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  1. MEOR APPLICATIONS—1:Microbes enhance oil recovery through various mechanisms

    Microbial enhanced oil recovery (MEOR) methods rely on microorganisms and their metabolic products to mobilize residual oil in several different ways. MEOR mechanisms include interfacial tension reduction, selective plugging, gas production, biodegradation, and wettability alteration. The process is environmentally friendly and easy to operate. This first of a two-part series summarizes the mechanisms, while the concluding part will discuss 10 field cases involving 221 producing wells in Malaysia, US, Argentina, and China. Enhanced oil production Oil and gas exist in porous rocks at depths from several hundred to several thousand meters. In the early life of an oil field, the reservoir pressure is high and oil and gas flow to the wellbore naturally. When reservoir pressure declines, field operators often inject water or gas into reservoirs to maintain pressure and sweep oil and gas to wellbores. Even after secondary recovery with water or gas flooding, the reservoir rocks because of capillary forces hold large amounts of residual oil. One estimate is that more than 50% of original oil remains underground at field abandonment. Operators employ tertiary recovery or enhanced oil recovery methods to produce residual oil. Most common EOR methods include surfactant flooding, polymer flooding, CO 2 flooding, and thermal recovery. MEOR is different from traditional EOR methods. The method injects live microorganisms and nutrients into the reservoir so that bacteria and their metabolic products mobilize the residual oil. If favorable bacteria already reside in the reservoirs, it is feasible to inject nutrients only. MEOR methods have many distinguishable advantages. It is environmentally friendly because it does not involve toxic chemicals. It is easy to carry out in the field because it does not require modification of existing water-injection facilities. MEOR is not a new concept, but field applications have become more common in the past 10 years. Certain bacteria can produce surfactants, polymers, gases, and solvents that contribute to mobilizing residual oil in a reservoir. IFT reduction Certain bacteria produce biosurfactants that reduce oil-water interfacial tension (IFT). Capillary pressure, which is proportional to the IFT between oil and water, holds the residual oil in porous rocks. Residual oil starts to flow with the reduction of IFT to a lower value. Table 1 lists published values for IFT with several biosurfactants. 1-3 The IFT between hydrocarbon and water is typically about 30-40 mN/m (milli-Newton/m). The biosurfactants must reduce IFT at least to below 0.4 mN/m to have any effect on oil recovery. Table 1 lists such biosurfactants. Most IFT measurements with existing biosurfactants, however, are above 1 mN/m. 4 Moreover, laboratory measurements, such as the spinning drop method, require high concentrations of biosurfactant. The expected concentration of biosurfactants in real reservoirs is lower because of dilution. As such, this may limit in practice the effectiveness of IFT reduction. Selective plugging A porous rock contains pores of various sizes. When undergoing waterflooding, larger pores receive most of the injected water, while residual oil remains in unswept small pores. When bacteria flow in reservoir rocks, they also tend to enter large pores. Certain bacteria can generate biopolymers that plug the high-permeability zones with large pores, thus forcing injected water to sweep the oil in low-permeability zones. One study injected pseudomonas aeruginosa strain into glass-bead packs and Berea sandstone cores to investigate the permeability reduction by bacteria and their metabolic products. 5 For the three bead packs with high permeability about 1,400 md, the permeability reductions ranged from 20% to 54% of the original permeabilities. For the cores with low permeabilities about 13 md, the reduction ranged from 45% to 72%. Another test injected bacteria solution into sandstone core and reduced by 80% the observed permeability. 