Biocorrosion in oil recovery systems. Prevention and protection. An update

Héctor A. Videla¹ and Liz Karen Herrera Quintero²

¹INIFTA, Universidad Tecnológica Nacional. FRLP.1900. La Plata, Argentina. Phone/fax: + 54 2214893142. hvidela@infovia.com.ar.

²Instituto de Ciencia de Materiales de Sevilla (CSIC). 41092 Isla de la Cartuja (Sevilla), España. Phone: +34954489527 - Fax: +34954460665. lkaren@icmse.csic.es

Abstract

Seawater injection systems are widely used in oil recovery. Biocorrosion of metallic structures and oil souring due to the action of sulfate-reducing bacteria are two of the most relevant problems affecting these systems. A proper understanding of the complex biological and inorganic processes occurring at the surface of metallic structures is needed to carry out effective treatments for biocorrosion prevention and protection. Moreover, monitoring of biocide treatments in real time is mandatory to achieve an effective control. Recently, a wide variety of innovative techniques to study biocorrosion and monitoring methods has been developed and are presently used with new biocide formula taking especially into account environmental preservation. An updated review of these advances in the field is briefly offered at the end of this presentation.

Key words: Sulfate-reducing bacteria (SRB), secondary oil recovery, water injection systems, biofilms,  biocorrosion.

Biocorrosión en sistemas de recuperación de petróleo. Prevención y protección. Una actualización

Resumen

Los sistemas de inyección de agua de mar son ampliamente utilizados en la producción del petróleo. La corrosión de estructuras metálicas y el agriamento del pozo productor son dos de los problemas más importantes que afectan a estos sistemas. Un entendimiento adecuado de los complejos procesos biológicos e inorgánicos que ocurren en la superficie de las estructuras metálicas es necesario para poder llevar a cabo tratamientos eficaces de protección y prevención de la corrosión. Más aún, un monitoreo en tiempo real de los tratamientos biocidas es imperativo para alcanzar un control eficaz del sistema. En los últimos años se han desarrollado una gran variedad de técnicas innovativas en el estudio de la biocorrosión así como métodos más eficientes para el seguimiento de los tratamientos de control en campo con el uso de nuevos biocidas con especial énfasis en el cuidado y la preservación del medio ambiente. En la última parte de este artículo se ofrece una revisión actualizada de estos avances en la materia.

Palabras clave: Bacterias sulfato-reductoras (BSR); recuperación secundaria de petróleo, sistemas de inyección de agua; biofilms; biocorrosión.

Recibido el 30 de Junio de 2006 En forma revisada el 30 de Julio de 2007

Biocorrosion fundamentals

Microorganisms influence corrosion by changing the electrochemical conditions at the metal/solution interface. These changes may have different effects, ranging from the induction of localized corrosion, through a change in the rate of general corrosion, to corrosion inhibition [1].

At the metal/solution interface a very active interaction between the corrosion product layers and the biofilms can be expected. The consequent corrosion behavior of the metal will vary according to the degree of this reciprocal interaction and a concept of a new biologically conditioned interface must be kept in mind [2]

The oil and gas industry is affected by biocorrosion in many of the activities involved in recovery, processing, transport and storage. Wherever free water is encountered in oilfield operations there have been reported incidents of bacterial activity resulting in souring of oil reservoirs and corrosion of pipelines and process equipment [3].

The most general case of biocorrosion in the oil industry is the sulfate-reducing bacteria (SRB) induced corrosion of carbon steel in a corrosive medium like injection water. In the presence or absence of biofilms and extra cellular polymeric substances (EPS), the protective characteristics of the corrosion product may vary. In the presence of sulfur species (either biogenic or abiotic) carbon steel firstly develop a film of mackinawite that later changes through several chemical and electrochemical paths to more stable iron sulfides [4] In all cases iron sulfides are characterized by strong cathodic effects on the hydrogen reduction reaction, causing an indirect increase in the corrosion rate. Thus, SRB induced corrosion of carbon steel must be interpreted mainly through the process of breakdown of steel passivity by corrosive metabolic products generated by SRB [5]. Depending on the sulfide concentration in the medium and on the presence or absence of biofilms and EPS, the protective characteristics of the corrosion products may change. Biogenic layers of corrosion products can offer enhanced protection by improving the adherence of the passive film to the metal, but can also increase corrosion by inducing heterogeneities at the metal surface [6].

