. there is a dire need to produce more crude oil.12–16]. recovery efficiencies in the secondary stage vary from 10% to 40% of the oil in place [3–5]. The greater the capillary number. in two stages. gases and solvents are used to increase the permeability through the porous network and to re-pressurize the oil reservoir [1–8. the lower the residual oil saturation in the reservoir and hence higher the oil recovery [10]. changes in cell membrane architecture. The following are the oil recovery agents used for CEOR. No single EOR or MEOR approach can address this problem. The capillary number is defined as the ratio of viscous to capillary forces as given in Eq. could be an efficient approach for the EOR [7. Of these methods. Polymers are used to increase viscosity of water-flood. Thus. are ubiquitous in reservoirs around the world. more advanced technologies are being implemented in the oil industry today to recover the trapped oil under the program of EOR. To meet the rising energy demand worldwide. of the displacing fluid. Capillary forces s cos y (1) where v and m are the velocity and viscosity. 1) shows the types of EOR processes that are currently employed in the oil industry.8]. Enhanced oil recovery The flow diagram (Fig. 3. 2. chemotaxis. Surfactants are used to reduce interfacial tension between oil and water. The normal microscopic view and a cross sectional view of a typical oil reservoir and the method of water-flooding are shown in Fig. Such reviews incorporating the in-depth analysis of the aspects of accession and transformation of hydrocarbons. Thus. and also of the production and quality improvement of petroleum feedstock and petrochemicals have been presented [1. and funded major programs to develop new technologies. Stagnant oil production and unimpressive recovery by conventional methods have been a major concern. Introduction Crude petroleum is present worldwide in the complex capillary network of oil reservoirs. s is the oil–water interfacial tension (IFT) and y is the contact angle. A combinatorial strategy that involves the injection of a fluid or fluids to supplement the natural pressure in a reservoir. These programs resulted in the creation of the enhanced oil recovery (EOR) technologies. thereby extending the life of the oil wells [1–5]. In the US alone the original oil in place has been estimated to be 650 billion barrels. Critical evaluation of the physical and biochemical mechanisms that control microbial responses to the hydrocarbon substrates and their mobility is a prerequisite for a comprehensive review on MEOR. biomass. oil recovery is challenging because the remaining oil is often located in regions of the reservoir that are difficult to access and the oil is held in the pores by capillary pressure [6–8]. Since these are petrochemicals. Traditional oil recovery technologies under the umbrella of chemically enhanced oil recovery (CEOR) can recover a maximum of 40–45% of the oil initially in place.3–7. where the injected fluids interact with the reservoir rock/oil/brine system to create favorable conditions for maximum oil recovery. respectively. are adding fuel to this fire of concern. Primary recovery produces oil and gas using the natural pressure drive of the reservoir [1–4]. constitutes an operation. which is an expensive option.ARTICLE IN PRESS R. namely. When water is the injection fluid. microbial enhanced oil recovery (MEOR) has the potential to be cost-efficient [1]. Flow sheet diagram showing the process steps in enhanced oil recovery (EOR). However. MEOR refers to petroleum recovery methods. attention has been focused on the EOR techniques for recovering more oil from the existing and abandoned oilfields. called pressure maintenance [4. the purpose of this review is to discuss the operating mechanisms of MEOR and to highlight the recent developments and future prospects in this important area of scientific research. the capillary number. Injection of natural gas. which involve the use of a mixed microbial population and the metabolic products including biosurfactants. cell-surface adhesion and hydrophobicity. gov/scngo/Petroleum/Exploration%20&%20Production/EOR/ eor. primary and secondary recovery. In the US. The effect of capillary forces on trapping of oil within the pores of reservoir rock is normally characterized by the use of a dimensionless number.5]. Secondary recovery involves stimulating the oil wells by the injection of fluids. Several techniques were used for injecting fluids into an oil reservoir to augment the natural forces in secondary recovery [4]. 1.10. solvents. the process is called water-flooding. (1) [9.netl. Thus. a major part of which is the target of improved and advanced oil recovery methods (Fig. . Acids. the EOR methods including MEOR have the potential to recover much of that remaining oil. these conventional oil recovery operations often leave two-thirds of the oil in the reservoir. acids. 2). which is estimated to be about 375 billion barrels. most major oil companies in the USA had set up their own research centers. The socio-political factors. biopolymers. which kept the industry going strong and competitive /http://www. During the turbulent times of the Arab oil embargo. harmful and helpful. Microbial enhanced oil recovery Microorganisms. 3. These microbes are increasingly being acknowledged for their ability to influence reservoir behavior and oil mobilization [11]. doe. showing targets for EOR /http://www. This has also been studied by nutrient resuscitation and growth of starved cells in sandstone cores [20]. but unrecoverable by conventional technologies. Genetic engineering tools and techniques are being used to develop microorganisms that cannot only survive and grow in extreme reservoir environment. MEOR scores over other EOR processes on two accounts. but can also subsist on inexpensive nutrients and produce substantial amounts of metabolic products as EOR agents including enzymes. MEOR. MEOR mechanisms The MEOR processes involve the use of reservoir microorganisms or specially selected natural bacteria to produce specific metabolic events that lead to enhanced oil recovery. In fact what is known today as MEOR finds its history dating back to 1947 [12]. oil swelling. gases and solvents to perform the job of recovering residual oil by fermenting cheaper raw materials. IFT and viscosity reduction petroleum feedstock after refining and downstream processing. Pseudomonas Polymers Bacillus.ARTICLE IN PRESS 716 R. It is the exponential nature of microbial growth. 