4 Oilfield Review

Microbes—Oilfield Enemies or Allies?

Microbes have a long history in the oil and gas industry. New molecular analysis

methods, coupled with increased knowledge of microbe identity and chemistry, have

led to advances in combating microbiologically influenced corrosion and reservoir

damage. Scientists are also using these advances to develop new methods for

microbiologically enhanced oil recovery and bioremediation.

Zdenko AugustinovicDONG E&PHoersholm, Denmark

Øystein BirketveitM-I SWACOBergen, Norway

Kayli ClementsMike FreemanM-I SWACOHouston, Texas, USA

Santosh GopiM-I SWACOAccra, Ghana

Thomas IshoeyGlori Energy, Inc.Houston, Texas

Graham JacksonHusky Energy Inc.Calgary, Alberta, Canada

Gregory KubalaSugar Land, Texas

Jan LarsenMaersk OilCopenhagen, Denmark

Brian W.G. MarcotteTitan Oil Recovery, Inc.Los Angeles, California, USA

Jan ScheieM-I SWACOStavanger, Norway

Torben Lund SkovhusDanish Technological InstituteAarhus, Denmark Egil SundeStatoilStavanger, Norway

Oilfield Review Summer 2012: 24, no. 2. Copyright © 2012 Schlumberger.For help in preparation of this article, thanks to Sonny Espey, M-I SWACO, Houston; and Dietmar Schumacher, Geo-Microbial Technologies Inc., Ochelata, Oklahoma, USA.AERO is a registered trademark of Glori Energy, Inc.

We live in a natural world of extremes in size and scale. Topographical extremes range from moun-tains to deserts to ocean trenches. These extremes include a place so small that we cannot directly view it: the unseen world of microbes.

Microbes are the most abundant life form on the planet—no other life form approaches them in terms of numbers, diversity or habitat. Microbes were the first link in the evolutionary chain and are an essential part of the Earth’s biota.1 Microbes catalyze important transforma-tions in the biosphere, produce key components of the atmosphere and represent a large fraction of the genetic diversity on this planet. The num-ber of microbial cells on Earth has been esti-mated at 4 to 6 × 1030 cells, and this aggregate mass contains 350 to 550 × 1015 g of carbon. Microbes are distributed everywhere, including places that are hotter, colder, drier and deeper than humans can tolerate. This wide distribution suggests that oil exploration and production operations must always contend with microbes.

Microbes have a long history in the oil field. Analysis of produced water from shallow reser-voirs in the 1930s and 1940s showed abundant populations.2 Despite these findings, scientists at the time believed that temperature, pressure and salinity in most reservoirs were too hostile for

microbes to thrive. Initiation of North Sea oil and gas production in the 1960s, however, demon-strated that early assumptions were incorrect. Microbes in these reservoirs not only lived in extreme conditions, they produced hydrogen sul-fide [H2S]. Souring, or increases in H2S, resulted from injection of sulfate-rich seawater in North Sea reservoirs and ultimately led to corrosion of both surface and downhole equipment. Formation plugging by biomass during waterflood operations was another early negative effect of microbes.3

However, not all of the early production-related experiences with microbes were nega-tive. Some operators found that injection of sugar-based materials that resident microbes could use as food caused an increase in oil pro-duction, although results were often temporary and inconsistent.4 In the past several decades, much of the oilfield research on microbes focused on short-term strategies to either mitigate nega-tive effects or enhance positive ones—but that work was based on a partial understanding of microbiological mechanisms.

The ability to control and harness microbes is key to some of the major advances in microbial oilfield science. This progress has been aided by new analytical methods that give a more complete picture of microbe identity, quantity,

1. Whitman WB, Coleman DC and Wiebe WJ: “Prokaryotes: The Unseen Majority,” Proceedings of the National Academy of Sciences 95, no. 12 (June 9, 1998): 6578–6583.

2. Bass C and Lapin-Scott H: “The Bad Guys and the Good Guys in Petroleum Microbiology,” Oilfield Review 9, no. 1 (Spring 1997): 17–25.

3. Chang CK: “Water Quality Considerations in Malaysia’s First Waterflood,” Journal of Petroleum Technology 37, no. 9 (September 1985): 1689–1698.

4. Rassenfoss S: “From Bacteria to Barrels: Microbiology Having an Impact on Oil Fields,” Journal of Petroleum Technology 63, no. 11 (November 2011): 32–38.

Summer 2012 55

positive characteristics. Case studies from Canada and the US demonstrate how these techniques are employed in production environments.

behavior and function. Advances include simple chemicals added to injection water that provide environmentally safe control of reservoir souring and associated corrosion as well as new direc-tions for microbiologically enhanced oil recovery (MEOR). Other progress includes bioremediation research that allows safe disposal of oilfield solid waste into ordinary soil.

This article focuses on microbes in the oil field and describes techniques for their analysis and for controlling their negative effects and harnessing

6 Oilfield Review

The Microbial WorldBiological classification systems have evolved in tandem with methods for microbe detection. In the early 1800s, what was not a mineral or plant was considered an animal. The discovery that bacteria could be considered either plant or ani-mal led to reformulations of the biological classi-fication system for living organisms that continue to the present era. Proposed more than 30 years ago, today’s accepted classification into three pri-mary domains has its roots in molecular analysis methods, including genome sequencing.5 The three domains are bacteria, archaea and eucarya (left). Bacteria and archaea—collectively called prokaryotes—are the organisms that affect the oil field (below left).

Fossils of prokaryotic organisms that lived 3.5 billion years ago have been found in Western Australia and South Africa; for 2 billion years they were the only form of life on Earth. Larger and more-complex eucaryotic cells did not appear until much later—about 1.5 to 2 billion years ago. The archaea and bacteria that consti-tute the prokaryotes differ from the cells in com-plex eucaryotic organisms such as plants and animals. Prokaryotic cells have no compartmen-talized nucleus, and each cell is capable of inde-pendent existence.6 Unlike plants and animals, prokaryotes are not typically thought of as organ-isms able to interact with their environment, but that view may be changing. Researchers have shown that many bacteria have cell to cell com-munication through signaling molecules called autoinducers. This signaling is called quorum sensing and allows the microbes to monitor and respond to their surroundings.7

Prokaryotes are found everywhere on Earth and thrive in extreme habitats. From hot springs, arid deserts and ocean depths to polar caps and underground formations, these single-cell organ-isms withstand conditions that humans cannot.8 These microbes may remain dormant for thou-sands of years but can reactivate rapidly—often in days or weeks. Their wide distribution in a vari-ety of habitats and conditions means that microbes are always present during E&P activi-ties. Some microbes are indigenous to reservoirs, while others may be introduced during drilling, workover or waterflood. These single-cell life forms have an innate tendency to cling to rock and metal surfaces and may assemble into masses called biofilms. Microbe-generated bio-films provide a safe harbor for growth and may eventually lead to serious problems in both equipment and reservoirs (next page).

