Microbial EOR

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  Spring 1997 17 Catherine BassHilary Lappin-Scott U n iversity of Exeter E  x e t e r, England  There is more diversity among microorgan-isms such as bacteria than there is in therange of life from artichokes to zebras. Bac-teria have successfully colonized virtuallye very environment on earth because theycan rapidly adapt to changing conditions,and use a large and varied number of nutri-ents to generate energy. How e ve r, until rela- t ively recently, the environments of manypetroleum reservoirs were considered toohostile for bacterial growth due to low ava i l-ability of wa t e r, and high tempera t u r e s ,pressures and salinities ( a b ove  ) . 1 Despite pioneering work during the 1930sand 40s in the USA by Claude Zobell,d e m o n s t rating a rich population of bacteria in water produced from shallow hy d r o c a r- bon reservoirs, the possibility of bacteriaexisting in larger, deep reservoirs was largelyignored. The start of North Sea production inthe 1960s led to the realization that bacteriacould produce hydrogen sulfide [H 2 S] as awaste product and cause reservoir souring. The Bad Guys and the Good Guysin Petroleum Micro b i o l o g y M i c ro o rganisms make up 15 of the 24 subdivisions of life on eart h  — t h e animal kingdom occupies just one.With so much diversity, it is not surprising that microbes can even be found in high-pre s s u re, hot, a n a e robic oil wells. Until re c e n t l y, the effects—both undesirable andbeneficial—of these organisms onre s e rvoirs were largely ignored. This attitude is gradually changing,h o w e v e r, and bacteria are now being h a rnessed to improve re c o v e ry.   Bacteria: diverse and adaptable. For help in preparation of this article, thanks to Chris Hall,S chlumberger Cambridge Research, Cambridge, England;and Jonathan Getliff, Dowell, Cornwall, England.1. For further background reading on this topic: Hurst CJ, Knudsen GR, McInerney MJ, StetzenbachLD and Walter MV (eds): Manual of Env i ro n m e n ta l  M i c ro b i o l o g y. Wa s h i n g t o n , DC, USA: American Soci-ety of Microbiology, 1996. Costerton JW, Lewa n d owski Z, Caldwell DE, Ko r b e r DR and Lappin-Scott HM: “Microbial Biofilms,” Annual Rev i ews of Micro b i o l o g y  49 (1995): 711-745.Bass CJ, Webb JS. Sanders PF and Lappin-Scott HM:1996. “Influence of Surfaces on Sulphidogenic Bacte-r i a ,” B i o f o u l i n g  10 (1996): 95-109. Campbell A: “Reservoir Biogenics and its A p p l i c a t i o n to Improved Oil Recove r y,” E m e rging Technology Sta-  tus Rev i ew  . Edinburgh, Scotland: Petroleum Scienceand Te chnology Institute ( PSTI), October 1996.  Spring 1997 19 A better understanding of subsurfacemicrobiology from research progra m s —s u ch as the British Geological Survey andUS Department of Energy Deep Microbiol-ogy Subsurface Progra m — n ow shows that microorganisms can grow in tempera t u r e s a b ove 125°C [257°F], at pH values of 1 to11, in the presence or absence of oxygen,and in up to 30% sodium chloride [NaCl]solutions. As this knowledge deve l o p s ,microbiologists are turning their attention topetroleum reservoirs as a habitat formicroorganisms ( p revious page  ). This articleexamines how bacteria affect oil productionby dividing them into two groups—the “badguys” and the “good guys.” The bad guys are those groups of bacteriathat use sulfur-based compounds present ins e awater—and sometimes in formation oraquifer waters—as part of their energychain, and use the simple carbon com-pounds that are present in formations asfood. Waste from this growth includeshydrogen sulfide, wh i ch is poisonous tohumans, and corrosive to tubulars and top-side tanks. Other detrimental microorgan-ism grow profusely around the wellboreregion—in mud filter cake and the forma-t i o n — b l o cking rock pores and reducing per- meability; still others break down and ren-der ineffective chemicals that are added tofacilitate production operations andincrease wellbore life. On the other hand, good microorganismsare those helpful bacteria wh i ch, duringtheir growth, produce useful compoundsthat improve oil recovery—for example, sol-vents, acids, gases, surfactants and biopoly-mers. It is these bacteria and their by p r o d- ucts that can be used constructively inr e s e r voirs. Petroleum microbiologists try to find ways of suppressing bad bacteria wh i l e e n c o u raging growth of good microbes. The World of Petroleum Micro b i o l o g y Bacteria carry out all their life functionswithin a single cell. Nevertheless, their huged iversity ensures ubiquity. Bacteria arefound virtually eve r y where: in the air webreathe, the food we consume and one verything we touch; and they thrive in themost extreme habitats—hot springs, ariddeserts, subterranean vents, aquifers, saltylakes and salt deposits. Microorganismsh ave a remarkable ability to survive adve r s e conditions and remain in a dormant statewaiting for favo rable growth conditions.M a ny microbiologists believe that this dor-m a n cy could last for thousands of ye a r s .  