SRB induced corrosion of carbon steel. Background

In our research group at INIFTA, University of La Plata, Argentina, the main efforts of our work during the 80’s were focused to elucidate the electrochemical aspects of the anaerobic corrosion of iron by developing a series of laboratory experiments using alkaline and neutral buffered solutions as well as SRB cultures in saline media under well defined experimental conditions [7, 8]. Given the results of these studies a bioelectrochemical interpretation of the biocorrosion process of carbon steel in anaerobic environments could be summarized as follows [10]: a) biogenic sulfides affect localized corrosion of carbon steel in a similar manner than abiotic sulfides. The type and intensity of sulfide effects are closely related to the nature of the protective film already present on the metal surface; b) in neutral media sulfide ions led to the formation of a poorly protective film of mackinawite; c) the anodic breakdown of passivity would be the first step of the corrosion reaction. Thus the role of SRB would be indirect through the production of final (sulfides, bisulfides, hydrogen sulfide) or intermediate metabolites (thiosulfates, polythionates) which are corrosive to carbon steel; d) cathodic effects such as cathodic depolarization attributed to SRB hydrogenase or to iron sulfide films would be developed later that passivity breakdown when the corrosion process is in progress; e) the corrosive action of biogenic sulfides can be enhanced by other corrosive ions already present in the medium (e.g. chlorides) of through the action of microbial consortia within biofilms on the metal surface.

Present understanding on SRB induced corrosion of iron

By the end of the 90’s and the beginning of the new century several surface analysis techniques, electrochemical experiments and microscopy observations were employed to clarify the role of biotic and abiotic sulfide films in the corrosion behavior of steel in saline media [9-13]. Microbiological experiments were performed under controlled laboratory conditions using a strain of Desulfovibrio alaskensis (D. alaskensis), known for its ability to produce EPS, isolated from a soured oil reservoir in Alaska [14]. Atomic force microscopy (AFM) was applied to image the SRB biofilms, current transients were measured to determine the electrochemical behavior of steel, and SEM observations coupled with EDAX, as well as XPS, XRD and electron microprobe analysis (EPMA) were carried out to examine the structure and composition of biotic and abiotic sulfide films on the surfaces of steel specimens. Further studies developed in our laboratory [9-13] allowed us to conclude that SRB influenced corrosion of steel is markedly affected by the nature and the structure of the sulfide films produced during the metal dissolution. The environmental characteristics of the metal / biofilm /medium interface and its surroundings (pH, ionic composition, oxygen levels, EPS distribution) control the chemical and physical nature of corrosion product layers and may change their effects on the metal behavior from corrosive to protective. In marine environments, the impact of sulfur compounds on corrosion is enhanced by other aggressive anions, such as the widely distributed chlorides, already present in the medium. 

The entrance of oxygen into the anaerobic environment accelerates corrosion rate, mainly through a change in the chemical nature of iron sulfides and elemental sulfur production. Both chemical species can provide additional cathodic reactants to the corrosion process, acting as electron carriers between the metal and the oxic  interface within the biofilm.

The main conclusions obtained from these studies have been: i) both abiotic and biotic iron sulfide films are related to the formation of tubercles on the steel surface. However, for biotic solutions FeS (mackinawite) predominates, whereas in abiotic media FeS2 (pyrite) is the major iron sulfide present, ii) the structure of the outer crust of iron sulfide acts as a barrier for the diffusion of ions towards and from the pit cavity; iii) biotic films are more adherent to the metal surface, while abiotic films are flaky and loosely attached; iv) the previous history of the sulfide film may play a relevant role in the corrosion behavior of steel. Depending on the sulfide concentration in the medium and on the presence or absence of biofilms and EPS, the protective characteristics of the corrosion product may change. Biogenic layers of corrosion products can offer enhanced protection by improving the adherence of the film to the metal, but can also increase corrosion, inducing the presence of heterogeneities at the metal surface.

