循环应力作用下水泥环密封性实验研究  

Experimental Study on Airtightness of Cement Sheath Under Alternating Stress

作　　者：刘仍光[1,2] 张林海[1] 陶谦[1] 周仕明[1] 丁士东[1] 

机构地区：[1]中国石化石油工程技术研究院,北京100101 [2]中国石油大学(北京),北京102249

出　　处：《钻井液与完井液》2016年第4期74-78,共5页Drilling Fluid & Completion Fluid

基　　金：国家自然科学基金面上项目(51374218);中国博士后科学基金资助项目(2015M571228)

摘　　要：利用自主研发的水泥环密封性实验装置研究了套管内加卸压循环作用下水泥环的密封性,根据实验结果得出了循环应力作用下水泥环密封性失效的机理。实验结果显示,在较低套管内压循环作用下,水泥环保持密封性所能承受的应力循环次数较多;在较高循环应力作用下,水泥环密封性失效时循环次数较少。表明在套管内较低压力作用下,水泥环所受的应力较低,应力水平处于弹性状态,在加卸载的循环作用下,水泥环可随之弹性变形和弹性恢复;在较高应力作用下,水泥环内部固有的微裂纹和缺陷逐渐扩展和连通,除了发生弹性变形还产生了塑性变形;随着应力循环次数的增加,塑性变形也不断地累积。循环压力卸载时,套管弹性回缩而水泥环塑性变形不可完全恢复,2者在界面处的变形不协调而引起拉应力。当拉应力超过界面处的胶结强度时出现微环隙,导致水泥环密封性失效,水泥环发生循环应力作用的低周期密封性疲劳破坏。套管内压力越大,水泥环中产生的应力水平越高,产生的塑性变形越大,每次卸载时产生的残余应变和界面处拉应力也越大,因此引起密封性失效的应力循环次数越少。The airtightness of cement sheath under alternating loading and unloading was studied using self-developed experiment facilities, and the mechanism of airtightness failure of cement sheath under alternating loading and unloading was understood based on the experimental results. Under the action of lower alternating pressure inside the casing, the time for the cement sheath to fail can be long; the action of high alternating pressure, on the other hand, will soon break down the airtightness of the cement sheath. This indicates that low pressure inside the casing string imposes low force on cement sheath which still retains its elasticity, and elastic deformation takes place in the cement sheath under alternating loading and unloading. Under the action of high stresses, on the other hand, the intrinsic micro fractures and deficiencies inside the cement sheath begin to connect with each other, and both elastic deformation and plastic deformation take place inside the cement sheath. As the alternating goes on, the plastic deformation becomes accumulated, to the extent that the plastic deformation does not vanish after unloading. Differences in the deformations of casing string and cement sheath at the interface result in a tensile stress, which, when exceeding the cementation strength at the interface, will cause the airtightness of the cement sheath to fail. This is the so-called low cycle fatigue failure of the airtightness of cement sheath under the action of alternating loading and unloading. Higher internal pressure in the casing string results in higher induced stress inside the cement sheath, larger plastic deformation, higher residual strain and tensile stress at the interface after unloading, and, the less of the number of cycles required to cause the airtightness of the cement sheath to fail.

关 键 词：水泥环 密封性装置 循环应力 微环隙 疲劳破坏 

分 类 号：TE256.9[石油与天然气工程—油气井工程]