Part 3: Oil Field Microbiology Flashcards
When were microorganisms in oil wells first detected?
Approximately 100 years ago (e.g. H2S identified in the 1920s in wells)
Which group of organisms are particularly problematic for oil production and why?
Sulphate reducing bacteria, because:
- oil souring
- production of toxic H2S
- microbially induced corrosion
What are the two groups of hydrocarbons?
1) Aliphatic compounds (alkanes, cycloalkanes)
2) Aromatic compounds (6C with alternating single and double bonds)
How does boiling point affect the applications of crude oil components?
Boiling point directly correlates to molecule size. This allows oil to be separated by fractional distillation.
e. g. methane (natural gas) =-161 C
e. g. 3 propane (LPG) = -42C
What is an isomer?
A molecule with the same molecular formula but a different spatial arrangement of atoms (exluding twisting or rotating around bonds).
Give examples of the number of isomers for hydrocarbons.
C3 = 1 C4 = 2 C5 = 3 C6 = 5 C7 = 9 C25 = 36,797,588
Why is crude oil so difficult to degrade?
There is a huge variation in the molecules of hydrocarbons. Enzymes are rarely capable of degrading more than one type of compound (including isomers), and so oil is difficult to degrade.
What is meant by the “fingerprint” of crude oil?
Each crude oil reservoir has a unique composition of alkanes, cycloalkanes and aromatics (and their isomers).
What is gas chromatography and what is it used for?
The identification of the relevant contributions of compounds within an oil sample through volatilising each compound and quantifying.
Used for: 1) identifying the potential uses of crude oil. 2) Identifying source of illegal spillages from tankers.
Describe the gas chromatography process,
1) Molecules are volatalised and transported by a carrier gas into a column.
2) Each molecule is retained according to its size and shape to separate them.
3) The samples are passed into a detector
4) The signal produced is dependant on the amount of each compounds present.
How much crude oil was spilled during the Amoco Cadiz (Normandy 1978) spill?
223,000 m3 or 233,000,000 litres (Gundlach et al 1983).
What proportion of crude oil was degraded by bacteria in the Amoco Cadiz oil spill (Normandy 1978)?
4.5 % - this was a surprise and prompted research into possible applications. (Gundlack et al 1983).
Why is microbial degradation of oil ineffective?
It is thought that microbes can only degrade one compound at a time. Therefore a community of specialist bacteria are required.
What enzyme is used for aerobic degradation of aliphatic compounds?
Monooxygenase
Explain the process of aerobic degradation of aliphatic compounds.
Monooxygenase (enzyme) is used with an oxygen free radical to convert the alkane to an alcohol. The OH bond can then be easily degraded.
What enzyme is used for aerobic degradation of aromatic compounds?
Dioxygenase
Explain the process of aerobic degradation of aromatic compounds.
Aromatic rings are very stable and hard to degrade. Dioxygenase (enzyme) produces two oxygen radicals which bond to the ring to create an easily degradable di-alcohol.
Which degrades faster - aliphatic of aromatic compounds?
Aliphatic (less stable).
To what does BTEX refer?
Another name for aromatics - Benzene, Toluene, Ethylbenzene, Xylenes.
How do anaerobic microorganisms degrade aromatic rings?
Anaerobes lack oxygen and so do not have a compound reactive enough to break down the aromatic ring. Instead a variety of methods must be used to attack the side chains.
Give an example of a method of degrading aromatic rings.
Reduction of the aromatic ring: 2 electrons and 2 ATP are used to remove one of the three double C=C bonds. Therefore the ring is less stable and can be more easily attacked.
Has an anaerobic benzene-degrading organism been cultured?
No, although it is known to exist since benzene can be degraded in nature.
How does formation of Benzoyl-CoA highlight the need for a number of different organisms to degrade oil?
Benzyl-CoA formation makes degradation easier. However, this can be formed by a number of pathways, meaning that a range of organisms are required. (Fuchs 2008).
What are the two methods of increasing the efficiency of microbial degradation of oil?
1) Biosurfectants - emulsifiers which increase the number of oil droplets, thereby increasing surface area and allowing bacteria to degrade more effectively.
2) Biostimulation - adding nutrients to stimulate growth of more bacteria.
Which is the smallest biodegradable alkane? (EXAM)
Methane.
