多角度辨识压裂套管变形的失效影响因素

Multiple angles identification on failure influence factors of fracturing casing de-formation

页岩气开发具有施工压力大、排量大、改造规模大的特点,使得压裂套管处于复杂力学环境中。挤压、剪切和弯曲等载荷共同作用,易引发套管挤毁变形,进而导致后续作业时井下工具下入遇阻。但目前研究多针对压裂套管的单一失效原因,难以保障其完整可靠性。鉴于此,针对页岩气大规模压裂作业特点,从不同薄弱位置(如垂直段、造斜段、水平段)、不同自身规格(如钢级、外径、壁厚)和不同约束条件(如内压、孔径、螺距)等多角度,系统辨识套管变形的失效影响因素。通过建立压裂套管三维模拟的有限元模型,分析套管内压变化引起套管应力、位移的变化规律及形态,明确套管变形的大小以及与载荷变化的关系,并揭示套管变形的位置及影响因素的临界值。结果表明:压裂套管的造斜段最大变形、水平段应力集中现象较为严重,属于危险脆弱点;且套管最大应力、最大位移随内压的增加而近似成线性降低关系。

In the process of shale gas exploitation,some characteristics such as large construction pressure,large displacement,and large transformation scale could lead the fracturing casing into the complicated mechanical environment. Under the combined effect of extrusion,shear and bend load,it is easy to cause the collapse and deformation of casing,thus result in the encountered resistance of the running downhole tools during the subsequent operation. However,many current studies focus on the single failure cause of fracturing casing,which is hard to guarantee its integrity and reliability. In view of this,aiming at the characteristics of large-scale fracturing operation for shale gas,the failure influence factors of casing deformation were identi-fied systematically from multiple angles,such as different weak points（ e. g. vertical,deflecting and horizontal sections）,different own specifications（ e. g. steel grade,external diameter and wall thickness） and different constraint conditions（ e. g. internal pressure,bore diameter and thread pitch）. Through establishing the finite element model for three-dimensional simulation of fracturing casing,the change rules and forms of casing stress and displacement caused by the change of internal pressure of cas-ing were analyzed. The amount of casing deformation and the relationship between it and the load change were clarified,and the location of casing deformation and the critical values of influence factors were revealed. The results showed that the phe-nomena of the maximum deformation in the deflecting sections and the stress concentration in the horizontal sections of fracturing casing were more serious,which belong to the risk weak points. In addition,the maxi-mum stress and the maximum displacement of casing presented linear decrease relationship with the increas-ing internal pressure approximately.