6 Most MEOR laboratory tests have been in sandstone cores. Very few experiments involved fractured system. One investigation etched fractures inside a glass to imitate a fractured rock and then injected leuconostoc mesenteroides and bacillus subtilis into this micromodel to investigate their effects in MEOR. 7 Leuconostoc mesenteroides is a biopolymer producer, while bacillus subtilis produces biosurfactant. The results showed that bacillus subtilis improved recovery in a fractured system, while leuconostoc mesenteroides was not as effective. This indicates that reduction in IFT is more effective than selective plugging in enhancing recovery for a fractured system. Viscosity reduction Certain bacteria produce gas and solvents in the reservoir, such as CO 2 . Gas and solvents can dissolve in crude oil and reduce crude oil viscosity, leading to an improved mobility ratio and oil recovery. The produced gas can also increase reservoir pressure, which leads to higher producing rates. One test involved the mixing of clostridium acetobutylicum with crude oil in a sealed cell. 8 The observed pressure in the cell started to rise because the culture generated CO 2 . The measured crude oil viscosity after shut-in tests indicated that the culture reduced crude viscosity to less than 50 cp from about 80 cp before the test. Clostridium acetobutylicum also effectively improved oil recovery in the flooding test. The bacteria's ability to produce gas, however, is limited as tested in laboratory. It is unlikely that bacteria can generate large quantities of gas in underground reservoirs. Biodegradation Certain bacteria can degrade crude oil, especially the paraffin contents in crude oil. When applied to reservoir, bacteria can remove the paraffin deposit in the near wellbore region, thus improving permeability and production rate. 9 Wettability alteration Rock wettability greatly influences the distribution of residual oil. In water-wet sandstones, water is in contact with sand grains, and oil droplets are in the center of the pore space. On the other hand, for oil-wet rocks, oil is in contact with grain surfaces and remains in the small pores. In other words, water wettability is better for oil recovery. The research on bacteria-induced wettability change is limited. One study treated Berea sandstone cores with rhodococcus sp. 094 solution. 10 The study used the Amott method 11 to evaluate the rock wettability before and after microbial treatments. The study showed that after injection of bacteria solution the originally water-wet cores became more water wet, while mixed-wet cores had insignificant changes in wettability. Another test measured the contact angle of water-wet limestone cores submerged in bacillus solution. 12 Observed was a reduction in contact angle, indicating wettability alteration towards water wettability. In practice, the concentration of bacteria in a reservoir is low. Therefore, MEOR is unlikely to alter significantly rock wettability. Bacteria delivery Reference 13 provides a more detailed description of MEOR mechanisms than in the previous paragraphs and Table 2 summarizes some bacteria for MEOR projects. Because reservoirs contain very little oxygen, these bacteria have to function in an anaerobic environment. Operators can inject bacteria into the reservoir through the tubing and annulus of oil and gas producing wells as well as water injection wells. One can classify most MEOR projects as huff and puff or bacteria flooding. Huff and puff Operators often inject bacteria solution into the reservoir through production tubing in a producing well. They can inject nutrient after or simultaneously with the bacteria. The common nutrient used in practice is molasses, an inexpensive by-product of sugar refining. After the bacteria injection, the well remains shut in for a period, usually from several days to weeks. Some bacteria can produce acid, solvent, or surfactant that helps to eliminate debris in the near-wellbore region. Other bacteria can generate polymers that seal high-permeability channels in porous media. After the operator puts the well back into production, the well may produce at higher rates. The huff-and-puff process repeats the injection and production cycle several times to maximize the gain. Bacteria flooding Operators can also inject bacteria and nutrient into a reservoir from an injector and then continue normal waterflooding operations. The injected water carries the bacteria deep into the reservoir. In many field cases, tests can detect bacteria at remote producers. While being transported inside the reservoir, bacteria can produce surfactants that improve oil recovery. Microbes can also plug the zones with high permeability and force water to sweep the low-permeability zones. In the more common huff-and-puff operations, bacteria only treat the near-wellbore region of producers, while bacteria flooding transports bacteria deep into the reservoir. Feeding existing bacteria In the third scenario, certain bacteria that can enhance oil recovery may exist already in the reservoir but not as the dominant bacteria colony. As such, operators only have to inject nutrition into reservoir to activate the bacteria. This operation is rare compared with bacteria flooding and huff-and-puff operations because the favorable strains may be unable to compete for the nutrition supplied with the other colonies. In some cases, operators