Hydrogen sulfide (H2S) scavengers have been used extensively in different field operations such as drilling and acid stimulation treatments. Typically, H2S scavengers are preliminarily designed to react effectively at different in-situ conditions. For example, Triazine-based scavengers are designed for neutral-high pH conditions, while aldehyde-based scavengers are intended for low pH conditions. However, reaction products of these scavengers with H2S could lead to potential formation damage.
The efficiency of two Triazine-based H2S scavengers were investigated over a wide-range of parameters: solution pH value, temperature and exposure time. Additionally, the effects of both scavenger concentration and its ratio to H2S gas on the scavenging capacity were explored. In this work, the scavenger concentration varied from 1 to 5 vol%, reaction temperatures up to 50°C.
Earlier studies have shown that, at low pH, triazine-based have a very low efficiency in scavenging H2S because the hydrolysis rate of triazine is faster than the reaction rate with H2S. Nonetheless, in this study, it was found that long exposure time between triazine-based scavengers and H2S can result in significant scavenging efficiency even at low pH values. Doubling the exposure time had almost doubled the amount of scavenged H2S in acidic solutions. In addition, this work, for the first time, highlights the possibility of calcium sulfide (CaS) precipitation in spent acid containing H2S scavengers. This precipitation has been observed when low scavenger concentrations were used in spent hydrochloric (HCl) acid. This paper provides optimum design parameters that allow for much effective use of H2S scavengers without causing the formation of calcium sulfide scale.