3. Biomass. Bacillus.16. which leads to the production of useful biochemical agents for MEOR at higher rates from inexpensive and renewable resources.com/titanprocess1. Zymomonas. Arthrobacter. target for new EOR technology (377 billion barrels) Fig.html). In an in situ process. Table 1 Microbial consortia and their metabolites with applications in MEOR Microbial product Biomass Example microbes Application in MEOR Fig. as shown in Table 1. Xanthomonas Solvents Clostridium. Enterobacter. Leuconostoc. Enterobacter Methanobacterium Selective plugging and wettability alteration Emulsification and de-emulsification through reduction of IFT Injectivity profile and viscosity modification.gov/scngo/Petroleum/Exploration%20&%20Production/EOR/ eor.11–18. A cross sectional view of an oil reservoir during water-flooding (adapted from http://www. Though MEOR is considered as an environmentally compatible tertiary oil recovery method and is a time tested and increasingly applied method of oil treatment in the industry as well. Original oil in place in the USA (649 billion barrels). albeit at a slower rate and with lower yields. aids in MEOR by selective plugging of oil-less zones [1–4. Firstly. 2. which remain viable at such reservoir conditions as temperatures up to 85 1C.23–27]. have been used in MEOR. Microbial consortia. which are not discussed here. pressure over 17. e. which exploits microorganisms for the production of all the chemicals as mentioned above. there are a few shortcomings of MEOR. selective plugging Rock dissolution for better permeability. which mostly secretes exopolysaccharides and forms biofilms. Bacillus.1. but recoverable by conventional technologies (30 billion barrels) Known Reserves (22 billion barrels) Not found and unrecoverable by conventional technologies (37 billion barrels) Current cumulative production (183 billion barrels) Found. Leuconostoc. the microbial cell factories need little input of energy to produce the MEOR agents and secondly.23 MPa. Mixed acidogens Gases Clostridium. This is why the scientist looked for a cost-effective alternative and discovered the same in MEOR. 4. emulsification Increased pressure.titanoilrecovery. to be economically viable. One major obstacle that has slowed the implementation of MEOR has been the difficulty in isolating and/or engineering microbial strains. extremes of pH and salinity. Genetically engineered MEOR (GEMEOR) and the enzyme enhanced oil recovery (EEOR) constitute the advanced MEOR methods. 3. acids.htmlS. Klebsiella Acids Clostridium. demands the use of microbial strains.19]. stimulation of the indigenous microflora by injecting suitable nutrients serves to enhance oil mobilization.3–5. They have the ability to produce biosurfactants. Xanthomonas Surfactants Acinetobacter. as shown in Fig. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 Not found. the application of microbial processes does not directly depend on the global crude oil price. there is also growing application of microorganisms for the treatment of petroleum-based products both in reservoirs and on earth’s surface for bioremedial cleanup of hydrocarbons [1. which can survive in the extreme environment of the oil reservoirs. The mechanisms by which MEOR processes operate can be quite . However. biopolymers. Brevibacterium.g.27–30].netl. 21. molasses [1–5.13. CEOR methods turned out to be economically unattractive as the finished products are utilized for the recovery of raw materials. oil viscosity reduction Permeability increase. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 717 Fig.govS sponsored projects at the California Institute of Technology to design new surfactant systems to make surfactant EOR process commercially attractive.26. conditions for microbial metabolism are supported via injection of nutrients. Success of in situ MEOR operations depends on developing microbial consortia that can survive and produce the desired metabolic products in reservoirs containing hydrocarbons and saline water [5.7–10. In selective plugging approaches.16.14–16. temperature.41.7. sophorolipid.7. the biosurfactants. lichenysin and lipid polysaccharide complexes like emulsan have also been found very effective. Surfactin. Role of biosurfactants Surfactant EOR represents one of the most promising advanced methods to recover a substantial proportion of the residual oil [5. Usually. Surfactants thus contribute positively to improve oil recovery by reducing IFT and also by altering the wettability of reservoir rock for water-flood to displace more oil from the capillary network.26.45]. production and application aspects of rhamnolipid. The concentration and purification parameters of surfactin when recovered by ultrafiltration method in one step after broth clarification have been characterized and reported recently [49]. either already present in or introduced into the formation to multiply using the oil as the main carbon source [25]. Another project sponsored by DOE was to employ advanced biotechnology methods to enhance biosurfactant production from selected bacterial strains acclimatized to reservoir conditions.22]. As biosurfactants play a major role in MEOR [5. 3.13–16. There are two major ways in which microbes may contribute to EOR: (a) they grow in reservoir rock to produce gases.14]. barophiles and thermophiles for better adaptation to reservoir conditions [13–16.32. In this technology.com/titanprocess1. A bioprocess was developed and optimized for the enhanced surfactin production in three stages. has been produced ex situ in the controlled environment of a fermenter and has been reported to be successfully used in MEOR [35]. 