INSTRUCTIONS: CHEMICAL ANALYSIS OF ACTIVE SULFIDES IN OIL-BASED DRILLING FLUID

Transcription

1 OFI Testing Equipment Chemical Analysis of Active Sulfides Instructions Page 1 of 4 INSTRUCTIONS: CHEMICAL ANALYSIS OF ACTIVE SULFIDES IN OIL-BASED DRILLING FLUID DESCRIPTION: 1. The Garrett Gas Train (GGT) separates gas from the liquid sample, thereby preventing contamination of the H2S detector by the liquid phase. A Draeger tube is the preferred H2S detector for quantitative sulfide analysis, although a lead-acetate paper disk can be accommodated in the GGT (at top of Chamber 3) for a positive or negative indication of the presence of H2S in a sample. If a positive indication for H2S is noted by darkening of the lead-acetate paper disk, a Draeger tube should be used to test the same sample again to obtain a quantitative analysis of active sulfides. A Draeger tube responds to HxS by progressively darkening in length as H2S reacts with the tube s reagent. Two ranges of Draeger tubes are redommended and will span a wide range of sulfide concentrations. The low-range tube, H2S 100/a, is white until H2S turns it brownish black. The high-range tube, H2S 0.2%/A, is a pale blue until H2S turns it jet black. No known common mud component for contaminant, other than H2S, causes this coloration of these Draeger tubes. 2. Whole oil-mud sample (not filtrate) should be analyzed in order to capture all of the socalled active sulfides. Whole oil mud consisting of liquid and solid phases contains the active sulfides (neutralization products of H2S by lime plus any unreacted H2S). Thus, by using the 2M citric acid (a weak acid) and the whole mud sample in this procedure inert sulfides (such as solid ZnS, CuS or PbS) are not measured and only active sulfides are measured. EQUIPMENT: 1. The following equipment and chemicals are required to perform the active sulfides analysis of an oilbased mud sample using a slightly modified Garrett Gas Train. a. The Garrett Gas Train as shown in Fig. X.1, consists of a transparent plastic train equipped with an inert gas supply and pressure regulator, a floating-ball flowmeter and H2S Draeger tube of proper range.

2 OFI Testing Equipment Chemical Analysis of Active Sulfides Instructions Page 2 of 4 PROCEDURE: b. Draeger H2S Analysis Tubes: (1) low range marked H2S 100/a CH29101 and (2) high range marked H2S 0.2%/A Ch28101 (or equivalent). c. Citric acid (2M), demulsifier and isopropyl alcohol solution. Prepared by dissolving 420 g reagent-grade citric acid (C6 H8 O7.H2O) to make 1000 cm3 solution. Dissolve into the citric acid solution 200 cm3 of isopropyl alcohol. Into this solution dissolve 25 cm3 of Dowell Schlumberger W-35 (or equivalent) demulsifier. NOTE: Other demulsifiers are Exxon Chemical Corexit 8592; ChemLink OFC 1057; Champion Special Products PRODUCT 1063; Witco 1298 Soft Acid. Lab test these substitutes with the oil mud being analyzed. d. Octanol : in a dropper bottle, as a defoamer. e. Hypodermic syringes : one 20-cm3 syringe for citric acid solution and several 2.5-, 5.0- and 10.0-cm3 disposable syringes for measuring oil mud sample into GGT. f. Magnetic Stirrer and plastic or glass coated stirring bar (1/4 x 1 in. [0.64 x 2.54 cm]) to fit Chamber 1. g. Sample injection tube so called Teflon needle KF 11 TF, (cut to 6-inch [15.2 cm]) length, in [2.4 mm] ID, in [0.3-mm] wall thickness, from Hamilton Co., Reno, NV (or equivalent). 1. Fig. X.1 shows how the GGT sits on the magnetic stirrer and how the sample injection tube is fitted into the GGT in Chamber 1. NOTE: Pass the plastic injection tube (Teflon needle) through the rubber septum on top of Chamber 1 being sure it seals in the septum. 2. Position dispersion tube in Chamber 1 high enough to allow it to be above the liquid level to avoid frothing during the air purging step, below. 3. If compressed and regulated N2, He or CO2 carrier gas is used as a carrier gas, connect that gas supply by a flexible hose onto top of the dispersion tube of Chamber 1 bypassing the built-in GGT cartridge assembly and regulator. (Do not use N2O or air.) Otherwise, install and puncture a CO2 cartridge in the GGT carrier gas assembly, being sure regulator has been backed off. 4. Place stir bar in Chamber Add 20 cm3 of 2M citric acid, demulsifier and isopropanol mixture into Chamber Add 10 drops of octanol defoamer to Chamber Select a low or high-range Draeger tube for expected amount of H2S to be released from the sample. Refer to Table X.1 for proper sample size (2.5, 5.0 or 10.0 cm3) and for proper Draeger tube. 8. Break the tips off both ends of the Draeger tube and install it into the bored receptacle with the arrow pointing downward. Similarly, install the flowmeter tube, with top upward. Be sure the O-rings seal around the body of each tube. 9. Install the top on the GGT body. Hand tighten all screws firmly to seal. 10. Attach the flexible tubing from carrier gas outlet to top of the dispersion tube. Also, connect flexible tubing from outlet of Chamber 3 to top of Draeger tube. Do not clamp tubing connections to provide pressure relief. 11. Place a more than adequate volume of the whole-mud to be tested into syringe and attach syringe to the Teflon Needle. (Hold the plunger to keep it form blowing out under gas pressure.)

