Downhole CO2 partial pressure calculation and tubing material selection – a case study of an offshore oil field in the South China Sea

Purpose This paper aims to establish the downhole CO 2 partial pressure profile calculating method and then to make an economical oil country tubular goods (OCTG) anti-corrosion design. CO 2 partial pressure is the most important parameter to the oil and gas corrosion research for these wells which contain sweet gas of CO 2 . However, till now, there has not been a recognized method for calculating this important value. Especially in oil well, CO 2 partial pressure calculation seems more complicated. Based on Dolton partial pressure law and oil gas separation process, CO 2 partial pressure profile calculating method in oil well is proposed. A case study was presented according to the new method, and two kinds of corrosion environment were determined. An experimental research was conducted on N80, 3Cr-L80 and 13Cr-L80 material. Based on the test results, 3Cr-L80 was recommended for downhole tubing. Combined with the field application practice, 3Cr-L80 was proved as a safety and economy anti-corrosion tubing material in this oil field. A proper corrosion parameter (mainly refers to CO 2 partial pressure and temperature) can ensure a safety and economy downhole tubing anti-corrosion design. Design/methodology/approach Based on Dolton partial pressure law and oil gas separation process, CO 2 partial pressure profile calculating method in oil well is proposed. An experimental research was conducted on N80, 3Cr-L80 and 13Cr-L80 material. A field application practice was used. Findings It is necessary to calculate the CO 2 partial pressure properly to ensure a safety and economy downhole tubing (or casing) anti-corrosion design. Originality/value The gas and oil separation theory and corrosion theory are combined together to give a useful method in downhole tubing anti-corrosion design method.