Comparison of Nitrate and Perchlorate in Controlling Sulfidogenesis in Heavy Oil-Containing Bioreactors

Nitrate and oxygen significantly changed the abundance rather than structure of sulphate-reducing and sulphur-oxidising bacteria in water retrieved from petroleum reservoirs

Sulphate-reducing bacteria (SRB) are the main culprits of microbiologically influenced corrosion in water-flooding petroleum reservoirs, but some sulphur-oxidising bacteria (SOB) are stimulated when nitrate and oxygen are injected, which control the growth of SRB. This study aimed to determine the distributions of SRB and SOB communities in injection-production systems and to analyse the responses of these bacteria to different treatments involving nitrate and oxygen. Desulfovibrio, Desulfobacca, Desulfobulbus, Sulfuricurvum and Dechloromonas were commonly detected via 16S rRNA gene sequencing. Still, no significant differences were observed for either the SRB or SOB communities between injection and production wells. Three groups of water samples collected from different sampling sites were incubated. Statistical analysis of functional gene (dsrB and soxB) clone libraries and quantitative polymerase chain reaction showed that the SOB community structures were more strongly affected by the nitrate and oxygen levels than SRB clustered according to the sampling site; moreover, both the SRB and SOB community abundances significantly changed. Additionally, the highest SRB inhibitory effect and the lowest dsrB/soxB ratio were obtained under high concentrations of nitrate and oxygen in the three groups, suggesting that the synergistic effect of nitrate and oxygen level was strong on the inhibition of SRB by potential SOB.

Burning question: Are there sustainable strategies to prevent microbial metal corrosion?

The global economic burden of microbial corrosion of metals is enormous. Microbial corrosion of iron-containing metals is most extensive under anaerobic conditions. Microbes form biofilms on metal surfaces and can directly extract electrons derived from the oxidation of Fe0 to Fe2+ to support anaerobic respiration. H2 generated from abiotic Fe0 oxidation also serves as an electron donor for anaerobic respiratory microbes. Microbial metabolites accelerate this abiotic Fe0 oxidation. Traditional strategies for curbing microbial metal corrosion include cathodic protection, scrapping, a diversity of biocides, alloys that form protective layers or release toxic metal ions, and polymer coatings. However, these approaches are typically expensive and/or of limited applicability and not environmentally friendly. Biotechnology may provide more effective and sustainable solutions. Biocides produced with microbes can be less toxic to eukaryotes, expanding the environments for potential application. Microbially produced surfactants can diminish biofilm formation by corrosive microbes, as can quorum-sensing inhibitors. Amendments of phages or predatory bacteria have been successful in attacking corrosive microbes in laboratory studies. Poorly corrosive microbes can form biofilms and/or deposit extracellular polysaccharides and minerals that protect the metal surface from corrosive microbes and their metabolites. Nitrate amendments permit nitrate reducers to outcompete highly corrosive sulphate-reducing microbes, reducing corrosion. Investigation of all these more sustainable corrosion mitigation strategies is in its infancy. More study, especially under environmentally relevant conditions, including diverse microbial communities, is warranted.

Bio-augmentation with dissimilatory nitrate reduction to ammonium (DNRA) driven sulfide-oxidizing bacteria enhances the durability of nitrate-mediated souring control

Biological souring (producing sulfide) is a global challenge facing anaerobic water bodies, especially the oil reservoir fluids. Nitrate injection has demonstrated great potential in souring control, and dissimilatory nitrate reduction to ammonium (DNRA) bacteria was proposed to play crucial roles in the process. How to durably control souring with nitrate amendment, however, remains undiscovered. Herein, Gordonia sp. TD-4, a DNRA-driven sulfide-oxidizing bacterium, was used to elucidate the effects of bio-augmentation with DNRA bacteria on the durability of nitrate-mediated souring control. The results revealed that nitrate amendment combined with bio-augmentation with TD-4 after souring could effectively control souring and enhance the durability of nitrate-mediated souring control, while nitrate amendment before souring failed to persistently control souring. Nitrate amendment before and after souring resulted in different evolution dynamics of nitrate-reducing bacteria. Denitrifying bacteria were enriched in reactors amended with nitrate before souring or in dissolved sulfide exhausted reactors amended with nitrate after souring. The heterotrophic denitrifying activity of denitrifying bacteria, however, decreased the durability of nitrate-mediated souring control. Comparative and functional genomics analysis identified potential niche adaptation mechanisms (autotrophic and heterotrophic nitrate/nitrite reduction, including DNRA and denitrification) of predominant SRB in nitrate-amended environments, which were responsible for the rapid resumption of sulfide accumulation after the depletion of nitrate and nitrite. Pulsed injection of nitrate combined with bio-augmentation with DNRA-driven sulfide-oxidizing bacteria was proposed as a potential method to enhance the durability of nitrate-mediated souring control. The findings were innovatively applied to simultaneous bio-demulsification and souring control of emulsified and sour produced water from the petroleum industry.

Exploiting Microbes in the Petroleum Field: Analyzing the Credibility of Microbial Enhanced Oil Recovery (MEOR)

Crude oil is a major energy source that is exploited globally to achieve economic growth. To meet the growing demands for oil, in an environment of stringent environmental regulations and economic and technical pressure, industries have been required to develop novel oil salvaging techniques. The remaining ~70% of the world’s conventional oil (one-third of the available total petroleum) is trapped in depleted and marginal reservoirs, and could thus be potentially recovered and used. The only means of extracting this oil is via microbial enhanced oil recovery (MEOR). This tertiary oil recovery method employs indigenous microorganisms and their metabolic products to enhance oil mobilization. Although a significant amount of research has been undertaken on MEOR, the absence of convincing evidence has contributed to the petroleum industry’s low interest, as evidenced by the issuance of 400+ patents on MEOR that have not been accepted by this sector. The majority of the world’s MEOR field trials are briefly described in this review. However, the presented research fails to provide valid verification that the microbial system has the potential to address the identified constraints. Rather than promising certainty, MEOR will persist as an unverified concept unless further research and investigations are carried out.

