Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
Biodegradation occurs when bacteria, fungi, or other organism or biological process chemically dissolves materials. The process can be beneficial or detrimental within the industry depending on the circumstances. For instance, biodegradation via bacteria can aid in the cleanup of oil spills.
During biodegradation, microbial organisms metabolize petroleum, degrading the hydrocarbon content. The process can take more or less time depending on the amount of type and amount of bacteria, the reservoir or ecosystem in which the bacteria are found, and the amount of oxygen present.
In reservoirs cooler than approximately 80°C, oil biodegredation is common and detrimental. Oils from shallow, cool reservoirs tend to be progressively more biodegraded than those in deeper, hotter reservoirs. Increasing levels of biodegradation generally cause a decline in oil quality, diminishing the producibility and value of the oil as API gravity and distillate yields decrease. Additionally, viscosity, sulfur, asphaltene, metals, vacuum residua, and total acid numbers increase. For a specific hydrocarbon system (similar source type and level of maturity), general trends exist for oil-quality parameters vs. present-day reservoir temperatures of <80°C. However, other controls on biodegradation may also have significant effects, making predrill prediction of oil quality difficult in some areas.
Organism order of preference
Petroleum biodegrading organisms have a specific order of preference for compounds that they remove from oils and gases. Progressive degradation of crude oil tends to remove saturated hydrocarbons first, concentrating heavy polar and asphaltene components in the residual oil. This leads to decreasing oil quality by lowering API gravity while increasing viscosity, sulfur, and metal content. In addition to lowering reservoir recovery efficiencies, the economic value of the oil generally decreases with biodegradation, owing to a decrease in refinery distillate yields and an increase in vacuum residua yields. Furthermore, biodegradation leads to the formation of naphthenic acid compounds, which increase the acidity of the oil (typically measured as Total Acid Number, or TAN). Increased TAN may further reduce the value of the oil and may contribute to production and downstream handling problems such as corrosion and the formation of emulsions.
Biodegradation of gas caps and solution gases
Reservoir gas caps and solution gases also undergo biodegradation in cool reservoirs. C2+ gas components, particularly propane (C3) and n-butane (n-C4), are preferentially removed from natural gas, making biodegraded gases drier through the enrichment of methane (C1). Most biodegrading organisms also generate carbon dioxide (CO2) as a byproduct when they degrade hydrocarbons, increasing the CO2 content of solution gas or gas caps. Elevated CO2 contents can impact development economics negatively by necessitating the use of special steels to resist corrosion.
Oil and gas quality
Oil and gas quality reflects the compositional characteristics of hydrocarbons that impact the economic viability of an exploration, development, or production opportunity. Compositions may affect the direct value of the product (e.g., crude valuation relative to a benchmark oil) or the development or facility costs (e.g., additional wells required, emulsion processing, use of special steels), or they may even cause the oil to be unrecoverable. Typical oil-quality properties include API gravity, viscosity, sulfur, asphaltene, and metals (e.g., vanadium, nickel, and iron content), residua (e.g., vacuum residua or Conradson carbon content), acidity (TAN), wax content or pour point, and sensitivity to emulsion formation upon production. Biodegradation impacts essentially all oil-quality properties.
Saline water and prevention of biodegradation
Fresh, oxygenated waters can cause extensive aerobic biodegradation when they are in contact with reservoir oil. More recently, it has been recognized that anaerobic sulfate-reducing and fermenting bacteria also can degrade petroleum. Highly saline formation waters may inhibit bacterial degradation and effectively shield oils from oil-quality deterioration. The timing of hydrocarbon charge(s) and the post-charge temperature history of the reservoir can have major effects on oil quality. Reservoirs undergoing current charging with hydrocarbons may overwhelm the ability of bacteria to degrade the oil, resulting in better-than-anticipated oil quality. Fresh charge to reservoirs containing previously degraded oil will upgrade oil quality. Calibrated methods of oil-quality risking, based on a detailed evaluation of reservoir charge and temperature history and local controls on biodegradation, need to be developed on a play and prospect basis.
When an unfortunate event such as Exxon Valdez or Deepwater Horizon, the environmental impact can be stark and longstanding. Biodegradation can help bring an ecosystem back to its original state. Billions of hydrocarbon-chewing microbes, including Alcanivorax borkumensis, are part of the process. To break down the oil and make it easier for microorganisms to consume, chemical dispersants are often poured over an oil spill, as was the case in Deepwater Horizon.