4). In a MEOR process. an aqueous surfactant formulation is injected into a mature oil reservoir. phosphates and an electron acceptor such as nitrate into the formation and allowing anaerobic bacteria.11.26]. It is of utmost importance to the petroleum industry that these microbial products should cause a series of very desirable changes in the physicochemical properties of the crude. and a marked improvement or a near-complete restoration of the lithological properties of the reservoir rock [3–8. in situ production and application of biosurfactants in a limestone reservoir has recently been reported [95]. a lipopeptide and the most potent microbial surfactant known so far. are injected with other chemicals in the water-flood to facilitate the emulsification of oil with the water jet for better recovery. it dramatically reduces the interfacial tension (IFT) and mobilizes this trapped oil by increasing the capillary number [7–11. Biosurfactants with potential applications in MEOR are listed in Table 2. non-pathogenic and are naturally occurring in petroleum reservoirs [3–5. The mechanics of MEOR at a molecular level must be thoroughly understood to assess both the efficiency and the economic viability of the process. doe. These strains alone can be used when oil viscosity reduction is not the primary aim of the operation. In other processes. However.10]. But a consortium of both types of bacteria would be more useful and preferred. rates of agitation and aeration [47] and optimization of the inoculum age and size for judicious dosing of seed culture [48]. optimization of the environmental parameters including pH.html).11–17. microbial cell mass and/or biopolymers plug high-permeability zones and lead to a redirection of the water-flood [27–30]. glycolipid and lipopeptide biosurfactants assume great industrial significance and thus have been extensively reviewed in literature [42–45]. The Department of Energy /http://www. (b) flooded nutrients stimulate indigenous and injected microbes to produce biosurfactants for emulsification (adapted from http://www.7–11. 4.5.1. Research activities are continuously focusing on anaerobic extremophiles including halophiles. .1. complex and may involve multiple biochemical process steps [1–8.31–41]. The core floods conducted in Berea sandstones using Yates stocktank oil and synthetic brine showed the ability of these surfactants to improve wettability for enhanced oil recovery [9. Where this solution contacts the small blobs of oil trapped in the pores of the reservoir rock. and (b) they can selectively plug high-permeability channels so that the sweep efficiency of the recovery process can be increased (Fig. A patented method involves injecting water containing a source of vitamins.31–34]. biosurfactants are produced in situ which leads to increased mobilization of residual oil and reduces oil viscosity [1.8. In situ MEOR processes (a) the thief zones are the targets for plugging by biomass or biopolymers. namely. which are produced in fermenters ex situ. In some processes.92–94]. Formation of an oil-in-water emulsion often leads to an improvement in the effective mobility ratio until the surfactant is diluted or otherwise lost due to adsorption on the rock. The microbes in MEOR are mostly hydrocarbon-utilizing.22–26]. biopolymers and other non-toxic biochemical to recover trapped oil. optimization of the nutritional factors in the production medium [46]. Some reservoirs also require inorganic nutrients as substrates for cellular growth or for serving as alternative electron acceptors in place of oxygen.26].22.titanoilrecovery.41.ARTICLE IN PRESS R. Though glycolipid biosurfactants produced by Pseudomonas strains have been extensively used in MEOR experiments.26.netl. lipopeptides like surfactin. injected with water-flood. this involves injecting a fermentable carbohydrate including molasses into the reservoir [11]. biosurfactants.11–16.3. It is important to note that various Bacillus strains can produce the MEOR agents when grown on glucose mineral salts medium. AB-2 Bacillus subtilis MTCC 2423 Bacillus subtilis MTCC 1427 Serratia marcescens Arthrobacter protophormiae Bacillus subtilis DSM 3256 Emulsification index (E24) 80–90 90 33.58. The results were compared with the literature values. The polysaccharides secreted by many strains of bacteria serve mainly to protect the bacteria against desiccation and predation. The emulsification index is thus defined as the height of the emulsion layer. [38] Arino et al. biopolymers are perhaps more efficient than the bacterial bodies. [62] Akit et al.30. Its physical properties of viscosity.27. and ferric chloride solutions to facilitate the thickening process [56. Selective plugging was initially thought to have occurred primarily due to sieving action which is best achieved when the average pore throat radius is less than twice the diameter of the bacteria [71]. Biomass growth in those laminae plugs the pore throats. The efficacy of this biopolymer as an oil recovery agent has been studied by injecting an aqueous mixture of the polysaccharide (xanthan gum) and sodium chloride in a model reservoir in the laboratory [57. As plugging and mobility control agents. or plug the media to a lesser extent than do similar strains that produce an exopolysaccharide layer.32. Application of these processes in field trials has made the feeding program effective in slowing the deterioration of hydrocarbon in wells undergoing water-flooding operations by diverting the water jet from high-permeability zones to oil-rich zones [1–8.15.34.26.7.27.5. The emulsification index (E24) was determined by adding 6 ml crude petroleum to 4 ml culture broth in a graduated tube.85–87]. Bacteria and/or nutrients preferentially enter the reservoir along high-permeability pathways. [63] Buller and Vossoughi [64] Kim