Technology Focus: Oilfield Chemistry (September 2014)

Abstract

Technology Focus Sulfide scales are becoming more prevalent in the oilfield environment. There are several reasons for this, including higher-pressure and -temperature fields being developed, which can be associated with the formation of zinc and lead sulfide; fields maturing, with iron sulfide being often associated with old mature waterflooded fields; and sour production becoming more commonplace, resulting in more-challenging integrity management. Little comprehensive research has been conducted on sulfide scales when compared to that concerning conventional carbonate- and sulfate-scale species. Perhaps this is partly because of the challenge of studying these materials in the laboratory and the perception that iron sulfide formation in the oil field is protective of tubulars and production equipment. As the prevalence of these scales increases, so too does the focus being placed on them by the industry, academia, and independent research groups. However, a lot of space remains for innovation and understanding. Today, the correct mitigation and cleanup methods can be selected most appropriately only on the basis of accurate root-cause-of-failure analysis. Generally, no single solution can be applied to all systems because the root causes differ from system to system. Commonality exists, however, insofar as a combination of dissolver/chelant and surfactant is nearly always needed for cleanup of the typically oil-wet scale. Control methods have focused very much on indirect techniques of removing one of the scaling species, such as chelation and sequestration of either the metal cation or the sulfide in order to reduce supersaturation. However, as the formation environments become more severe, these methods are being found to be increasingly challenging as they typically involve stoichiometric amounts of preventive chemicals. More recently, however, threshold inhibitors are starting to be reported in the literature, including some very promising chemistry evaluations from several research groups. Critical to success here is the accurate testing in the laboratory that best approximates field conditions. Unlike with conventional scale types, this is very easy to get wrong, so great care must be taken when intending to realize conformity between laboratory testing and success in the field. One thing is certain: More work needs to be performed by the industry before it can be said that control of these scales is understood and is as easy to manage as it is for conventional scale species. JPT