Threat to Oil & Gas Sector of India: A Microbial Perspective
Dr Amit Bhattacharya Dow Microbial Control, Dow Chemical International Pvt Ltd

Indian crude contains high levels of sulphides, causing 'souring' which is a constant threat to the country's precious assets of oil and natural gas reservoirs, refiners, pipelines and transportation facilities. This article provides in-depth insight into the cause of souring and its impact to the industry and various new technologies evolved that are practiced worldwide successfully and how it may be adapted to Indian scenario too.

A sustainable source of energy is the dream of all nations. Rapid Industrialisation and modernisation of traditional processes has so far only resulted in enhancing the need for a constant source & supply of energy for developed and still developing economies. In fact energy import has been the biggest impact on the Current Account Deficit (CAD) for majority of economies. The changing global scenario and ever increasing burden of energy import has now influenced many countries to undertake major ventures of local oil & gas exploration. India, a country with immense natural resources , has embarked on this mission for developing indigenous source of oil and gas. India is 6th largest consumer of oil in the world and 9th largest crude oil importer, with the industry contributing more than 15 per cent to the Gross Domestic Production (GDP). In Addition to this, the fact that India is one of the least explored countries in the world and the discovery of quite a few new gas fields along Eastern coast, the Oil & Gas industry in India seems set to be steering itself on an exciting new path. Exploration and production spent in the country has been almost doubled in recent years. The country's gas pipeline coverage has increased substantially and domestic gas supplies are expected to increase. Through various onshore and offshore projects, the oil and gas sector for India is one of the six core industries .

Today India can stake claim to giant offshore projects, ultramodern environment friendly refineries and high-tech pipelines and transportation facilities. Unfortunately the country's precious assets of oil and natural gas deposits are facing tremendous threat of 'souring' due to high levels of sulphides in crude oil. In offshore oil fields, deoxygenated seawater is often injected into the reservoir in order to sustain reservoir pressure and enhance secondary recovery. An anoxic condition combined with high numbers of Sulphate Reducing Bacteria (SRB) in oil reservoirs, pipelines, and installations is resulted in the production of Hydrogen Sulphide (H2S) (Vance and Thrasher, 2005). It is a toxic and corrosive gas that is responsible for a variety of environmental hazards and economic losses due to reservoir souring and the consequently low production of oil and Microbial Induced Corrosion (MIC) (Davidova et al., 2001; Eckford and Fedorak, 2002). The rate of pitting corrosion has been attributed to sulphate and thiosulphate reducing bacteria (Crolet, 2005). New technologies based on synergistic blends of biocides which provide more heat stable, long lasting with fact acting, broad spectrum control are available. These technologies are practiced worldwide successfully and should be adapted to Indian scenario too.

Microbiology of H2S Generation and Microbial Induced Corrosion (MIC )
MIC can locally enhance corrosion and cause pits due to generation of sulphide. It happens much faster, can be more difficult to detect and to treat. This extensive sulphide generation in the crude pipeline is directly correlated with microbiological profile of the reservoir and biofilms associated with reservoir and pipelines. Microbiological studies revealed the presence of huge diversity of thermophilic and hyperthermophilic anaerobic microorganisms from high temperature, petroleum rich strata from a number of geographically distant sites (Orphan et al., 2000). Many reservoir studies have also confirmed that there are microorganisms present in extreme conditions (high temperature and pressure) prevalent in reservoirs (Myhr et al., 2002). Petroleum reservoirs constitute a group of unique terrestrial sites, because they present an unusual combination of extreme environmental conditions including temperature, pressure, and salinity. Petroleum composition varies widely between reservoirs, which might have an impact on the microbial biodiversity of such environments (Tello et al., 2004). Some 'ancient lineage' bacteria and archea (extremophiles; which can survive at extreme temperature of 80°C-90°C and pressures of ~200 bars; Figure 1) collectively known as Sulphate Reducing Prokaryotes (SRPs) have been 'waiting' down-hole for many, many years (Pederson, 2000). It is proved that bacteria like sulphate reducing bacteria commonly known as SRBs (Figures 2, 3 & 4) are universally present in oil reservoirs (Nilsen et al., 1996).

Microbial studies revealed the presence of a rich and diverse community of bacteria and archea including (i) fermentative, (ii) sulphate, thiosulphate, and sulphur-reducing, and (iii) methanogenic species in petroleum reservoirs (Salinas et al., 2004). Sulphate-reducing bacteria are physiologically unique among living organisms in being able to reduce sulphates to sulphides; they are also capable of reducing sulphites, thiosulphates and elementary sulphur to sulphides by following mechanism shown in Figure 5.

Practically every type of soil and natural water contains SRPs and they are widely distributed in the seas and oceans. In earlier times they have been identified as essential agents in the anaerobic corrosion of buried ferrous pipes (Butlin et al., 1948). Sulphate reducing bacteria were thought to be strict anaerobes (Pfenning et al., 1981), but it was later demonstrated that some SRBs grow in the presence of O2 as well as reduce O2 to H2O. Their growth may occur in the presence of 2-3 ppm of oxygen (Bultin et al, 1948). They can be re-activated from their dormancy by the perturbation generated by the extraction work (chemicals, water, and reduced temperature). Now a days it is common knowledge that injection of sea water stimulate growth of thermophilic sulphate reducers because of high concentrations of sulphate are introduced with the injected water (Nilsen, 1996) and causes reservoir souring. To date, most petroleum microbiological work has centred on water flooded reservoirs that offer a cooled, oxygen-free, saline environment, which meets the environmental requirements of many different groups of bacteria. As discussed earlier, microbiologically influenced corrosion or simply MIC is the deterioration of a metal by a corrosion process that occurs directly or indirectly as a result of the metabolic activity of microorganisms. MIC can be considered in two categories – anaerobic and aerobic. The sulphate reducing bacteria are considerably most critical microbes in anaerobic MIC. They reduce sulphate to sulphide and promote formation of sulphide film i.e., their characteristics form of respiration uses sulphate and results in sulphide formation (Postgate, 1984). The petroleum production environment is particularly suitable for the metabolism of SRB because it handles large volumes of de-aerated water from underground reservoirs. This water is rich in nutrients and due to H2S dissolution it can become very sour. SRBs can cause corrosion of a wide range of metals including of low grade carbon steels stainless steels.

