提升偏心环空注水泥顶替效率的浆柱结构优化分析

套管偏心是决定注水泥环空流速与流态演化的关键因素，进而制约了顶替效率的提升；优化浆柱结构是改善顶替效率的重要途径，而关于相关的理论机理未见报道。借助于计算流体力学Fluent软件，建立了不同套管偏心下水平环空3D模型，探讨了不同套管偏心下的顶替效率变化特性，结合套管偏心0.4时顶替效率差的案例，通过探究不同浆柱结构下的环空流体滞留体积分数，以推荐最佳注水泥浆柱结构。研究表明：(1)套管偏心0.1时，偏心效应与浮力作用下顶替效率优于居中工况；偏心度大于0.1后，在套管偏心与质量扩散效应耦合作用下，顶替效率逐渐降低；(2)先注入隔离液后注入冲洗液有利于充分发挥隔离液在正密度差下的顶替效果，再利用低密度冲洗液的冲刷作用，进而提高了顶替效率；(3)套管偏心时，宽边流速高于窄边，引起水泥浆从宽边提前返出，而窄边滞留大量钻井液的问题。模拟成果为偏心环空下浆柱结构设计与优化提供了重要理论依据。

Optimization Analysis Of Slurry Column Structure To Improve Cement Displacement Efficiency In Eccentric Annulus

Casing eccentricity is the key factor that determines the evolution of flow rate and flow pattern in the cementing annulus, which restricts the improvement of displacement efficiency. It is an important way to improve displacement efficiency by optimization of slurry column structure, and the related theoretical mechanism has not been reported. Using computational fluid dynamics of Fluent software, 3 D model horizontal annulus with different eccentric casing were developed, and the changes of displacement efficiency in different eccentric casing were discussed. Combined with the poor displacement efficiency with casing eccentricity of 0.4, the best slurry column structure was recommended through analysis of annulus fluid retention volume fraction in different column structure. The results showed that: ① Due to the combined effect of eccentricity and buoyancy, the displacement efficiency of casing eccentricity of 0.1 is better than that of casing concentricity; When the eccentricity is greater than 0.1, the displacement efficiency decreases gradually with the increase of eccentricity under the coupling effect of casing eccentricity and mass diffusion. ② injecting spacer first and then flushing fluid is beneficial to give full play to displacement effect of spacer under positive density difference, and the flushing effect of low density flushing fluid is utilized to improve displacement efficiency. ③ the velocity of wide side is higher than that of narrow side in the casing eccentricity, which causes the cementing slurry to return from the wide side in advance, while the narrow side retains a large amount of drilling fluid. The simulation results provide an important theoretical basis for the structure design and optimization of slurry column under eccentric annulus.