Denitrification Biokinetics: Towards Optimization for Industrial Applications

Denitrification is a microbial process that converts nitrate (NO₃^–) to N₂ and can play an important role in industrial applications such as souring control and microbially enhanced oil recovery (MEOR). The effectiveness of using NO₃^– in souring control depends on the partial reduction of NO₃^– to nitrite (NO₂^–) and/or N₂O while in MEOR complete reduction of NO₃^– to N₂ is desired. Thauera has been reported as a dominant taxon in such applications, but the impact of NO₃^– and NO₂^– concentrations, and pH on the kinetics of denitrification by this bacterium is not known. With the goal of better understanding the effects of such parameters on applications such as souring and MEOR, three strains of Thauera (K172, NS1 and TK001) were used to study denitrification kinetics when using acetate as an electron donor. At low initial NO₃^– concentrations (∼1 mmol L^–1) and at pH 7.5, complete NO₃^– reduction by all strains was indicated by non-detectable NO₃^– concentrations and near-complete recovery (> 97%) of the initial NO₃-N as N₂ after 14 days of incubation. The relative rate of denitrification by NS1 was low, 0.071 mmol L^–1 d^–1, compared to that of K172 (0.431 mmol L^–1 d^–1) and TK001 (0.429 mmol L^–1 d^–1). Transient accumulation of up to 0.74 mmol L^–1 NO₂^– was observed in cultures of NS1 only. Increased initial NO₃^– concentrations resulted in the accumulation of elevated concentrations of NO₂^– and N₂O, particularly in incubations with K172 and NS1. Strain TK001 had the most extensive NO₃^– reduction under high initial NO₃^– concentrations, but still had only ∼78% of the initial NO₃-N recovered as N₂ after 90 days of incubation. As denitrification proceeded, increased pH substantially reduced denitrification rates when values exceeded ∼ 9. The rate and extent of NO₃^– reduction were also affected by NO₂^– accumulation, particularly in incubations with K172, where up to more than a 2-fold rate decrease was observed. The decrease in rate was associated with decreased transcript abundances of denitrification genes (nirS and nosZ) required to produce enzymes for reduction of NO₂^– and N₂O. Conversely, high pH also contributed to the delayed expression of these gene transcripts rather than their abundances in strains NS1 and TK001. Increased NO₂^– concentrations, N₂O levels and high pH appeared to cause higher stress on NS1 than on K172 and TK001 for N₂ production. Collectively, these results indicate that increased pH can alter the kinetics of denitrification by Thauera strains used in this study, suggesting that liming could be a way to achieve partial denitrification to promote NO₂^– and N₂O production (e.g., for souring control) while pH buffering would be desirable for achieving complete denitrification to N₂ (e.g., for gas-mediated MEOR).

Advances in heavy metal removal by sulfate-reducing bacteria

Industrial development has led to generation of large volumes of wastewater containing heavy metals, which need to be removed before the wastewater is released into the environment. Chemical and electrochemical methods are traditionally applied to treat this type of wastewater. These conventional methods have several shortcomings, such as secondary pollution and cost. Bioprocesses are gradually gaining popularity because of their high selectivities, low costs, and reduced environmental pollution. Removal of heavy metals by sulfate-reducing bacteria (SRB) is an economical and effective alternative to conventional methods. The limitations of and advances in SRB activity have not been comprehensively reviewed. In this paper, recent advances from laboratory studies in heavy metal removal by SRB were reported. Firstly, the mechanism of heavy metal removal by SRB is introduced. Then, the factors affecting microbial activity and metal removal efficiency are elucidated and discussed in detail. In addition, recent advances in selection of an electron donor, enhancement of SRB activity, and improvement of SRB tolerance to heavy metals are reviewed. Furthermore, key points for future studies of the SRB process are proposed.

Use of Microorganisms in the Recovery of Oil From Recalcitrant Oil Reservoirs: Current State of Knowledge, Technological Advances and Future Perspectives

The depletion of oil resources, increasing global energy demand, the current low, yet unpredictable, price of oil, and increasing maturity of major oil fields has driven the need for the development of oil recovery technologies that are less costly and, where possible, environmentally compatible. Using current technologies, between 20 and 40% of the original oil in a reservoir can be extracted by conventional production operations (e.g., vertical drilling), with secondary recovery methods yielding a further 15-25%. Hence, up to 55% of the original oil can remain unrecovered in a reservoir. Enhanced oil recovery (EOR) is a tertiary recovery process that involves application of different thermal, chemical, and microbial processes to recover an additional 7-15% of the original oil in place (OOIP) at an economically feasible production rate from poor-performing and depleted oil wells. EOR can significantly impact oil production, as increase in the recovery rate of oil by even a small margin could bring significant revenues without developing unconventional resources. Microbial enhanced oil recovery (MEOR) is an attractive, alternative oil recovery approach, which is claimed to potentially recover up to 50% of residual oil. The in situ production of biological surface-active compounds (e.g., biosurfactants) during the MEOR process does not require vast energy inputs and are not affected by global crude oil prices. Compared to other EOR methods, MEOR can be an economically and more environmentally friendly alternative. In this review, the current state of knowledge of MEOR, with insights from discussions with the industry and other stakeholders, is presented and in addition to the future outlook for this technology.