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

Aug. 17, 2009
Microbial enhanced oil recovery (MEOR) methods rely on microorganisms and their metabolic products to mobilize residual oil in several different ways.

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, CO2 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 CO2. 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 CO2. 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.


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 method11 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.


  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. Behlulgil, K., and Mehmetoglu, M.T., "Bacteria for improvement of oil recovery: a laboratory study," Energy Sources, Vol. 24, 202, pp. 413-21.
  9. 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.
  10. 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.
  11. Amott, E., "Observation relating to the wettability of porous rock," Trans. AIME, Vol. 216, 1959, pp. 156-62.
  12. 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.
  13. 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 ([email protected]) 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.

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