Mechanism of collapse failure and analysis of yield collapse resistance of casing under combined load

Abstract

In studies of casing strength in oil and gas wells, the focus has largely been on the influence of the non-uniformity of the external load on the casing strength. However, most equations for calculating casing strengths in the analysis of non-uniformities focus on the radial non-uniform load while neglecting the effect of shear load. To address this, the possible effects of shear load on the outer casing is discussed. A study is carried out on the hoop stress, which is the primary factor affecting casing yield failure, and this stress can be obtained by the energy method. As a result, the yield failure mechanism for the inner wall of the casing is determined under combined load. Based on these results, a new equation for calculating medium-thickness casing collapse strength under combined load is proposed. In addition, the results indicate that the casing collapse strength reaches a maximum when the shear load constant is close to the corresponding critical shear load constant of the casing, which can effectively eliminate the negative effects on casing strength caused by the radial non-uniform load. Compared with the traditional methods of increasing the casing grade and wall thickness, the results presented here provide a new concept for casing design and casing rehabilitation in complicated strata of deep and ultra-deep wells (e.g., salt and gypsum rock).