Microbial enhanced oil recovery
||This article has been nominated to be checked for its neutrality. (January 2012)|
Microbial Enhanced Oil Recovery (MEOR) is a biological based technology consisting in manipulating function or structure, or both, of microbial environments existing in oil reservoirs. The ultimate aim of MEOR is to improve the recovery of oil entrapped in porous media while increasing economic profits. MEOR is a tertiary oil extraction technology allowing the partial recovery of the commonly residual two-thirds of oil, thus increasing the life of mature oil reservoirs.
MEOR is a multidisciplinary field incorporating, among others: geology, chemistry, microbiology, fluid mechanics, petroleum engineering, environmental engineering and chemical engineering. The microbial processes proceeding in MEOR can be classified according to the oil production problem in the field:
- well bore clean up removes mud and other debris blocking the channels where oil flows through;
- well stimulation improves the flow of oil from the drainage area into the well bore; and
- enhanced water floods increase microbial activity by injecting selected microbes and sometimes nutrients. From the engineering point of view, MEOR is a system integrated by the reservoir, microbes, nutrients and protocol of well injection.
- 1 Introduction
- 2 MEOR outcomes
- 3 Relevance
- 4 Bias
- 5 History
- 6 MEOR advantages
- 7 MEOR disadvantages
- 8 Primary production
- 9 Secondary production
- 10 Tertiary production or Enhanced Oil Recovery (EOR)
- 11 The environment of an oil reservoir
- 12 Environmental constraints
- 13 MEOR mechanism
- 14 Clogging mechanism
- 15 MEOR strategies
- 16 Field studies
- 17 Models
- 18 Grounds of failure
- 19 Trends
- 20 Results and Discussion
- 21 Ventures working in MEOR
- 22 References
- 23 External links
Microbial enhanced oil recovery (MEOR) is a tertiary oil recovery process where microorganisms and/or their metabolic by products are utilized for the mobilization of crude oil trapped in mature oil formations. The proposed MEOR mechanisms leading to oil recovery fall into two broad categories: (Mechanism 1) alteration of oil/water/rock interfacial properties and (Mechanism 2) changes in flow behavior due to bioclogging. Numerous reports show the efficacy of MEOR at the lab-scale; however, a complete understanding of the mechanisms involved is lacking, and the effectiveness of each mechanism for different reservoir parameters (such as wettability) is unknown.
As stated by, three general strategies exist for the implementation of MEOR: (1) injection of nutrients to stimulate indigenous microorganisms, (2) injection of exogenous microorganisms(s) and nutrients, or (3) injection of ex situ produced products. The first two strategies have the added difficulty of dealing with subsurface bacterial transport, competition for nutrients among the desired organism and other indigenous microorganisms, and maintaining nutrient levels throughout a reservoir for extended periods of time. Therefore, it is likely that the third strategy is the simplest, and thus, the most likely for success at the field-scale. This third strategy is the approach used in the experiments presented in this article. In particular, we focus on the injection of ex situ generated products produced by Bacillus mojavensis JF-2 and the effect of these products on oil mobilization in fractional-wet systems.
Bacillus mojavensis JF-2 is a gram-positive, biosurfactant producing, facultative aerobe, isolated from oil reservoir brine in Oklahoma. As categorized above, the possible MEOR mechanisms associated with JF-2 include the following: reduction of interfacial tension (IFT) via biosurfactant production (Mechanism 1), changes in wettability (Mechanism 1), and bioclogging (Mechanism 2). Mobilization of crude oil in a sand-packed column after addition of JF-2 biosurfactant was initially demonstrated by. studied the production of biosurfactant by JF-2 under anaerobic and aerobic conditions, biosurfactant structural characterization (i.e., the chemical structure of the biosurfactant), and biosurfactant yield. From these studies, a growth medium, referred to as Media E, was formulated, which optimizes biosurfactant production and, therefore, maximizes reduction. Evidence of wettability alterationwas reported by who found that for oil-wet flow cells where residual oil saturation decreased from 0.18 to 0.14, the Amott wettability indices increased from −0.269 to −0.10 indicating more water-wet conditions after MEOR. Similarly, forwater-wet sandstone, found that JF-2 shifted the USBM wettability indices significantly in the positive direction toward a more water-wet condition. Currently, no literature exists on oil recovery with JF-2 via bioclogging. However, most organisms including JF-2 can form biofilm, and the effect that biofilm formation can have on oil recovery should not be overlooked.
Wettability is a major factor controlling residual oil saturation, and thus, it is essential to characterize reservoir wettability. Reservoir rock wettability can be altered by contact with absorbable crude oil components (e.g., asphaltenes), which can lead to heterogeneous forms of wettability characterized by the term fractional wettability. A fractional-wet system is where a portion of the reservoir rock is strongly oil-wet, while the rest is strongly water-wet. Fractional-wet systems have previously been studied by packing columns with different ratios ofwater-wet sand and sand rendered oil-wet by treatment with an organic silane solution found that nonuniform wettability can distort the capillary pressure curve such that it no longer represents the true pore-size distribution. The findings of indicate that wettability can have a dramatic effect on residual oil entrapment. Residual oil blobs increase in size and length as the porous medium is composed of fewer oil-wet surfaces. In addition, simulation results using pore-network models developed by support the experimental trends found by. Using X-ray microtomography, found that the distribution of residual water phase is less uniform in fractional-wet columns than water-wet columns and that fractional-wet columns contained fewer and larger residualwater blobs. developed a technique to modify the hydrophobicity of carbonate cores, such that, well-defined wettability states could be obtained. Pore-scale images acquired with CMT after spontaneous imbibition in the fractional-wet cores showed that the oil phase was concentrated in the larger, presumably oil-wet pores, suggesting that imbibition occurred preferentially through the water-wet regions.
