|Content Type||Conference Paper|
|Title||Inhibition of Corrosion of Carbon Steel in the Presence of CO2, H2S and S|
|Authors||Dharma Abayarathna, Ali R. Naraghi, and Nihal U. Obeyesekere, Champion Technologies Inc.|
|Source||CORROSION 2003, March, 2003 , San Diego Ca|
|Copyright||2003. NACE International|
|Keywords||corrosion inhibitors, C02 corrosion, H2S and sulfur corrosion, sulfur deposition|
In the oil and gas production environments, the corrosivity of produced water depends on the nature of the dissolved substances. The mitigation of corrosion of carbon steel in the presence of CO2, HzS and elemental sulfur was investigated in this study. Laboratory autoclave tests were conducted to study the efficiency of selected corrosion inhibitors. Based on the laboratory test results, water-soluble organic corrosion inhibitors are found to be effective in controlling corrosion of carbon steel in the presence of CO2, H2S and S.
The control of corrosion is a major concern in the oil and gas production environments. The corrosion failures can lead to costly repairs, plant shutdowns, as well as health and environmental hazards. As the production of sour fields increases these concerns become paramount. The co-production of elemental sulfur in sour gas wells causes extreme dangerous conditions. At high temperature and pressure, sulfur dissolves in H2S. 1"2 As temperature and pressure decrease the solubility of sulfur in H2S decreases resulting in precipitation of sulfur in the production tubing and flowlines. The precipitation of sulfur eventually leads to plugging of the production lines. 3-4 Furthermore, sulfur deposits on metal surfaces cause accelerated corrosion in aqueous environments.
The problems related to corrosion in the presence of C02 and sulfur compounds can be solved by the use of appropriate chemicals. 59 The continuous injection of sulfur solvents prevents the deposition of sulfur on metal surfaces. 10-12 The sulfur corrosion can be mitigated with the application of corrosion inhibitors. Organic corrosion inhibitors are found to control oilfield corrosion problems successfully. The effectiveness of organic corrosion inhibitors in controlling corrosion in the presence of CO2, HzS and S was investigated in this study. Corrosion inhibitors with two types of chemistries were selected for the study. The inhibition properties of the chemicals were evaluated using high temperature and high pressure autoclave tests.
The chemical reactions that take place in an aqueous solution containing C02 are detailed below:
The corrosion of carbon steel in C02 media has been studied by many researchers. Based on these research studies several mechanisms for CO2 corrosion have been proposed. Among the various proposed corrosion mechanisms, the generally accepted anodic reaction mechanism for Fe dissolution in acidic environments is as follows: 16
The overall C02 corrosion reaction for Fe in an aqueous medium is
Fe + 2H2CO3 --~ Fe2++ 2HCO3- + H2 (10)
A number of researches have investigated the corrosion of carbon steel in the presence of H2S. The chemistry of HzS-H20 system is fairly complex. The stability of the different sulfur species present (H2S, HS or S z) depends on the solution pH and the redox potential. The corrosion of carbon steel in H2S media is influenced by the formation of salts of iron with sulfur anions. The types of iron sulfide scales that are formed include kansite (Fe9S8), troilite (FeS), pyrrhotite (Fel.xS), mackinawite (Fel+xS), markasite FeS2 (S-deficient) and pyrite FeS2 (S or Fe deficient). 17 These scales may or may not be protective depending on the conditions such as temperature, pH and H2S concentration. The iron sulfide scale formed under most oil and gas conditions at temperatures up to 90 C is mackinawite. At higher temperatures the scale consists predominantely of troilite and/or pyrrhotite. The overall H2S corrosion reaction for Fe in an aqueous medium is written as
Fe + H2S ~ FeS + H2 (11)
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