CO_(2)腐蚀-应力耦合下固井水泥环密封完整性

Sealing Integrity of Cement Sheath under the Condition of CO_(2) Corrosion-Stress Coupling

CO_(2)地质封存过程中,与地层围岩中的水反应后腐蚀着固井水泥环,腐蚀损伤和套管内压(应力)耦合作用极大地影响着水泥环的密封完整性。基于CO_(2)腐蚀实验,获得不同腐蚀程度水泥石材料力学性能参数,采用混凝土损伤塑性(CDP)本构模型和Mohor-Coulomb准则描述腐蚀前后水泥环的应力-应变行为,利用ABAQUS软件建立考虑CO_(2)腐蚀与应力耦合作用的井筒组合体(套管-水泥环-地层围岩)有限元分析模型,分析和探讨了套管内压和腐蚀时间对水泥环完整性的影响。结果表明,较高套管内压下,井筒水泥环发生弹塑性变形,出现结构损伤,套管与水泥环界面易形成微间隙;受腐蚀和套管内压的耦合作用,水泥环更易于出现完整性失效问题,相比较于未腐蚀水泥环,腐蚀水泥环受压后径向应力、等效塑性应变、微间隙以及拉伸和压缩损伤均较大,与之相反,塑性半径是减小的;微间隙与拉伸和压缩损伤受腐蚀时间的影响不明显。

In geologically sequestrating CO_(2),the reaction between the CO_(2) and the water in the confining rocks produces a chemical that causes the cement sheath to corrode,the coupling of the corrosion damage and the pressure(stress)inside the casing string greatly affects the seal integrity of the cement sheath.In CO_(2) corrosion test,the mechanical parameters of the set cement with different corrosion severities are obtained.The stress-strain behavior of the cement sheath before and after corrosion is described using the concrete damaged plasticity(CDP)constitutive model and the Mohor-Coulomb criterion.Using the ABAQUS software,a finite element analysis model for the wellbore assembly(casing-cement sheath-confining formation rocks)is established taking into account the coupling of CO_(2) corrosion and stress.The effects of internal pressure of the casing and corrosion time on the integrity of cement sheath are investigated.It is found that at high internal pressure of casing,the cement sheath undergoes elastic-plastic deformation,structural damage is found in the cement sheath,and micro-gaps are easy to be generated in the interfaces between the casing and the cement sheath.The coupled action of corrosion and internal pressure of casing causes the integrity of the cement sheath to fail easily.Compared with the cement sheath that undergoes no corrosion,the cement sheath with corrosion damage after being pressurized has higher radial stress,higher equivalent plastic strain,wider micro-gap and more serious tensile and compressive damages.On the other hand,the plastic radius decreases,and the corrosion time does not significantly affect the micro-gap and the tensile and compressive damage.