The differences between biotic and abiotic media containing identical levels of corrosive compounds (i.e. iron sulfides) may be mainly attributed to the presence of extra cellular polymers and to the heterogeneities created at the metal surface by the formation of a biofilm. In addition, the physicochemical parameters of the corrosion product layers may change in the absence or a presence of a biofilm, rendering these layers either protective or corrosive. This feature can explain the observed differences in corrosion behavior of steel between abiotic and biotic environments with respect not only to localized corrosion but also to the hydrogen attack developed as embrittlement and crack growth. 

This updated interpretation of SRB induced corrosion of carbon steel has been supported by recent publications on the subject [9-15].

Biocorrosion prevention and control. An update

One of the classic concepts for maintaining an industrial system free of biocorrosion deleterious effects is "to keep the system clean" [16]. Although this is a very difficult task to accomplish in practice, there are several general methods that can be used. These methods can be broadly classified in: i) physical and, ii) chemical. Among the former, flushing is perhaps the most simple, although of limited efficacy. A special case is the use of flushing supported by cleaners or jointly with chemical agents that induce biofilm detachment. Abrasive or non-abrasive sponge balls are frequently employed in the industry. The former could present problems related with protective passive films that can be damaged, and the second is not very effective with thick biofilms. 

With reference to chemical methods, the most common approach for controlling biofouling problems in industrial water systems is the use of biocides. These substances can be either oxidizing or non-oxidizing toxicants. Chlorine, ozone and bromine are three typical oxidizing agents of industrial use. Non-oxidizing biocides are claimed to be more effective than oxidizing biocides for an overall control of algae, fungi, and bacteria. They have a greater persistence and many of them are pH independent. Often a combination of oxidizing and non-oxidizing biocides or of two non-oxidizing biocides is used to optimize microbiological control of industrial water systems. Typical biocides of the second type are formaldehyde, glutaraldehyde, isothiazolones and quaternary ammonia compounds.

Increasing legislative requirements and the necessity for greater environmental acceptability, have contributed to restrict the use of some traditional biocides and to develop either new compounds or carefully selected blends of existing biocides.

Among the most promising non-oxidizing biocides the THPS (tetra kis-hydroximethil phosphonium) is a new compound of wide spectrum effective on bacteria, fungi and algae. It is being widely used in the oil industry due to its capacity of dissolving the ferrous sulfide. Its main advantage is its low environmental toxicity [17].

Monitoring Biocorrosion

Monitoring programs for biocorrosion have been mainly focused in the assessment of planktonic population in samples and generalized corrosion by using corrosion coupons or some kind of resistance or polarization resistance probes. 

The main objections to these monitoring programs are: i) the planktonic population does not properly reflect the type and number of organisms living in the biofilm and causing biodeterioration problems; ii) susceptibility of planktonic microorganisms to antimicrobial agents markedly differs from that of sessile microorganisms within the biofilm, mainly because of the protective action of their EPS. Thus, the monitoring methods adopted must provide information of well-established biofilms like those developed in water systems. For corrosion assessment, the electrical resistance method that has been widely used in the industry, is only appropriate for indicating a change in the general corrosion rate, but the results are difficult to interpret in the presence of localized corrosion like pitting, the most frequent form of attack found in biocorrosion cases [18]. If biofilms or localized corrosion is present, the polarization resistance will reveal that something is happening, but may not give an accurate measure of the corrosion rate. Only the use of any of these techniques jointly with other electrochemical methods or parameters assessing localized corrosion hazard can provide valuable data for monitoring the deleterious effects of biocorrosion and biofouling. 

