With the increasing exploitation of oil and natural gas, the corrosion and protection of oil and gas pipelines have also received more and more attention. CO2 exists in oil and gas formations as a component of oil, natural gas or formation water. When CO2 miscible technology is used to improve oil recovery, it will also bring CO2 into the crude oil production system. After CO2 is dissolved in water, its total acidity is higher than that of hydrochloric acid at the same pH value, so the corrosion of pipes in the well is more serious than hydrochloric acid. In addition, some oil and gas wells contain H2S gas, and the flow state, temperature, pH value, and materials of the mixed liquid in the pipe also have a great influence on the corrosion, which makes the corrosion process more complicated.
At present, the research on corrosion under the sole action of CO2 or H2S is relatively sufficient at home and abroad, while the research on the coexistence of H2S and CO2, especially in the multiphase flow medium of H2S and CO2 under high temperature and high pressure, is relatively small. The research on sex is even less, and it is still unable to meet the needs of actual anti-corrosion applications. To this end, this article reviews the current CO2 and H2S corrosion research status of oil and gas fields in order to provide reference for corrosion protection programs and research directions in oil and gas fields.
CO2 corrosion is a common type of corrosion that plagues the world's petroleum industry as well as the development of my country's oil and gas industry. The most typical feature of CO2 corrosion is pitting, moss-like corrosion and mesa-like corrosion in the local pipeline. Among them, mesa-like corrosion is the most serious corrosion process.
Regarding the corrosion mechanism of CO2, it is generally believed that CO2 dissolved in water reacts with distilled water to form H2C03, and then reacts with Fe to make it corroded:
But most of the H2C03 in the solution is H+ and HC03-. Therefore, most of the reaction products are Fe(HCO3)2, which decomposes into:
In fact, the corrosion product carbonate (FeCO3, CaCO3) or fouling product film covers different areas on the steel surface to different extents. The areas with different coverage degrees form a strong corrosion couple with strong autocatalysis, and CO2 local corrosion It is the result of this corrosion galvanic effect. This mechanism is also a good explanation for the water chemistry and once the above process occurs on site, local corrosion will suddenly become very serious.
There are many factors affecting CO2 corrosion, such as temperature, CO2 partial pressure, flow rate, alloying elements, CI-, HCO3-, Ca2+ and Mg2+,bacteria, Fe3C concentration, FeCO3 solubility, protective film, heat treatment of pipes, and microstructure have certain effects on corrosion, and the corrosion situation under the influence of multiple factors is relatively complicated.
Some oil and gas fields contain a large amount of H2S gas, which has a relatively large solubility in water and is highly corrosive. When FeS is dense and closely combined with the metal matrix, it has a certain slow-down effect on corrosion. But when the generated FeS is not dense , It can form a strong galvanic couple with a potential difference of 0.2~0.4V with the metal base, which will promote the corrosion of the base metal. In addition, when there are sulfides in the solution or on the surface of the metal substrate, the sulfides prevent the conversion of hydrogen atoms to hydrogen molecules to a certain extent. These hydrogen atoms combine to form hydrogen molecules at the defects and other parts of the surface layer of the pipe rod and gather and expand. Hydrogen pressure is generated, and under the superposition and synergistic effect of the service tension of the pipe and rod, SSCC (H2S stress corrosion cracking) is formed. The working conditions of the pipe and rod in the oil well, such as the production fluid flow rate, working temperature, stress state, surface defects, etc. can accelerate the corrosion of steel by H2S and SSCC.
Regarding the corrosion of oil and gas pipelines in the H2S-CO2 coexistence system, there are relatively few studies at home and abroad, especially in the high-temperature and high-pressure H2S-CO2 multiphase flow medium. Therefore, it is more important to study the corrosion under the coexistence of H2S and CO2.
Hunan Gaoxing Steel Development Zone, No.1888 Purui South Rd, Wangcheng District,Changsha, Hunan, China