|Microbial Corrosion in the oil and gas sector; and BART tests|
One of the very important and major concerns in the oil and gas sector is corrosion. This is often linked to the sulfate reducing bacteria (SRB).
Both of these BART™ tests are described in the section on the BARTs and so this site will be used to describe particular protocols expressly for the oil and gas industry.
If there is gas in the water sample at greater than 20 ppm then there is a possibility that the tests will react to that gas causing a precipitation of the black sulfide that is taken to indicate a positive. To vent off the surplus hydrogen sulfide gas from the water sample, add 30ml of the sample to the outer tube for the SRB-BART™ test. Cap the outer tube without the inner test vial inside and shake the outer tube for ten seconds. This will cause the hydrogen sulfide to move out of the water sample into the air. Allow 20 seconds for the water sample to settle down and then add 15ml of the water sample to the inner test vial (up to the fill line). Drain the remaining water sample from the outer test tube and insert the inner test vial back in and screw both caps down tightly.
Examine the test on a daily basis and there are three reactions that can be observed:
The time lag to the observation of the reactions is critical to understanding how serious the corrosion problem is relating to the biogeneration of hydrogen sulfide gas. If there is a short delay before a positive detection (BB, BT or BA) of less than 5 days, that means that the SRB are very aggressive and likely to generating corrosive levels of gas. If the time lag to seeing a positive reaction is in the 6 to 8 days then the SRB are moderately aggressive. Time lags of greater than 8 days indicate a low level of aggressivity.
There are a range of treatment strategies that are sometimes used to control SRBs in corrosive waters. If these treatments are effective, this will be observed in increases in the time lags to a positive detection occurring in the water sample taken after treatment. It should be remembered that water samples taken straight after a treatment are likely to be unreliable since there may be high populations of displaced and highly aggressive SRB in the water! A minimal time lapse before post-treatment testing for the effectiveness of a treatment should be at least 14 days after the treatment with 42 days a suitable lapse period.
The APB-BART requires that the ware sample to be tested has a pH of at least 5.8 in order to ensure that the test can be reliably conducted. Water sample known, or thought to have, pH values of less than 6.2 should go through the following pre-treatment steps using the outer tube of the APB-BART:
In the event that the water has a high salt content such as produced (connate) water from an oil well, this can interfere with the test. To correct for the problems that greater than 6% salt can have on the test, the following procedure should be followed:
APB are highly aggressive if there is a positive detection of these bacteria in 4 days or less. Moderate aggressivity occurs when the time lag is between 5 and 8 days and a loss level of aggressivity occurs when the time lag is greater than 8 days.
There are a range of treatment strategies that are sometimes used to control APBs in corrosive waters. If these treatments are effective, this will be observed in increases in the time lags to a positive detection occurring in the water sample taken after treatment. It should be remembered that water samples taken straight after a treatment are likely to be unreliable since there may be high populations of displaced and highly aggressive APB in the water! A minimal time lapse before post-treatment testing for the effectiveness of a treatment should be at least 14 days after the treatment with 42 days a suitable lapse period.
Corrosion costs money, time and failures. Preventative maintenance using the APB- and the SRB- BARTs can save that money and time and also prevent failure through the appropriate treatment applications. The effectiveness of these treatments can be monitored using the BARTs judiciously after treatments. It has to be remembered that it is virtually impossible to "sterilize" a gas or oil well and so the bacteria will keep coming back. Preventative maintenance can provide a tool to control these invasions and minimize the consequences.
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