Biofilms and Its Impact on H2S Levels and MIC
Another major contributor to increased levels of H2S in reservoir and crude pipelines is the ‘biofilm’ formation. A biofilm is a group of microbial cells that is associated with a surface, enclosed in a matrix of primarily polysaccharide material, and cannot be easily removed by rinsing (Figure 6). Materials such as clay or silt particles, mineral particles, corrosion particles, depending on the environment in which the biofilm has developed, may also be found in the biofilm matrix. Biofilm-associated organisms (sulphate reducers, sulfidogens, fermentative bacteria, manganese and iron reducers, methanogens, and acetogens; Figure 7) also differ from their planktonic (freely suspended) counterparts with respect to the genes that are transcribed. Biofilms may form on a wide variety of surfaces, including living tissues, indwelling medical devices, industrial or potable water system piping, or natural aquatic systems (Donlen et al., 2002). These biofilm microorganisms have been shown to bring forth specific mechanisms for initial attachment to a surface, development of a community structure and ecosystem, and detachment.

One theory about the mechanism of MIC depicts that biofilms promote corrosion by inducing the formation of ‘corrosion cells’. This is thought to occur as a consequence of aerobic respiratory activity within biofilms that leads to the establishment of local cathodic and anodic regions on the steel surface, which promotes electron flow (Neria-Gonza´ lez et al, 2006). Another explanation is that MIC is promoted by anaerobes such as sulphate-reducing and iron-reducing bacteria (IRBs). In the biofilm, the sulphate reducers consume hydrogen and induce corrosion by the formation of ferrous sulphide and the iron reducers promote corrosion by reductively dissolving the protective ferric oxide coat that forms on the steel surface (Potekhina et al, 1999). Bacterial communities in biofilms developed on the surface of materials in natural environments are heterogeneous, and therefore there is significant uncertainty concerning how these communities affect corrosion in a given environment. The knowledge of bacterial diversity in the biofilms is helpful to understand the interactions between corrosive bacteria and metal surface, as well as with other micro-organisms, and provides the basis for the development of new and better means for the detection and prevention of corrosion.

Measures to Control Souring and H2S
Many studies have been done to understand the origin and impact of Biogenic H2S gas in the reservoir and in the crude oil which have a direct correlation with MIC (Neria-Gonza´ lez, et al, 2006). It should be the top priority to take necessary measures to protect those invaluable natural resources. There are sufficient studies and data available through various scientific journals, electronic media and various symposia every year which suggests why and how this over increasing problem can be tackled. Many examples from across the globe are available on various new technologies and treatment regimes which can be employed in Indian scenario to suppress this burning issue of souring of crude oil. New concepts and innovative products are now available which can treat the problems from their root. These innovative products have shown their remarkable competency over old and conventional treatments in the field worldwide. Variations in the factors like geochemistry of wells, microbial diversity in the subsurface environment, water and substratum chemistry, and the presence of biofilm on surfaces within water flow zones magnifies the complexity of the problem. Thus, mitigating soured wells and preventing bio-souring is challenging. Because of the heterogeneity in the physico-chemical parameters of oil and gas wells and the added complexity of the subsurface microbiological communities, it is unlikely that one microbial control programme will be applicable to mitigate all wells; it is more likely that a site-specific solution will be needed. Developing optimal microbial control programmes first requires a thorough diagnosis and understanding of the microbial problems affecting a production site. This often requires the use of advanced microbiological methods like ATP count, biofilm studies, high throughput systems and molecular biology techniques. Once a thorough understanding of the problem is made, advanced solutions can then be developed and applied. This requires the use of advanced biological testing techniques, field application knowledge and a broad portfolio of biocidal actives to choose from, as well as close collaboration with site operators. Then new technologies based on synergistic blends of correct biocides (heat stable, long lasting with fact acting, broad spectrum) are available to control the souring problem due to H2S generation.

There are infrequent but alarming indications that sulphide levels have crossed the threshold permissible limit which is a warning signal of asset deterioration. If not controlled with immediate effect, this may lead to major economical loss to the country. On the contrary, if we take a proactive approach and equip ourselves to cope up with this situation by employing new innovative products and following the same for new projects, then positive, money-saving impacts may be realised. For example, non-sour grade materials and weld procedures can be used in systems where sulphide levels are well controlled. In addition, the demand for chemical corrosion inhibitors can be lower in low-sulphide systems and the need for corrosion monitoring and other testing may be lessened. Finally, health and safety concerns for a low-sulphide system are mitigated relative sour systems.

These benefits can save costs that are difficult to quantify because each project tends to be different. However, sour service-grade material is reported as being 15 per cent or more expensive than nonsour-service carbon steel (McElhiney, 1996).

Conclusion
The oil and gas industry has had a positive economic impact on India but in order to maintain sustainable production, microbial associated souring must be controlled via the use of efficient, targeted treatments of both reservoirs and infrastructure.

References:
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