The effect microorganisms can have on wettability and approaches taken to understand such wettability alteration during MEOR vary in the literature. Traditionally, surface wettability has been quantified by placing a liquid drop on a solid surface and then measuring the resulting contact angle, known as the wetting angle which is defined by the Young–Laplace equation. Other than directly measuring contact angle, porous media wettability is often quantified using macro-scale indices (obtained with techniques such as the Carter or Amott methods. Changes in these macro-scale indices due to microbial activity have been shown by Afrapoli et al. who found more water-wet conditions after MEOR using these measurements.However, the direction in which microorganisms change wettability is not consistent. For example, changes in surface wettability toward more water-wet conditions have been reported by, while reported the opposite trend.
MEOR is a large-scale outcome driven by pore-scale processes. Therefore, to better understand the various MEOR mechanisms facilitating oil recovery, pore-scale investigations are needed. In particular, high-resolution 3-dimensional (3D) images of the pore-space and the immiscible fluid distributions are needed to understand pore-scale temporal and spatial changes in interfacial curvature and oil blob morphology during MEOR. X-ray computed microtomography (CMT) has been available for more than three decades and has been a powerful tool for studying a wide array of multi-phase processes in porous media systems. Using CMT, numerous researchers have distinguished multiple fluids and their menisci, quantified interfacial areas, and measured oil blob size distributions. However, similar CMT analyses have not been applied toMEOR. Thus, it is unclear how MEOR affects interfacial curvature and residual oil blob morphology, both essential parameters needed for understanding and predicting the complex physical phenomena that occur during MEOR.
So far, the outcomes of MEOR are explained based on two predominant rationales:
Increment in oil production. This is done by modifying the interfacial properties of the system oil-water-minerals, with the aim of facilitating oil movement through porous media. In such a system, microbial activity affects fluidity (viscosity reduction, miscible flooding); displacement efficiency (decrease of interfacial tension, increase of permeability); sweep efficiency (mobility control, selective plugging) and driving force (reservoir pressure).
Upgrading. In this case, microbial activity acts may promote the degradation of heavy oils into lighter ones. Alternatively, it can promote desulphurization due to denitrification as well as the removal of heavy metals.
Several decades of research and successful applications support the claims of MEOR as a mature technology. Despite those facts, disagreement still exists. Successful stories are specific for each MEOR field application, and published information regarding supportive economical advantages is however inexistent. Despite this, there is consensus considering MEOR one of the cheapest existing EOR methods. However, obscurity exists on predicting whether or not the deployment of MEOR will be successful. MEOR is, therefore, one of the future research areas with great priority as identified by the “Oil and Gas in the 21st Century Task Force”. This is probably because MEOR is a complementary technology that may help recover the 377 billion barrels of oil that are unrecoverable by conventional technologies.
Before the advent of environmental molecular microbiology, the word “bacteria” was utilised indistinctively in many fields to refer to uncharacterized microbes, and such systematic error affected several disciplines. Therefore, the word “microbe” or “microorganism” will therefore be preferred hereafter in the text.
It was in 1926 when Beckam proposed the utilisation of microorganisms as agents for recovering the remnant oil entrapped in porous media. Since that time numerous investigations have been developed, and are extensively reviewed. In 1947, ZoBell and colleagues set the basis of petroleum microbiology applied to oil recovery, whose contribution would be useful for the first MEOR patent granted to Updegraff and colleagues in 1957 concerning the in situ production of oil recovery agents such as gases, acids, solvents and biosurfactants from microbial degradation of molasses. In 1954, the first field test was carried out in the Lisbon field in Arkansas, USA. During that time, Kuznetsov discovered the microbial gas production from oil. From this year and until the 1970s there was intensive research in USA, USSR, Czechoslovakia, Hungary and Poland. The main type of field experiments developed in those countries consisted in injecting exogenous microbes. In 1958, selective plugging with microbial produced biomass was proposed by Heinningen and colleagues. The oil crisis of 1970 triggered a great interest in active MEOR research in more than 15 countries. From 1970 to 2000, basic MEOR research focused on microbial ecology and characterization of oil reservoirs. In 1983, Ivanov and colleagues developed the strata microbial activation technology. By 1990, MEOR achieved an interdisciplinary technology status. In 1995, a survey of MEOR projects (322) in the USA showed that 81% of the projects successfully increased oil production, and there was not a single case of reduced oil production. Today, MEOR is gaining attention owing to the high prices of oil and the imminent ending of this resource. As a result, several countries are willing to use MEOR in one third of their oil recovery programs by 2010.
There is a plethora of reviewed claims regarding the advantages of MEOR. However, they should be cautiously regarded due to the lack of published supportive evidence. In addition, assessments of both full live cycle analysis and environmental impact are also unknown.
- Injected microbes and nutrients are cheap; easy to handle in the field and independent of oil prices.
- Economically attractive for mature oil fields before abandonment.
- Increases oil production.
- Existing facilities require slight modifications.
- Easy application.
- Less expensive set up.
- Low energy input requirement for microbes to produce MEOR agents.
- More efficient than other EOR methods when applied to carbonate oil reservoirs.
- Microbial activity increases with microbial growth. This is opposite to the case of other EOR additives in time and distance.
- Cellular products are biodegradable and therefore can be considered environmentally friendly.
- The oxygen deployed in aerobic MEOR can act as corrosive agent on non-resistant topside equipment and down-hole piping
- Anaerobic MEOR requires large amounts of sugar limiting its applicability in offshore platforms due to logistical problems
- Exogenous microbes require facilities for their cultivation.
- Indigenous microbes need a standardized framework for evaluating microbial activity, e.g. specialized coring and sampling techniques.
- Microbial growth is favoured when: layer permeability is greater than 50 md; reservoir temperature is inferior to 80 0C, salinity is below 150 g/L and reservoir depth is less than 2400m.