To date, most petroleum microbiologicalwork has centered on waterflooded reser-voirs that offer a cooled, oxygen-free, salinee nvironment, wh i ch meets the env i r o n m e n- tal requirements of many different groups of bacteria. And bacteria certainly do grow inthese conditions, thriving when nutrients—including reservoir chemicals or seawa t e r — are available, and entering dormant stateswhen food is scarce. Resuscitation from dor-m a n cy is rapid—perhaps taking two day safter months of inactiv i t y. Most bacteria have a natural tendency tog r ow attached to rock surfaces rather thanfree-floating in the liquid phase. In apetroleum reservo i r, bacteria may attach tothe rock, start to grow and then produceexopolymers—sugars—that help them attachto each other and rock surfaces. Such grow t his termed a biofilm and offers the adva n t a g e s of protection from biocides while encoura g-ing the bacteria to interact to best use nutri-ents and other resources ( a b ove  ) .Bacteria that are introduced to reservo i r s through waterflooding will flow over preex-isting biofilms; some bacteria will attacht h e m s e l ves to these biofilms and grow. Fr o m time to time, some bacteria detach from thebiofilm and move with the liquid flow or bytheir own motility and colonize other areasdeeper in the reservo i r.Complete analysis of all potential foods o u rces supporting bacterial growth is still in progress. How e ve r, analysis of formationwater from many reservoirs has demon-s t rated the presence of short-chain fattya c i d s — s u ch as acetate, propionate andb u t y rate—that may be utilized by some bac- teria to provide energy. Possible locations of micro o rganisms. Active and dormant bacteria are foundt h roughout the oil and gas production cycle, at and below the surface, on- ando ff s h o re, and in shallow zones as well as deep, hot, high-pre s s u re re s e r v o i r s .   E l e c t ron micrograph of a biofilm inside rock. The blocking of pores by bacteria can clearly be seen.  20 Oilfield Review B i o f i l m - dwelling bacteria coating rock sur- faces and pore spaces not only use externalnutrients from formation fluids, but also uti-lize chemicals from other dead and dy i n gparts of the biofilm by breaking them dow nwith enzymes to release essential nutrients,wh i ch are then recycled. This process occursto a significant degree in many biofilms,ensuring that energy sources entering, ora l r e a dy present in, a reservoir are used effi- ciently and economically seve ral times ove rby many different opportunistic bacteria.E nvironmental microbiologists haved e m o n s t rated that bacteria exist in—andm ay have originated from—the Earth’s sub-s u r f a c e . 2 In carrying out this work, manyproblems were encountered obtaining sub-surface fluid samples and rock cores thatcontain bacterial cells from the target env i-ronmental niche alone.Drilling equipment carries chemical, min-e ralogical and biological material through-out the borehole, contaminating otherwisepristine environments and making it difficultfor microbiologists to obtain genuine sam-ples at a specific depth in sediments or rockcores. How e ve r, technology is now ava i l- able to obtain cores in presterilized sleeve s ,sealed for examination in sterile conditionsat surface. Methods for ch e cking theintegrity of these cores have also beend e veloped—for example, fluorescentmarker beads included in drilling muds canindicate contamination if they show up inthe core. There are seve ral ways of determining themicrobial content of cores. Direct observa-tion of core material using microscopict e chniques can show cells associated withr o ck pore spaces. Cell cultures from subsur-face environments are possible by enrich i n g portions of rock that have been crushedaseptically and then mixing these with liq-uid nutrient to see what grows. How e ve r, this method does not necessarily result ing r owth of representative organisms—merelythose that could grow in the enriched mediap r ovided. To d ay, molecular techniques are e m p l oyed to relate the genetic materialfound in recovered bacteria cells with well-documented organisms or with geneticmaterial that is associated