ABSTRACT

Many high temperature sour gas wells, world-wide, have accumulated heavy iron sulfide deposit on downhole tubulars and in surface flowlines near wellheads. The scale deposits decrease well productivity, restrict well intervention, and interfere with downhole surveillance. Efficacious scale removal has become an important part of field operation for the reliable, efficient, and safe recovery of condensate-rich natural gas from these prolific wells.

Mechanical methods, such as milling, jetting, or a combination of both, are currently used for downhole remedial descaling. Meanwhile, many efforts have been devoted in recent years to identify and develop satisfactory scale dissolvers with the aim of reducing operation cost and enhancing cleaning efficiency. This paper summarizes the latest developments in iron sulfide dissolver studies. Laboratory tests were performed to evaluate some newly developed products for thermal stability, corrosivity to mild steel, compatibility with formation water, and dissolving capacity to authigenic mineral and field deposits at different temperatures. It is hoped that this paper will not only serve as a useful summary of recent advancements, but also clearly illustrate the remaining challenges, which need to be addressed.

INTRODUCTION

Iron sulfide (FeS) scale is a common problem in sour oil and gas wells. It can be formed through the corrosion reaction of hydrogen sulfide (H2S) with the carbon steel components in production systems or mixing of incompatible waters. The H2S is produced either by the activity of sulfate reducing bacteria (SRB) or thermochemical sulfate reduction (TSR) under reservoir conditions. Unlike carbonate and sulfate scales, iron sulfides consist of several different forms with distinct crystal structures, chemical compositions (sulfur to iron ratios), and chemical and physical properties, depending on their formation conditions such as temperature, H2S content, and age. They are notoriously difficult to inhibit and, once formed, pose serious challenges to remove economically and safely.

Many wells producing from a sour carbonate reservoir world-wide are experiencing heavy iron sulfide scale deposition on downhole tubular and surface flowline.1-3 Mechanical descaling is often required to improve well deliverability and restore downhole accessibility.4-6 These operations are costly and may also cause damage to downhole completion systems. The use of HCl based dissolvers is limited due to severe corrosion to tubulars, generation of toxic H2S gas, and potential formation damage associated with FeS re-precipitation.7 Other types of dissolvers, such as organic acids, iron chelators, and biocide, have also been evaluated.8,9 The performance of these commercially available products were relatively poor and was uneconomical to use. Effective alternative discovers are highly desirable.

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