Oil exists in the small pores and in the narrow fissures and interstices within the body of the reservoir rocks underneath the surface of the earth. The natural pressure of the reservoir causes the oil to flow up to the surface and provide the so-called primary production, which depends upon the internal energy and the characteristics of the reservoir rock and the properties of the hydrocarbon fluids. In some reservoirs, which are the part of a much large aquifer system, a natural flow of underground waters may be the drive force (aquifer drive) to push and displace oil. The initial reservoir pressure is usually high enough to lift the oil up to surface; however as oil production progresses, the reservoir pressure is continually depleted to a point in which artificial lift or pumping is required to maintain an economical oil production rate. In other reservoirs, there may be other recovery mechanisms, such as the expansion of dissolved gas during the pressure decline. As the reservoir pressure falls below the bubble point during production, some of the more volatile components are released and come out of solution to form small gas bubbles. Initially the bubbles are trapped in the pores and then their expansion causes oil displacement (dissolved gas drive). Furthermore in some reservoirs, as the pressure fall, gas bubbles increase in size and eventually coalesce forming a continuous gas phase that flows towards the upper part of the reservoir forming a gas cap. The gas cap constantly expands as the reservoir pressure continually decreases displacing more oil (gas cap drive) to the production wells.
As the reservoir pressure declines during primary production, a critical point is reached when it is necessary to provide external energy for the reservoir to achieve additional oil recovery, which is termed secondary recovery. The extra energy can be introduced by injecting gas (gas injection) and/or water (water flooding).
Gas injection is usually only applied to reservoirs which have a gas cap where gas drive would be an efficient displacement mechanism. In Water flooding, which nowadays is one of the most common methods of oil recovery, keeps the reservoir pressure around the bubble point, thus preventing the pores to be blocked by dissolved gases. Also, according to the hydrocarbon thermodynamics, at the bubble point, the oil will have its lowest viscosity. So that, for a specific pressure gradient, the maximum amount of the oil will be displaced under this condition. After some years of operation in a field, due to the reservoir heterogeneity, the injected fluids (water or gas) flow preferentially along high permeable layers that cause these fluids to by-pass oil saturated areas in the reservoir. Therefore, an increasingly large quantity of water (or gas) rises with the oil, and by decreasing the ratio of oil to water, eventually it becomes uneconomic to continue the process and the field must be abandoned. In this situation, due to the low proportion of the oil production in both primary and secondary stages (about 30%), attention will be focused on the third stage of the oil recovery, so-called tertiary production or Enhanced Oil Recovery (EOR) for recovering more oil from the existing and abandoned oil fields.
Tertiary production or Enhanced Oil Recovery (EOR)
Generally, tertiary or enhanced oil recovery involves the extraction of residual oil after the primary and secondary phases of production. At this stage, modern and technically advanced methods are employed to either modify the properties of reservoir fluids or the reservoir rock characteristics, with the aim of gaining recovery efficiencies more than those obtained by conventional recovery methods (primary and secondary recovery stages). This can be achieved based on different mechanisms such as reducing the interfacial tension between oil and water, reducing oil viscosity (thermal methods), creating miscible displacement and increasing viscosity of the displacing fluid to be more viscous than the oil. The applied EOR method for each reservoir depends on its specifications, and requires a great deal of rocks and fluids sampling and also laboratory investigations. In general, EOR processes can be classified into four main categories as thermal methods, chemical methods, miscible or solvent injection, and microbial methods.
The general principle of thermal processes which are mostly used for recovery of heavy or viscous oils is to supply the reservoir with heat energy in order to increase the oil temperature and reduce its viscosity increasing the mobility of the oil towards production wells. Thermal processes can be conducted by two different methods: steam flooding and in-situ combustion. In steam flooding, steam at about 80% quality is injected into an oil reservoir, in which by condensing the steam, its heat energy transfers to reservoir rocks and fluids. This leads to the thermal expansion of the oil and the consequently reduction in its viscosity, and the release of dissolved gases. Steam flooding is the most widely used EOR method and probably the most profitable from an economic standpoint. In the in-situ combustion method (fire flood), which is theoretically more efficient than steam flood, burning some of the reservoir oil results in heating the reservoir and displacement of the remaining oil to the producing wells. But generally, due to the complex operational problems of this method, it is not widely applied.
Chemical methods (chemical flooding) are claimed to have significant potential based on successful laboratory testing, but the results in field trials have not been encouraging. Furthermore, these methods are not yet profitable. In these processes, chemicals such as surfactants, alkaline solutions, and polymers are added to the displacing water in order to change the physicochemical properties of the water and the contacted oil making the displacement process more effective. In surfactant flooding, by reducing the interfacial tension between the oil and the displacing water and also the interfacial tension between the oil and the rock interfaces, residual oil can be displaced and recovered. Moreover, in caustic flooding, the reaction of the alkaline compounds with the organic acids in the oil forms insitu natural surfactants that lower the oil-water interfacial tension. In addition to surfactant and alkaline flooding, polymers are used to increase the viscosity of the displacing water to improve the oil swept efficiency.
Miscible displacement processes
The underlying principle behind miscible displacement processes is to reduce the interfacial tension between the displacing and displaced fluids to near zero that leads to the total miscibility of the solvent (gas) and the oil, forming a single homogeneous moving phase. The displacing fluid (injected solvent or gas) could be carbon dioxide, nitrogen, exhaust gases, hydrocarbon solvents, or even certain alcohols.
Microbial processes (MEOR)
Another tertiary method of oil recovery is microbial enhanced oil recovery, commonly known as MEOR, which nowadays is becoming an important and a rapidly developed tertiary production technology, which uses microorganisms or their metabolites to enhance the recovery of residual oil.
