Finite element analysis and control measures on casing failure in sand shale interbeds

According to the incomplete statistics of Dagang Oilfield, the casing failure in sand shale interbeds accounts for 63%. To solve this problem, an axisymmetric finite element model was established for sandstone-shale-sandstone three-layer section according to the theories of plasto elasticity and fracturing mechanics. Then, it was solved by means of the finite element software to analyze the stress distribution inside the casing on the three-layer section by varying steel grade and wall thickness. It is indicated that the Von Mises stress inside the casing is the highest at sandstone-shale interface and shale interval, and the stress changes abruptly at sandstone-shale interface, resulting in sharp edges, so the casing at sandstone-shale interface and shale interval gets failed easily. Based on the characteristics of sand shale interbeds, the control measures on casing failure in shale interval were analyzed. And it is shown that the casing failure in shale interval can’t be controlled effectively by thickening the casing wall if the steel grade is kept the same. And the effective way is to increase the steel grade. This paper provides the beneficial reference for the study on casing failure control at sandstone-shale interface.