Corrosion of C110 carbon steel in high-pressure aqueous environment with mixed hydrocarbon and CO2 gas

Corrosion is a major issue in the oil industry, especially in presence of aqueous carbon dioxide under high-pressure high-temperature (HPHT) conditions. Exposing tubular materials to aqueous CO2 (carbonic acid) environment causes severe corrosion. This paper presents results of an experimental study conducted to investigate effects of CO2 partial pressure ratio (i.e. ratio of CO2 partial pressure to the total pressure) and total pressure on corrosion behavior of C110 carbon steel. In addition, effect of CO2 corrosion on mechanical strength was investigated to identify presence of microscopically undetectable localized corrosion such as intergranular attack that can cause substantial mechanical degradation. To perform corrosion experiments, test specimens were cut from C110 casing and machined. In order to improve accuracy and reduce machining-induced defects, water-jet cutting and milling machines were utilized. Corrosion tests were conducted by exposing the specimens (three specimens in one batch) to 2% NaCl solution saturated with mixed gas containing CH4 and CO2. CO2 partial pressure ratio (CPPR) and total system pressure were varied. Tests were performed at a constant temperature (38 +/- 1 degrees C) for one week duration. Corrosion rate was measured using weight loss technique. In addition, corroded specimens were examined using a digital microscope to inspect corrosion product and detect presence of localized corrosion. Localized corrosion is known for degrading mechanical properties of carbon steel. To identify existence of undetectable localized corrosion, the specimen load-carrying-capacity (LCC) was measured using tensile strength measuring apparatus. When CPPR was varied from 0 to 100%, similar corrosion rate trends were observed at 41.37 and 62.05 MPa. Corrosion rate increased to a maximum value and subsequently decreased as the CPPR was increased to 100%. A dense and compact corrosion product was formed at 41.37 MPa and 100% CPPR. To detect presence of localized corrosion, total reduction in Load Carrying Capacity (LCC) and reduction in LCC due to uniform corrosion were determined and compared. Results show no mechanical property degradation after exposure indicating absence of localized corrosion. The total reduction was slightly higher than the reduction due to uniform corrosion. One possible explanation for this observation could be minor variation of corroded specimen thickness, which resulted in underestimation of LCC reduction due to uniform corrosion. (C) 2016 Elsevier B.V. All rights reserved.