A New Field Method for the Quantitative Determination of Sulfides in Water-Base Drilling Fluids
Authors: Garrett R.L. 
Published: December 2007

This paper presents a new analysis method for measuring sulfides in water-base drilling fluids. The method is simple, accurate, and rapid. It can measure sulfide concentrations ranging from about 1 to 10,000 mg/liter. Laboratory, work to verify, the new method also is presented.

Introduction
A satisfactory method for analyzing sulfides in drilling muds and filtrates at the rig site has been a long-standing need in the drilling industry. The main requirements are speed, accuracy, reliability, and simplicity. The newly developed Garrett Gas Train procedure has been proven to meet these field requirements. The method has been tentatively adopted as an API procedure and was published recently. It is identified by the API as the Garrett published recently. It is identified by the API as the Garrett Gas Train Sulfide Analysis and is abbreviated here as GGT/S=.
The old API Hach Paper method for sulfide determination has not shown sufficient accuracy nor wide enough analytical range to be acceptable for current H2S drilling situations. Quantitatively accurate laboratory sulfide methods are not fieldworthy, while simple methods are inaccurate and often respond falsely to components other than sulfides in a mud. The GGT/S= method does not respond falsely to other fluid components, primarily because it eliminates most of the interference problems by using a gas train to separate H2S gas from the complex liquid phase. The method's accuracy, reliability, and simplicity comes from using well known, commercially available, Drager H2S-detector tubes in conjunction with the gas train.
Research has shown that this general gas train and detector tube principle of analysis can be applied to other volatilizable components in' drilling fluids. Two uses currently being applied experimentally in the laboratory and the field are analyses for filtrate carbonates, GGT/CO3=, and sulfites, GGT/SO3=.

Sulfide Chemistry in Water-Base Muds
Use of the chemical reactions between H2S and alkaline fluids is fundamental to how the GGT/S= method works and how its results can be applied at the rig to treat hazardous sulfides from a mud. H2S is a highly toxic and corrosive acidic gas having at low concentrations the distinctive odor of rotten eggs. If H2S enters a drilling fluid down hole, it reacts quickly as an acid with the alkaline mud. This neutralization at high pH converts the noxious H2S into the odorless, but not harmless, sulfide salts and water. From this di-basic acid, two salts in equilibrium are formed: bisulfides (HS-) and sulfides (S=). The hydroxyl ion (OH-) in the mud is the neutralizing agent, forming H2O as a byproduct. The process can be expressed as two separate neutralization reactions at equilibrium:
H2S + OH- P H2O + HS-................... (1) HS- + OH- P H2O + S=.................... (2)
The positive ion in this reaction is not indicated, but is Na+ in muds using caustic soda for pH control, or Ca++ when lime is used. For solutions in the pH range of 7 to 13 (most muds fall in this range), sulfides in solution would exist predominately in the form of HS-ions. If the pH is below about 7, the H2S gas becomes the predominant sulfide species, and above pH 13 the S= predominant sulfide species, and above pH 13 the S= predominates. predominates. The term "soluble sulfides," used frequently here, refers to the sum of all three sulfur-bearing components shown in Eqs. 1 and 2: HS- and S-ions plus any unreacted H2S that also might be present.
JPT
P. 1195