水泥环环状缺失套损机理及防控措施

固井质量差，后续作业使油气井某处水泥环小段缺损，致使套管与水泥环之间出现间隙而使套管受力变化，造成套损。基于大港油田枣南东油田现场资料及弹塑性力学原理，建立了套管-缺失水泥环-地层对称有限元模型，研究水泥环全部缺失不同高度、缺失后套管与水泥环间隙大小及不同内压对套管受力的影响，从而研究水泥环全部缺失套损的防控措施。研究表明:在水泥环完好与水泥环缺失部分交界面套管出现最大应力，在水泥环缺失段出现较大应力，且处于稳值高应力状态；此外，内压越高，在环状缺失段，套管的最大应力越大；不同的水泥环缺失高度，在缺失段应力均相等，而不同的第一界面间隙对套管受力影响小；且研究发现仅增加壁厚，不能有效防控水泥环缺失段套损，增加钢级为有效方法。通过以上分析研究对水泥环全部缺失防控套损的研究提供有益借鉴。

Mechanical and control measures on casing damage due to annular absence of cement sheath

If cementing is of poor quality, a small section of cement sheath will be absent in a certain part of oil and gas well in the subsequent operation. As a result, clearance occurs between the casing and the cement sheath, so casing stress changes and the casing is ultimately damaged. A symmetric fnite element model of casing-absent cement sheath-strata was established based on the feld data of Zaonandong Oilfeld in Dagang Oilfeld and the principle of plasto elasticity. By virtue of this model, the effects of the height of complete cement sheath absence, the size of clearance between the casing and the cement sheath after the absence of cement sheath and the internal pressure on the casing stress were investigated, and the control measures for complete absence of cement sheath were proposed correspondingly. It is shown that the stress on the casing at the interface between complete cement sheath and incomplete cement sheathis the maximum value. The stress in the section where cement sheath is absent is higher and stable. The higher the internal pressure is, the higher the maximum stress on the casing in the annular absence section is. The stress on the sections with absent cement sheath is equal no matter what the absence height of cement sheath is. Casing stress is less affected by the clearance at the frst interface. It is demonstrated that the casing damage in the section with absent cement sheath cannot be controlled effectively only by increasing the wall thickness, and the effective method is to increase the steel grade. These researches and analysis can be used as the benefcial reference to study casing damage control while cement sheath is completely absent.