and Fogler [65] Sandford [66] Source: Acinetobacter sp. biopolymers plug high-permeability thief zones to redirect the water-flood to oil-rich channels. In order to show the efficacy of some of the biosurfactant producing strains.11–16. Xanthan gum (Xanthomonous sp.30]. Xanthomonas species has been used to ferment carbohydrates and produce a thermally stable heteropolysaccharide called xanthan gum. for permeability modification [3. [33] Yakimov et al. This is an adjunct to water-flooding operations.45.) Levan (Bacillus sp. Table 4 Exopolysaccharide biopolymers used in EOR with their microbial sources [57–66] Biopolymer and microbial sp.11–16.5–6. [40] Tango and Islam [41] Table 3 Results of laboratory scale simulation experiments are given Microbial strain Bacillus sp.6 after repeatedly washing with distilled water.21. Emulsan Alasan Source: Pseudomonas Rhamnolipid Source: Rhodococcus sp.13–15.21. Though xanthan gum is an ex situ product of fermentation of carbohydrates.64–69]. In selective plugging approaches.5.30. [35] Banat [26] Jenneman et al.) Dextran (Leuconostoc sp.27.57–66]. using the biopolymer with other chemicals or adsorption agents to prevent adsorption of the polysaccharide on to rock surfaces. in which water is pumped into injection wells in the reservoir in order to force the oil up to the surface bypassing the oil-depleted zones in the reservoir [23.) Pullulan (Aureobasidium sp.27. 3. Surfactin Rhamnolipid Lichenysin Reference Schaller et al.18. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 Experimental studies have been performed to assess the potential of biosurfactants for their applications as MEOR agents [22.20–24. thus decreasing the permeability in what had once been the high-permeability zones.21]. Dead bacteria or non-slime producing bacteria either do not plug porous media.23.) Curdlan (Alcaligeness sp.7.7.71].3 94 60 57–63 % Oil recovered from sand packed column 90–100 62 56 82 90 87–94 References Banat [50] Makkar and Cameotra [51] Makkar and Cameotra [52] Pruthi and Cameotra [53] Pruthi and Cameotra [54] Sen (unpublished data) [55] The glass column that was used for surfactin from B. The emulsion stability was determined after 24 h.50–54]. Role of biopolymers and biofilms Biopolymers have been used in MEOR experiments mainly for selective plugging of oil-depleted zones and hence. [31] McInerney et al. The effects of biopolymers on fluid conductivity in sand columns and sandstone permeability have been well studied in laboratory trials [17. using multivalent cations to bind the polysaccharide molecules together for preferential partial plugging and applying with gel particles. The packing was subsequently dried in an oven at 60 1C overnight before being used for the oil recovery experiment. multiplied by 100. The use of bacterially produced polysaccharides as floodwater thickening agents is well known.68. [39] Neu et al. subtilis DSM 3256 was 36  12. followed by vortexing at high speed for 2 min.18. the results of laboratory scale simulated experiments on emulsification index and percentage of oil recovered from sand packed columns are presented in Table 3. Crude surfactin (approximately 1 g/L in broth) was used in this study.5 cm in dimension and was packed with 100 g acid washed sand.ARTICLE IN PRESS 718 R. Viscosin Trehaloselipids Rubinovitz et al.7.57–66]. This tends to equalize the permeability across the reservoir Table 2 Various potential microbial surfactants for application in MEOR Biosurfactant and microbial source Source: Bacillus sp. as well as to assist in adhesion to surfaces [3.) Reference Pollock and Thorne [59] Cho et al. Xanthan gum has been used in water-flooding systems. like hydrolyzed polyacrylamides.30.30.31.85].71–72. [34] Horowitz and Griffin [36] and restore the sweep efficiency of water-flooding operations [5.11. the use of it comes under chemical enhanced oil recovery.11. .69].65. divided by the total height.64–69.2.1.) Scleroglucan (Sclerotium sp.23. Processes based on the uses of xanthan gum in EOR involve thickening the injection waters. The pH of the column packing was adjusted to 6.32. Bacterial plugging usually involves the feeding of injected or in situ bacteria within the reservoir [3. [37] Navon-venezia et al. Some biopolymers that have potential applications as mobility control agents in MEOR are listed in Table 4. the related financial burden and the threat to health and safety of the operators. Enterobacter. Though xanthan gum has been the most effective biopolymer.71. a pressure less than 500 atm and a salt concentration of 4% [63]. Such gases produced in situ can contribute to pressure build-up in a pressure-depleted reservoir.106. which jeopardizes the economical calculations. Bacteria can ferment carbohydrates to produce gases such as CH4. Another biopolymer.71. But starved SRB are less affected by standard biocide treatments than actively growing populations [3. In this dormant state. There are reports on the effects of hydrocarbon utilizing SRB on corrosion of metallic parts of the drilling pipes in injection and production wells and also on nitrate-mediated microbial control of reservoir souring on the efficiency of MEOR [74.75]. The control of iron and H2S has been tested in an oil field by employing BCX process [77]. butanol. Use of gases and solvents as MEOR agents The use of bacterial consortia to produce gases. Klebsiella) Propionic and butyric acids (Clostridium.pdfS exploits this potential and targets indigenous de-nitrifying bacteria (DNB) and manipulates the entire reservoir microflora to release trapped oil for commercial production. corrosion caused by H2S. applications of other biopolymers like curdlan. dextran and pullulan in MEOR have also been reported [1. called bioweb at a certain stage of growth [72]. acids and gases with their microbial sources Acetone. hot and highpressured reservoirs has been confirmed from souring of the formations. Extensive research has been carried out in understanding the biofilm growth in porous media