Production Technology

Microbial enhanced oil recovery techniques improve production

Bacteria may be valuable in offering cost-effective and environmentally benign EOR.

Saeid Mokhatab, University of Wyoming, and Leo A. Giangiacomo, Extreme Petroleum Technology, Inc., Casper, Wyoming

A potentially inexpensive method, Microbial Enhanced Oil Recovery (MEOR), may prove useful and economical. Most conventional oil recovery processes are only able to retrieve from 15 to 50% of the available oil in the reservoir. The utilization of this technology can extend the average well life without increasing excessive lifting costs. As MEOR effects chemical changes in the reservoir, it is an environmentally compatible method of carrying out tertiary oil recovery. MEOR will become increasingly economically feasible as genetic engineering develops more effective microbial bacteria that may subsist on inexpensive and abundant nutrients. This article will review how the microbe works to improve oil recovery, evaluating the potential gain in oil production versus treatment cost, and identify applications and potential advancement of MEOR.

TERTIARY RECOVERY

Recovering oil usually requires three stages. Primary recovery typically recovers 10 to 35% of a reservoir’s oil-in-place. Secondary recovery, which most often involves waterflooding, can increase recovery by 20% or more. Enhanced oil recovery (EOR), also called tertiary oil recovery, enables oil producers to extract as much as 30 to 60% of a reservoir’s original oil content. EOR may be accomplished through several different methods. Thermal recovery, chemical flooding, miscible displacement (gas injection) and MEOR all have been explored as tertiary techniques.

MEOR is actually a family of processes that involves the use of microorganisms for enhanced recovery. There are six ways in which microorganisms may contribute to EOR: 1) microorganisms can produce biosurfactants and biopolymers on the surface; 2) microorganisms grow in reservoir rock pore throats to produce gases, surfactants, and other chemicals to recover trapped oil; 3) microorganisms can selectively plug high-permeability channels in reservoir rock, so that sweep efficiency increases; 4) biocracking, where microbes metabolize carbon atoms from the interior of an alkane chain; and 5) biocompetitive exclusion,¹ where a microbial population, such as denitrifying bacteria, is stimulated to outcompete an undesirable population, such as sulphate reducers. Table 1 presents microbial products and their contribution to EOR.

TABLE 1. Microbial products and their contribution to EOR

Although thermal and gas injection methods find the widest commercial applications, MEOR has two distinct advantages: 1) microbes do not consume large amounts of energy, and 2) the use of microbes is not dependent on the price of crude, as compared with other EOR processes.² In some reservoirs, beneficial microbes are indigenous and only need nutrients to stimulate growth. Because microbial growth occurs at exponential rates, it should be possible to produce large amounts of useful products rapidly from inexpensive and/or renewable resources. Thus, MEOR has the potential to be more cost-effective than other EOR processes.

Reported operational costs of MEOR range from $2 to $4 per incremental barrel of oil.³ Studies have shown that several microbially produced biosurfactants compare favorably with chemically synthesized surfactants. The ability to produce effective surfactants at a low price may make it possible to recover substantial amounts of residual oil. Not every technique can be used in every oil reservoir. Most are highly application-specific. But even a few percent of added recovery can be an enormous amount, cumulatively, across a region.

THEORY

Figure 1 illustrates microbes inside an oil drop. To get microbes to grow and multiply fast enough, scientists are testing ways to inject food into a reservoir for the microbes to eat. Some microbes feed on nutrients in a reservoir and release gas as part of their digestive process, creating a gas reservoir or gas cap over oil.

Fig. 1. Microbes inside an oil drop. (Source: U.S Department of Energy, 2006)

Microbes can also be used to block off flow channels within a reservoir. After many years of waterflooding, most of the water eventually finds the easiest path through the oil reservoir, bypassing other parts of the reservoir. To send the water to other parts of the reservoir, microbes and their food are mixed together and injected into the waterflood. By multiplying their numbers, they block off the short-circuiting water pathways, improving water-flood efficiency in other parts of the reservoir.

