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1. Introduction
CO2 corrosion in oil and gas industry has caused huge economic losses and introduced safety hazards (Kermani and Morshed, 2003; Dugstad et al., 2000; Dugstad, 1992; Smart, 1990; Tang et al., 2008; Zhang et al., 2000). This problem has caught the attention of many oil production companies. CO2 corrosion usually occurs on the tubing and casing of pipelines under the effect of multiphase formation fluid (Deng et al., 2011). It is related not only to the metallurgy and electrochemistry (Omar, 2014) but also to the fluid mechanics and oil and gas separating theory (Zhu et al., 2015).
CO2 partial pressure is one of the main factors of the CO2 corrosion, so calculating CO2 partial pressure correctly is important for downhole tubing anticorrosion design. Unfortunately, most corrosion research institutes have not investigated how to calculate the CO2 partial pressure, and there is no recognized method to calculate downhole CO2 partial pressure. Some researchers even believe that the CO2 partial pressure is equal to the hole-bottom liquid column pressure multiplied by the mol% of the CO2 in the gas phase, with the result reaching to 10 MPa or more. This is obviously inconsistent with the actual situation; it cannot reflect the actual corrosion situation of the downhole tubing, so the anticorrosion design for CO2 corrosion may not be economical. This paper presents a set of methods for calculating the CO2 partial pressure distribution in oil and gas wells.
2. CO2 partial pressure calculation method for gas well
Definition of partial pressure (Wang and Zhou, 2009): if the volume and temperature of the gas mixture remain constant and all the gases except gas i are excluded from the system, the pressure is called the partial pressure of gas i in the gas mixture. Partial pressure is the pressure of the isolated gas i at given temperature and volume.
For an ideal gas, Dalton’s law of Partial pressure states that the total pressure of the gas mixture is equal to the sum of the pressures of each individual component, or that the partial pressure of each component is equal to the product of mole fraction of the component multiplying total...