Microbial Enhanced Oil Recovery: An Overview and Case Studies

Abstract

After traditional oil recovery processes, large amount of residual oil are still trapped in the pore spaces of the complex capillary network of the reservoir. MEOR (microbial enhanced oil recovery), a promising tertiary oil recovery method involves the utilization of indigenous microbial species capable of producing various secondary metabolites which further enhances the recovery of oil through their plugging, viscosity or interfacial tension reduction mechanisms. The chapter represents the overview of MEOR, mechanism involved in the process and field trials. Furthermore, microbial based mechanisms are widely demonstrated. The chapter confirms the credibility of MEOR process towards the enhanced oil recovery.

1. Introduction

Generally, after the primary (natural pressure) and secondary recoveries (gas injection, water flooding etc.) of crude oil, around 35–55% of oil is left behind in the reservoir that needs to be extracted through other improved or enhanced oil recovery techniques [1]. There are many IOR and EOR techniques currently in practice such as miscible gas injection, polymer flooding and thermal EOR methods [2]. The selection of recovery methods is strongly influenced by the country’s economy. Therefore, the development of low-cost technologies that bring maximum oil reserves to production is a major topic of energy research today [2].

2. Microbial enhanced oil recovery (MEOR)

An economical approach for the recovery of unrecovered oil is MEOR. MEOR often refers to injecting live microorganisms containing essential nutrients into oil reservoirs through injection wells. Under favorable environmental conditions in the reservoir, the infused microorganisms grow exponentially within the reservoirs and develop a variety of metabolites that play a crucial role in the mobilization of unrecovered oil that leads to enhanced oil recovery (Figure 1).

Figure 1.

Schematic representation of microbial enhanced oil recovery.

Various factors affect the growth of the microorganisms in oil reservoir viz. pressure, porosity, permeability, temperature, pH, dissolved solids, the salinity of the reservoir and availability of nutrients [3]. Therefore it becomes necessary to develop suitable microorganisms which can sustain the reservoir environment and can produce useful metabolites for recovering the oil. MEOR is believed to be able to extract up to 50% of the unrecovered oil remaining in a reservoir after primary and secondary recovery processes have been exhausted [1].

3. Requisite for MEOR

Instead of the recent renewable energy source available in the market, the world is still dependent on crude oil and petroleum-based products. To establish a green economical process viable research has been going to overcome various drawbacks such as extraction of crude oil from the reservoirs, reducing the harmful effect of crude oil on the environment etc. [1].

There are various types of recovery processes involved in oil extraction as shown in Figure 2. Firstly, primary oil recovery where the well pressure allows the gushing of oil from the reservoir led to less than 30% recovery of oil [4]. Once, the natural pressure of drilled well goes down other enhanced recovery methods were incorporated such a process is called secondary oil recovery methods. Secondary methods involve an injection of water at the wellhead that pushes the oil from the reservoirs. Secondary process accounts for 30–60% oil recovery [5]. To extract the remaining oil from the reservoir, an enhanced oil recovery process also known as a tertiary process can be used which involves surfactants, polymers, solvents, emulsifiers, acids, and dispersants along with the secondary method [6].

Figure 2.

Represents the type of recovery process for the extraction of oil.

Microbial enhanced oil recovery involves two distinct approaches: First, bio augmentation where the exponentially grown microorganisms were injected in the reservoir that is capable of surviving and producing metabolites under harsh conditions. Eventually, the microbial species were occupied the metabolic niche within the oil reservoir. The second approach is an in-situ simulation where the nutrients were only added in the well that allows the inhabitant of the well to grow and proliferate to improve the recovery. Before finalizing the approach that needs to be implemented, researchers tend to explain the microbial niche found in the reservoir whether the niche is detrimental or beneficial [7].

4. Mechanisms involved in MEOR

The diverse types of mechanisms, metabolites are generated in the process of MEOR that make it complex and comprehensive. Insight knowledge of the MEOR mechanism is the source for determining the feasibility and efficiency of MEOR. In the MEOR process, nutrients are injected into the well in order to promote the proliferation of indigenous microbes. When the inorganic salts are added, the microorganism utilizes the crude oil as a carbon source and converts the complex form into the simple form. Conversely, when the exogenous carbon sources such as sucrose, glucose and molasses are added this led to the generation of more metabolites [1, 8].

The MEOR mechanism can be divided into two parts viz. utilizing biomass or biopolymer of microorganisms that selectively block the high permeable zones and facilitate the recovery of oil; another method is utilizing the solvents or biosurfactants that reduces the interfacial tension within the reservoir. If only the viscosity of crude oil is taken into account then recovery can be done in two ways, either reducing the oil-water interfacial tension or by reduction of viscosity through microorganisms or enzymes such as alkyl-monooxygenase/hydroxylase that degrade the heavy fraction of the oil [9].

