Basic chemistry, thermochemistry, and chemical structure discussion of hydrogen sulfide and soluble sulfide scavengers is followed by topics of reaction rates, effects on fluid rheology and an economic note in addressing the problem of drilling encounters with sour gas.
It is absolutely necessary to remove hydrogen sulfide and soluble sulfides from drilling fluids as quickly as possible because of the destructive effects of H2S on personnel and materials.
Breathing air containing more than 700 ppm H2S for 15 minutes may result in death. Damage to equipment, both metallic and nonmetallic, can be extremely severe. Since Henry's Law constant for hydrogen sulfide in aqueous mixtures tends to favor the vapor state over the dissolved state in the distribution of the gas between head space and fluid, it is essential that drilling crews reduce the concentration of H2S in the fluid to an extremely low value very quickly. A target would be the range of around 10 milligrams per liter (mg/l), although at this time this is arbitrary and is NOT a defined value in our business. Besides the danger to rig personnel, the presence of even small amounts (perhaps 10 to 100 mg/l) of H2S can cause sulfide stress cracking (SSC) of very hard steels; with drillpipe harder than Grade E in a hole, considering that tool joints may run one to two hardness numbers above the body of a joint, serious thread damage can occur when H2S is allowed to remain in the drilling fluid. NACE Standards tend to place the burden of pipe and equipment protection on the drilling environment meaning the drilling fluid.
A number of excellent papers have been written previously in this area, and these should be consulted.
This paper describes the REMOVAL of hydrogen sulfide and soluble sulfides from water-based drilling fluids; when removal succeeds, safe and economic drilling into sour gas formations becomes routine.
Scavenging can be confined to three types of chemical reactions:
(a) alkalinity control,
(b) formation of zinc sulfide, and
(c) formation of iron sulfides.
A wider range of potentially useful chemical scavengers will be discussed under the topic of Thermochemistry, below.
Alkalinity control must be used ALONG WITH one or both of the other scavenger types. But alkalinity control by itself will result in conversion of H2S to soluble sulfide: due to the equilibrium or H2S with large concentrations of soluble sulfides. the buildup of soluble sulfides (such as sodium sulfide, Na2S) concentration will eventually result in high levels of both the sulfide and H2S. The head space above a fluid with high concentrations of soluble sulfides will certainly contain dangerous levels of gaseous H2S.
Consequently, alkalinity control, using either sodium hydroxide or potassium hydroxide, maintains control of H2S as a temporary expedient, but iron or Zinc scavenger are essential for actual REMOVAL of all soluble sulfide forms from the fluid.