to harness indigenous microorganisms that promote increased oil recovery from depleted oil reservoirs at low cost. Importance of mathematical modeling in MEOR—a qualitative analysis Structured mathematical models are required to describe the MEOR processes in a better way. This oil-mobilizing and sulfide-removing microbial system was first introduced to industry as Max-Well 2000.org/res/dl/rest0302p. The dynamics of microbial control of H2S production in oil reservoirs has been elucidated [76]. The simplified models were developed based on fundamental conservation laws along with growth. Additional benefits of this bacteria-induced fermentation process include the production of acids. are regularly injected as slug doses in an intermittent treatment program to kill SRB. anaerobic bacteria. with the starting of seawater injection in previously so-called sweet fields is largely due to SRB activities [11. Clostridium acetobutylicum.106] shear resistance.78]. play a very negative role in MEOR [11. On the other hand. In order to develop a proper field strategy. microbial physiology.15. formulation of an efficient reservoir simulator capable of predicting bacterial growth and transport through porous network and the in situ production and action of the metabolites called MEOR agents is of paramount importance. it is more expensive and more susceptible to bacterial degradation [67]. such as acetic and propionic acids and the production of solvents.14. 3.71] [57. Methanobacterium) Operating mechanisms References Methanobacterium have been the microbes of choice for their availability as indigenous natural reservoir microflora. biofilms are heterogeneous systems of bacteria. is produced by a species of the fungus Sclerotium and was examined for possible use in EOR [66.68. their exopolysaccharides and water channels.11. These two phenomena are influenced by such parameters as water chemistry. the morphology may assume a web like structure. plugging by iron sulfide. ethanol.70]. Studies on biofilms indicated that they were composed of less than 27% bacterial bodies and the remainder (73–98%) was assumed to be composed of extracellular products. reduction of IFT and oil viscosity [12.11. propan-2-diol (Clostridium. a pH between 6 and 9. Sulfate-reducing bacteria—Notorious villains in MEOR The sulfate-reducing bacteria (SRB).64–66]. However. Several studies have been reported on mathematical modeling of biomass growth. Enterobacter cloacae and 5.68.ARTICLE IN PRESS R.12. Biopolymers may occur in several different morphologies forming biofilms within porous media like oil reservoirs. mixed acidogens) Methane and hydrogen (Clostridium.66. SRB are traditionally known to be active in shallow wells.71] 4. pH. retention kinetics of biomass and biomass concentration in aqueous and oil . which has proven its efficacy in the real time field tests involving the injection of low-cost nutrients [73].14. Chemicals.3–5. nutrients and fluid flow [3.3.85]. Within the porous media.71]. such as biocides. such as acetone. Enterobacter. Improved permeability by carbonate rock dissolution and oil viscosity reduction Enhancement of permeability and degree of emulsification Rock re-pressurization. probably exopolysaccharides and void space [69. Both gases and solvents can dissolve the carbonate rock. The microbial consortia and the operating mechanisms are enlisted in Table 5. as well as inhibit the sulfide production by SRB almost permanently [89]. 1-butanol and butanone. acids and gases with their producing microorganisms and MEOR mechanisms Solvents. selective plugging and bacterial transport in porous media [79–83].78]. The major concerns of the global oil industry include souring.1. temperature and salt tolerance make it almost an ideal polymer for use in EOR [59–61].57. but the existence of significant SRB populations in deep. oil swelling. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 719 Table 5 Solvents.ptac. The continuous biogenic sulfide contamination of huge rock formations and production facilities has belied the theory that microorganisms cannot significantly influence the micro-environment of the oil reservoirs [73].5. The SRB are extremely sturdy and can survive prolonged starvation in seawater at both surface and reservoir temperatures. CO2 and H2. The so-called Bio-Competitive Exclusion (BCX) technology /http://www. It is considered superior to polyacrylamide for the above-stated reasons. solvents and acids for enhancing oil recovery by exploiting the mechanisms of reservoir re-pressurization and carbonate rock dissolution has been an old practice [1.68.71. Biopolymer levan was found to be potentially suitable in oil reservoirs which have a temperature of less than 55 1C. thereby increasing its permeability and porosity.114].78]. Biopolymer production and consequent biofilm formation are both important for MEOR operations to be effective.62. [15. which have relatively simple growth requirements in sulfate and carbon as energy sources. surface charge.68.72. The association of the onset of reservoir souring. scleroglucan. Zymomonas. These gases may dissolve in the crude oil and reduce its viscosity. starved SRB tend to be smaller in size than their growing counterparts and may travel longer distances through the porous network during waterflooding. success or failure of field applications of microbial technology is often hindered by the absence of specific and quantitative understanding of microbial activity. Field trials to determine and document the effectiveness of microbial processes and to assess the validity of laboratory studies and models have been conducted. A compilation of very relevant information and data of field trials conducted in the USA and Romania is presented in Table 6. The simulation results showed that the biofilm models based on Bingham yield stress predicted the biomass accumulation and channel breakthrough reasonably well [81]. optimum slug size and the application time constitute the major governing factors