固井水泥环在川东气田地层水中的腐蚀特征

针对川东气井地层水中的固井水泥，采用电化学测试、扫描电镜观察和能谱分析，研究了水泥环腐蚀的电化学、微观结构和元素分布特征。结果表明：浸泡初期，钢的交流阻抗的高频容抗增大，低频出现Warburg阻抗，水泥/套管钢界面由于水化作用而变得更加稳定，钢的电位在26d后负移约200mV，此时腐蚀介质已经到达钢表面，在水泥未被腐蚀解体时，界面遭到破坏，使交流阻抗的高频容抗减小，低频出现容抗弧；腐蚀后水泥表层的Ca、O含量减少，而C、S、Mg的含量增加，水泥环发生了H₂S腐蚀、CO₂腐蚀和Mg腐蚀H₂S腐蚀较严重，外表面呈溶出型腐蚀，而在内部，腐蚀产物滞留其中，腐蚀主要使孔隙和胶结性发生劣化；Cl^-在水泥中具有较强的渗透性。研究成果对含H₂S和CO₂的酸性气田的固井工作具有指导意义。

CORROSION PERFORMANCE OF WELL CEMENT ANNULUS IN FORMATION WATER OF EAST SICHUAN GAS FIELD

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.