Use of Reactive Iron Oxide To Remove H2S From Drilling Fluid
Authors: Ray J.D. , Randall B.V. , Parker J.C. 
Published: December 2007

During a gas kick in a Wyoming wildcat well, blowout prevention procedures were complicated by equipment failure and the presence of H2S. This paper describes the use of a specially prepared magnetic iron oxide to remove the H2S chemically from drilling fluids. This mud treatment protected the drillpipe from corrosion and prevented the release of toxic gases, allowing the well to be killed successfully.

Introduction
Drilling a well in an area where gas containing hydrogen sulfide (H2S) can escape poses grave risks. The complications of dealing with H2S at the surface, as indicated by Goolsby, emphasize the importance of good blowout prevention procedures and of precipitating out any H2S entering the wellbore. The combination of treating the drilling fluid with NaOH to absorb H2S and with zinc compounds to precipitate H2S has proven successful. The combined treatment has disadvantages, particularly the relatively large quantities of treating particularly the relatively large quantities of treating chemicals required and the harmful effects on drilling fluid properties, especially those of low-solids, nondispersed polymer muds. These disadvantages encouraged us to search for an alternative sulfide scavenger.

Reactions
H2S reacts with water and is extremely destructive to steel. Typically, in basic drilling fluids, H2S is neutralized by this reaction:
H2S + NaOH NaHS + H2O
This reaction theoretically requires 1.174 kg NaOH to neutralize each kilogram of H2S. The reaction is essentially complete at pH higher than 9, but lowering the pH below 9 would allow free H2S to escape.Zinc carbonate reacts with sulfide ions in solution to form insoluble zinc sulfide:
ZnCO3 + NaHS ZnS + NaHCO3
This reaction theoretically requires 3.679 kg zinc carbonate to precipitate each kilogram of H2S. Zinc sulfide (wurtzite or sphalerite) has a specific gravity of about 4.0. Commercial zinc compounds are either basic zinc carbonate, which contains up to 50 wt% zinc, requiring 3.8 kg material to precipitate each kilogram of H2S, or organic zinc chelates, which may require 10 kg or more for each kilogram of H2S. Precipitated zinc sulfide has an extremely low solubility in water and does not pose any hazard in neutral or basic drilling fluid, but zinc sulfide is soluble in dilute acids. Muds containing zinc sulfide could release H2S at any time if the mud should become acidic (Table 1).While searching for a more effective material, we noted that ferrous hydroxide reacts with H2S under the fight conditions to form iron pyrite, which is insoluble even in concentrated hydrochloric acid. The proposed reaction is
Fe(OH)2 + 2 H2S FeS2 + 2 H2O + H2
This reaction requires 1.32 kg ferrous hydroxide to precipitate each kilogram of H2S. This reaction appeared precipitate each kilogram of H2S. This reaction appeared to be almost three times as effective on a weight basis as zinc compounds are in removing H2S from solution in mud. The precipitate is a naturally occurring, stable compound that would not have adverse environmental effects. However, ferrous hydroxide is not readily available nor very soluble in drilling fluids and it reacts slowly with H2S.
JPT
P. 797