CO2, when it enters an alkaline mud, immediately ionizes and is neutralized forming carbonate (CO3=) and bicarbonate (HCO3-) ions. When the mud’s filtrate is put into a GGT and acidified, the original CO2 gas is reformed, liberated and measured by the stain on the CO2 Dräger tube (after first being collected in a 1-liter gas bag).
Note:If sulfites are present in the high-range Dräger tube, an orange color (caused by SO2 may appear ahead of the black front. This orange region should be ignored. Record only the dark-ened length.
Note:For best Dräger tube accuracy, the “Darkened Length” should fill more than half the tube’s length.
Sulfide, Mg/L (Darkened Length*)(Tube Factor) Sample Volume, cm3
---=
A lead-acetate paper disk fitted under the o-ring of Chamber 3 can be substituted for the Dräger 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 Dräger tube for a quantitative analysis.
Carbonate/bicarbonate contamination is often difficult to detect. It can cause poor filter cake quality and excessive mud rheology which can lead to problems such as loss of circulation and the sticking of pipe and logging tools. The Garrett Gas Train is a reliable means of measuring carbonate/bicarbonate contamination.
Step 1. Be sure the Garrett Gas Train is clean, dry and on a level surface, with the top removed.
Step 2. Add 20 cm3 of deionized water to Chamber 1, then add 5 drops of octanol* defoamer. (*Avoid inhalation or skin contact.)
Step 3. Install the top on the gas train and hand-tighten all screws evenly to seal the o-rings.
Step 4. Turn regulator handle counterclockwise to avoid pressurization. Connect flexible tubing to the dispersion tube of Chamber 1. Install and puncture N2O cartridge.
Step 5. Adjust the dispersion tube in Chamber 1 to approximately ¼ in. off bottom using white adjust-ment sleeve.
Step 6. See Table 9 for sample volume required for the expected carbonate range and for the Dräger tube identification.
Step 7. Gently flow carrier gas for 1 minute by turning handle on regulator clockwise to purge air from the system. Check for leaks. Shut off carrier gas.
Step 8. Connect the gas bag and stopcock to hand pump. Use a discarded Dräger tube as a connec-tion and start with an empty gas bag (Figure25).
Step 9. Fully depress and release hand pump. When the bag is completely empty and free of leaks, the pump will remain depressed for several minutes. If leakage is detected, first check the pump and all connections. (See note below.) If the bag is found to leak, discard it.
Note:If CO2 has been used as a carrier gas in the previous test, purge GGT with N2O carrier gas for at least 1 minute.
Table 9.
Dräger Tube Identification Sample Volumes and Tube Factors to be Used for Various Carbonate Ranges Carbonate
Note:To check the pump, insert a sealed Dräger tube into pump opening and depress bellows. It will remain depressed if pump does not leak.
Step 10. With the bag fully collapsed, install rubber tubing from the stopcock and bag onto the outlet of Chamber 3. Close stopcock.
Step 11. Inject a measured volume of solids-free filtrate into Chamber 1 through the rubber septum, using a hypodermic syringe and needle. Then slowly inject 10 cm3 of 5N sulfuric acid. Gently shake the gas train to mix acid with sample.
Step 12. Open the stopcock on the gas bag and restart gas flow by turning regulator handle clockwise.
Allow gas bag to gradually fill until the bag is firm to the touch (do not burst it). Try to fill bag over approximately a 10-minute interval. Shut off flow (turn handle counterclockwise) and close the stopcock.
Step 13. Carefully break the tip off each end of the Dräger tube.
Figure 25
Checking for Leaks in Gas Bag
Figure 26
GGT with Gas Bag in Place for Carbonate Test
Step 14. Remove the rubber tubing from Chamber 3 outlet and reinstall it onto the upstream end of the Dräger tube. Attach hand pump to downstream end of the Dräger tube.
Step 15. Open the stopcock on the bag. With steady hand pressure, fully depress the hand pump.
Allow pump to suction gas out of the bag and through the Dräger tube. When chain on pump is fully extended, once again depress the hand pump. Continue this procedure until the bag is empty. Ten strokes (one liter) should empty the bag. More than ten strokes indicates leak-age has occurred and the test must be repeated.
Step 16. A purple stain on the Dräger tube indicates the presence of CO2. Record stain length in units marked on the Dräger tube including the faint blue tinge ahead of the purple stain. Using the measured Sample Volume, the Dräger tube Stain Length and Tube Factor (25,000) Table 4, calculate total soluble carbonates in the filtrate as follows:
Step 17. To clean the GGT apparatus, remove the flexible tubing and top. 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 gas. Rinse the GGT apparatus with deionized water and allow to dry. Periodically replace the gas bag to avoid leaks and contamination.
Potassium
This test method is used to determine the concentration of potassium ions in mud filtrates at levels above 3.5 lb/bbl potassium chloride. K+ ion is precipitated as the perchlorate salt. The potassium ion concentration is determined by measuring the volume of precipitate in a graduated (Kolmer) tube and reading the concentration of potassium ions from a previously prepared standardization curve.
Equipment