> Tree of life. Life on Earth is divided into three primary domains—bacteria (left, blue), archaea (middle, pink) and eucarya (right, green). This classification encompasses the entire realm of living organisms, from the proteobacteria that contain the digestive disease salmonella to the more familiar plants and animals. Branch order and length are based on genetic sequencing.

Spirochetes

Bacteria Archaea Eucarya

Methanosarcina

Methanobacterium

Methanococcus

Thermococcus

Thermoproteus

Pyrodictium

Halophiles

Entamoeba Slimemolds Animals

Fungi

Plants

Ciliates

Flagellates

Trichomonads

Microsporidia

Diplomonads

Proteobacteria

Cyanobacteria

Planctomyces

Thermotoga

Aquifex

Gram-positivebacteria

Greenfilamentous

Bacteroides

> Bacteria. The bacterial cell is enclosed by a capsule, cell wall and plasma membrane. The interior of the cell is filled with cytoplasm, a homogeneous, gel-like substance. The primary interior component is the nucleoid, which contains the chromosome material. Plasmids, containing deoxyribonucleic acid (DNA), and ribosomes, containing ribonucleic acid (RNA), are other essential interior components. Although not all bacteria are motile, many use a whip-like flagellum to move in aqueous media. Bacteria and other prokaryotes range in size from 10–5 to 10–6 m.

Flagellum

CapsuleCell wallPlasma membrane

Cytoplasm

Ribosome

Plasmid

NucleoidEukaryotes

Viruses

Small moleculesAtoms

Proteins

Prokaryotes

10–3

10–4

10–5

10–6

10–7

10–8

10–9

10–10

Size

, m

Summer 2012 7

New methods for enumeration and identifi-cation of bacteria and archaea have led scien-tists to a better understanding of microbial behavior and chemistry. Their efforts, in turn, have led to more accurate identification of prob-lems caused by microbes as well as better means for their solution. Taken together, these meth-ods give the operator tools to control microbes in places where their effects are harmful and to exploit their positive characteristics.

Enemy and AllyAlthough microbes and humans have existed as both enemy and ally for ages, the roles of microbes in those relationships have been recog-

nized only in the recent past. In the fight against infectious diseases, bacteria were identified about 150 years ago as one of the culprits.9 As industrial allies, microbes played a key role in the leaching of copper from mine drainage water, a practice in the Mediterranean region about 1000 BCE, but their role in the process was unknown until the 1950s.10

The bulk of experience with microbes in oil and gas exploration and production has occurred in the past 75 years. One of the early encounters with microbes in a production environment occurred in the late 1950s during waterflooding.11 Microbes produce high molecular weight polysac-charides that deposit on the sandface and other

formation surfaces as a biofilm.12 This biofilm is the glue that holds the microbes together. Given the right conditions, the microbes will continue to grow, divide and plug rock pores, thus decreas-ing the effectiveness of water injection in displac-ing oil. As a result, early water quality control methods included membrane filtration and the use of strong oxidizing agents as biocides.13 Later waterflood control applications employed nonox-idizing biocides.

Shortly after operators learned to manage microbe plugging during waterflooding, they encountered another significant problem—microbiologically influenced corrosion (MIC)—corrosion caused by microbial action.14 This type

5. Woese CR and Fox GE: “Phylogenetic Structure of the Prokaryotic Domain: The Primary Kingdoms,” Proceedings of the National Academy of Sciences 74, no. 11 (November 1, 1977): 5088–5090.

Woese CR, Kandler O and Wheelis ML: “Towards a Natural System of Organisms: Proposal for the Domains Archaea, Bacteria and Eucarya,” Proceedings of the National Academy of Sciences 87, no. 12 (June 1, 1990): 4576–4579.

Todar K: “Todar’s Online Textbook of Bacteriology,” http://www.textbookofbacteriology.net (accessed May 24, 2012).

6. Although the bacteria and archaea that make up the prokaryotes are similar in size and structure, their genome structures and metabolism differ.

7. Taga ME and Bassler BL: “Chemical Communication Among Bacteria,” Proceedings of the National Academy of Sciences 100, supplement 2 (November 25, 2003): 14549–14554.

> Biofilm formation. The growth of biofilms is a stepwise process that begins with the transport of microbes to a metal or rock surface (A). The microbes absorb organic molecules from their surroundings to form a film (B) composed of exopolymers—sugars—that allow the microbes to stay attached to the surface as well as to each other (C). As the biofilm expands (D), its size gives the interior microbes protection from biocides. Eventually, when the biofilm grows to a certain size, some microbes are released (E) to form new areas of growth.

Biofilm

Metal or rock surface

Microbes

A B C D E

8. Bass and Lapin-Scott, reference 2. Danish scientists have discovered microbes living in

undisturbed sediments that are more than 86 million years old. The microbes consume oxygen in quantities too small to be directly measured. For more: Bhanoo SN: “Deep-Sea Microbes That Barely Breathe,” The New York Times (May 21, 2012), http://www.nytimes.com/2012/05/22/science/deep-sea-microbes-that-barely-breathe.html (accessed May 22, 2012).

9. Santer M: “Joseph Lister: First Use of a Bacterium as a ‘Model Organism’ to Illustrate the Cause of Infectious Disease of Humans,” Notes & Records of the Royal Society 64, no. 1 (March 2010): 59–65.

10. Brierley CL: “Microbial Mining,” Scientific American 247, no. 2 (1982): 42–50.

11. Lee D, Lowe D and Grant P: “Microbiology in the Oil Patch: A Review,” paper 96-109, presented at the Annual Technical Meeting of The Petroleum Society, Calgary, June 10–12, 1996.

12. Polysaccharides are long carbohydrate molecules composed of repeating units and are common sources of energy for bacteria. For more: Todar, reference 5.

13. Mitchell RW and Bowyer PM: “Water Injection Methods,” paper SPE 10028, presented at the SPE International Petroleum Exhibition and Technical Symposium, Beijing, March 17–24, 1982.

14. The literature on microbes associated with oilfield environments uses numerous acronyms for microbe-driven processes such as MIC or MEOR. It is not uncommon to encounter both “microbial” and “microbiologically” as the initial term in these acronyms, depending on the reference—the terms are essentially equivalent.