with specific,w e l l - ch a racterized functions. Obtaining uncontaminated fluid samplesat depth also poses problems, and muchr e s e a rch and development have gone into devising equipment that will capture andhold separate sterile fluid samples from dif-ferent depths. Extremely deep samples mayrequire slow decompression to reduce therisk of physically damaging bacterial cells. There has been little evidence, how e ve r, in the scientific literature supporting the exis-tence of genuinely barophilic organisms—those that actively require high pressure—socultures of cells obtained from great depthneed not be maintained at pressure.One of the biggest issues facing subsurfacesampling is the expense of drilling.I n e v i t a b l y, petroleum reservoir microbiolo- gists often have to be content with samplesd e r ived from produced fluids. These contain a mixture of oil, formation waters, andinjection fluids plus all the added reservo i rchemical treatments—many of wh i ch areused to mitigate the effects of microbiala c t ivity in the reservo i r. I n t roducing the Bad Guys  The most notorious villains on the reservo i rscene are sulfate-reducing bacteria (SRB),wh i ch have relatively simple requirementsfor growth and energy genera t i o n — s u l f a t e and carbon. Seawater contains about 2800ppm sulfate and formation waters may con-tain up to 2000 ppm short-chain fattya c i d s . 3 G iven suitable conditions, with nooxygen and favo rable temperatures, thec o cktail recipe is simple: inject seawa t e r,and expect sulfide souring.Although it has been known for manydecades that SRB are active in shallowwells, the existence of significant bacterialpopulations in deep, hot, high-pressure oilformations was not considered until souringbecame economically significant. To d ay, theassociation of the onset of reservoir souringwith commencement of seawater injectionin previously “sweet” fields is all too appar-ent and is largely due to SRB activ i t y. As a group, reservoir SRB have a widerange of temperature tolerances, and theirsrcin has been the subject of much debate.It is likely that thermophilic bacteria(tSRB)—those most active between 55 and70°C [129 and 158°F] or higher—havea l ways been present in the hot, deep subsur-face environment, existing in porous rockmatrices and maintaining viable populationsusing nutrients from deep aquifers andd e g raded hy d r o c a r b o n s . Work carried out by researchers at theHatherly Laboratories, University of Exeter,England, demonstrated that living tSRB havebeen recovered from open North Sea wa t e r s at 10 to 16°C [50 to 60°F]. Th e r m o p h i l i c SRB are most likely brought to surface dur-ing production operations and introducedinto the sea when separated fluids aredumped. These tSRB are extremely hardyand many of them are able to survive pro-longed periods of starvation in seawater atboth surface and reservoir tempera t u r e s .G iven the resilience of these organisms, it ispossible that they may subsequently be rein- jected into another reservoir where theym ay also grow if conditions are favo ra b l e . S t a r ved SRB can exist by just surviving at a very low metabolic level, waiting for theright conditions in order to revitalize. Dur-ing this dormancy, SRB tend to be smallerthan their growing counterparts and mayt ravel greater distances through the rockmatrix pores during waterfloods. Starve dSRB are less susceptible to standard reser-voir biocide treatments than actively grow-ing populations, making them particularlydifficult to treat effective l y. The conse-quences of flushing these bacteria to thesurface during oil recovery and then laterreintroducing them during waterflooding arepotentially grave .In cooler areas of the oil production pro-c e s s — s u ch as surface equipment—themajority of SRB are mesophilic bacteria(mSRB) that prefer temperatures of 20 to40°C [68 to 104°F]. Since they are not well 2. Long PE, Onstott TC, Fr e d r i ckson JK, Stevens TO, Gao G, Bjorstad BN, Boone, DR, Griffiths R, Hallett RBand Lorenz JC: “Origin of Subsurface Microorganisms:Evidence from a Volcanic Thermal Au r e o l e ,” presented at the International Symposium on Subsurface Micro-b i o l o g y, Davos, Switzerland, September 15-21, 1996. 3. Biofilms in oil reservoirs frequently contain mixturesof different groups of bacteria, including SRB. Some of the organisms are invo l ved in processes that comple-ment the sulfate reduction activity of SRB by slow l yreoxidizing the reduced sulfur products, thus complet-ing the natural cycling of sulfur in the env i r o n m e n t .


Jul 23, 2017
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