Home arrow News arrow Microbial Corrosion in the oil and gas sector; and BART tests
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).

  • One reason for this is that the very reductive conditions encourage the SRB to generate hydrogen sulfide (H2S) gas. This gas will start off the process of electrolytic corrosion which can rapidly corrode steel.
  • There is another group of bacteria that can also cause corrosion. These are the acid producing bacteria (APB). Under the same very reductive conditions that the SRB operate in, the APB can begin to degrade organics with the releases of short chain fatty acids that can also be corrosive. This activity happens when there is water present in the oil or gas and can be detected using the two barts designed to detect the aggressivity of SRB and APB in the water.

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.

SRB-BART™ test.
The first problem is that the water sample may contain hydrogen sulfide gas.

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:

  • Blackening in the Base of the BART test (BB reaction). When this happens it means that the bacteria have been growing under very reductive conditions. If a CL reaction also occurs before (more commonly) or after (rarer) the BB reactions then this would indicate that the SRB are associated with other bacteria growing in a biofilm.
  • Presence of black specks growing in a slime around the ball (more commonly on the underside than right up at the water line, as a BT reaction). This reaction is rarer in the oil and gas wells but does occur when the SRB are growing in less reductive conditions where there is some organic matter that is causing heterotrophic bacteria to grow. The reason for the BT reaction is that there is a slime-like biofilm formed under and around the ball and, once this has formed, conditions are now sufficiently reductive to allow the SRB to grow dispersed through the biofilm slime. Generally, these black specks form at only one or two locations but then rapidly spread as black speckles throughout the slime. Eventually a black band will form around the ball and the blackening may extend through out the bart (BA reaction).
  • Cloudy growths (CL reaction) that are usually white but can sometimes be colored yellow, pink or orange. This reaction does not indicate the presence of SRB but does indicate that there are anaerobic bacteria in the water sample. Some of these bacteria may be APB and so the APB-BART™ test should be performed at the same time as the SRB-BART.

 

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.

APB-BART™ test
The acid producing bacteria grow in waters which are reductive and also contain organic material that can become fermented with acidic products. These acidic products are usually the shorter chained fatty acids that cause the water to become mildly acidic (pH commonly ranging from 4.2 to 5.8).

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:

  • Add 30ml of the water sample to the outer tube which is then capped.
  • Shake the tube gently for five seconds. This will allow the acid neutralizing chemicals time to dissolve in the water and bring the pH up by 2.0 units.
  • Allow the water to settle for 20 seconds.
  • Pour 15ml of the water sample into the inner test vial (water up to the fill line with the ball floating) and screw cap back on. Invert the tube for ten seconds to allow the dried reagent in the cap to dissolve in the water turning it purple.
  • Tip the residual water out of the outer and place the inner test vial back into the outer and cap. Observe daily for signs of acid production (purple color moves to a yellow - dirty orange). Note time lag to this event.

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:

  • Dispense 14ml of sterile distilled or deionized water into the inner test vial.
  • Add 1ml of the water sample to be tested so that the water line is now at the fill line.
  • Tightly cap the inner vial. Invert the tube for ten seconds to allow the dried reagent in the cap to dissolve in the water turning it purple.
  • Observe daily for any changes in the color of the purple water to a yellow or dirty orange color. This may be observed occurring often in a narrow band (2 to 6mm) at any point up, or down the water column or the purple may clear completely to a yellow. Note the time lag to the first clear observation of the color change.

 

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.

Reference: www.dbi.ca