Owing to the variables of dissimilar nature involved in biofouling and biocorrosion, an effective monitoring program, either for the laboratory or the field, must necessarily supply information on water quality, corrosive attack, sessile and planktonic bacteria populations, biofilms characteristics, and chemical composition of inorganic and biological deposits [19].

Sampling devices for monitoring biocorrosion and biofilms can be simultaneously used to assess corrosion attack after the removal of biological and inorganic deposits, giving a wider and more useful information. Sampling devices fall into two main types: a) directly implanted and, b) side-stream implanted. Metal coupons, generally made with the same structural material of the system, present a known surface area, to enable an accurate count of sessile bacteria per square cm after biofilm detachment. Coupons are mounted in holding assemblies which are inserted in the pipe work of the laboratory or industrial system.

Recent advances in monitoring and control of biocorrosion in oil production

Despite the great advances in environmental microbiology the methods used to monitorbacteria in oil field systems have remained unchanged for nearly 30 years. The current recommended standard [20] is mainly based on the enumeration of a limited range of groups of bacteria by using viable culturing techniques [21].

Regarding culturing techniques recent advances in molecular microbial ecology have shown that only a small proportion of the microorganisms present in a natural ecosystem is cultivable, ranging from 0.001 to 15% depending on the environment [22].

Promising techniques for the future advances in biocorrosion are molecular techniques involving the direct analysis of natural bacterial populations from their genetic material (no culture required) mainly through the sequence analysis of their 16S rRNA gene. These techniques offer the potential to a) identify dominant bacteria in natural and industrial ecosystems without the limitations of viable culture techniques; b) determine the proportion of corrosion contributing bacteria in the total population; c) identify susceptible and persistent bacteria to biocides; d) assess the changes in the overall composition of bacterial populations caused either by the use of biocides or fluids composition modifications. The classical molecular analysis sequence involves: total DNA extraction, amplification of 16S rRNA related genomic sequences by PCR, separation of these PCR products by a community fingerprint technique such as DGGE or cloning in plasmid libraries, sequencing of the separated DNA fragments and identification by comparison with DNA sequence databases.

Although molecular biology methods were used to assess corrosive microorganisms in oil production at the beginning of the 1990’s [23] an active research is being developed in this field mainly in relation to its use in water injection systems [24-26]. A large bank of 16S rRNA sequence data on microorganisms is presently available due to the progress of microbial ecology and the advances in molecular techniques.

Recent work of LeBorgne et al. [27] showed that the interpretation of microbial presence and its effects in the system were misled by the use of artificial culture media. These results suggest that the microbiological analysis of waters in industrial systems by culturing methods may not be adequate. Facultative anaerobic bacteria from the Shewanella and Vibrio genera were isolated and repeatedly detected from the injection water suggesting that these bacteria might be persistent and less susceptible to the biocide treatments. Evaluation of the corrosive activity of isolated microorganisms indicated that they were more aggressive in mixed than in pure cultures.

The use of Biocompetitive Exclusion (BE) strategies is increasing in the oil industry to inhibit the SRB activity that causes reservoir souring and biocorrosion. The addition of nutrients that stimulate the growth of competing bacteria populations (namely nitrate-reducing bacteria (NRB)) would effectively displace SRB from a microbial community by BE. Thus, the addition of nitrate can induce a population shift from SRB domination to NRB domination [3, 28]. 

The use of nitrate to control SRB and hydrogen sulfide production in oil fields is nowadays a new biotechnology which effectiveness has been demonstrated in laboratory investigations and in several field studies [29]. Although the microbiological basis is well understood it is still not clear whether heterotrophic or autotrophic NRB play the most important role. From the present knowledge nitrate amendment (in some cases phosphates or organic acid) stimulates NRB in the oil field waters and there appears that an inoculum of NRB would not be necessary. 

This replacement of generally toxic biocides by this environmentally friendly approach has proven to be successful in the oil fields of the North Sea [3, 29] not only to control sulfide levels but also to reduce biocorrosion effects. 