4.1 Selective plugging

The major issue related to the recovery is the high porosity of the media found in the reservoirs. The oil saturates the media and accumulated in the inaccessible regions of the media such regions are called thief zones [10, 11]. The aim of the process is to release the entrapped oil from the thief zones, selective plugging involves the clogging of the media of high permeability that prevent the accumulation of oil and also after water flooding divert the water directly into the oil-rich zones that push the oil out of the reservoir. The biomass and biopolymers are used that are attached to the surface of the media where they proliferate. This led to the generation of biofilm and cluster that prevent the seepage of oil into the high permeable regions [12].

Biopolymer is the high molecular weight molecules metabolized by diverse microorganisms that contain hydroxyl groups which make it dipole, ion-dipole and hydrogen bonds with itself or with other substances to develop the network-like structure. These networks form a barrier to enhance the recovery of oil as reported in recent studies [13, 14].

4.2 Reduction of interfacial tension (IFT)

Biosurfactants are amphipathic molecules that contain both hydrophobic and hydrophilic moieties that can be produced in situ by microorganisms. Biosurfactants are effectively used in the recovery of oil from recalcitrant reservoirs [15]. The poor oil recovery is either due to the high viscosity of oil or low permeability of rock formation can be improved by utilizing the biosurfactants that have the capability to reduce the interfacial tension between the aqueous phase and oil saturation [16].

Biosurfactants are capable of reducing the capillary forces that halt the oil movement through the rock pores. Viscous forces are opposing force generated by the capillary forces that promote the flow in the reservoir. The parameter between the two forces is defined as a capillary number that accesses the chances of mobilization of residual oil within the reservoir. There is a direct correlation between the capillary number and mobilization of oil. Therefore, the biosurfactant can enhance the capillary number by reducing the IFT that facilitate the improved oil recovery [17].

4.3 Biogases, solvents and biogenic acids

MEOR mechanism involves the various metabolites such as solvents, gases and organic acids. Various thermophiles bacteria were reported that have the capability of synthesizing volatile fatty acid, biomass and gases that collectively help in the recovery of around 19% of oil in a core flood assay [13]. Different gases such as CO₂, H₂, CH₄ and other gases were produced during the fermentation of carbohydrates and other hydrocarbons. These gases help in pressurizing the crude oil and also reduce the viscosity of the oil.

Organic acids such as formic acid, acetic acid propionate that are low molecular weight were produced by the microorganisms during the fermentation process. These organic acids are capable of dissolving carbonate rocks that improves the reservoir permeability, whereas, gases and solvents can be reduced in viscosity by dissolving in crude oil [9].

4.4 Biodegradation

One of the most attractive mechanisms is biodegradation in the MEOR process. In this process, the microorganisms utilizes the crude oil as a carbon source and convert the heavy fraction into the light components that fundamentally alters the viscosity, fluidity and properties of the crude oil thus, improving the oil recovery. Biodegradation is of two types such as aerobic and anaerobic biodegradation. In aerobic biodegradation initiated when the bacteria have taken up the crude oil that was oxidized by the oxygenase and peroxidases enzymes (Figure 3). The peripheral degradation pathway converts the organic components into various intermediates of central metabolism which led to the synthesis of the cell biomass from the precursor (Acetyl-CoA, succinate, and pyruvate) [18].

Figure 3.

Schematic diagram of steps involved in aerobic degradation of crude oil.

In general, the environment in a reservoir is anaerobic means no accessibility to oxygen, so various studies have reflected that hydrocarbon degradation was carried out by anaerobic microbes [19]. The in-depth investigation of the process proved that biochemical processes and genes involved belong to the anaerobic biodegradation [20].

On a laboratory scale, microbial-based enhanced oil recovery processes were also reported that showed diverse methanogens (Methanothermobacter thermoautotrophicus), bacterial species (Thermoanaerobacter brockii, Thermoanaerobacter italicus, Thermoanaerobacter mathranii, Thermoanaerobacter thermocopriae, Clostridium sp.) that significantly appeared an efficient model in core flood studies and can be implemented in the field trials [13, 16, 21].

5. Case studies

Worldwide, numerous MEOR field trials have been implemented that attain varying degrees of success. Statistical dataset showed that in field test more than 90% of MEOR trials represent encouraging affected [22]. United States was the first country to conduct MEOR field trial in 1954 with Clostridium acetobutylicum. The microbes were injected along with molasses in the Lisbon oil field of Arkansas which showed the generation of various metabolites such as gases, acid and biosurfactant [22, 23]. A field trial carried out in the Bebee field in Oklahoma showed the nine times lower biosurfactant concentration that was lower than the minimum concentration required for improved oil recovery, Bacillus strain was introduced in the field trail [24].

In China, field trials of MEOR had conducted in various oil fields such as Daqing, Shengli, Xinjiang, Jilin, Liaohe, Qinghai, and Changqing. In MEOR application, microbial wax removal was one of the major types used report suggested that a total of 11 fields have d in 1739 wells [25]. In Shengli Oilfield, huff and puff was carried out in 1640 wells, which led to an incremental production of 219,000 tons of crude oil, whereas in Daqing oilfield, microbial huff and puff were performed in 518 wells in 10 blocks that showed an increase in production up to 64,000 tons. In 2012, Daqing oilfield, microbial flooding was carried out in 45 injection wells that led to the total recovery of 56,837 tons of crude oil [26].

