Abstract: The corrosion of cement in the formation water of east Sichuan gas field,including electrochemical performance,microstructure feature and element distribution,is investigated by the methods of electrochemical test,scanning electron microscope (SEM) and energy spectrometer.The results shows that:the hydration enhances the stability of the interface of cement and casing steel at the initial immersion period due to the increasing of capacitive loop in high frequency region and appearance of Warburg resistance in low frequency region.However,the potential rapidly decreases by 200 mV after 26 days immersion,which means the corrosive medium reach the surface of steel.Though the integrality of cement is not broken,the stability of interface is lost.Thus,the capacitive loop in high frequency region decreases,and capacitive loop also occurs in the low frequency region.The concentration of Ca and O decrease in the surface layer,but the inversed change is found for C,S and Mg.It can be concluded that the cement is eroded by hydrogen sulfide,carbon dioxide and magnesium salt.The hydrogen sulfide plays a very important role in the corrosion.The soluble type corrosion is observed at the surface of the cement,but the corrosion products are inside the cement in the deep zone.Corrosion degrades the pore space and cementing property.Cl^- has strong transfer ability in cement.The research results are of instructive meaning to well cementation in sour gas field which contains H₂S and CO₂.

Key words: well cement, sour gas field, electrochemistry, microstructure, corrosion