Most bacteria have a natural tendency to attach to rock surfaces rather than free-float in liquid. In a petroleum reservoir, bacteria may attach to rock, start to grow, and then produce exo-ploymers that help them attach to each other, as well as rock surfaces. Such growth is termed a biofilm and offers the advantages of protection from biocides, while encouraging the bacteria to best use nutrients and other resources, Fig. 2. Bacteria that are introduced to reservoirs through waterflooding will flow over pre-existing biofilms; some bacteria will attach themselves to these biofilms and grow. Occasionally, some bacteria detach from the biofilm and move with the liquid, or by their own motility and colonize other areas deeper in the reservoir.

Fig. 2. Electron micrograph of a biofilm inside rock.42

Fundamentally, the mechanisms that govern oil release due to beneficial microbial growth are very much the same as the proven and demonstrable chemical and physical effects derived from well-known conventional EOR methods and products that are introduced at the surface. However, even though microbial and conventional systems are similar in terms of oil-releasing mechanisms, they differ significantly in other ways.

The unique attributes of microbial systems are not shared by conventional, non-biological systems that rely on massive additions of product at the injection wellhead. Instead, microbial systems function in-situ throughout the aqueous phase of the reservoir, including the water-rock, water-oil interfaces, and can be deliberately manipulated and directed to continually produce at the molecular and pore level, gases, solvents, surfactants and other bio chemicals. These bio-products are well-known oil-releasing mechanisms that have a chemical and physical effect on the oil.

MEOR TECHNOLOGY

In MEOR (Table 2), microbial growth can be either in situ or on the surface, where by-products from microbes grown in vats are selectively removed from the nutrient media, and injected into the reservoir.^4,5,6

TABLE 2. Types of microbial processes for oil recovery

From a microbiologist’s perspective, MEOR processes are somewhat akin to in-situ bioremediation processes. Injected nutrients, together with indigenous or added microbes, promote in-situ microbial growth and/or generation of products that mobilize additional oil and move it to producing wells through reservoir depressurization, interfacial tension/ oil viscosity reduction, and selective plugging of the most permeable zones.^7,8 Alternatively, the oil-mobilizing microbial products may be produced by fermentation and injected into the reservoir.

For in-situ MEOR processes, the microorganisms must not only produce the chemicals necessary for oil mobilization, but must also thrive in the reservoir environment. In a MEOR process, conditions for microbial metabolism are frequently supported by nutrient injection. In some processes, this involves injecting a fermentable carbohydrate into the reservoir. Some reservoirs also require inorganic nutrients as substrates for cellular growth or for serving as alternative electron acceptors in place of oxygen or carbohydrates.

RESERVOIR CHARACTERISTICS

Many reservoir characteristics must be determined before applying MEOR. This technology requires consideration of the physicochemical properties of the reservoir in terms of salinity, pH, temperature, pressure and nutrient availability.^9,10 Rock factors are also important. Natural fractures may alter how microbes can effectively be introduced to the reservoir. The presence of clays may preferentially adsorb biopolymers and biosurfactants, rendering them useless. Carbonates may quickly utilize acids and produce larger quantities of beneficial gasses, such as carbon dioxide.

Only bacteria are considered promising candidates for MEOR. Molds, yeasts, algae and protozoa are not suitable due to their size or inability to grow under the conditions present in reservoirs. Many petroleum reservoirs have high NaCl concentrations¹¹ and require the use of bacteria that can tolerate these conditions.¹² Bacteria-producing biosurfactants and polymers can grow at NaCl concentrations of up to 8% and selectively plug sandstone to create a biowall to recover additional oil.¹³

It is unlikely that a single MEOR method can be applied to all types of reservoirs. One MEOR approach, thermophilic isolates, successively limits the carbon sources and increases the temperature, pressure, and salinity of the media to select microbial strains capable of growing on crude oil at 70 to 90°C and 2,000 to 2,500 psia, and a salinity range of 1.3 to 2.5%.^14,15 Extremely thermophilic anaerobes that grow at 80 to 110°C have been isolated and cultured. All of these organisms belong to the arachaebacteria, living autotrophically on sulfur, hydrogen and CO₂ by methanogenesis, and heterotrophically on organic substrates by sulfur respiration or anaerobic fermentation.

