Theoretical analysis of casing collapse in locally expansive mudstone

Abstract

Expansion of mudstone through absorbing water can increase local stress in information, which is one of the main reasons for casing damage in mudstone. To establish a reasonable mechanical model of local stress increase is the key problem for casing damage in locally expansive mudstone. Many models of uniform stress or linear stress are established to simulate the local stress increase. However, the accuracy and scientificity for those models are insufficient. In this paper, a study on casing strength in locally expansive mudstone is carried out based on theoretical analysis and numerical calculation. Firstly, a mechanical model of casings under parabolic-like loads is established to simulate the local stress increase. Then, an analytical solution of casings is obtained based on the stress function method. Next, a finite element calculation of casings under the same boundary condition is carried out. In the end, the comparisons between analytical solution and finite element method (FEM) are conducted. The results reveal that the theoretical solutions are in good agreement with the FEM. In addition, a novel predicting method of the casing stress in locally expansive mudstone is presented. Finally, our theoretical analysis can be the standard for numerical and approximate calculations. It is helpful to explore the casing damage mechanism in mudstone layer and reduce the casing collapse in complex underground environment.