may inject the favorable microbe to maximize their chance of dominating the underground environment. References   Kowalewski, E., et al., "Analyzing microbial improved oil recovery processes from core floods," Paper No. IPTC 10924, International Petroleum Technology Conference, Doha, Qatar Nov. 21-23, 2005. Hung, H.C., and Shreve, G.S., "Effect of the hydrocarbon phase on interfacial and thermodynamic properties of two anionic glycolipid biosurfactants in hydrocarbon/water systems," J. Physical Chemistry B, Vol. 105, 2001, pp. 12596-600. Makkar, R.S., and Cameotra, S.S., "Structural characterization of a biosurfactant produced by Bacillus subtilis at 45 degrees C." J. Surf. Deter., Vol. 2, 1999, pp. 367-72. Gray, M.R., et al., "Potential microbial enhanced oil recovery processes: a critical analysis," Paper No. SPE 114676, SPE ATCE, Denver, Sept. 21-24, 2008. Gandler, G.L., et al., "Mechanistic understanding of microbial plugging for improved sweep efficiency," Paper No. SPE 100048, SPE/DOE Symposium on Improved Oil Recovery, Tulsa, Apr. 22-26, 2006. Cheng, J., et al., "Studies on the pilot test with microbial profile modification after polymer flooding in Daqing oil field," Paper No. IPTC 11227, International Petroleum Technology Conference, Dubai, Dec. 3-6 2007. Soudmand-asli, A., et al., "The in situ microbial enhanced oil recovery in fractured porous media," Journal of Petroleum Science and Engineering, Vol. 58, 2007, pp. 161-72. Behlulgil, K., and Mehmetoglu, M.T., "Bacteria for improvement of oil recovery: a laboratory study," Energy Sources, Vol. 24, 202, pp. 413-21. Bailey, S.A., et al., "Microbial enhanced oil recovery: diverse successful applications of biotechnology in the oil field," Paper No. SPE 72129, SPE Asia Pacific Improved Oil Recovery Conference, Kuala Lumpur, Oct. 6-9, 2001. Shabani, M., et al., "The effect of bacterial solution on the wettability index and residual oil saturation in sandstone," 10th International Symposium on Evaluation of Wettability and Its Effect on Oil Recovery, Abu Dhabi, Oct. 26-28. 2008. Amott, E., "Observation relating to the wettability of porous rock," Trans. AIME, Vol. 216, 1959, pp. 156-62. Zekri, A., et al., "Carbonate rocks wettability changes induced by microbial solution." Paper No. SPE 80527, SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, Sept. 9-11, 2003. Sen, R., "Biotechnology in petroleum recovery: the microbial EOR," Progress in energy and combustion science, Vol. 34, 2008, pp. 714-24.   The authors Chang Hong Gao (cnusau@yahoo.com.au) is assistant professor of petroleum engineering at UAE University. His research interests include enhanced oil recovery and formation damage. Gao has a PhD in petroleum engineering from Curtin University of Technology in Perth, Australia. Abdulrazag Zekri is professor of petroleum engineering at UAE University. He has extensive research experiences in enhanced oil recovery. Zekri has a PhD in petroleum engineering from the University of Southern California. Khaled El-Tarabily is associate professor of biology at UAE University. His research focuses on the applications of microbiology in agriculture and petroleum processing. El-Tarabily has a PhD in microbiology from Murdoch University in Perth, Australia.   MEOR literature survey The survey of the literature provided the following points regarding MEOR: MEOR methods active during the past 10 years are environmentally safe, easy to operate, and economical. Among the proposed MEOR mechanisms, field observations indicate that selective plugging may be the main contributor for better recovery. In practice, IFT reduction may be less effective than shown in laboratory tests. For the selected MEOR field cases in the past 10 years, more than 60% of wells treated by bacteria increased oil production rates. Most treated reservoirs had temperatures below 85° C. Moreover, most successful cases were for reservoirs below 55° C. The survey provided no clear relationship between the success of MEOR projects and reservoir permeability. Many field cases indicate that MEOR reduced IFT, crude oil viscosity, and paraffin content, as well as modified the injection profile. MEOR methods are more suitable for low temperature, low production rate, and high water cut wells. A better practice is to inject both bacteria and nutrient into the reservoir. Injection of nutrient only is not very effective because the favorable indigenous bacteria may not compete effectively for the nutrients with other existing bacteria colonies. The current MEOR success rate is not very satisfactory. Developments in biotechnology and petroleum technology will, however, deepen the understanding of MEOR methods to lower project costs and improve success rates. More Oil & Gas Journal Issue Articles