8 Oilfield Review

of corrosion can occur anywhere in the produc-tion environment—in downhole tubulars, in top-side equipment and in pipelines. This type of corrosion can cause ruptures that seriously impede operations (above). Reports in the 1980s showed that sulfate-reducing bacteria (SRB) were the cause of MIC.15 SRB typically live in an anaerobic, aqueous environment and use organic acids and hydrogen from decomposing biomass

as nutrients, oxidizing the nutrients while reduc-ing the sulfate in the water to H2S. The role of SRB in initiating MIC is complex and involves not only biofilms that trap corrosive microbial waste products but also electrochemical reactions at the metal surface. Early explanations pointed toward an SRB produced enzyme that removes cathodic hydrogen from steel, which causes rapid pitting of the surface.16

MIC is a common occurrence in the oil field, and to control it, operators usually treat injected and produced water to mitigate microbial action. Complete sterilization of the water is impossible, and microbe control strategies have usually been directed toward disinfection—that is, reduction of microbe numbers to acceptable levels by killing a large portion of the population with a biocide.

>Microbiologically influenced corrosion (MIC). Operated by DONG E&P, the Siri platform (center) is located in the North Sea 220 km [137 mi] west of the Danish coast and is flanked by the smaller Cecilie (left) and Nini (right) satellite platforms. Five fields—Siri, Nini, Nini East, Cecilie and Stine—produce from reservoirs 1,800 to 2,200 m [5,900 to 7,220 ft] below sea level. Seafloor lines between the three structures and wells carry oil and gas, gas for lift, and injection water for pressure support. In 2007, a 25.4-cm [10-in.] water injection line ruptured (inset) 3 km [2 mi] from the Siri platform. Subsequent investigation revealed that the MIC deposit at the rupture site was a mixture of iron sulfide and other corrosion by-products plus microbes and polysaccharide slime. These deposits allow sulfate-reducing prokaryotes (SRPs) and other troublesome microbes to grow protected from biocides. (Adapted with permission from DONG E&P.)

Cecilie

NiniSiri

Water injection

Stine

Water injectionGas liftMultiphaseOilUmbilical

13 km9 km

32 km

Oil storage

15. Cord-Ruwisch R, Kleinitz W and Widdel F: “Sulfate-Reducing Bacteria and Their Activities in Oil Production,” Journal of Petroleum Technology 39, no. 1 (January 1987): 97–106.

16. Lee et al, reference 11.

17. Campbell S, Duggleby A and Johnson A: “Conventional Application of Biocides May Lead to Bacterial Cell Injury Rather Than Bacterial Kill Within a Biofilm,” paper NACE 11234, presented at the NACE Corrosion Annual Conference and Exposition, Houston, March 13–17, 2011.

18. Maxwell S and Campbell S: “Monitoring the Mitigation of MIC Risk in Pipelines,” paper NACE 06662, presented at the NACE Corrosion Annual Conference and Exposition, San Diego, California, USA, March 12–16, 2006.

19. Eckert R and Skovhus TL: “Using Molecular Microbiological Methods to Investigate MIC in the Oil and Gas Industry,” Materials Performance 50, no. 8 (August 2011): 50–54.

Summer 2012 9

If the biocide is a strong oxidizing agent such as chlorine, it is added to the injection water con-tinuously. Nonoxidizing biocides, typically used in current operations, are added intermittently (right). Each batch of biocide kills a portion of the microbe population, but the survivors may recover between doses. Recent research has shown biocides may not be as effective as previ-ously thought—they may only injure or inhibit but not kill microbes.17

Although biocides can be effective in combat-ing MIC, there are reports of equipment failure in spite of treatment, and examination of these inci-dents revealed that the biocide application was erratic and haphazard.18 Until recently, unlike with common corrosive agents, there were no effective tools to predict and quantify the risk of corrosion influenced by microbes. The juncture of genome-based test methods and the inadequa-cies of biocides and their risks has led to a new approach in managing MIC in production envi-ronments. This approach uses molecular micro-biological methods (MMMs) and represents a fundamental change in assessing microbe effects.19 These methods—fluorescence in situ hybridization (FISH), quantitative polymerase chain reaction (qPCR) and a microbe staining technique using a fluorescent stain known as 4’,6-diamidino-2-phenylindole (DAPI)—allow scientists to gain a more complete understanding of identities, quantities and behaviors of the microbes involved in MIC.

To appreciate the significance of these meth-ods, it is important to understand how microbes were handled in the laboratory prior to the intro-duction of MMMs. Traditional microbiological methods for identification and enumeration relied on serial dilution and cultivation in nutri-ent media for relatively long periods—often up to 30 days. Even after these long periods, less than 10% of the viable microbes could be cultured. It is no surprise that laboratory results based on tradi-tional serial dilution and culturing methods did not correlate well with field results.

In contrast, results from application of FISH, DAPI and qPCR techniques reveal nearly com-plete identities and distribution of the microbes of interest in oil production systems. These new methods utilize a combination of microscopy, analysis of cell genetic material and enzymatic reactions to give a complete enumeration of microbes present in the sample (right). In addi-tion, the results are available in days rather than weeks. These methods permit scientists to more completely understand the chemistry of MIC on a

> Biocide treatment. Offshore topside equipment is commonly treated with biocides to prevent MIC and precipitation of iron sulfide from produced H2S. In the North Sea, engineers treated a reclaimed oil sump tank with glutaraldehyde to obtain data showing how the biocide and H2S concentrations changed with time. The sump tank effluent was analyzed for residual glutaraldehyde and sulfide as a marker for H2S. Data from the study show expected results after biocide treatment. As the high concentration of biocide kills troublesome microbes, the sulfide concentration drops sharply. At the highest concentration of biocide, the sulfide concentration reaches a minimum. Both trends reverse as the biocide is flushed from the system. Biocide is reapplied when the sulfide returns to a threshold level.

1 2 3Time, d

Sulfide

Biocide

Sulfi

de, p

pm

Bioc

ide,

ppm

0 0

200

400

600

800

1,000

1

5

4

3

2

Start StopBiocide

>Molecular microbiological methods. These laboratory methods allow characterization and determination of the relative proportions of microbes present in oil production systems. Traditional microbe culturing using serial dilution produces the most probable number (MPN) of microbes, which may represent only a small fraction of the total number actually present. In contrast, MMMs represent a trio of new methods—FISH, DAPI and qPCR—that divide the microbe population into organisms that are active, inactive and dead. FISH analysis involves staining and microscopy to examine living, or active, microbes. The fluorescent stain, DAPI, binds to DNA and quantifies both active and inactive microbes. The qPCR method employs an enzymatic reaction that gives additional information on all the microbial groups. When these methods are used together, scientists obtain a complete enumeration and characterization of the microbes in a sample. (Adapted with permission from DTI Oil & Gas, Danish Technological Institute.)