A one-dimensional model was developed to simulate the MEOR process.¹⁶ The model involved five components (oil, bacteria, water, nutrients, and metabolites), with adsorption, diffusion, chemotaxis, growth and decay of bacteria, nutrient consumption, permeability damage and porosity reduction effects. Comparison between the experimental and simulated results emphasized the validity of the simulator and determined its degree of accuracy (average absolute relative error, 8.323%). Oil recovery was found to be sensitive to variations in the concentration of injected bacteria, the size of the bacterial culture plug, incubation time and residual oil saturation.

Quantitative modeling of microbial reservoir processes merges the disciplines of reaction engineering with reservoir engineering.¹⁷ The application of reaction-rate equations provides a unique insight into how these processes work. It treats the microbially active area as a bioreactor with some defined radius, where the residence time in the bioreactor must be greater than the reaction time. More data is required from the behavior of field trials to further understand how to accurately apply these models.

MEOR-participating microorganisms produce a variety of fermentation products, e.g., carbon dioxide, methane, hydrogen, biosurfactants and polysaccharides from crude oil, pure hydrocarbons, and a variety of nonhydrocarbon substrates. Xanthan gum, a microbial biopolymer, is frequently used in MEOR field testing,^12,18 often with base-hydrolyzed polyacrylamide as a copolymer. Xanthan, an exo-polysaccharide, is molecularly composed of many different sugars and is externally secreted. Xanthan may be used in well drilling to lubricate drillstrings, to help remove rock cuttings from the borehole, and in MEOR, to compensate for decreased pressure in depleted oil wells, thereby aiding production.

Desirable properties of polymers for MEOR include shear stability, high solution viscosity, compatibility with reservoir brine, stable viscosity over a wide pH range, temperature, pressure and resistance to biodegradation in the reservoir environment.^11,18,19 Organic acids produced through fermentation readily dissolve carbonates and can greatly enhance permeability in limestone reservoirs, and attempts have been made to promote their anaerobic production.⁵ Organic solvents and dissolved CO₂ can decrease oil viscosity. Fermentation gases can repressurize wells, leading to displacement and production of light or conventional crude oil through a revitalized gas-driven mechanism.⁵

Residual oil in reservoirs can be recovered when highly permeable, watered-out regions of oil reservoirs are plugged with bacterial cells and biopolymers.²⁰ Bacteria and nutrients are injected into the reservoir, and the system is shut in to allow the biomass to plug the more permeable region as it grows.^21,22 Water is then injected (water flooding) to force oil, trapped in less permeable regions of the reservoir, out into the recovery well.

A porous glass micromodel has been used to simulate biomass plugging with Leuconostoc mesenteroides under nutrient-rich conditions.^20,23,24,25 As nutrients flow through the porous glass, a biomass plug establishes at the nutrient-inoculum interface. High substrate loading and high pH promote plug development.²⁵ The residual oil remaining after waterflooding is a potential target for selective reservoir plugging of porous rocks with in-situ bacterial growth on injected nutrients.^{18, 26}

Bacteria may exert a much greater plugging effect when they multiply within the reservoir rock rather than when they are injected and accumulate at the surface. This process is technologically relatively simple and is a dynamically stable system.¹⁷ As water enters the reservoir rock, biomass grows vigorously, diverting flow to the next permeable layer, which then promotes more growth. The process can be limited by restricting nutrients at any time. It is therefore one of the more promising areas for near-term applications.