In this method, nutrients and suitable bacteria, which can grow under the anaerobic reservoir conditions, are injected into the reservoir. The microbial metabolic products that include biosurfactants, biopolymers, acids, solvents, gases, and also enzymes modify the properties of the oil and the interactions between oil, water, and the porous media, which increase the mobility of the oil and consequently the recovery of oil especially from depleted and marginal reservoirs; thus extending the producing life of the wells. In MEOR process, different kinds of nutrients are injected to the reservoirs. In some processes, a fermentable carbohydrate including molasses is utilized as nutrient. Some other reservoirs require inorganic nutrients as substrates for cellular growth or as alternative electron acceptors instead of oxygen. In another method, water containing a source of vitamins, phosphates, and electron acceptors such as nitrate, is injected into the reservoir, so that anaerobic bacteria can grow by using oil as the main carbon source. The microorganisms used in MEOR methods are mostly anaerobic extremophiles, including halophiles, barophiles, and thermophiles for their better adaptation to the oil reservoir conditions. These bacteria are usually hydrocarbon-utilizing, non-pathogenic, and are naturally occurring in petroleum reservoirs. In the past, the microbes selected for use, had to have a maximum growth rate at temperatures below 80°C, however it is known that some microorganisms can actually grow at temperatures up to 121°C. Bacillus strains grown on glucose mineral salts medium are one of the most utilized bacteria in MEOR technologies, specifically when oil viscosity reduction is not the primary aim of the operation.
The environment of an oil reservoir
Oil reservoirs are complex environments containing living (microorganisms) and non living factors (minerals) which interact with each other in a complicated dynamic network of nutrients and energy fluxes. Since the reservoir is heterogeneous, so do the variety of ecosystems containing diverse microbial communities, which in turn are able to affect reservoir behaviour and oil mobilization.
Microbes are living machines whose metabolites, excretion products and new cells may interact with each other or with the environment, positively or negatively, depending on the global desirable purpose, e.g. the enhancement of oil recovery. All these entities, i.e. enzymes, extracellular polymeric substances (EPS) and the cells themselves, may participate as catalyst or reactants. Such complexity is increased by the interplay with the environment, the later playing a crucial role by affecting cellular function, i.e. genetic expression and protein production.
Several factors concomitantly affect microbial growth and activity. In oil reservoirs, such environmental constraints permit the establishment of criteria to assess and compare the suitability of various microorganisms. Those constraints may not be as harsh as other environments on Earth. For example, with connate brines the salinity is higher than that of sea water but lower than that of salt lakes. In addition, pressures up to 20 MPa and temperatures up to 80 °C, in oil reservoirs, are within the limits for the survival of other microorganisms.
Some environmental constraints creating selective pressures on cellular systems that may also affect microbial communities in oil reservoirs are:
Enzymes are biological catalysts whose function is affected by a variety of factors including temperature, which, at different ranges, may improve or hamper enzymatic mediated reactions. This will have an effect over the optimal cellular growth or metabolism. Such dependency permits classification of microbes according to the range of temperatures at which they grow. For instance: psychrophiles (<25 °C), mesophiles (25-45 °C), thermophiles (45-60 °C) and hyperthermophiles (60-121 °C). Although such cells optimally grow in these temperature ranges there may not be a direct relationship with the production of specific metabolites.
The effects of pressure on microbial growth under deep ocean conditions were investigated by ZoBell and Johson in 1949. They called those microbes whose growth was enhanced by increasing pressure, barophilic. Other classifications of microorganisms are based on whether microbial growth is inhibited at standard conditions (piezophiles) or above 40 MPa (piezotolerants). From a molecular point of view, the review of Daniel shows that at high pressures the DNA double helix becomes denser, and therefore both gene expression and protein synthesis are affected.
One study has concluded that substantial bacterial activity is achieved when there are interconnections of pores having at least 0.2µ diameter. It is expected that pore size and geometry may affect chemotaxis. However, this has not been proven at oil reservoir conditions.
Changes in cellular surface and membrane thickness may be promoted by pH due to its ionization power of cellular membrane embedded proteins. The modified ionic regions may interact with mineral particles and affect the motion of cells through the porous media.
Embedded cell proteins play a fundamental roll in the transport of chemicals across the cellular membrane. Their function is strongly dependent on their state of ionisation, which is in turn strongly affected by pH.
In both cases, this may happen in isolated or complex environmental microbial communities. So far the understanding on the interaction between pH and environmental microbial communities remains unknown, despite the efforts of the last decade. Little is know on the ecophysiology of complex microbial communities and research is still in developmental stage.
The oxidation potential (Eh, measured in volts) is, as in any reaction system, the thermodynamic driving force of anaerobic respiration, which takes place in oxygen depleted environments. Prokaryotes are among the cells that have anaerobic respiration as metabolic strategy for survival. The electron transport takes place along and across the cellular membrane (prokaryotes lack of mitochondria). Electrons are transferred from an electron donor (molecule to be oxidised anaerobically) to an electron acceptor (NO3, SO4, MnO4, etc.). The net Eh between a given electron donor and acceptor; hydrogen ions and other species in place will determine which reaction will first take place. For instance, nitrification is hierarchically more favoured than sulphate reduction. This allows for enhanced oil recovery by disfavouring biologically produced H2S, which derives from reduced SO4. In this process, the effects of nitrate reduction on wettability, interfacial tension, viscosity, permeability, biomass and biopolymer production remain unknown.
Electrolytes concentration and other dissolved species may affect cellular physiology. Dissolving electrolytes reduces thermodynamic activity (aw), vapour pressure and autoprotolysis of water. Besides, electrolytes promote an ionic strength gradient across cellular membrane and therefore provides a powerful driving force allowing the diffusion of water into or out to cells. In natural environments, most bacteria are incapable of living at aw below 0.95. However, some microbes from hypersaline environment such as Pseudomonas species and Halococcus thrive at lower aw, and are therefore interesting for MEOR research.
They may occur on pH and Eh. For example, increasing ionic strength increases solubility of nonelectrolytes ('salting out') as in the case of dissolution of carbon dioxide, a pH controller of a variety of natural waters.
Although it is widely accepted that predation, parasitism, syntrophism and other relationships also occur in the microbial world, little is known in this relationships on MEOR and they have been disregarded in MEOR experiments.