in MEOR. and nutrient consumption during growth and metabolism and also to estimate permeability reduction. From an order of magnitude analysis. A biosurfactant flooding process using a very low concentration (35–41 ppm) of biosurfactant produced by Bacillus mojavensis strain JF-2 has been recently reported to be very effective in recovering about 35–45% residual oil from Berea sandstone cores [91]. The field assessment of a MEOR technology applied in the Alton field in Australia showed an approximate 40% increase in net oil production.89. Conversely. explaining success or failure of field applications of microbial technology is often hindered by the absence of specific and quantitative understanding of microbial activity. In addition to the bottom-hole conditions and the nature of the formations. Literature reports discussing in situ applications of MEOR in field trials with analysis of the results are available [17. porosity. Some of the relevant scaling up criteria and numerical simulation results were discussed in order to explain the difficulties of scaling up laboratory results before planning a field application [83]. Microbial enhanced water-flooding technology has also been shown to be an economically feasible technology in the United States [16]. one-dimension model was developed to simulate microbial growth and transport. multiple-species. A reservoir engineering perspective. 5. a field study involving in situ production and application of biosurfactants by a consortium of Bacillus strains demonstrated that approximately nine times the minimum concentration of biosurfactant required to mobilize oil was produced in situ and resulted . The model was validated by applying to static (sand packs) and core-flooding (sandstone cores) experiments to describe microbial movement.18. reservoir characteristics and operating conditions [87]. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 phases in order to predict porosity reduction as a function of distance and time or based on a filtration model to express bacterial transport as a function of pore entrance size and also to relate permeability with the rate of bacteria penetration by applying Darcy’s law [80–82]. In most of the cases. An analysis of the data from a data base of information collected from 322 projects employing the same MEOR methods resulted in the evaluation of the technical effectiveness and process economics of this particular technology and provides a source of information useful for predicting treatment response in any given reservoir [89]. The basic equations governing the transport of oil. there are differences in the extent of improvement in oil recovery. This is the major reason behind the formulation of mathematical models of MEOR process based on these two mechanisms. Convection dispersion equations and microbial kinetics were incorporated in the model system to characterize and quantify biomass production. An unfortunate consequence is that the relevance of microbial performance to reservoir engineering design is often obscure. permeability. crude oil gravity and the drive types. focusing on issues such as scale-up of laboratory results. their adaptation time. etc. The oil reservoirs worldwide represent very complicated biological systems for which laboratory simulations of microbial activities become very challenging. it is known that only biosurfactant flooding and selective plugging mechanisms have the potential to recover more oil in MEOR process. required product concentrations. in the reservoir these are likely to be out-competed by the better adapted indigenous species. The application of MEOR in these trials has resulted in a substantial and sustained increase in production compared to control operations on the same reservoir [88]. The application of MEOR in these trials has resulted in a substantial and sustained increase in production compared to control operations on the same reservoir. biosurfactant production rates and biosurfactant yields [95]. product formation. But a more comprehensive approach based on artificial neural network modeling would be more useful to develop suitable models for describing the in situ MEOR processes. Very recently. however.98]. which is influenced by various factors including individual reservoir characteristics like lithology. southeast Turkey) were utilized in a mathematical model that describes the transport of bacteria and its nutrients by convective and dispersive forces.ARTICLE IN PRESS 720 R. While it may be possible to show beneficial effects in laboratory conditions with tailored microbial cultures. bacterial composition and concentration of the inocula. and metabolism processes involved in MEOR and to predict permeability modification that results from these microbial activities in porous media. Efforts to explain these differences are severely limited by the lack of quantitative measures of microbial performance in terms of the reaction rates. A recent study that generated essential data for modeling in situ MEOR processes included specific growth rates. A pictorial presentation of the state of the oil droplets in a porous sandstone rock before and after the MEOR process is illustrated in Fig. nature of the sands. and field implementation and operation provides a consistent framework for comparing MEOR with other EOR processes [87]. nutrient and metabolites in MEOR are based on component mass balance and overall mass conservation. process design.16]. In the last decade of the last millennium.92–95. a number of MEOR field projects have been conducted in various parts of the world with varying degrees of success. This could be extended to provide numerical predictions for the purposes of design and evaluation of MEOR field projects [82]. which sustained for considerable period of time [88]. stoichiometry. In an MEOR field study in the Southeast Vassar Vertz Sand Unit salt-containing reservoir in Oklahoma. This justifies the rationale behind the pilot scale field trials that are undertaken after successful development of MEOR processes through R&D in laboratory settings. The biomass-plug propagation and channel breakthrough using Bingham yield stress of biofilm were described well by a biofilm removal model. and nutrient utilization in the MEOR process [82]. to demonstrate quantitative relationships between microbial performance. bacteria. thereby decreasing the effective permeability by 33% [90]. carbon balances. metabolite production. nutrient injection stimulated growth of the microbial populations. Deliberations on information and papers presented by companies and researchers all over the world on field applications using microbes and also the details of MEOR projects have been in public domain [11. It is possible. reservoir temperature. 