There are several methods commonly used or available to determine the presence of H2S in drilling fluids. Gas detectors or monitors are used on drilling rigs to detect unreacted H2S gas at the surface. The Hach test is used to detect H2S in the acid form, or as soluble sulfides in the makeup water, filtrate or mud. The Garrett Gas Train also analyzes the soluble sulfides in the filtrate of the mud. In addition, both tests can also determine the total sulfides, both soluble and insoluble, in a mud. It is important that the engineer have a complete understanding of the testing procedures and of the test results being reported.
In a slightly acid solution (below 7pH), H2S gas will ionize and exist as an acid or acid gas; or in a basic solution (above 7pH) as soluble sulfides. These soluble ions can go through filter paper and will be detected in the filtrate. The Hach soluble sulfide test is performed to determine the amount of H2S present in the drilling fluid in the soluble sulfide form. The test is performed on the mud or mud filtrate in which H2S gas has been liberated by a CO2flow from an Alka-Seltzer tablet. This test determines the amount of H2S present as an acid, or present as soluble bisulfide or sulfide ions. The initial result is uncorrected for pH. To accurately report the results, corrections must be made for pH of the drilling fluid.
The Garrett Gas Train test for sulfides uses a controlled CO2 flow from a gas cartridge to force H2S from an acidized sample of fluid or filtrate. Acidification of the sample ensures all of the sulfides in the sample are in the acid gas (H2S) form.
If H2S in the form of an acid gas, or the soluble bisulfide, or sulfide ions, has reacted with a sulfide scavenger (such as zinc carbonate), it will precipitate out as insoluble zinc sulfide. These insoluble sulfides cannot pass through filter paper, and therefore will not be present in the filtrate. Insoluble sulfides revert to soluble ions only at a very low pH (1-3). Therefore, to determine the amount of total sulfides in a mud, it becomes necessary first to change the insoluble sulfides back to soluble sulfides by acidizing the mud to a low pH (1) before running the test.
The test for total sulfides in the drilling fluid indicates what amount of H2S gas in the acid form or in the soluble sulfide form has reacted with an H2S scavenger. The test is performed on a mud sample which has been acidized below a pH of one. At this low pH range, all the sulfides in the mud, including those which have reacted with the H2S scavenger, are made soluble and measured by the test. To determine the amount of insoluble sulfides in the mud, the soluble sulfides are subtracted from the total sulfides.
Use of this procedure gives an engineer relative guidelines as to how much scavenger has reacted with H2S. For example, if the concentration of excess scavenger has decreased and the insoluble sulfides check indicates no decrease, then it is very likely the scavenger did not react with the H2S, but was discarded, or diluted, or mechanically settled out in the pits.
If hydrogen sulfide gas is encountered or expected to be encountered, the pH of the mud should be adjusted above 10 and a scavenger added to the system, either to remove the existing sulfides or as a pretreatment. If a water-based mud system is used, a calcium or lime system is preferred. Basic zinc carbonate is normally used as an H2S scavenger in water-based systems. If used as pretreatment, in anticipation of H2S gas, no more than 5 lb/bbl (14.27 kg/m3) should be added to the system. To effectively control the alkalinity from the carbonate added, 0.25 lb/bbl (0.71 kg/m3) lime should be added for every 1.0 lb/bbl (2.85 kg/m3) zinc carbonate. Approximately 1 lb/bbl (2.85 kg/m3) zinc carbonate is needed to remove 500 mg/L of the sulfide ion. Additional amounts of lignosulfonate can help control rheology. Zinc carbonate in excess of 5 lb/bbl (14.27 kg/m3) can cause rheological problems, especially in deep, hot holes (above 250°F, 121 °C); therefore, rheological and alkalinity properties should be closely monitored for adverse effects. Other scavengers on the market include zinc chelate, zinc chromate, ironite sponge (Fe3O4), zinc oxide and copper carbonate. All these scavengers have certain limitations. Ironite sponge performs best in the lower pH range (below 10.0). At least 5 lb/bbl should be added to the system as pretreatment and maintained as excess, the correct amount to be determined by the ironite-magnetic test. Copper carbonate should not be added to a system as pretreatment; it should only be added after the sulfide ion is measured in the mud system. Zinc chelate should be used in unweighted mud systems, and can be used in weighted mud systems. The zinc chelate does not affect mud properties as do the zinc carbonates. They react quickly with HS- but require more of the product to complete the reaction.
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