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    Mon, 17 Aug 2009

  2. Thermal heavy-oil recovery projects succeed in Egypt, Syria

    To increase oil recovery from existing fields, operators in both Egypt and Syria have started producing heavy oil with thermal enhanced oil recovery processes.

    Magazine Articles

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    Mon, 22 Dec 2008

  3. QC updates carbon dioxide projects in OGJ's enhanced oil recovery survey

    Every 2 years, Oil & Gas Journal conducts a survey of carbon dioxide enhanced oil recovery activity, including providing detailed field by field reports of performance and oil production.

    Magazine Articles

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    Mon, 2 Jul 2012

  4. SPECIAL REPORT: PDO initiates various enhanced oil recovery approaches

    To sustain long-term oil production from its contract area in Oman, Petroleum Development Oman has turned to diverse methods for enhancing oil recovery from its fields that contain many billions of barrels of oil in place.

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    Mon, 5 Nov 2007

  1. Special Report: EOR/Heavy Oil Survey: CO 2 miscible, steam dominate enhanced oil recovery processes

    Magazine Articles

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    Mon, 19 Apr 2010

  2. OTC: Aramco pursues 70% oil recovery rate

    Aramco wants to improve its oil recovery rate to 70% from 50% over the next 20 years by focusing on EOR and other technologies, said Amin H. Nasser, Aramco senior vice-president, E&P.

    Online Articles

    Online Articles

    Tue, 5 May 2009

  3. Shell’s interest in enhanced oil recovery grows

    As easy oil becomes more difficult to find and produce, Royal Dutch Shell PLC sees enhanced oil recovery (EOR) contributing more to the world’s growing energy needs.

    Magazine Articles

    Magazine Articles

    Mon, 20 Nov 2006

  4. TECHNOLOGY Economics, new technology improve Danish offshore oil recovery

    Aksel Mortensgaard Danish Energy Agency Copenhagen Cost-efficient development concepts and technologies, such as horizontal wells and water injection, have almost tripled the expected ultimate oil recovery from Danish offshore fields. All currently produced Danish oil and gas is from chalk ...

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    Magazine Articles

    Mon, 10 Jun 1996

  5. Shell's interest in enhanced oil recovery grows

    As easy oil becomes more difficult to find and produce, Royal Dutch Shell PLC sees enhanced oil recovery (EOR) contributing more to the world's growing energy needs.

    Online Articles

    Online Articles

    Thu, 28 Sep 2006

  6. Tulsa conference discusses advances in improving oil recovery

    Oil recovery factors for many of the world's oil fields have improved over the years, but implementing new technologies, expanding existing technologies to other reservoirs, and understanding the processes better can improve recovery further. That can be gleaned from many of the papers presented at ...

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    Tue, 16 Apr 2002

  7. Producing Horizontal Wells Horizontal wells prove versatile for improved oil recovery

    Hemanta K. Sarma, Kenji Ono Japan National Oil Corp. Chiba, Japan The diversity of Canadian horizontal well applications illustrates the improved oil recovery potential of this technology. Canada has been a pioneer in IOR with horizontal wells since the late 1970s. Improved oil recovery (IOR) ...

    Magazine Articles

    Magazine Articles

    Mon, 11 Dec 1995

  8. Statoil to try bacteria-driven oil recovery at Norne field

    Statoil AS aims to produce an extra 30 million bbl of oil over the next 15 years from its Norne development in the Norwegian Sea through the pioneering use of a 'bacteria-aided' method of improved oil recovery , it said Thursday. Kjell Arne Jakobsen, staff engineer for petroleum technology on Norne, ...

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    Thu, 8 Feb 2001

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