In Romania, various field trials were performed in the oil field that produced an average of 100 and 200% crude oil [22]. After the injection of hydrocarbon-degrading bacteria and anaerobic bacteria in Piedras Coloradas oil field in Argentina for 1 year, the average production of the wells was increased to 66% and the viscosity of the oil was also reduced effectively. Researchers were also tried to inject facultative anaerobic bacteria along with the nutrients for more than a year and there was a 20% incremental recovery of crude oil which accounts for 27,984 m³ [27].

In Canada, endogenous microbial flooding was performed in the Saskatchewan oil field and results showed a 10% reduction in the water content in the first phase. In the second phase of the test (after 3 weeks), the production of crude oil was increased from 10.18 to 16.7 m³/d [28]. In the Loco filed of Canada, a specially adapted strain of Clostridium was injected along with the nutrients showed promising results such as reduction of oil viscosity due to the production of carbon dioxide; it tends to improve the mobility and sweep efficiency [29].

In India, research on MEOR started in the nineties and since then MEOR processes involving in-situ stimulation and augmentation of microorganisms have been tested successfully with the production of enhanced production of oil. Oil & Natural Gas Corporation Ltd. (ONGC) had developed a thermophilic anaerobic bacterial consortium comprising Clostridium and Bacillus species and suitable for MEOR having reservoir temperature between 45°C and 65°C. Field trials were carried out in the Kosamba and Badarpur oil fields of ONGC with oil gain of 1150 m3 and 1200 m3 respectively. Further ONGC initiated research with The Energy and Resources Institute (TERI) and hyperthermophilic and halophilic anaerobic bacteria suitable for MEOR in oil reservoirs up to 90°C could be developed. The process has been patented in India, USA and Canada. A Joint Venture company ONGC-The Energy and Resources Institute Biotechnology Limited (OTBL) is implementing the MEOR process in huff and puff mode on a commercial scale. So far MEOR has been done in more than 125 oil wells mostly stripper wells with an average oil gain of about 300 m³ per job/well. Further, research on the development of reservoir and oil specific bacterial consortia led to a yield of 2–8% additional oil recovery over and above waterflood recovery.

ONGC and TERI are in the process of developing the MEOR process for heavy oil reservoirs. Laboratory investigations and core/sand column flood tests are very impressive with a 13% recovery over waterflood recovery with 86% viscosity reduction after treatment of Becheraji oil. Another consortium could give a 13% recovery with a 42% reduction in oil viscosity of Lanwa oil. Field tests in five wells reported viscosity reduction of 17–25% in Becheraji wells and 11–18% in Lanwa wells. These fields were injected with strict anaerobic bacterial consortia habitant of the reservoirs. Statistically, 12 wells in four fields showed a threefold increase in crude oil production and efficient reduction in the water cut [16].

6. Advantages and limitations of the MEOR process

Numerous advantages associated with the MEOR process are cost-effective process as it involves the bacteria, nutrients and/or other natural products that are easily accessible, it is an economically attractive alternative, consume less energy as compared to the other EOR processes, the benefits of bacterial activity within the reservoir are amplified with time, whereas the opposite is true for other EOR technologies, the involvement of biodegradable products which make the process environmentally friendly [30]. A variety of metabolites such as acids, gas, solvents, surfactants, polymer etc. can be produced by bacterial consortia in the reservoir itself that can work simultaneously to recover the oil through their cumulative effect.

Another advantage including that the microbial processes can be simulated in situ within the reservoir, thus minimizing or eliminating the need to accommodate large storage facilities onsite/offshore (Figure 4). No extra infrastructure facility is required to set up for the execution of the MEOR process in the field, particularly for the in-situ simulation process.

Figure 4.

Advantages of overall MEOR process compared to other recovery processes.

Various limitations of MEOR processes are it is a complex process as specific bacterial activities are dependent on the condition of reservoirs. Majorly, MEOR field trials were conducted on the stripper wells reduces the microbial enhance recovery thus, impacted the oil recovery. MEOR is a slower process as it takes weeks or even months for proper outcomes. The production of microorganisms in the laboratory under desired reservoir conditions has been proven difficult.

7. Conclusion

Microbial enhanced oil recovery (MEOR) is advantageous over other recovery processes because of numerous reasons such as being environmentally friendly, economical; requiring fewer amounts of energy and operationally simple. In this chapter, the types of recovery, mechanisms of action and various fields’ trials were reviewed that confirms the ultimate oil recovery using MEOR. To gain complete insight into microbial action assisted recovery, extensive field trials in different reservoir environment conditions are needed to generate datasets that will lead to the development of sustainable microbial systems and the defined mechanisms that are effective in the recovery of unrecovered crude oil which is left in the reservoir for a variety of reasons. Most of the trials have been done in huff and puff mode but large scale applications can better be done in flooding mode. The choice of in-situ bio-simulation and bio-augmentation approaches largely depends on the type of in-situ microflora present in the reservoir. The in-situ approach is more cost-effective with minimal or no effect on the ecosystem.