3 OFI Testing Equipment Chemical Analysis of Active Sulfides Instructions Page 3 of With dispersion tube above the liquid level in Chamber 1 (to avoid frothing), flow the carrier gas for 10 seconds to purge air from the GGT. Check for leaks. Stop gas flow. (Don t restart flow until Step 15.) 13. Operate magnetic stirrer at moderate speed. Carefully lower the gas dispersion tube into the liquid to a point just above the rotating stir bar. 14. Slowly inject the correct sample volume (see Note) of oil mud form the syringe, allowing it to mix into the vortex from the stir bar. Stir for at least 2 minutes. (Try to prevent oil mud from sticking to walls of GGT.) NOTE: Sample volume can best be accurately attained by measuring the injected volume by difference in a syringe. Be sure to compensate for the mud volume held up inside the injection tube, which can be about 0.5 cm Restart carrier gas flow. Adjust flow rate at 200 to 400 cm3/min by keeping flowmeter ball between the marks. NOTE: A CO2 cartridge should provide 10 to 15 minutes of flow at this rate. 16. Observe the Draeger tube for changes, which will begin at the inlet end. Record the maximum distinct stain length (in units marked on the tube) before the front starts to feather and smear. Continue gas flow at least 15 minutes, although maximum length may occur before that time. NOTE: The high-range (pale blue) tube may show an orange colored stain ahead of the jet black H2S front. Orange color indicates that SO2 gas is coming off the mud (from sulfite salts in the mud). Ignore the orange region record only the black stain length, in units marked on the tube. 17. Calculations. Using the measured Sample Volume, the Draeger tube s maximum Stain Length, and Tube Factor (found in Table X.1) calculate the Active Sulfides in the oil mud. Active Sulfides, mg/l = (Stain Length) (Tube Factor) (Sample volume, cm3) 18. Clean the GGT apparatus immediately after each use to avoid plastic deterioration from contact with the isopropyl alcohol. To clean, remove the flexible tubing and the top. Take the Draeger tube and flowmeter out of their receptacles and plug the holes. Clean the chambers and passages with warm water and mild detergent, using a soft brush and pipe cleaner. Wash the dispersion tube frits and injection tube with a paint thinner and then with water. Blow out frits with gas. (Occasionally, the dispersion tube may need to be soaked in strong acid to remove CaCO3 deposits.) Rinse the entire GGT unit with deionized water and allow it to drain dry. Table X.1 Draeger Tube Identifications Sample Volumes And Tube Factors for Various Sulfide Ranges Draeger Tube Tube Sulfide Range Sample Volume Identification Factor (mg.l) (cm3) (See tube body) (Step 17) 1.5 to H2S 100/a 12 3 to

4 OFI Testing Equipment Chemical Analysis of Active Sulfides Instructions Page 4 of 4 60 to 1, H2S 0.2%/A 600* 120 to 2, to 4, *Tube Factor 600 is based on a manufacturers batch factor of 0.40 (stenciled on box). For any other batch factor (on box) use a Corrected Tube Factor calculated as follows: Corrected Tube Factor = 600 (New Batch Factor) 0.40 Note: For highest accuracy, a Draeger tube should have over 50% of its length filled with the stain. If not, repeat the test using larger mud sample, up to 10.0 cm3 maximum. SPECIFICATIONS: Specifications for GGT materials and nominal dimensions are as follows: 1. Body dimensions are: Chamber 1 depth 3.54 in. [90 mm] and diameter 1.52 in. [39 mm]. Chambers 2 and 3 depth 3.54 in. [90 mm] and diameter 1.18 in. [30 mm]. Passages between chambers should have a diameter of 0.08 in. [2.0 mm] and the material of manufacture recommended is Lucite or equivalent transparent plastic or glass that is inert to acid, sulfides and gases used. 2. Dispersion tube stem diameter of in. [8.0 mm] and length 5.9 in. [150 mm]. The dispersion frit (ASTM 4-5.5) diameter 1.18 in [30 mm] and recommended material is Pyrex or equivalent type of glass. 3. Carrier gas should be inactive with H2S or sulfide salts, citric acid, demulsifier and isopropanol. Preferred gasses are N2, CO2, He. Avoid using air, N2O or oxidative gases. 4. Flowmeter, floating ball type is preferred, capable of measuring between 200 and 400 cm3/min of CO2. 5. Flexible tubing for connections to glass tubing must be inert to H2S and carrier gas. Latex rubber or Tygon are preferred. Flexibility should allow easy connections and blow-off if over pressured. 6. Syringes and injection tube should be glass or a plastic inert to oil mud and to sulfides.

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