Dead

Active

Inactive

Microbe Characterization and Relative Proportions

MPN: most probable number

FISH: flourescence in situhybridization

DAPI: 4’,6-diamidino-2-phenylindole

qPCR: quantitative polymerasechain reaction

10 Oilfield Review

metal surface. By using MMMs, scientists discov-ered that corrosion involves not only SRB but also other microbes that contribute to H2S and meth-ane [CH4] production (above).20

This complexity in MIC is illustrated by recent laboratory work on topside equipment in the Danish sector of the North Sea. In 2008, a pro-duced water separator in the Halfdan field

showed high corrosion rates in the carbon steel water outlet piping. The pipe showed severe metal pitting and scaling, and scientists deter-mined that MIC was the cause (next page).21 The microbes responsible for corrosion problems at Halfdan field are not the only varieties that can cause MIC in production systems. Acid-producing bacteria (APB) are microbes that produce organic acids under certain conditions. These acids can cause the pH to drop enough to create conditions favorable to corrosion on metal surfaces such as those of submersible pumping components.22 Control of APB is typically accom-plished by biocide disinfection that may aid in control of SRB as well.23

New Theories and SolutionsThe H2S produced during MIC in the wellbore and reservoir by sulfate-reducing prokaryotes (SRPs) contributes to reservoir souring.24 There are new and effective ways to control souring, but souring is not a new problem for producers. Some reservoirs are sour as a result of high levels of H2S that have been present over geologic time frames. Many reservoirs are sour, however, as a conse-quence of seawater injection for secondary recov-ery.25 The SRPs that live near the wellbore and in the reservoir have simple needs for growth—they require sulfate, carbon and nutrients. Seawater is rich in sulfate, and reservoir formation water usually contains abundant short-chain fatty acids that supply the carbon and other nutrients.26 Add a suitable temperature regime, inject seawater and the end result is inevitable—souring.

Exactly how and to what extent souring occurs have been recently challenged, and the picture may not entail simple microbe growth from water injector all the way to produced water

outlet.27 New research relies on data that show the amount of H2S produced is consistent with production only in the immediate vicinity of the wellbore but not throughout the formation. Scientists have concluded that elevated levels of heavy metals, water-soluble hydrocarbons and microbe activity by-products inhibit microbe growth in the reservoir. Another result of this research is a model showing how H2S produced in the vicinity of the wellbore moves through the reservoir. Early theories relied on a simple mixing-zone model that predicted rapid H2S breakthrough. Data show the opposite—several reservoir pore volumes must be displaced before H2S breakthrough. This newer model assumes that most of the H2S generation takes place in a biofilm near the injector and that the reservoir is merely a zone for transport and adsorption.

Regardless of how it occurs, souring creates many problems for the industry, including corro-sion of pipelines and topside equipment, reser-voir plugging from sulfides, health risks from H2S toxicity and increased refining costs.28 The effects of souring are serious enough that oilfield pro-ducers have investigated several ways to control it. These include biocides, nanofiltration to remove sulfate and manipulation of injection water salinity levels to inhibit microbe growth. Because processes such as nanofiltration have high capital costs, biocides have usually been the first choice for controlling microbial growth to prevent souring.

New methods employed for control of reser-voir souring have helped scientists further under-stand microbial identities and their chemistry during secondary recovery. Although earlier inves-tigations into souring focused almost exclusively on SRB, seawater and reservoir formations con-

20. Larsen J, Rasmussen K, Pedersen H, Sørensen K, Lundgaard T and Skovhus TL: “Consortia of MIC Bacteria and Archaea Causing Pitting Corrosion in Top Side Oil Production Facilities,” paper NACE 10252, presented at the NACE Corrosion Annual Conference and Exposition, San Antonio, Texas, USA, March 14–18, 2010.

21. Skovhus TL, Holmkvist L, Andersen K, Pedersen H and Larsen J: “MIC Risk Assessment of the Halfdan Oil Export Spool,” paper SPE 155080, presented at the SPE International Conference and Exhibition on Oilfield Corrosion, Aberdeen, May 28–29, 2012.

22. Adams DL: “Microbiologically Influenced Corrosion of Electrical-Submersible-Pumping-System Components Associated With Acid-Producing Bacteria and Sulfate-Reducing Bacteria: Case Histories,” paper SPE 136756, presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Lima, Peru, December 1–3, 2010.

23. Bagchi D, Periera AP, Chu J, Smith JP and Scheie J: “Successful Mitigation of Microbiologically Influenced Corrosion in Waterflood Pipelines and Process Equipment,” in Blackwood DJ (ed): Proceedings of Corrosion Asia 2000. Singapore: Corrosion Association Singapore (2000): 55–65.

> Corrosion reactions. A complex set of reactions underlies the production of MIC on a metal surface. These reactions are strongly influenced by sulfate-reducing prokaryotic and methanogenic respiration (blue and red paths, top). This set of reactions is best illustrated by listing the net reactions for sulfate reduction and CH4 production (bottom). In the net sulfate reduction reaction, iron [Fe], H2S and the sulfate ion [SO4

2–] combine to give FeS and water. In the net CH4 production reaction, Fe, H2S and carbon dioxide [CO2] combine to give FeS, water and CH4. (Adapted from Larsen et al, reference 20.)

FeS

Net reactions

H2S

CO2

Methanogens

Metal

WaterSulfate-reducingprokaryotes

SO42–

CH4

H2Fe2+

H+

Fe0

S2–

e–

4Fe0 + 3H2S + SO42– + 2H+ 4FeS + 4H2O

n4Fe0 + 4H2S + CO2

4FeS + 2H2O + CH4

24. Larsen J, Sørenson K, Højris K and Skovhus TL: “Significance of Troublesome Sulfate-Reducing Prokaryotes (SRP) in Oil Field Systems,” paper NACE 09389, presented at the NACE Corrosion Annual Conference and Exposition, Atlanta, Georgia, USA, March 22–26, 2009.

25. Kuijvenhoven C, Bostock A, Chappell D, Noirot JC and Khan A: “Use of Nitrate to Mitigate Reservoir Souring in Bonga Deepwater Development Offshore Nigeria,” paper SPE 92795, presented at the SPE International Symposium on Oilfield Chemistry, Houston, February 2–4, 2005.

26. Bass and Lapin-Scott, reference 2.27. Sunde E and Torsvik T: “Microbial Control of Hydrogen

Sulfide Production in Oil Reservoirs,” in Ollivier B and Magot M (eds): Petroleum Microbiology. Washington, DC: ASM Press (2005): 201–214.

28. Youssef N, Elshahed MS and McInerney MJ: “Microbial Processes in Oil Fields: Culprits, Problems, and Opportunities,” in Laskin AI, Sariaslani S and Gadd GM (eds): Advances in Applied Microbiology, vol. 66. Burlington, Vermont, USA: Elsevier (2009): 141–251.

29. SRB use anaerobic respiration while NRB use anoxic respiration.

30. Thorstenson T, Bødtker G, Lillebø B-LP, Torsvik T, Sunde E and Beeder J: “Biocide Replacement by Nitrate in Sea Water Injection Systems,” paper NACE 02033, presented at the NACE Corrosion Annual Conference and Exposition, Denver, April 7–11, 2002.