The fate of oil compounds in the seawater is strongly influenced by microbial biodegradation processes, especially for compounds with a significant dissolution potential in the water phase.
During biodegradation in the seawater column, oil compounds are transformed primarily through aerobic oxidation, ending up as inorganic carbon by complete mineralisation. Compounds in the water-accomodated fraction (WAF) will be easily available to pelagic seawater organisms, including the biodegrading microbes, while the compounds with low dissolution potentials will mainly be accessible to organisms digesting oil droplets. Thus phase distribution will have strong implications both for the fate and effects of the oil compounds.
Most acute toxicity studies of oils with pelagic organisms have been performed with the WAF preparations. The dissolution potentials of various components are influenced by the oil loading rate, and studies have shown that the relative toxicity increases at lower oil/water ratio. The CROSERF approach has been established for exotoxicity testing of WAFs and dispersed oils.
- Wenger, L.M., Davis, C.L., and Isaksen, G.H. 2002. Multiple Controls on Petroleum Biodegradation and Impact on Oil Quality. SPE Res Eval & Eng 5 (05): 375 – 383. SPE-80168-PA. http://dx.doi.org/10.2118/80168-PA.
- Biello, D. 2010. Slick Solution: How Microbes Will Clean Up the Deepwater Horizon Oil Spill. Scientific American. http://www.scientificamerican.com/article/how-microbes-clean-up-oil-spills/.
- Brakstad, O.G., Faksness, L.-G. 2000. Biodegradation of Water-Accommodated Fractions and Dispersed Oil in the Seawater Column. Presented at the SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Stavanger, Norway, 26-28 June SPE-61466-MS. http://dx.doi.org/10.2118/61466-MS.
Noteworthy papers in OnePetro
Bland, R.G., Clapper, D.K., Fleming, N.M., et al. 1993. Biodegradation and Drilling Fluid Chemicals. Presented at theSPE/IADC Drilling Conference, Amsterdam, 22-25 February. SPE-25754-MS. http://dx.doi.org/10.2118/25754-MS.
Kraus, L.A.S., Rabke, S.P., Marcia V. Reynier, et al. 2010. Application of Biodegradation Testing in Brazil. Presented at the SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Rio de Janeiro, 12-14 April. SPE-127014-MS. http://dx.doi.org/10.2118/127014-MS.
Rabion, A., Perie, F., Basseres, A. 1997. Biodegradation of Synthetic Muds: Oxidative Pretreatments. Presented at the SPE/UKOOA European Environment Conference, Aberdeen, United Kingdom, 15-16 April. http://dx.doi.org/10.2118/37862-MS. IDSPE-37862-MS.
Robichaux, T.J., Myrick, N.H. 1972. Chemical Enhancement of the Biodegradation of Crude-Oil Pollutants. http://dx.doi.org/10.2118/3392-PADocument IDSPE-3392-PA J Pet Tech 24 (01): 16 – 20.
Oswald, R.J., Hille, M. 1997. Biodegradation on the Seafloor - Science or Speculation? Presented at the International Symposium on Oilfield Chemistry, Houston, 18-21 February. SPE-37262-MS. http://dx.doi.org/10.2118/37262-MS.
Atlas, R.M., Hazen, T.C. 2011. Oil Biodegradation and Bioremediation: A Tale of the Two Worst Spills in U.S. History. Environmental Scientific Technology. 45 (16): 6709–6715. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3155281/.
Olivieri, R., Bacchin, P., Robertiello, A., et al. 1976. Microbial degradation of oil spills enhanced by a slow-release fertilizer. Applied Environmental Microbiology 31 (5): 629–634. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC291167/.
Earth Gauge. Biodegradation: Microbes at Work. http://www.earthgauge.net/wp-content/EG_Gulf_oil_spill_Microbes.pdf.
Wilhelms, A., Larter, S.R., Head, I., et al. 2001. Biodegradation of oil in uplifted basins prevented by deep-burial sterilization. Nature 411: 1034-1037. http://www.nature.com/nature/journal/v411/n6841/abs/4111034a0.html.