However, after an expertise of six years in the use of this technology its major drawbacks can be briefly enumerated as follows:

a. Stimulated population may be dominated by uncultivable species

b. SRB are not killed and activity suppression might not be indicated by enumeration methods

c. Some SRB are also NRB and may be enumerated by both SRB and NRB growth media d. Increase of localized corrosion could be caused by nitrate treatments when polysulfide and thiosulfate through simultaneous oxidation of sulfide and reduction of nitrate are produced.

Another innovative approach to improve the effectiveness of biocide treatments for the control of corrosive bacteria in oil fields is the use of modelling of biocorrosion risks. 

On this matter, an interesting approach for monitoring mitigation of biocorrosion risk in pipelines has been recently presented [30]. 

Nowadays the only available corrosion risk assessment models have been only focused on electrochemical corrosion where main rate influencers are pH, CO₂ and H₂S.

Now an assessment to predict pitting rates due to biocorrosion in oil field environments mainly based on: a) quantify bacterial numbers and activity; b) qualitative assessment of bacterial growth and activity, would be available. 

The fundamentals of this new approach would be to combine simple bacterial growth and sulfide production kinetics to model the development of a sulfidogenic biofilm as the essential first stage for biocorrosion process and later to apply an amended abiotic corrosion model as the predictor of biocorrosion [31].

The main goal will be to allow routine bacterial monitoring data to be interpreted in terms of biocide efficiency and biocorrosion mitigation. One of the major drawbacks of biocorrosion monitoring using field monitors has been until now the incapacity to show in real time biocorrosion and biofilms interactions. 

On this respect corrosion sensors and electrochemically based corrosion assessment technology have received special attention nowadays in order to find effective and reliable tools to detect in real time biocorrosion effects. 

As an example, an electrochemical sensor for monitoring biofilms on metallic surfaces in real time has been presented [32]. The system provides an immediate indication of the condition  of biological activity on probe surfaces and it is a powerful tool to optimize biocide treatment.

This type of probe consists [33] of a stack of nominally identical stainless steel discs (or other passivemetals such as titanium) that are configured as a right circular cylinder, the electrodes beingelectrically isolated from each other and from the body of the probe. One set of discs is polarized negative to the other for a short period of time each day. The externally applied potential creates different local conditions on the stainless steel electrodes and provides a current that can be readily measured. If the applied current is tracked on a daily basis, a significant increase in that current provides a method to detect the onset of biofilm formation. Thus, the difference in the magnitude of the applied current from the baseline (the applied current in the absence of a biofilm) provides a measure of biofilm activity. 

This kind of device has been successfully used jointly with integrated data acquisition and data analysis capabilities for monitoring biofilm activities on metallic surfaces to optimize biocide additions in a plant. The use of this real - time monitors of biofilms in oil production activities has been recently reported [34, 35]. The conclusions of these tests have been: a) biofilm activity detector was sensitive to biofilm formation (especially for SRB biofilms); b) the bioprobe gave fast information about microbial activity providing an on-line assessment of biofilm growth and treatment effectiveness; c) the biofilm activity probe detected biofilm activity earlier than the conventional coupons; d) on-line monitoring of physicalchemical parameters was fundamental to distinguish between SRB growth and oxygen intake.

Concluding remarks

Biofilms mediate the interaction between metal surfaces and the liquid environment leading to an important modification of the metal solution interface by drastically changing the types and concentrations of ions, pH and oxygen levels. As a consequence of these changes the electrochemical behavior of the metal can be modified from active to passive and even a microbial inhibition of corrosion can be reached.

The use of new analytical tools, molecular biology methods and innovative electrochemicaldevices is increasing in biocorrosion research and these new advances can allow to reach a sound interpretation of bioelectrochemical phenomena at the metal surface, to develop in real time monitoring devices and to develop methods to control microbial deleterious effects through environmentally friendly approaches such as be.

In the future the application of updated biofilm monitoring strategies using a combination of molecular and microscopic techniques will be necessary to achieve an effective control of SRB activity in water injection lines.

References

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