Sulfide

If hydrogen sulfide gas (H2S) or soluble sulfides exist in a mud they are very corrosive to the drill string and may lead to catastrophic pipe failure. H2S, even in low concentrations, presents both health and environmental hazards. Therefore, the monitoring of soluble sulfides in a mud system is critical, especially when drilling in areas known to contain H2S. Soluble sulfides can be quantitatively determined in a mud filtrate sample using a Garrett Gas Train (GGT).

H2S, when it enters an alkaline mud, immediately ionizes and is neutralized forming sulfide, S", and bisulfide, HS- ions. When the mud's filtrate is put into a GGT and acidified, the original H2S gas is reformed, liberated and measured by a stain on the Drager tube in the GGT.

Figure 20 Assembled Garret Gas Train (Sulfide Analysis)

Figure 20 Assembled Garret Gas Train (Sulfide Analysis)

Garrett Gas Train

Step

  1. Be sure the Garret Gas Train is clean, dry and on a level surface with the top removed.
  2. To Chamber 1, add 20 cm3 of deionized water and 5 drops of defoamer (Figure 21).
  3. 6/94 3-32

Figure 21 Preparing GGT for Sulfide Analysis

Chamber 1

Gas Train Base

Chamber 1

Garret Gas Train

3. To select the correct Dragertube and sample volume required for the expected sulfide range, use Table 8 below.

Table 8 Dräger Tube Identification

Sample Volumes and Tube Factors to be Used for Various Sulfide Ranges

Sulfide Range (mg/L)

Sample Volume (cm3)

Dräger Tube I.D. (See Tube Body)

Tube Factor

1.2 - 24

10.0

H2S 100/a

12

1.5 - 48

5.0

4.8 - 96

2.5

60 - 1020

10.0

120 - 2040

5.0

H2S 0.2% A

600*

240 - 4080

2.5

Note: *Tube Factor 600 is based on a Batch Factor (stenciled on box) of 0.40. For another Batch Factor (as stenciled on box), a corrected Tube Factor should be calculated:

„ . t . . .„-.-../Batch Factor^ Correct Tube Factor = (600)1-g-^g-1

4. Select the correct Drager tube and sample volume, then carefully break the glass tip off each end of the Drager tube (Figure22).

Figure 22 Breaking Drager Tube Tip

Figure 22 Breaking Drager Tube Tip

5. Place the Drager tube with arrow pointing downward into the bored receptacle. Install flowmeter tube with the word "Top" in the upward position in Chamber 3. Make sure o-rings seal properly

(Figure 23).

  1. Install the top on the gas train and hand-tighten all screws evenly to seal the o-rings.
  2. With the regulator backed off (turn regulator handle counterclockwise), connect the carrier gas to the dispersion tube of Chamber 1 using rubber tubing. Install and puncture a new CO2 cartridge.

Connect the flexible tubing from Chamber 3 outlet to the Drager tube (Figure 24).

  1. Adjust the dispersion tube in Chamber 1 to be approximately 1/4 in. above the bottom using the white adjustment sleeve.
  2. To purge air from the GGT, gently flow carrier gas for 30 seconds by turning handle on regulator clockwise. Check for leaks. Shut off carrier gas (CO2) by turning regulator handle counterclockwise.
  3. Collect a sufficient volume of solids-free filtrate obtained from low-pressure/low-temperature filtration test. If a low concentration of soluble sulfides is anticipated, a larger volume of filtrate will be needed (as shown in Table 8).
  4. 6/94 3-34

Figure 23 Drager Tube Installed in GGT Base

Figure 23 Drager Tube Installed in GGT Base

  1. Using a hypodermic syringe and needle, inject a measured volume of the solids-free filtrate sample into Chamber 1 through the rubber septum. Next, with another syringe, slowly inject 10 cm3 5N sulfuric acid.
  2. Immediately restart the carrier gas flow by turning handle clockwise. The flow rate should be maintained between 200-400 cm3/minute (keep ball between two lines on flowmeter). Continue flowing for a total of 15 minutes. One CO2 cartridge should provide 15-20 minutes of flow at this rate.
  3. Observe changes in appearance of the Drager tube. Note and record the maximum darkened length (in units marked on the tube) before the stain front starts to smear.

Note: If sulfites are present in the high-range Drager tube, an orange color (caused by SO2 may appear ahead of the black front. This orange region should be ignored. Record only the darkened length.

Note: For best Drager tube accuracy, the "Darkened Length" should fill more than half the tube's length.

14. Using the measured sample volume, the Drager tube's maximum Darkened Length and the Tube Factor (Table 3), calculate mg/L sulfide as follows:

Sulfide Mg/L = (Darkened Length*)(Tube Factor)

Sample Volume, cm3

*In units marked on the tube

15. To clean the gas train, remove the flexible tubing and remove the top. Take Drager tube and flow-meter out of the receptacles and plug the holes with stoppers to keep them dry. Wash the GGT with a soft brush, warm water and mild detergent. A pipe cleaner may be used to clean the passages between the chambers. Wash, rinse and blow out the dispersion tube with a dry air.

Rinse the GGT apparatus with deionized water and allow to dry.

A lead-acetate paper disk fitted under the o-ring of Chamber 3 can be substituted for the Drager tube in the gas train. The lead-acetate paper will qualitatively show the presence or absence of sulfides in the sample. A dark discoloration of the paper is a positive indication of sulfides. After a positive indication of sulfides, the test should be repeated using a Drager tube for a quantitative analysis.

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