31. Rassenfoss, reference 4.32. Zahner RL, Tapper SJ, Marcotte BWG and Govreau BR:

“What Has Been Learned from a Hundred MEOR Applications,” paper SPE 145054, presented at the SPE Enhanced Oil Recovery Conference, Kuala Lumpur, July 19–21, 2011.

33. Brisbane PG and Ladd JN: “The Role of Microorganisms in Petroleum Exploration,” Annual Review of Microbiology 19 (October 1965): 351–364.

34. Tucker J and Hitzman D: “Detailed Microbial Surveys Help Improve Reservoir Characterization,” Oil & Gas Journal 92, no. 23 (June 6, 1994): 65–68.

Summer 2012 11

tain several other species of microbes, including nitrate-reducing bacteria (NRB). SRB and NRB can live and thrive in the wellbore and formation, provided they have a sufficient carbon source such as short-chain fatty acids.

On the molecular level, SRB reduce the sul-fate to sulfide, and NRB reduce the nitrate to nitrogen.29 SRB and NRB compete for food, and where it is limited, that competition is intense. In the wellbore and reservoir, if oxygen is absent, introduction of nitrate, via injection water, favors NRB growth over SRB growth. Nitrate, in the form of calcium nitrate [Ca(NO3)2], is added to injection water to prevent souring.30 This form of nitrate may be used in place of biocide and has minimal health and environmental impacts. Although nitrate may not completely eliminate the need for biocides, it can reduce the amount of other chemicals needed.31 Using their increased knowledge of SRB and NRB microbe species and chemistry, scientists have improved treatment techniques for souring—the same is true for microbiologically enhanced oil recovery.

Operators have employed MEOR for decades. Much of the past work relied on trial and error and produced mixed results. A more complete understanding of microbe behavior and chemistry is resulting in a resurgence of MEOR field trials. These trials have two common objectives—iden-tify the indigenous microbes in the formation and design formulas for nutrient injection to stimulate their growth; that is, find the helpful microbes and feed them what they like.

Just as there are many types of indigenous microbes, there are several mechanisms microbes employ that may stimulate oil produc-tion from mature wells. First, natural microbe metabolic processes produce downhole gases that may increase pressure and decrease oil vis-cosity. Second, microbes produce surfactants that decrease the surface tension between oil and water. In addition, biomass and polymers from microbes selectively plug oil-depleted areas in the reservoir, diverting fluids into oil-rich zones. Successful MEOR projects typically depend on a combination of these mechanisms rather than any single one.

After reviewing MEOR field trial results, sci-entists have made important observations regarding its application.32 Although MEOR tech-nology has been used in both oil-producing and water injection wells, it probably has the best chance of success in reservoirs with active water injection programs for secondary recovery. Not only does the water provide the energy to push oil

out, but it also distributes the nutrients through-out the system. Data show that MEOR can enhance recovery in reservoirs with a wide range of oil densities—from 16 to 41 degree API gravity and with reservoir temperatures as high as 93°C [200°F] and salinities as high as 142,000 parts per million (ppm) total dissolved solids. It is also possible to apply MEOR to dual-porosity reservoirs if the added nutrients are able to penetrate the matrix and not bypass the formation via high-permeability streaks. In some cases, MEOR treatment may reduce reser-voir souring in addition to stimulating produc-tion. Scientists theorize that the added

nutrients stimulate microbes that outcompete SRPs for food and thereby depress SRP growth. These trials demonstrate that MEOR processes can economically free up oil trapped in mature fields. Although most work to date has been on mature, noneconomic wells, there is potential for application of MEOR at an earlier stage in the life of a producing reservoir.

In addition to using microbes to stimulate pro-duction and mitigate reservoir souring and corro-sion, scientists are using them in exploration via biomonitoring.33 In one application of biomonitor-ing, a grid of shallow soil samples was analyzed for specific microbes.34 Elevated amounts of the target

> Halfdan corrosion. Visual examination of a cross section of the produced-water separator pipe (top) revealed a 2- to 3-cm [0.8- to 1.2-in.] layer of corrosion scale (middle). The scale had an orange outer layer and a black inner layer adjacent to the metal (bottom). Scientists observed areas of severe pitting corrosion at various points on the inner layer. Laboratory studies, including examination by the new MMMs, showed that the outer scale layer was composed of salts, iron oxides and decomposed biomass—primarily SRB and sulfate-reducing archaea (SRA). The inner scale layer was composed of salts, iron sulfides and decomposed biomass that had high levels of methanogens. (Adapted from Skovhus et al, reference 21.)

Outer solidsInner solids

Innersurface

Water separator pipe

2 to 3 cm of corrosion scale

Pitting corrosion

Pipe metal

12 Oilfield Review

microbes indicated microseepage of oil and gas from underground formations (above). Operators use this type of data to rank drilling prospects, characterize heterogeneities and iden-tify bypassed oil.

Controlling and Harnessing MicrobesArmed with these new insights on microbe behavior and chemistry, producers are putting this knowledge to work in the oil field. Statoil is using nitrate to control corrosion and H2S at its Gullfaks platforms in the North Sea.35

The Statoil Gullfaks field is located 175 km [109 mi] northwest of Bergen, Norway. Discovered

in 1979, this field produces about 30,000 m3/d [189,000 bbl/d] of oil from three large plat-forms—Gullfaks A, B and C.36 The platforms started production in the late 1980s and cur-rently use seawater injection for pressure sup-port. The seawater intake depth for Gullfaks A and B is 70 m [230 ft] below the surface; the Gullfaks C intake depth is at 120 m [394 ft]. Injection water volumes vary from 30,000 m3/d [189,000 bbl/d] to 70,000 m3/d [440,000 bbl/d]; the pressure downstream of the pumps is about 20 MPa [2,900 psi]. Injection water undergoes vacuum deaeration to remove oxygen, and the final water temperature downstream of the deaerator is 25°C [77°F].

Although Statoil employed stringent filtration and biocides to control injected water quality at Gullfaks, those approaches were not entirely effective. In the early 1990s, Gullfaks A experi-enced high H2S levels in produced gas and water. The high H2S levels, coupled with laboratory data that showed rapid increases in the Gullfaks SRB population from 1994 to 1996, gave Statoil reason to rethink the microbe control strategy.37 A suc-cessful application of nitrate added to injected seawater at Statoil’s Veslefrikk platform in early 1999 provided engineers with the confidence to use it at Gullfaks.38

>Microbial surveys. Soil samples in Osage County, Oklahoma, USA, were analyzed to detect the abundance of butane-utilizing microbes. More than 1,200 samples were analyzed from a grid measuring 3.5 mi [5.6 km] by 7.5 mi [12.1 km] (left). The orange circles indicate samples with the highest 30% of the microbe concentration; the size of the circle is proportional to concentration. The smoothed data are contoured to provide a more informative picture of the microbe distribution (right). The strongest microbial anomaly (purple) corresponds to structural data from a 3D seismic survey covering the same grid area. Several years after the microbial survey was conducted, an operator drilled and completed a producing oil well at the microseepage anomaly. (Adapted with permission from Geo-Microbial Technologies Inc.)