In other cases, some microorganisms can thrive in nutrient deficient environments (oligotrophy) such as deep granitic and basaltic aquifers. Other microbes, living in sediments, may utilise available organic compounds (heterotrophy). Organic matter and metabolic products between geological formations can diffuse and support microbial growth in distant environments.
Understanding MEOR mechanism is still far from being clear. Although a variety of explanations has been given in isolated experiments, it is unclear if they were carried out trying to mimic oil reservoirs conditions.
The mechanism can be explained from the client-operator viewpoint which considers a series of concomitant positive or negative effects that will result in a global benefit:
- Beneficial effects. Biodegradation of big molecules reduces viscosity; production of surfactants reduces interfacial tension; production of gas provides additional pressure driving force; microbial metabolites or the microbes themselves may reduce permeability by activation of secondary flow paths.
- Detrimental effects. Biologically produced hydrogen sulphide, i.e. souring, causes corrosion of piping and machinery; consumption of hydrocarbons by bacteria reduces the production of desired chemicals.
- Beneficial or Detrimental. Permeability reduction can be beneficial in some cases but detrimental in others. Negatively, microbial metabolites or the microbes themselves may reduce permeability by activation of secondary flow paths by depositing: biomass (biological clogging), minerals (chemical clogging) or other suspended particles (physical clogging). Positively, attachment of bacteria and development of slime, i.e. extracellular polymeric substances (EPS), favour the plugging of highly permeable zones (thieves zones) leading to increased sweep efficiency.
One method of microbial improving oil recovery is by modifying the fluid flow through the reservoir by shifting fluid flow from the high permeability zones in a reservoir to the moderate or low permeability zones thus increasing the sweep efficiency by forcing the injected water to pass through previously by-passed oil zones of the reservoir. The changes in flow pattern can be achieved by an increase in microbial cell mass within the reservoir. Stimulating either indigenous microbial populations or injecting microorganisms together with nutrients produce biomass and hence microbial plugging. The injected nutrient and microbes preferentially flow into the high permeability zones of the reservoir and as a result of cell growth, the biomass selectively plugs these zones to a greater extent than the moderate or low permeability zones. Experiments using brine-saturated sandstone cores showed that injecting nutrients and viable bacterial cells resulted in clogging of 60-80% of the pore space.
Changing oil reservoir ecophysiology to favour MEOR can be achieved by complementing different strategies. In situ microbial stimulation can be chemically promoted by injecting electron acceptors such as nitrate; easy fermentable molasses, vitamins or surfactants. Alternatively, MEOR is promoted by injecting exogenous microbes, which may be adapted to oil reservoir conditions and be capable of producing desired MEOR agents (Table 1).
|MEOR agents||Microbes||Product||Possible MEOR application|
|Biomass, i.e. flocks or biofilms||Bacillus sp.||Cells and EPS (mainly exopolysaccharides),||Selective plugging of oil depleted zones and wettability angle alteration|
|Surfactants||Acinetobacter||Emulsan and alasan||Emulsification and de-emulsification through reduction of interfacial tension|
|Bacillus sp.||Surfactin, rhamnolipid, lichenysin|
|Rhodococcus sp.||Viscosin and trehaloselipids|
|Biopolymers||Xanthomonas sp.||Xanthan gum||Injectivity profile and viscosity modification, selective plugging|
|Solvents||Clostridium, Zymomonas and Klebsiella||Acetone, butanol, propan-2-diol||Rock dissolution for increasing permeability, oil viscosity reduction|
|Acids||Clostridium||Propionic and butyric acids||Permeability increase, emulsification|
|Gases||Clostridium||Methane and hydrogen||Increased pressure, oil swelling, reduction of interfacial section and viscosity; increase permeability|
This knowledge has been obtained from experiments with pure cultures and some times with complex microbial communities but the experimental conditions are far from mimicking those ones prevailing in oil reservoirs. It is unknown if metabolic products is cell growth dependent, and claims in this respect should be taken cautiously, since the production of a metabolite is not always dependent of cellular growth.
Biomass and biopolymers
In selective plugging, conditioned cells and extracellular polymeric substances plug high permeability zones, resulting in a change of direction of the water flood to oil-rich channels, consequently increasing the sweep efficiency of oil recovery with water flooding. Biopolymer production and the resulting biofilm formation (less 27% cells, 73-98% EPS and void space) are affected by water chemistry, pH, surface charge, microbial physiology, nutrients and fluid flow.
Microbial produced surfactants, i.e. biosurfactants reduce the interfacial tension between water and oil, and therefore a lower hydrostatic pressure is required to move the liquid entrapped in the pores to overcome the capillary effect. Secondly, biosurfactants contribute to the formation of micelles providing a physical mechanism to mobilise oil in a moving aqueous phase. Hydrophobic and hydrophilic compounds are in play and have attracted attention in MEOR research, and the main structural types are lipopeptides and glycolipids, being the fatty acid molecule the hydrophobic part.
Gas and solvents
In this old practice, the production of gas has a positive effect in oil recovery by increasing the differential pressure driving the oil movement. Anaerobically produced methane from oil degradation have a low effect on MEOR due to its high solubility at high pressures. Carbon dioxide is also a good MEOR agent. The miscible CO2 is condensed into the liquid phase when light hydrocarbons are vaporised into the gas phase. Immiscible CO2 helps to saturate oil, resulting in swelling and reduction of viscosity of the liquid phase and consequently improving mobilization by extra driving pressure. Concomitantly, other gases and solvents may dissolve carbonate rock, leading to an increase in rock permeability and porosity.
Worldwide MEOR field applications have been reviewed in detail. Although the exact number field trials is unknown, Lazar et al. suggested an order of hundreds. Successful MEOR field trials have been conducted in the U.S., Russia, China, Australia, Argentina, Bulgaria, former Czechoslovakia, former East Germany, Hungary, India, Malaysia, Peru, Poland, and Romania. Lazar et al. suggested China is leading in the area, and also found that the most successful study was carried out in Alton field, Australia (40% increase of oil production in 12 months).
