局部载荷对页岩气井套管变形的影响

Effects of local load on shale gas well casing deformation

对四川盆地长宁区块页岩气水平井固井质量的分析结果认为:压裂过程的温度应力及由套管内压周期性变化导致的局部载荷是页岩气井套管变形的主要因素。为此建立了套管在局部载荷作用下应力计算模型,借助有限元数值方法分析了该区块目前使用的套管在局部载荷作用下的受力与变形情况,并讨论了在局部载荷线性增加时,局部载荷范围、套管壁厚、套管外径对现场使用的P110套管受力与变形的影响。结果表明:①载荷范围为45°时的套管变形最大,增加套管壁厚及减小套管外径有利于减小套管变形;②局部等效载荷为40 MPa时的套管径向变形量为4.8 mm,且套管壁厚应大于13 mm才能保证套管不发生屈服变形。通过对现场页岩气井数据的分析,考虑局部载荷作用,认为该区块目前使用的套管壁厚不能满足要求,需要增加壁厚。结论认为,为防止局部载荷的产生、减小套管变形,需要优化井眼轨道设计以合理钻遇天然裂缝发育的岩层,提高固井质量。该研究成果对于页岩气水平井套管设计具有参考价值。

In this paper, the cementing quality of shale gas horizontal wells in the Changning Block, Sichuan Basin, was analyzed. It is indicated that shale gas well casing deformation is dominantly caused by the local load induced by the periodic variation of casing internal pressure and the temperature stress in the process of fracturing. Therefore, a stress calculation model for the casing under local load was established. Then, the stress and deformation of in-operation casings under local load was analyzed by using the finite element numerical method. Finally, the effects of local load range, casing wall thickness and casing external diameter on the stress and the deformation of P110 casing which was used in this block were investigated by increasing the local load linearly. It is shown that the casing deformation is the maximum when the load range is 45°. Increasing casing wall thickness and reducing casing outer diameter are favorable for the reduction of casing deformation; and that the casing radial deformation is 4.8 mm when the local equivalent load is 40 MPa, and yield deformation will not occur if the casing wall is thicker than 13 mm. Actual data analysis of shale gas wells shows that the wall thickness of the casings currently used in Changning Block cannot meet the requirement if the effect of local load is taken into account. Therefore, the casing wall should be thickened. In order to prevent the generation of local load and reduce casing deformation, it is recommended to optimize the design of well trajectory to drill the strata with developed natural fractures appropriately and improve cementing quality. The study results can be used as a reference for the casing design of shale gas horizontal wells.