Added or in situ-produced biosurfactants, which aid oil emulsification and detachment of oil films from rocks, have considerable potential.^6,27 Emulsion reduced the viscosity of Boscon heavy crude oil from 200,000 cP to 100 cP, facilitating heavy oil pumping.²⁸ Biosurfactant from the thermo- and halotolerant species, Bacillus licheniformis, isolates, while thermotolerant Bacillus subtilis strains have been tested for in reservoirs, with various levels of success, and in lab simulations.^{29,30,31,32,33}

In a field MEOR study in the Southeast Vassar Vertz Sand Unit, a salt-containing reservoir in Oklahoma, nutrient injection stimulated growth of microbial populations, including several aerobic and anaerobic heterotrophic bacteria, sulfate-reducing bacteria and methanogenic halophiles. Nutrient-stimulated microbial growth produced a 33% drop in the effective permeability in an injection well at North Burbank Unit in Oklahoma, plugging off high-permeability layers and diverting injection fluid to zones of lower permeability and higher oil saturation.³⁴

In contrast to the poor experience with exogenous organisms for bioremediation (bioaugmentation), injection of selected microbial species into oilfield pilots in Japan and China resulted in improved oil recoveries of 15 to 23%.^{35, 36} In one case, microbial treatment caused some degradation of long-chain aliphatic hydrocarbon chains but with no apparent degradation of aromatic ring structures. MEOR technology has advanced from lab-based studies in the early 1980s, to field applications in the 1990s. Field MEOR projects have been conducted in the US, Australia, China, Rumania, Peru and Russia. These projects report beneficial results in most cases. Reported EOR resulting from MEOR processes vary from no impact, to 13%, 19%, 36%, 50 – 65% and even 204%. In addition to increased oil production, some projects report decreased water production, increased GOR and improved injectivity.³ While the potential use of MEOR has been of great interest, field use has been limited. However, Petronas has been at the forefront of using MEOR in their Bokor field to increase oil recovery.³⁷

More than 400 MEOR field tests have been conducted in the US alone, mostly as single-well stimulation treatments on low-productivity wells, so that reliable data are sparse.^5,9,10 Reservoir heterogeneity significantly affects oil recovery efficiency. MEOR technology may be particularly attractive to small, independent oil producers, operators of the approximate 470,000 “stripper wells” in the US. A single-well stimulation treatment might increase the production rate to 2.8 bopd from 1.4 bopd and sustain the increased rate for 2 to 6 months without additional treatments. The incremental cost per barrel ranges from $1.30 to $7.92 in a recent application in Peru.³⁸

LIMITATIONS OF MEOR

Bacteria have numerous limitations, especially when applied in situ. Limitations include substrate availability, substrate toxicity and clogging. Also, once the oil is recovered, it is important to remove the substances produced by the bacteria, as well as the bacteria themselves, to prevent further modification. Bacterial enzymes are mostly intracellular, so the oil would have to be absorbed through the relatively impermeable cell membrane.

The cell membrane is especially impermeable to large or charged molecules. This greatly reduces the range of hydrocarbons on which the bacteria can act. Also, because of their size and their tendency to clump together, it has been reported that in-situ microbial growth drastically reduced permeability and, consequently, oil-production.

The MEOR process may modify the immediate reservoir environment in ways that could also damage the production hardware or the formation itself.²¹ Certain sulfate reducers can produce H₂S, which can corrode pipeline and other components of the recovery equipment. This has commonly been observed when injecting water, rich in sulfates, for waterflooding. The addition of nitrate and nitrite nutrients to an indigenous denitrifying bacteria population has been shown to reverse this effect.³⁹

Despite numerous MEOR tests, considerable uncertainty remains regarding process performance. The chances for success are increased if a specific objective is targeted. Wellbore stimulation treatments are technologically simpler and consequently have a higher chance of success. Using microbes to generate and deliver specific EOR agents deep within the reservoir is much more complex.

The ability to manipulate environmental conditions to promote growth and/or product formation by the participating microorganisms is critical. Exerting such control over the microbial system in the subsurface is a serious challenge. In addition, reservoir conditions vary, which calls for reservoir-specific customization of the MEOR process. This alone has the potential to undermine microbial process economic viability.⁴⁰

CURRENT RESEARCH AREAS

The processes that facilitate oil production are complex and usually involve multiple biochemical processes. MEOR systems represent high-risk processes to oil producers looking for efficient and predictable oil recovery. Microbial biomass or biopolymers might: plug high-permeability zones and lead to a redirection of the waterflood; produce surfactants, leading to increased mobilization of residual oil; increase gas pressure by producing CO₂ or methane; or reduce oil viscosity due to digestion of large molecules.