The majority of the field trials were done in sandstone reservoirs and very few in fractured reservoirs and carbonates. The only known offshore field trials were in Norne (Norway) and Bokor (Malaysia).
As reviewed by Lazar et al., field application followed different approaches such as injection of exogenous microorganisms (microbial flooding); control of paraffin deposition; stimulation of indigenous microbes; injection of ex situ produced biopolymers; starved selected ultramicrobes (selected plugging); selected plugging by sand consolidation due to biomineralization and fracture clogging in carbonate formations; nutrient manipulation of indigenous reservoir microbes to produce ultramicrobes; and adapted mixed enrichment cultures.
Reported MEOR results from field trials vary widely. Rigorous controlled experiments are lacking and may not be possible due to the dynamic changes in the reservoir when oil is being recovered. Besides, the economical advantages of these field trials are unknown, and the answer to why the other trials were unsuccessful is unknown. General conclusions can not be drawn because the physical and mineralogical characteristics of the oil reservoirs reported were different. The extrapolation of such conclusions is therefore unviable.
A plethora of attempts to model MEOR has been published. Until now, it is unclear if theoretical results reflect the scarce published data. Developing mathematical models for MEOR is very challenging since physical, chemical and biological factors need to be considered.
Published MEOR models are composed of transport properties, conservation laws, local equilibrium, breakdown of filtration theory and physical straining. Such models are so far simplistic and they were developed based on:
(A) Fundamental conservation laws, cellular growth, retention kinetics of biomass, and biomass in oil and aqueous phases. The main aim was to predict porosity retention as a function of distance and time.
(B) Filtration model to express bacterial transport as a function of pore size; and relate permeability with the rate of microbial penetration by applying Darcy’s law.
Chemical kinetics is fundamental for coupling bioproduct formation to fluxes of aqueous species and suspended microbes. Fully numerical approaches have also been followed. For instance, coupled nonlinear parabolic differential equations: adding equation for the rate of diffusion of microbes and their capture by porous medium; differential balance equations for nutrient transport, including the effect of adsorption; and the assumption of bacterial growth kinetic based on Monod equation.
Monod equation is commonly used in modelling software but it has a limited behaviour for beirng inconsistent with the law of mass action that form the basis of kinetic characterization of microbial growth. Application of law of mass action to microbial populations results in the linear logistic equation. If the law of mass action is applied to an enzyme-catalysed process it results in the Michaelis-Menten equation, from which Monod is inspired. This makes things difficult for in situ biosurfactant production because controlled experimentation is required to determine specific growth rate and Michaelis-Menten parameters of rate-limiting enzyme reaction.
Modelling of bioclogging is complicated because the production of clogging metabolite is coupled nonlinearly to the growth of microbes and flux of nutrients transported in the fluid.
The ecophysiology of the entire microbial microcosms at oil reservoir conditions is still unclear and thus not considered by the available models. Microorganisms are a kind of catalyst whose activity (physiology) depends on the mutual interplay with other microbes and the environment (ecology). In nature, living and non living elements interact with each other in a complicated network of nutrients and energy. Some microbes produce extracellular polymeric substances and therefore its behaviour in pours media needs to consider both occupation by the EPS and the microbes themselves. Knowledge is lacking in this respect and therefore the aim of maximizing yield and minimizing cost remains unachieved.
Realistic models for MEOR at the conditions of the oil reservoir are missing, and reported parallel-pore models had fundamental deficiencies that were overcome by models considering the clogging of pores by microbes or biofilms, but such models have also the deficiency of being two-dimensional. The utilisation of such models in three-dimensional models has not been proven. It is uncertain if they can be incorporated to popular oilfield simulation software. Thus, a field strategy needs a simulator capable of predicting bacterial growth and transport through porous network and in situ production of MEOR agents.
Grounds of failure
- Lack of holistic approach allowing for a critical evaluation of economics, applicability and performance of MEOR is missing.
- No published study includes reservoir characteristics; biochemical and physiological characteristics of microbiota; controlling mechanisms and process economics.
- The ecophysiology of microbial communities thriving in oil reservoirs is largely unexplored. Consequently, there is a poor critical evaluation of the physical and biochemical mechanisms controlling microbial response to the hydrocarbon substrates and their mobility.
- Absence of quantitative understanding of microbial activity and poor understanding of the synergistic interactions between living and none living elements. Experiments based on pure cultures or enrichments are questionable because microbial communities interact synergistically with minerals, extracellular polymeric substances and other physicochemical and biological factors in the environment.
- Lack of cooperation between microbiologists, reservoir engineers, geologists, economists and owner operators; incomplete pertinent reservoir data, in published sources: lithology, depth, net thickness, porosity, permeability, temperature, pressure, reserves, reservoir fluid properties (oil gravity, water salinity, oil viscosity, bubble point pressure, and oil-formation-volume factor), specific EOR data (number of production and injection wells, incremental recovery potential as mentioned by the operator, injection rate, calculated daily and total enhanced production), calculated incremental recovery potential over the reported time.
- Limited understanding of MEOR process economics and improper assessment of technical, logistical, cost, and oil recovery potential.
- Unknowns life cycle assessments. Unknown environmental impact
- Lack of demonstrable quantitative relationships between microbial performance, reservoir characteristics and operating conditions
- Inconsistency in in situ performance; low ultimate oil recovery factor; uncertainty about meeting engineering design criteria by microbial process; and a general apprehension about process involving live bacteria.
- Lack of rigorous controlled experiments, which are far from mimicking oil reservoir conditions that may have an effect over gene expression and protein formation.
- Kinetic characterization of bacteria of interest is unknown. Monod equation has been broadly misused.
- Lack of structured mathematical models to better describe MEOR.
