ABSTRACT

Sulfide scales, such as ZnS or FeS, are not as common as carbonate and sulfate scales, but an effective way to control them has not been fully developed. Solubility of ZnS and FeS is extremely low. As a result, it does not require a great amount of metal ions to precipitate sulfide scales. Once they are deposited on the surface, it is difficult to remove them due to their low solubility. The objectives of this study are to identify more effective and efficient chemicals for prevention and removal of sulfide scale deposits and to understand the sulfide scale control mechanism of these tested chemicals. We found that carboxymethyl cellulose, which is a cellulose derivative, was an effective sulfide dispersant in our tested conditions and combining with diethylenetriamine penta (methylene phosphonic acid) increased its capability to prevent sulfide scale deposition. Moreover, the effectiveness of sulfide scale dispersing was not affected by the presence of other scales, such as barite. We also found that FeS scales were effectively dissolved using tetrakis(hydroxmethyl) phosphonium sulfate combining with D-amino acid as well as aminiopolycarboxylic acid. The dissolution rate was faster at the early stage then slow down as the dissolution reaction proceeded.

INTRODUCTION

Mineral scales are a ubiquitous problem in water distribution system as well as commercial and industrial operation systems. Millions of dollars are spent throughout the world due to scale deposits in water line, boiler, cooling, membrane, etc. The common scale prevention approach is to apply scale inhibitors that kinetically inhibit the solid precipitations. Carbonate and sulfate scales are the most common encountering scale problem, but there are several threshold inhibitors controlling them effectively in various applications. On the contrary, sulfide scales, such as ZnS or FeS, are not as common as carbonate and sulfate scales, but an effective way to control them has not been fully developed. Sulfide could be naturally produced through sulfur reducing bacteria (SBR) and Fe can be provided from various corrosion processes, including microbiologically influenced corrosion which commonly take place in water and wastewater distribution system as well as oil and gas pipelines.1 The potential sources of Zn are mineral dissolution in aquifer and zinc bromide completion fluid which has been lost into the formation during drilling.2

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