Until recently, the use of a high specific surface iron oxide ("ironite sponge") for H2S control in sour gas drilling was widespread; however, a published study of the chemical kinetics of H2S removal projected low scavenging rates at a mud pH greater than 10.
In the present work, this rate law is reviewed, and the data synthesized with other published work. It is concluded that the projected H2S removal above pH 10 is unlikely to be correct, and that the H2S control mechanism is more complex than originally assumed. A governmental policy review on the use of iron oxide scavenger is recommended.
The scavenging of free hydrogen sulphide from drilling mud, necessary to prevent stress corrosion cracking of drill pipe during sour gas drilling, has in the past been effected by maintenance of a high mud alkalinity, or by addition of some scavenger that under mud conditions reacts irreversibly with H2S.
The most common additives at present are zinc compounds, the most popular being a basic zinc carbonate with a formula approximating 3Zn(OH)2. 2 Zn CO3 (1), and a high specific surface iron oxide (magnetite) of the formula Fe3O4. The latter material is often referred to as "iron sponge".
The basic zinc carbonate is thought to act through the presence of a small equilibrium concentration of zinc ion, which precipitates in neutral to alkaline sulphide solution.
Zn++ + S = → ZnS ↓
The reaction of iron sponge is considered to be considerably more complex, and as a result the data concerning its reactions have been interpreted in widely varying ways. This paper is intended primarily as a review article, and deals with a synthesis of available data and their reinterpretation. Iron sponge was also used in the mud system at Alberta's "Lodgepole Blowout", an uncontrolled sour gas flow at the well Amoco Dome Braz R 13-12-48-12 W5M, and its use is considered in the context of this review.
The only attempt to establish a rate law for H2S removal with iron sponge is due to Wendt2, although kinetic studies with more limited objectives have been reported by others1, 3, 4.
Wendt's experiments were performed in a pH range 8–10 under laboratory pressure in a blender equipped with a double wall to permit the circulation of a temperature controlled fluid. Samples were periodically withdrawn from the agitated reaction mixture, the pH raised to stop the reaction, solids removed by centrifugation, and the supernatant analyzed for residual sulphide.
The rate law is reported as:
Equation (1) (Available in full paper) where the total sulphide in solution, [ST], is in mg/L, [H+] in moles/L, iron sponge in kg/m3 and time in minutes. With these units5,
Equation (2) (Available in full paper) provided the reactivity of the iron sponge is 0.7 g H2S/g iron sponge.
The second order term in [ST] was established by the observation that, with [I] in large excess and [H+] constant, a plot of reciprocal [ST] was linear with time.