- Lack of understanding of microbial oil recovery mechanism and deficient mathematical models to predict microbial behaviour in different reservoirs.
- Surfactants: biodegradable, effectiveness affected by temperature, pH and salt concentration; adsorption on to rock surfaces.
- Unfeasible economic solutions such as the utilization of enzymes and cultured microorganism.
- Difficult isolation or engineering of good candidate strains able to survive the extreme environment of oil reservoirs (up to 85 °C, up to 17.23 MPa).
- Wellbore microbial plugging and consequent lost of injectivity (clogging).
- Dispersion of components necessary to the target.
- Control of indigenous microbial activity.
- Mitigation of unwanted secondary activity due to competitive redox processes such as sulphate reduction, i.e. control of souring.
- Microbial paraffin removal.
- Microbial skin damage removal.
- Water floods, where continuous water phase enables the introduction of MEOR.
- Single-well stimulation, here the low cost makes MEOR the best choice.
- Selective plugging strategies.
- MEOR with ultramicrobes.
- Genetically engineered MEOR microorganisms able to survive, grow and produce metabolites at the expense of cheap nutrients and substrates.
- Application of extremophiles: halophiles, barophiles, and thermophiles.
- Artificial neural network modelling for describing in situ MEOR processes.
- Competition of exogenous microbes with indigenous micro flora, no understanding of microbial activity.
Results and Discussion
Prior growth curve results for JF-2 show that after ∼20 h the bacteria stop doubling and go into a self-maintenance stationary phase (Online Resource 5). Both pH and IFT were monitored in the bioreactor for the duration of the MEOR experiment. Results show that during the first ∼20 h, a rapid drop in pH and IFT occurs (Online Resource 5), indicating that both fermentation and biosurfactant production are occurring. The pH reduction can be explained by fermentative respiration of the organism, which creates excess protons and fatty acids in the growth media which reduce pH. The composition of these fermentation byproducts was partially characterized by where the main fermentation byproduct was identified as acetate with no detection of methanol, ethanol, propanol, or butanol during fermentation.
However, the exact composition of the metabolic by products produced by JF-2 during fermentation has not been characterized. Apart from biosurfactant (which is not a strictly fermentative by product), it is unclear if other compounds produced during fermentation are essential for oil recovery.
Oil recovery versus time curves for MEOR using either the JF-2 or biosurfactant flooding solutions are shown in Fig. 1. Residual oil saturations reported at 0 h correspond to oil saturations measured after water flooding as averages of six replicate columns, and the error bars correspond to a confidence interval of 90%. Residual oil saturation values reported at times >0 h are average values of two different regions in the same column, and thus, no error bars are associated with these data points. Results show that the majority of oil recovery occurred in the first ∼80 h, while only marginal recovery occurred over the remainder of time. The 50% oil-wet columns had the largest residual oil saturations after water flooding, while the 0% oil-wet columns had the lowest residual oil saturations after water flooding. However, after MEOR, residual oil saturations for all of the columns approached a similar level. The control columns, as expected, showed essentially no change in residual oil saturation over the duration of the test, demonstrating that the oil recovery observed in the MEOR columns was in fact due to flooding with bacteria and/or their metabolic by products (i.e., biosurfactants).
Additional oil recovered (AOR) values are shown in Table 2. The results show that the overall effectiveness, in terms of AOR, for the flooding solutions tested were quite similar. AOR results for the 0% oil-wet columns are reported as n/a in Table 2 since residual oil saturations after MEOR treatment were slightly higher in these columns than after water flooding.
It is likely that residual oil upstream from the imaged volume was mobilized into the imaged volume causing these spurious results in the water-wet columns. This would obviously also be likely to happen during MEOR in the fractional-wet columns, but the effect is masked by the overall larger recovery in these columns. It is expected that if larger volumes were imaged, reasonable AOR values would have been obtained for the 0% oil-wet columns as well. However, since residual oil saturations in the control columns remained constant during the experiments, this suggests that the pumping of three pore volumes during water flooding was sufficient for obtaining residual oil saturation. Thus, the spurious AOR results in the 0% oil-wet columns were not due to premature discontinuation of the water flood.
To better understand what drives oil mobilization, oil/water interfacial mean curvature (reported as mean radius of curvature) was analyzed after water flooding and after MEOR. Themean radius of curvature values for the columns treated with either the JF-2 or biosurfactant flooding solutions are shown in Fig. 2a, b. Considering that grain diameter (0.50–1.20mm diameter) is a reasonable proxy to pore radius (0.25–0.60mm), the reported mean radius of curvature values (0.23–0.34mm) appear reasonable for these experiments. After water flooding, the smallest average radius of curvature value was observed in the 0% oil-wet columns, while the fractional-wet columns had consistently larger average radius of curvature values. These results suggest that the surface tension between the oil and the oil-wet bead surfaces may play a role in trapping since different mean radius of curvature values (and residual oil saturations) exist depending on the fraction of oil-wet surfaces that are present in the porous bead pack. Similar results are recorded by where their grainbased pore-scale model demonstrated that the existence of a single oil-wet grain in the model domain can change trapped phase topology and that, in a fractional-wet system, the range of stable curvatures decreases and tends toward larger radius of curvatures.
It should be noted, that during MEOR, residual oil saturation and radius of curvature values in the 25% oil-wet columns are consistently above the 50% oil-wet columns. Potentially, this could be explained in terms of percolation where the 50% oil-wet columns have enough interconnected oil-wet pores to percolate across the column causing lower residual oil saturations. This can be demonstrated by generating a distance map of the pore space with respect to the water-wet surfaces. Once again assuming that the average pore radius is approximately 0.25mm and then removing any pore space that is within one pore radii distance of a water-wet surface, a volume rendering of the most oil-wet regions in the pore space can be created (Fig. 3). As evident in Fig. 3, the 50% oil-wet column has interconnected oil-wet regions from the top to the bottom of the imaged volume, while the 25% oil-wet column has no interconnected oil-wet regions. This interconnectedness is critical for mobilization, as pore-network models by have demonstrated that oil located in oil-wet pores surrounded by water-wet pores cannot escape and thus becomes trapped during imbibition. Also, as fractional wettability increases, residual oil saturation begins to decrease since oil-wet percolation networks become established across the model domain.