0 km

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High

0 mi 1

1

Contoured Microbial Concentrations

Relative microbe

concentration

0 km

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1

Presence ofbutane-utilizing

microbes

Smoothed Microbial Concentrations

Summer 2012 13

In late 1999, Statoil switched from biocide to nitrate to treat the injected seawater for reser-voir microbe control at the Gullfaks B and C plat-forms.39 Nitrate was added to the injection water at 30 to 40 ppm as a 45 weight percent solution of Ca(NO3)2. At both platforms, scientists observed decreases in SRB counts about one month after the start of nitrate injection. Decreases in the SRB population were accompanied by increases in the NRB counts. These changes in microbe dis-tribution are consistent with how these two microbe groups compete for nutrients. As nitrate injection continued, the changes in microbe dis-tribution led to major changes in the corrosion rate (right). Engineers also noted decreases in H2S levels in the produced water at Gullfaks (below right). Tangible benefits for Statoil include reduction of H2S in the produced water in most parts of the field and a 50% decrease in cor-rosion rates on metal coupons in the seawater injection system.

Nitrate controls certain undesirable aspects of microbe behavior, but MEOR does the oppo-site—it exploits microbes’ positive characteris-tics. The rationale for bringing microbes into the oil recovery process is simple—about 80% of the oil currently produced is from fields discovered in the early 1970s. More than 50% of the oil in these fields remains trapped and cannot be economi-cally recovered.40 In the last few years, scientists have developed MEOR processes that use new analytical technology to selectively identify and exploit beneficial microbes living in oil reser-voirs. These MEOR processes have given opera-tors a new tool to inexpensively free oil trapped in mature reservoirs. Husky Energy Inc. used this approach in a pilot oil recovery project in Canada.

35. Sunde E, Lillebø B-LP, Bødtker G, Torsvik T and Thorstenson T: “H2S Inhibition by Nitrate Injection on the Gullfaks Field,” paper NACE 04760, presented at the NACE Corrosion Annual Conference and Exposition, New Orleans, March 28–April 1, 2004.

36. Hesjedal A: “Introduction to the Gullfaks Field,” http://www.ipt.ntnu.no/~tpg5200/intro/gullfaks_introduksjon.html (accessed May 24, 2012).

37. Statoil collected samples of injection water downstream of the vacuum deaerator using a bioprobe. Bioprobes allow collection of samples from a biofilm that deposits on a metal surface within the probe. These instruments are commonly used in oil and gas systems to detect corrosion-causing organisms.

38. Thorstenson et al, reference 30.39. Statoil started nitrate injection at Gullfaks B in

October 1999 and about a month later at Gullfaks C.40. Marcotte B, Govreau B and Davis CP: “MEOR Finds Oil

Where It Has Already Been Discovered,” E&P, (November 4, 2009), http://www.epmag.com/Exploration-Wildcats-Stepouts/MEOR-finds-oil-it already-discovered_47917 (accessed July 15, 2012).

> Gullfaks microbe activity. Before nitrate was used in injection water at Gullfaks B, enrichment cultures from water and biofilm showed a stable and diverse SRB population. Although at lower concentrations, NRB were also present (not shown) and used the same carbon sources as nutrients. After nitrate addition, SRB activity significantly decreased and NRB numbers in the biofilm increased by three orders of magnitude (not shown). Corrosion measurements on carbon steel coupons in the water injection system showed similar trends. Beginning in early 1994, corrosion rates at Gullfaks B rose, peaking shortly before nitrate addition was started. After nitrate addition, corrosion rates trended downward and were reduced by at least half. (Adapted from Sunde et al, reference 35.)

Corro

sion

rate

, mm

/yr

Date

Corrosion rateSRB activity

Nitrate added, Gullfaks B

April1994

May1997

February2000

February2003

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activ

ity,

μg H

2S/c

m2 /

d

0 0

5

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15

20

25

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Gullfaks field

North Sea

> Gullfaks H2S production. Statoil engineers measured H2S in the produced water before and after nitrate addition. At Gullfaks C, produced water H2S levels were slowly increasing prior to introduction of nitrate. After nitrate addition, H2S levels fell significantly but only after a delay. This delay is a result of the time it takes H2S to equilibrate in the reservoir. Statoil scientists estimate that several pore volumes must be displaced in the reservoir before a new or reequilibrated H2S value is observed in the producers. Statoil researchers have also developed a reservoir souring model—predicted H2S values are shown for Gullfaks C. The predicted values indicate levels that would have been experienced had there been no nitrate addition. (Adapted from Sunde et al, reference 35.)

Date

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2

4

6

8

10Nitrate added, Gullfaks C

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gw

ater

, L

14 Oilfield Review

The Husky pilot MEOR project is located within a field in the southwest corner of Saskatchewan, Canada (left).41 This field, discov-ered in 1952, has a reservoir depth of about 1,200 m [3,940 ft] and an average temperature of 47°C [117° F]. Current production from the field is 60 m3/d [380 bbl/d] of 22 to 24 degree API gravity oil and 4,250 m3/d [150 Mcf/d] of gas. Husky began waterflooding in 1967, and current water injection is 1,300 m3/d [8,200 bbl/d]. In 2010, cumulative oil production from this field reached 3.3 million m3 [21 million bbl] since discovery—estimated to be about 29% of the original oil in place (OOIP).

Husky teamed with Titan Oil Recovery to investigate the feasibility of using MEOR to recover crude oil trapped in this field. The Titan technology is simple—identify and quantify microbes that are indigenous to the reservoir.42 Using these data plus results from other field tests, Titan engineers formulated a nutrient mix-ture to release in the reservoir by way of the water injection system. The engineers theorized that the injected nutrients would stimulate changes in certain species of resident microbes, allowing the microbes to affect the interfaces between oil, water and rock to release small droplets of oil into the active flow channels.

Husky applied the Titan process in two steps—batch nutrient treatment of a single well followed by water injection to spread nutrients to nearby wells. For the single-well treatment, Husky injected 1.3 m3 [8.2 bbl] of nutrients and 13 m3 [82 bbl] of injection water through the wellbore, then shut in the well for a week. When the well was returned to production, results were encouraging—oil production increased from 1.2 to 4.1 m3/d [7.5 to 25.8 bbl/d] and water cut decreased from 94% to 80%. Because these results showed that the nutrients were appropriate for the reservoir and its resident microbes, Husky shifted its focus to the water injector for treating nearby wells in the pilot area.

