Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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52 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 CHEMICAL TREATMENT O A caustic scale dissolver to be used for cleaning offshore gas lift valves was evaluated in the laboratory through corrosion and stress corrosion crack- ing (SCC) testing for generic tubing and casing steels. Although slow strain rate testing and electrochemical test- ing showed no evidence of SCC, weight loss tests indicated very high general corrosion rates. Based on these results, the scale dissolver was not used for this application. O i l f i el d s cal e i s a h ard , cr y st al lin e deposit resulting from the precipitation of i n o rga n i c c o m p o u n d s i n o i l a n d ga s pro duction . 1-10 Thi s can b e caused by p re ssu re - t e m p e ra tu re f l u c tu a t i o n s o r mixing of incompatible brines. The scale can deposit on tubing, casing, topside pipin g, p er forations, and e ven on th e formation face itself, thereby restricting fluid flow and lowering production rates. It is one of the biggest challenges that the industry faces, especially with maturing reservoirs and increased water/oil ratios. The most common scales encountered are sulfates (gypsum, barite) and carbonates (c a l c i t e) . T h e s e c o m p o u n d s c a n b e removed using chemical treatments with acids, ethylenediaminetetraacetic acid (EDTA), or caustic solutions. ConocoPhillips was considering a high- pH scale dissolver to clean gas lift valves at an offshore platform. Operations had previ- ously been successful in cleaning downhole safety valves with this chemical, since the product was diluted immediately after exposure as the well was put into produc- tion. However, there were corrosion and cracking concerns when adding the prod- uct to clean gas lift valves. The scale dis- solver was being injected through the gas lift valves into the casing-tubing annulus lift gas. The problem was that some of this product came down under the gas lift valves and got trapped on top of the packer between the casing and tubing. Unless it dried out, there was a risk that this chemi- cal could cause corrosion or cracking on the tubing (external) or casing (internal). This would lead to gas lift valve leakage and consequently loss of well integrity. Therefore, corrosion and stress corro- sion cracking (SCC) tests at 120 °C (248 °F) using API Grades L80 13Cr (tubing ) and Q125 (casing ) steels were conducted in order to characterize and qualify continued use of this scale dissolver. Experimental Design Th e three exp erim ental t echniqu es used in this study were the slow strain rate test (SSRT), electrochemical tests (cyclic polarization, electrochemical impedance sp ectroscopy [EI S], and pot entiostatic testing), and weight loss corrosion testing. UNS S41000 martensitic stainless steel (SS) was used in lieu of API Grade L80 13Cr, while UNS G41400 low-alloy carbon steel (CS) was used instead of API Grade Q125. The microhardness of the CS was Corrosion Studies of Scale Dissolver in a Carbon Dioxide- Containing Environment Sudhakar MahajanaM, rayMundo CaSe, karen Cloke, jordan danielS, and jere My dunn, ConocoPhillips USA, Houston, Texas Frode Bredal, ConocoPhillips Norway, Tananger, Norway

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