The presence of bacterial cells in the flooding solution had little effect on interfacial curvature (Fig. 2a, b). However, this may be a result of using metabolically inactive bacterial cells during flooding, since it is unclear if metabolically active bacteria are required for interfacial attachment. Thus, different results may occur depending on the MEOR strategy taken:
- injection of nutrients to stimulate indigenous microorganisms
- injection of exogenous microorganisms(s) and nutrients
- injection of ex situ produced products
In general, after MEOR, mean interfacial radius of curvature decreases (Fig. 2a, b). This decrease in mean radius of curvature after MEOR suggests that the remaining residual oil is strongly trapped in the smallest pores and that duringMEOR, mainly residual oil blobs held under relatively low capillary pressure (i.e., large radius of curvature) are being mobilized. Oil/water interfacial isosurfaces after MEOR treatment show that the remaining residual oil is indeed trappedmostly in oil-wet pore necks since mostly positive curvatures (i.e., oil-wet curvatures) are seen in the imaged regions, and in some cases, the residual oil exists as pendular rings (Fig. 4). When comparing images of the oil/water interfaces after water flooding and after MEOR in Fig. 4, it is apparent that, in the fractional-wet columns, interfacial curvature shifts toward more positive values indicating more oil-wet curvatures. This shift in curvature is not seen in the 0% oil-wet columns (Fig. 4c1, c2). These images further suggest that during water flooding, the oil-wet regions of the bead pack do not drain since mostly water-wet curvatures are present. Not until after MEOR, when IFT was reduced, do the oil-wet regions drain as indicated by the presence of oil-wet curvatures. Figure 4 also illustrates distinct differences in the oil-water interface morphology from post water flood to post MEOR for fractional-wet systems.
The residual oil blob size distributions for MEOR columns flooded using the JF-2 and biosurfactant flooding solutions are shown in Fig. 5a, b, respectively. These distributions demonstrate that after water flooding, residual oil blob size decreases consistently as the number of oil-wet surfaces decreases and that residual oil blob size distributions shift to smaller blob sizes afterMEOR. These observations are supported by the results of, where fractional-wet columns contained fewer and larger residual phases. These results suggests that during MEOR, the largest oil blobs are mobilized and/or broken up into smaller residual blobs that remain trapped in the pore-space. These results are consistent with the results of and where residual oil blob size decreased as capillary number increased. The blob size distributions for JF-2 MEOR appear to be less dependent upon wettability than for biosurfactant MEOR, since, in the JF-2 flooding solution, final blob size distributions are quite similar for the fractional wettabilities tested. Comparing blob size distribution and fractional wettability for JF-2 MEOR and biosurfactant MEOR, we see conflicting trends (Fig. 5). This suggests that the presence of bacterial cells in the flooding solution may have an effect on the morphology of residual oil blobs; however, little difference is seen in the overall recovery of oil (Table 2). Visual inspection of the CMT images captured after MEOR indicate that the smaller trapped residual oil blobs are primarily located in oil-wet pores (Fig. 6), which is also supported by the curvature results. In Fig. 6, the water-wet beads are significantly larger than the oil-wet beads and can be visually identified. These images show that during water flooding, preferential flow paths developed through the water-wet pores and that a portion of the oil-wet pores drained after MEOR; however, residual oil still remained in the smallest oil-wet pore regions.
The relationship between average residual oil blob size and the fraction of oil-wet surfaces in a column is shown in Fig. 7a, b. The experimental results show opposite trends post water flooding and post MEOR. After water flooding, our experimental results contradict experimental and pore-network model simulations reported in the literature. Both studies of and report that residual oil saturation and residual oil blob size decrease with increasing percentage of oil-wet surfaces in a porous medium. However, the capillary number in the experiment is two orders of magnitude higher than our capillary number (1.6 × 10−8) obtained during water flooding. During MEOR, the capillary number in our experiments increases by one order of magnitude because of IFT reduction at which point we obtain a similar trend as that for.
This finding suggests that the relationship between residual oil blob size (and the amount of oil recovered) and fractional wettability is not universal and could be dependent on the interplay between viscous and interfacial forces (i.e., capillary number).
Based on the presented results, we are able to suggest a conceptual model and summarize the prevalent mechanisms that control water flooding and MEOR in our fractional-wet systems. Each porous medium can be divided into two domains: (1) a water-wet domain and (2) a oil-wet domain. During water flooding, the water-wet domain proceeds through dynamic forced imbibition where either snap-off or frontal displacement can occur, while the oil-wet domain proceeds through dynamic forced drainage where an entry pressure must be overcome before drainage can occur. At low capillary number, the oil-wet domain does not drain since viscous forces are not large enough to overcome the entry pressure required to flood the oil-wet domain. Thus, flooding proceeds through the water-wet domain resulting in (1) large residual oil blobs, (2) mostly water-wet curvatures, and (3) large interfacial radius of curvature. Simulation results presented by where menisci movement in fractional-wet media was investigated show that advancement of the water phase toward the oil-phase during imbibition can be impeded by the presence of an oil-wet grain, which is consistent with our experimental results at low capillary number (i.e., during water flooding). At higher capillary number (e.g., duringMEOR through a reduction in IFT), the oil-wet domain drains and flooding proceeds through both the water-wet and oil-wet domains. Thus, reduction in IFT reduced the entry pressure needed to initiate drainage of the oil-wet pores resulting in (1) small residual oil blobs, (2) more oil-wet curvatures, and (3) a decrease in interfacial radius of curvature.
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