What did results from the International Ocean Drilling Programme show for the Peru Margin?
The flow of sulphate-rich waters from the ocean crust causes formation of a methane-sulphate transition zone at 90 m depth. This zone has incredibly high cell numbers and must therefore be a zone of synergistic use of sulphate and methane.
What is the syntrophic consortium of methane oxidising archaea and sulphate-reducing bacteria?
A methanotrphic archaea and sulphate reducing bacteria work cooperatively to utilise crustal seep at depth. This facilitates anaerobic oxidation of methane (AOM).
What is AOM?
Anaerobic oxidation of methane.
What sea features does AOM form?
Carbonate sea mounts - examples in the Black Sea have some of the highest biomass concentration in the world (Michaelis et al 2002).
What are the findings of Zengler et al (1999)? Hint: hexadecane.
A stable culture was established that could convert hexadecane into methane. Hexadecane was initially converted into acetate and hydrogen and then into methane. Means that 5 or 6 different organisms were working cooperatively to degrade hexadecane without any oxygen or external electron acceptor.
Which group of bacteria are especially problematic in oil reservoir contamination?
Sulphate reducing bacteria.
What proportion of oil reservoirs experience some level of bacterial contamination?
70 - 80%
Which oil reservoirs do not experience bacterial contamination?
Those which are deep enough to effectively be autoclaved due to high pressure and temperature. (Wilhelms et al 2001).
What are the ranks of oil bio-degradation?
Light - 1 - Short-chain alkanes depleted Moderate - 4 - No n-alkanes remaining Heavy - 6 - Sterones partly degraded Very heavy - 8 - Hopanes partly degraded Severe - 10 - Long-chain aromatics degraded
How do oil wells become contaminated?
Water injection is necessary to maintain pressure needed for oil recovery. This could be waste water from production or seawater if offshore. Water injection leads to inoculation with microorganisms,
Why is seawater injection into oil reservoirs more problematic than freshwater?
Seawater has a higher sulphate concentration (around 30 mM), meaning that sulphur-reducing bacteria are more likely to be stimulated.
What type of oil degradation is used in the absence of sulphate? How is this economically useful?
Methanogenic degradation, converting long chain aromatics to methane (and some CO2). Offers a method to economically utilise long-chain hydrocarbons.
How does the synthrophic microbial community in oilfields facilitate sulphate-reducing bacteria?
The synthrophic microbial community produces waste hydrogen that can be utilised by SRB.
What is “souring” of oil?
The enrichment of oil with H2S.
Why is souring of oil a problem?
When burning the oil, any H2S present will become sulphur dioxide which is responsible for acid rain. SO2 may also react further with water to produce sulphurous acid or with oxygen to form SO3 (and then sulphuric acid).
Which bacteria are responsible for souring?
Sulphate reducing bacteria.
Why is souring particularly problematic in gas reserves?
In summer, excess gas is pumped below ground for storage to be used in winter for heating. H2S forms in these deposits.
What chemicals has the gas industry attempted to use to reduce souring?
Methanol
Triethylene glycol
Formaldehyde
Why have methanol, triethylene glycol and formaldehyde been unsuccessful in preventing souring?
Sass and Cypionka (2004) found that SRB could utilise methanol and triethylene glycol as growth substrates e.g. Desulfotomaculem spp.
Formaldehyde is very reactive and forms alcohols and other compounds which can then be used by SRB.
Why are nitrate reducing bacteria the most effective choice for preventing SRB in oil?
Nitrate has a higher redox potential than sulphate, and so SRB cannot compete against nitrate-reducers.
Benefits:
1) Nitrate reducers outcompete SRB for shared electron donors
2) Nitrate reducers oxidise H2S since they are chemolithoautotrophs.
What is the name for the commercial product containing nitrate reducing bacteria?
Petrocare
What are the problems with nitrate reducing bacteria in oil?
1) Do not exclusively oxidise H2S - also damage oil
- however still better than SRB!
Why did microbial communities degrade the Exxon Valdez spill so effectively?
In the Gulf of Mexico, more than 600 natural seeps are known to exist, seeping more than 100 million litres of oil per year. This has meant that bacteria have evolved to effectively degrade oil.
What are the two special cases of hydrocarbon seeps?
1) Methane seeps
2) Asphalt volcanoes.