While methanogenic bacteria have been shown to be active in oil and coalbed methane reservoirs, they must function as part of a reaction chain to effectively produce methane. This includes breaking down the large chain hydrocarbon, converting the molecules to simple acids and alcohols, and then finally converting the simple molecules to methane. Each part of the chain is processed by different microbial consortia. All of these consortia are sensitive to environmental changes, such as the introduction of oxygen, which could sterilize the population, break the chain, and shut down the entire process.⁴¹

Biogenesis is being looked at for converting coal, shale and residual oil to methane in commercial quantities. This in-situ microbial conversion to methane has the potential to create significant amounts of natural gas in days rather than millions of years. In experiments administered on Dietz coals of the Powder River basin, tested samples of coal and unamended formation water resulted in a steady increase in methane in the headspace, slightly exceeding 3% by 135 days. However, when a nutritional enhancer was added, methane levels reached nearly 9% in a comparable time- frame, with methane representing nearly 100% of the newly generated gas.⁴¹

Researchers are working to create strains of bacteria that are better able to survive harsh environmental conditions in oil wells but still retain the ability to carry out the chemistry needed for MEOR. Genetic engineering is being used to develop microorganisms that not only live in high temperatures, but can also subsist on inexpensive nutrients, remain chemically active and produce substantial amounts of biosurfactants. Some researchers are developing bacteria that can be grown on inexpensive agricultural waste material, which is abundant in supply and is environmentally friendly.

Areas of MEOR research that merit more investigation are:

• Data on the performance of biosurfactants, compared with that of synthetic surfactants under reservoir conditions
• Techniques for the bio-emulsification of oil within the reservoir
• Parameters relating to transport, growth and metabolite production by microorganisms in petroleum reservoirs
• Potential environmental effects of introduction of microorganisms into reservoirs are important, and it appears that they have been overlooked in zealous efforts to try to find “super microorganisms” for EOR.

Research on microbial transport and activity in the reservoir, performed under reservoir conditions, should help alleviate environmental concern; such research is also necessary for adequate design of MEOR field projects and more sophisticated data is required from existing field trials.

Research in other areas is also essential to further MEOR, such as increasing the salt and heat tolerance of biopolymers in the reservoir. One disadvantage of biopolymers that further research may mitigate is their extreme biodegradability. Scleroglucan is a new biopolymer showing promise in this area.

CONCLUSION

MEOR is an environmentally friendly technology that reduces or eliminates the need to use harsh chemicals. MEOR will become increasingly economically feasible as genetic engineering develops more effective bacteria that may subsist on inexpensive and abundant nutrients. Methods for developing and growing MEOR bacteria are improving, thereby lowering production costs and making it a more attractive alternative to traditional chemical methods of tertiary oil recovery.

As with in-situ bioremediation systems, the environment, over which the microbiologist has little control, influences optimal performance. Clearly, microbial products reducing oil viscosity could be produced above ground, under optimal conditions, and injected with high chances of efficacy. Research on finding microbial products with universal applications in this area is worth pursuing.

A more robust, universal microbial system for increasing the oil recovery of porous reservoirs is desirable and should be aided by ongoing modeling studies directed to manipulating simulated porous reservoirs in columns. These approaches will facilitate implementation of microbe-based research to determine the most desirable strain types, nutritional, metabolic and physiological characteristics needed to achieve high success rates in applying of microbial technology in oil recovery. 

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THE AUTHORS
	Saeid Mokhatab is an advisor of natural gas engineering research projects in the Chemical and Petroleum Engineering Department of the University of Wyoming.
	Leo A. Giangiacomo is president of Extreme Petroleum Technology, Inc., in Casper, Wyoming. He started Extreme Petroleum Technology, Inc. in 1998, where he has continued to be active screening fields for CO2 floods. He is a registered professional engineer in the state of Wyoming. He holds a BS degree in Petroleum and Natural Gas Engineering from Penn State.