Starting in early 2008 and using procedures similar to treatment of the single well, Husky injected the custom nutrient via the water injector in the pilot area. After three weeks, the closest producing well showed a significant oil production increase and a corresponding decrease in water cut (left). After an appropriate interval to allow underground transit of nutri-ents, engineers observed these positive results at

> Husky-Titan pilot location. The pilot area encompassed four producing wells and one water injector in the Saskatchewan, Canada, field. Nutrient injection was carried out in two steps. Husky first used Well A to confirm the laboratory-derived nutrient formula by batch treating the well. The operator next used Injector B to deliver the nutrients while production was monitored at nearby Wells C, D and E. (Adapted from Town et al, reference 41.)

Well C

Well A

Well D

Well E

Pilot area

Injector B

Water injectorProducing well

C A N A D A

U N I T E D S T A T E S

MEOR pilot

Saskatchewan

0 km

0 mi 1

1

> Husky-Titan pilot results. From early 2007 until the beginning of 2008, oil production at Well C in the Saskatchewan MEOR pilot was reasonably steady, between 2 and 4 m3/d [13 and 25 bbl/d]. The water cut for the same period was about 95%. After the first and subsequent nutrient injections at Injector B, oil production at Well C increased to 7 to 9 m3/d [44 to 57 bbl/d]. For the same period, water cut fell to about 88%. Because Well C was not treated directly, the pilot confirmed response through the reservoir from injector to producer. (Adapted from Town et al, reference 41.)

Date

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875

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65

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90

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Mar2009

Wat

er c

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Nutrient Injections at Injector BTreatment

1Treatment

2Treatment

3

Oil p

rodu

ctio

n, m

3 /d Oil production

Water cut

Summer 2012 15

other wells in the pilot area. In addition, engi-neers used the same treatment in wells and injectors outside the pilot area, achieving posi-tive results, which confirmed the response from injector to producer.

Success with microbe stimulation to enhance oil production from a mature waterflood was also seen at a field in Kansas, USA.43 The Stirrup field, discovered in 1985, is located in the southwest corner of Kansas. The reservoir depth is about 5,200 ft [1,600 m], and current production is 490 bbl/d [78 m3/d] of 38 to 41 degree API gravity oil. The initial reservoir pressure was 1,650 psi [11.4 MPa] and had declined to less than 100 psi [0.69 MPa] at the start of waterflood in 2003. Primary recovery from the Stirrup field was cal-culated at 19.1 million bbl [3.04 million m3] of oil, and waterflood is estimated to add another 2.8 million bbl [0.44 million m3], for an ultimate recovery of about 15% of the OOIP. In mid-2010, Glori Energy, in collaboration with Statoil, tested the AERO activated environment for recovery of oil technology in the Stirrup field to see if there was potential to boost recovery based on micro-bial stimulation (right).

Detailed characterization of the existing microbe population using both traditional and genome-based techniques was the first step in implementing the AERO technology at Stirrup. Once Glori Energy had characterized the indig-enous microbe population, engineers devel-oped a custom nutrient formulation and micro- bial inoculant.

Glori Energy started the AERO technology pilot at Stirrup in May 2010 by continuously injecting the custom nutrient using two of the water injectors. The initial test pattern for the pilot included two injectors and five producing wells. After several months of operation, it was clear that water from one of the injectors was not entering the test pattern, so that injector was withdrawn from the pilot. Some of the five test wells experienced similar problems when follow-up work showed that injectors not included in the test pattern program were influencing perfor-mance. Since this field does not have a dedicated system for separation and testing, evaluation can be made only on a well-by-well basis. Stirrup Well 12-2 demonstrated the predominant response. Analysis of the data from Well 12-2 suggests that

41. Town K, Sheehy AJ and Govreau BR: “MEOR Success in Southern Saskatchewan,” paper SPE 124319, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, October 4–7, 2009.

> AERO technology. Glori Energy has theorized that the AERO technology stimulates oil production in four steps. The microbes in the reservoir use existing oil as a carbon source to produce surfactants that reduce the oil-water surface tension, helping to release trapped oil (top left). The microbes then multiply and block some existing water flow paths, thereby forcing the opening of new flow paths that move trapped oil out of the reservoir (top right). When some of the trapped oil has been released and the local carbon source depleted, the microbes disperse and former water flow paths reopen (bottom left). If the stimulated microbes are active and have sufficient nutrients, the process is continuously repeated until trapped oil is brought to the surface and production increases (bottom right). (Adapted with permission from Glori Energy.)

Microbes reduce oil-water tension.

Microbes

Water

Flow path

Rock grain

Oil releasedinto pore

Oil trappedin pore

Microbes affect preferential flow.

Microbes disperse. New water flowpaths open.

42. Analysis for resident microbes is typically carried out on produced water samples.

43. Bauer BG, O’Dell RJ, Marinello SA, Babcock J, Ishoey T and Sunde E: “Field Experience from a Biotechnology Approach to Water Flood Improvement,” paper SPE 144205, presented at the SPE Enhanced Oil Recovery Conference, Kuala Lumpur, July 19–21, 2011.

16 Oilfield Review

the AERO treatment, when properly applied, is capable of significantly boosting ultimate recov-ery (below).

Microbes on the SurfaceWhile many microbe applications are designed for the subsurface, others contribute to shallow and surficial processes such as the management of oilfield waste or remediation of production from oil sands and spills.

Oil and gas production generates a variety of vapor, liquid and solid waste that not only must meet governmental regulations but also be dis-

posed of in a safe and environmentally responsi-ble manner. Microbes are currently playing an important role in disposal of these waste streams, particularly in the realm of solid waste. Two solid waste streams that arise from oil and gas produc-tion may be amenable to bioremediation: hydro-carbon-impacted soil and drilling waste.

During the last 100 years, some production facilities have experienced contaminated soils because of leaks or uncontrolled discharges of crude oil and other liquids. Natural weathering can significantly reduce the hydrocarbon con-centration in soil but does not eliminate it.

Although researchers have shown that heavily weathered, high–molecular weight hydrocar-bons are essentially nonbiodegradable, these same hydrocarbons can be rendered less detri-mental by treatment with a mixture of nutrients and cultured microbes.44 Currently, bioremedia-tion is usually the preferred method for dealing with crude oil–impacted soil.45 Because not all crude oils respond to bioremediation in the same way, engineers have developed predictive models to allow quick assessment of ex situ bio-remediation without resorting to time-consum-ing laboratory trials.

