Application of a New H2S Scavenger with Improved Performance in The Field

Abstract

The presence of hydrogen sulfide (H2S) gas occurs naturally, or can be introduced via bacteria contamination, in oil and gas reservoirs worldwide. There are several options for the removal of H2S from produced oil and gas ranging from fixed assets that scrub H2S to chemical injection at the wellsite. The area of interest for many operators is in the continuous application of non-reversible chemical H2S scavengers as an easy, reliable and cost-effective solution. The majority of the non-reversible chemical H2S scavenger market is based on triazine technology derived from the reaction products of formaldehyde and amines. In recent past, there has been an active industry wide search to improve the overall performance of H2S scavengers. Major topics for improvement include: Increased H2S scavenging capacityReduction of nitrogen contamination of crude oilReduction of scale formationElimination of by-product depositionAddressing existing environmental, health and safety concernsMinimization of products/reaction by-products disposal Conversely, some of the biggest hurdles with new H2S scavengers are ensuring fast kinetic reaction rates, system compatibility, consumption rates, minimal precipitation of scavenger/by-products, scalable manufacturing and competitive economics. Many new products have been proposed by chemical manufactures but often are not able to deliver enough benefits to warrant a change from the industry standard triazine. One potential solution is to pull through a technology from a different industry that already has established production, in significant volumes, for use in oilfield applications. Ideally, the new product would offer better performance versus the incumbent, a reduction in nitrogen content and minimize solids formation and deposition. A product identified several years ago as a potential replacement was an oxazolidine derivative referred to as MBO (3,3’-methylenebis(5-methyloxazolidine)). However, MBO has had limited application in the field until recently. MBO offers some of the same benefits as triazine but outperforms the incumbent technology by increasing the consumption of H2S per mole of scavenger, reducing the nitrogen content in crude oil, reducing the by-product deposition potential. Moreover, MBO is already produced in large manufacturing quantities. In this paper we will discuss details about the chemistry and increased formaldehyde content, laboratory results related to performance, system compatibilities, decreased transportation cost and confirmation of field application on large scale that supports the usage of this alternative H2S scavenger to standard triazine. H2S scavengers are used to mitigate the risks presented by H2S. They react with H2S in the liquid phase to form non-hazardous, non-reactive species that are often water soluble and thus disposed with water. Monoethanolamine (MEA) triazine (hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine) is the most widely used scavenger. It is less toxic than most aldehyde scavengers and reacts very quickly with H2S. MEA triazine reacts irreversibly with H2S to form dithiazine (5-hydroxyethylhexahydrodithiazine). One of the major concerns with MEA triazine is that there is a strong possibility of the by-product MEA reacting with excess H2S to form an ethanolammonium sulfide species that in turn reacts with the dithiazine to form a largely insoluble polymer, commonly referred to as amorphous dithiazine. An alternative triazine used in oil and gas production is monomethylamine (MMA) triazine (1,3,5-trimethyl hexahydro-s-triazine). MMA triazine has greater volatility than MEA triazine so is more suitable for dry gas applications. In the cases on MEA triazine and MMA triazine the ratio to amine:formaldehyde is 1:1.