Abstract
Abstract
Hydrogen Sulfide (H2S) is problematic in oilfield systems because of its high toxicity and corrosive nature. There are a number of technologies available to remediate sour production streams containing water. However, technologies to effectively and quickly scavenge water-deficient, oil systems are limited.
In this paper we review the properties of a family of new, fast-acting, oil-soluble H2S scavengers based on carboxylates of zinc and iron. Laboratory and field performance data are presented. This technology's efficacy is demonstrated in a plant trial treating a 1200 ppm hydrogen sulfide contaminated oil-stream. A range of competing oil- and water-soluble scavengers was shown to be less effective under these conditions.
Introduction
Hydrogen sulfide is naturally occurring in many oil and gas production streams. It is produced by geochemical processes, similar to that which converts biomass to useful hydrocarbons. It can also be produced by microbiological processes, for example the reduction of sulfates by anaerobic bacteria such as sulfate reducing bacteria (SRBs).
Regardless of its origin, H2S contamination of oil and gas can be problematic. It can lead to corrosion such as sulfide stress corrosion cracking. It is extremely toxic, with a PEL (Permissible Exposure Limit) of 20 ppm according to OSHA and MMS standards. Its "Immediately Dangerous to Life or Health" concentration is established at 100 ppm.1 Finally, H2S degrades the value of the hydrocarbon stream to the end user. US EPA has promulgated regulations minimizing the sulfur content of fuels, driven by concerns of acid rain. Removing H2S from so-called "sour" streams is critical.
There are a number of techniques to reduce the H2S content of oil and gas. Physical/ engineering techniques include oil contactors, water contactors, and the like. These technologies are excellent a removing the bulk of the H2S content through a stripping process, typically with sweet gas. Chemical techniques, which are more common, use reactive scavengers. These are either injected directly into a production stream or sprayed into a contactor tower such as an amine unit. Scavengers react out the H2S content to afford nonproblematic derivatives. Chemical techniques are often used as a final "polishing" treatment in conjunction with physical/engineering techniques.
One must consider several criteria when choosing an H2S scavenger. First it must effectively scavenge H2S at an acceptable rate. Its kinetics must be sufficient to the residence time of the employed equipment. In most cases, the chemistry should be non-reversible, so that the H2S does not re-emerge further down the production line.2 Second, the reaction products of the scavenger plus H2S should not lead to additional problems, such as scaling, foaming, or gelling. Third, the scavenger and its reaction products should have a favorable environmental and safety profile. This is important because of the possibility of fugitive emissions. Fourth, the scavenger should be easy to handle. In many systems, freeflowing liquids are the best, as they can be easily pumped from drums and totes. In other systems, granular solids are preferred, as they can be efficiently packed into treatment columns. And finally, the scavenger technology must be cost - effective.
Various scavenger chemistries have been applied in the oilfield.3 These sulfide sequestering technologies include:Zn-based scavengers (by formation of zinc sulfides),Fe-based scavengers (by formation of iron sulfides),Cu-based scavengers (by formation of copper sulfides),Triazines (by formation of complex cyclic thio/nitrogen compounds)Amines (by formation of thioamine compounds),Acrolein (by formation of thio-addition adducts),4Formaldehyde/methanol (by formation of cyclic thio compounds), andNitrates (by oxidation of sulfide to sulfur).