Drilling waste, a by-product of oilfield opera-tions, is mostly nonhazardous, although the vol-umes of such waste are significant for both marine and onshore operations. For example, a midsize operator in the Gulf of Mexico may routinely gen-erate 250 tonUS [227,000 kg] of waste monthly.46 Some operators dispose of water-base drilling wastes from marine operations directly to the ocean. Although harm to the ecosystem from this type of disposal has not been demonstrated, it remains a controversial practice.47

As in marine environments, onshore drilling generates a significant volume of waste. A 509-m [1,670-ft], 61/2-in. hole produces 21 m3 [130 bbl] of cuttings, and disposal of onshore cuttings pres-ents a different challenge from that of marine environments. Scientists are designing synthetic drilling muds that, when added to soil, enhance soil quality and stimulate more rapid bioremedia-tion.48 In addition, engineers have developed a standardized screening protocol for drill cut-tings. This protocol compares rates of bioreme-diation using greenhouse-scale models to simulate field conditions (next page). Scientists at M-I SWACO, a Schlumberger company, use results from the greenhouse simulations to pre-dict the length of time for treatment, final condi-tion of the material following treatment, the capability of the material to comply with environ-mental targets and the likelihood of technique effectiveness.

> AERO technology results. Data from the Stirrup Well 12-2 are plotted as water cut versus cumulative production from the well with approximate trend lines drawn for periods both before and after nutrient injection. When these trend lines are extrapolated to constant 95% water cut, they imply a 50,000- to 55,000-bbl [7,950- to 8,740-m3] oil production increase as a result of the treatment. (Adapted with permission from Glori Energy.)

Wat

er c

ut, %

Cumulative production, 1,000 bbl

95

85

75

75 100 125 225150 250175 275200

90

80

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100

AERO nutrient start

Before AERO nutrient additionAfter AERO nutrient addition

Estimated increase50,000 to 55,000 bbl

44. Adams RH, Díaz-Ramírez IJ, Guzmán-Osorio FJ and Gutiérrez-Rojas M: “Biodegradation and Detoxification of Soil Contaminated with Heavily Weathered Hydrocarbons,” presented at the 13th Annual International Environmental Petroleum Conference, San Antonio, Texas, October 16–20, 2006.

45. Hoffman R, Bernier R, Smith S and McMillen S: “A Four-Step Biotreatability Protocol for Crude Oil Impacted Soil,” paper SPE 126982, presented at the SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Rio de Janeiro, April 12–14, 2010.

46. Louviere RJ and Reddoch JA: “Onsite Disposal of Rig-Generated Waste via Slurrification and Annular Injection,” paper SPE/IADC 25755, presented at the SPE/IADC Drilling Conference, Amsterdam, February 22–25, 1993.

47. Neff JM: “Composition, Environmental Fates and Biological Effects of Water Based Drilling Muds and Cuttings Discharged to the Marine Environment: A Synthesis and Annotated Bibliography.” Report prepared for the Petroleum Environmental Research Forum and API, January 2005, http://perf.org/pdf/APIPERFreport.pdf (accessed August 2, 2012).

48. Curtis GW, Growcock FB, Candler JE, Rabke SP and Getliff J: “Can Synthetic-Based Muds Be Designed to Enhance Soil Quality?,” paper AADE 01-NC-HO-11, presented at the AADE National Drilling Conference, Houston, March 27–29, 2001.

Clements K, Rabke S and Young S: “Development of a Standardized Screening Procedure for Bioremediation of Drill Cuttings,” presented at the 14th International Petroleum Environmental Conference, Houston, November 6–9, 2007.

49. Orwig J: “Scientists Grow Bacteria to Improve Oil Sands Remediation,” EARTH 57, no. 4 (April 2012):18.

50. Phan CM, Allen B, Peters LB, Le TN and Tade MO: “Can Water Float on Oil?,” Langmuir 28, no. 10 (March 13, 2012): 4609–4613.

Summer 2012 17

Microbe FrontiersThe ubiquity of microbe distribution on the planet ensures that scientists associated with the oil and gas industry have an abundance of oppor-tunities ahead of them. These opportunities include oil sands remediation and cleanup of ocean oil spills.

Production of hydrocarbons from Canadian oil sands has been successful in adding to the world’s sources of petroleum, but such hydrocarbon pro-duction is not without environmental cost. Tailings ponds, which must be fenced off to protect wild-life, are one consequence of oil sands production. Biologists and engineers have found that certain microbes thrive on potentially hazardous com-pounds in that environment.49 These scientists theorize that if the microbes could be cultured, identified and grown into biofilms, they could be

reintroduced to shorten the current 20- to 30-year compound breakdown time. Researchers are making progress; by simulating tailings pond con-ditions, they have reproduced 30% to 60% of the microbes in the sludge and expect to have pilot bioreactors running in a few years.

Drillers and producers in offshore operations must take significant precautions to avoid spills and must be prepared to deal with them if they occur. Use of dispersants remains controversial because the dispersants may have environmental impacts. Researchers in Australia, studying the physical chemistry of oil-water interactions, have reported a seemingly contradictory finding that may improve the odds in oil spill cleanup. These researchers found that, given certain values of interfacial tension, oil density and water droplet

volume, water droplets can float on an oil sur-face.50 Acceleration of aerobic biodegradation of spills is one application of this finding—small water droplets that float on the oil surface have more contact with airborne oxygen, thus acceler-ating the microbe-driven cleanup.

New analytical methods and scientists’ increased understanding of microbes have led to developments in controlling souring and corro-sion and improving oil recovery from mature wells. These advances are stimulating further work in the use of microbes for bioremediation in both onshore and offshore settings. Today, engi-neers are bringing the negative characteristics of microbes in the oil field under tighter control and are increasingly able to harness their positive aspects for improved hydrocarbon production and solutions to environmental concerns. —DA

> Bioremediation. Scientists at M-I SWACO in Houston use a greenhouse to study bioremediation rates by composting tub-sized samples of drill cuttings (right). These specialists have developed bioremediation protocols using 2.7- to 4.0-ft3 [0.08- to 0.1-m3] size samples of a drill-cutting mix containing sand, bentonite clay, an additional nonswelling silica clay and water, followed by coating with hydrocarbons at the 10 weight percent level. Typical soil amendments and nutrients are added to the resultant hydrocarbon-compost mix before it is allowed to sit for long periods under greenhouse conditions. During this extended period, constant conditions for the compost are maintained by introducing oxygen via periodic mixing and adding water and nutrients as required. Bioremediation as measured by total petroleum hydrocarbons is plotted for three hydrocarbon proxies (left). These data show that after 30 days, linear paraffins and mixed olefins have nearly completely dissipated, while diesel oil is significantly reduced but does not fall below about 1 weight percent. M-I SWACO uses this test to screen onsite remediation as well as to train field personnel to maintain optimal compost conditions.

Tota

l pet

role

um h

ydro

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ons,

wei

ght p

erce

nt

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Linear paraffinsDiesel oilMixed olefins

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