6. The experimental conditions of the MEOR technique applied for Garzan oil (261 API. Obviously. which represents the stability of biofilm against shear stress. The microbial consortia that are introduced into an oil field would have to compete with the indigenous microflora. 12 months after treatment. A three-phase. Field trials Microbial enhanced oil recovery technologies have progressed from laboratory-based studies in the early 1980s to field applications in the 1990s. water. including bacterial decay and growth [84]. After MEOR process: microbes surround the oil droplets. Before MEOR process: oil is trapped in porous sandstone. Table 6 Details of some field trials showing the efficacy of MEOR methods (adapted from Bass and Lappin-Scott [11]) Field test details Year of initiation Oil field Formation Depth (m) Permeability (md) Salinity (%) Oil viscosity (cp) Injection wells Production wells Nutrients used Microorganism(s) MEOR-Agents Water flood Test length Pre-MEOR production Post-MEOR production Comments Phoenix Pilot Project.html).3 9 1 treated Information not available Molasses Bacillus. and were considered profitable and economically feasible of further expansion [97]. USA [95]. Pontotoc City. Arthrobacter. respectively. Micrococcus Biosurfactants.9 6 19 47 Cane molasses Bacillus.4 B/D per well Cyclic microbial recovery. NIPER 1990 Chelsea-Alluwe Bartlesville sandstone 122 16 2.5% and 46. Pseudomonas. Clostridium. in the recovery of substantial amount of oil entrapped in the limestone reservoir of the Bebee field.2 B/D per well 20% increment in oil production SE Vasser Vez Sand Pilot Project. acids and gases Yes 1. termed as microbial permeability profile modification (MPPM) technology. Clostridium Biosurfactans. USA. A new MEOR technology. OU: Oklahoma University. Water flows without dislodging oil droplets from tiny pore spaces in the rock. well-bore clean up NIPER: National Institute of Petroleum and Energy Research. acids. The technical and economical feasibility studies of MEOR were carried out in mature water-floods in continuation of a pilot study of controlled microbial colonization in producer wells in La Ventana oil field of Argentina [96].9 5 treated 19 Molasses.com/titanprocess1.ARTICLE IN PRESS R. causing them to dislodge from the microscopic pore spaces between the tiny sandstone and carbonate rock particles in the reservoir for enhanced recovery (adapted with permission from http://www. NH4NO3 Indigenous microflora Biomass and gases Yes Approximately 9 months No oil production before injection 83 barrels produced (January 92–June 92) Decreased permeability Institute of Biology. 5. Oklahoma.06–0. has been . Romanian Academy 1990 Bragadiru Information not available 780 150–300 0. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 721 Fig. OU 1991 SE Vasser Vertz sand unit Vertz sandstone 550 60–181 11–19 2.5 Years 1 B/D per well 1.5 B/D per well 7. biopolymers.5% increments in oil production.titanoilrecovery. The MEOR pilot projects carried out in Providencia and Lobitos oil fields in Peru resulted in 36. gases and solvents Yes 4 Years 1. which was carried out for about 35–40 days in 1997. In spite of microbial processes holding great promise and prospect for EOR. the Oil and Natural Gas Corporation (ONGC) Limited. acclimatized in simulated oil reservoir conditions. This MPPM technology has been claimed to have extended the economic life of the field called North Blowhorn Creek Oil Unit (NBCU) located in Alabama by 60–137 months. Pseudomonas. had great potential for application of MEOR processes by activating the stratal microflora [109]. Rhodococcus. A laboratory study involving growth of some selected strains in the crude oil samples and consequent reduction of oil viscosity indicated that the results could be translated into an effective strategy for recovering residual oil from Daqing oil field. have been reported [92. conducted some field trials by employing a Huff and Puff process and using an indigenously developed MEOR technology based on a consortium of anaerobic extremophiles isolated from the candidate reservoirs. uncertainty about meeting the engineering design criteria by microbial processes and a general apprehension about processes involving live bacteria. mainly on . increase production rates and technical efficacy of the recovery systems. In Malaysia. in collaboration with The Energy and Resources Institute (TERI. for example in Chinese. New Delhi) and the Institute of Reservoir Studies (IRS). a study to assess the potential applicability of MEOR techniques in the Kongdian reservoirs of the Dagang oilfield in China showed that the oil field was inhabited mostly by anaerobic thermophilic fermentative. the biggest oil field in China [111]. The recovery of oil could be doubled after 6-month MEOR treatment from 14 production wells by injecting a bacterial consortium containing Enterobacter cloacae and Bacillus licheniformis as major bacterial strains and a nutrient package mainly comprising of molasses (10%) and cellulase enzyme (0. There is one structured report on MEOR field trial in the Fuyu oil field.108]. coupled with the bioaugmentation and stimulation of indigenous microflora have been successful in enhancing both the oil productivity