固井水泥环不连续对套管力学性能的影响分析

INFLUENCE OF DISCONTINUOUS CEMENT SHEATH ON CASING MECHANICAL PROPERTIES

固井过程中,固井质量不合格的情况经常发生。以青海油田某口井作为研究对象,建立平面应变力学模型,假设研究截面套管与地层之间的环空中有1/4未被水泥充填,采用有限元软件ABAQUS模拟套管的变形过程。计算结果表明:①由于固井水泥环不连续,该侧套管没有水泥的支撑和保护作用,在非均匀地应力作用下,套管极易发生变形损坏。②套管内压Po和水平地应力P1固定,改变水平地应力P2,当P2增大到一定值时,B点附近的套管内壁率先发生塑性变形,如果P2继续增大,套管内壁的塑性区域会逐渐增加,并向套管外壁延伸,直到套管整体发生塑性破坏。④水泥环的不连续改变了套管内壁的应力分布,从而使得等效应力曲线上最大的等效应力值不是在B点,而是发生在B点附近套管内壁处。该分析为改善固井质量采取有效措施提供了可靠的理论依据。

During cementing, well cementation failure often take place. One well in Qinghai oilfield is taken as object, and mechanical model is established. Assuming 1/4 of the high side of the casing/formation annulus is uncemented on the studying cross section,the ABAQUS program is used to simulate casing deformation. The results of analysis are as follows: (1) Because of cement sheath is discontinuous, no support and protection are provided to casing. Under the nonhomogeneous in-situ stress, casing is damaged easily. (2) Casing internal pressure Po and in-situ stress P1 are unchanged and in-situ stress P2 is varying, when P2 increase to a value, internal wall of casing in the vicinity of point B start plastic deformation. If P2 continue to increase, plastic area also increase and extend to external wall of casing until the entire casing happen to plastic failure. (3) Stress distribution of internal wall of casing is changed because of discontinuous cement sheath, the maximum equivalent stress on equivalent stress curve is not point B but internal wall of casing in the vicinity of point B. The analysis provides the dependable criteria to adopt effective measures in order to improve cementing quality.