and ultimate oil recovery factor.ARTICLE IN PRESS 722 R. Conclusions MEOR represents a truly eco-friendly petroleum recovery process employing biotechnological resources and techniques that can be used to replace and augment the traditional EOR processes and flooding chemicals. with a cumulative production of 4. with an expected recovery of (4–6)  105 bbl of additional oil. Malaysian and Indian oil fields. A three-fold increase in oil production and a significant reduction in water cut were achieved by applying this technology in 9 wells out of 12 wells treated in 4 oil fields. Many countries have envisaged that one-third of their oil recovery programs will utilize MEOR techniques by the year 2010.1 BSTB (billion stock tank barrels). biochemical and physiological characteristics of microbial consortia. Economically attractive alternative EOR technologies including microbial EOR. the reasons that retard the implementation of MEOR include inconsistency in in situ performance. an urge and urgency in developing novel MEOR technologies in the wake of an impending energy crisis seem to be imminent. Jilin province.. 7. In India. sulfate-reducing and methanogenic bacteria and hence. low ultimate oil recovery factor. which showed significant increment in crude oil recovery and prolongation of the cycle of oil well washing to increase total oil production [112]. a US patent has been granted on the same process and microbial consortium. Most of the MEOR work leading to field trials has been completed in the last two decades and now the technology has advanced from a laboratory-based evaluation of microbial processes. Another laboratory study using a model reservoir simulating the conditions of Daqing oil field and a pilot study in some wells of the same oil field employed the metabolic products (PIMP) from a strain of Pseudomonas aeruginosa. Advanced MEOR technologies based on the use of genetically engineered organisms and some specific enzymes may prove to be very effective and successful in the future. In situ application of a judicious consortium of aerobic and anaerobic bacteria. Though research initiatives and endeavors in MEOR have progressed rapidly. Microbiological and chemical characteristics of the production fluids of the high-temperature Liaohe oil field in China were examined to investigate the potential for MEOR [107. It was concluded that the Liaohe oil field contained a diverse thermophilic microbial community having a high biotechnological potential for petroleum recovery [107]. etc. Clavibacter. to field applications internationally. Nevertheless. Applications of MEOR processes in the oil fields in Asian countries. Ahmedabad. However. Sen / Progress in Energy and Combustion Science 34 (2008) 714–724 reported to improve oil recovery by adding nitrogenous and phosphorus-containing nutrients to the injection water of a conventional water-flooding operation [99]. the estimated oil-in-place from the 47 producing fields stands at about 20.9 BSTB till 2003 [105]. which was developed in TERI in collaboration with IRS and was evaluated in field trials by ONGC [114]. a better understanding of the MEOR processes and mechanisms from an engineering standpoint based on economics. thus improving the process efficiency. applicability and performance is the key to further improve the process efficiency for writing more success stories. with the exception of an MEOR stimulation project in Bokor field [105]. It is now known that manipulation of microbial consortia activity is a potentially powerful process that can profoundly and beneficially affect the microenvironment of a reservoir to mobilize more oil. Though it has been repeatedly encountered that the benefits of MEOR projects applied to one well have positively affected the recovery performance of neighboring wells. better understanding of the reservoir characteristics. mostly in the state of Gujarat [113]. In an exploratory study with the formation water from Daqing oil field.01%) [106]. Recently. Comprehensive R&D studies and annual reports on the development of microbial strains with improved transport and biosurfactant activity for EOR and on the biosurfactant-mediated oil recovery in model porous systems with its computer-aided simulations have tremendously contributed to the design and development of effective MEOR strategies and at the same time have enormously enriched the scientific literature on MEOR [102–104]. was found to have increased production performance and decreased water cut in 17 wells out of 25 wells treated. have been discussed and numerical simulation results have been presented to demonstrate the difficulties in scaling up and translating the laboratory results to real-time oil field settings [101]. though published literature does not have field trial data in an organized format as shown in Table 6. China. still some questions remain unanswered. which were capable of producing effective oil-releasing agents including biosurfactants [110].105–110]. The trial was carried out in 2001 in the sandstone formation with an average permeability of 180 md at a depth of 300–500 m and an oil viscosity of 6–8 cp. However. This bacterium produces spores that can propagate easily in Berea cores with permeability 4500 md and can reduce the rock permeability by producing biofilm on germination under suitable nutritional and environmental conditions [100]. Recently. there were no published reports on full-fledged field application of MEOR in Malaysia. Dietzia. Another microbial profile modification method employing a spore-bearing halotolerant mesophilic bacterium has been reported. China indicated that the oil field was found to be inhabited by aerobic bacteria like Bacillus. The MEOR squeeze treatment. controlling mechanisms and process economics are essential before MEOR becomes a viable process for general field applications. [33] McInerney MJ. 1990. USA: ASM Press.3:229–36. [41] Tango MSA. [56] Sheehy A. 10789. 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