Casing Capacity Degradation of Burst and Collapse of a Non-Uniform Casing with Wear under Bending Conditions
Casing wear during drilling and work-overs is a major concern in deep or extended reach wells. Wear by drill string results in thinner portion of casing wall which causes reduction of casing capacity. Wall thinning could also be caused by bending of casing at steep trajectories. It is important not only to estimate the casing wear but also to reassess the strength of the residual casing due to combined effect non uniform thickness due to bending and wear. The API equations describing casing burst and collapse capacities do not address casing response with geometrical defects such as wear, wall thinning and combined loads. A common methodology is to estimate the reduction of strength from API burst and collapse equations with a linear reduction by remaining wall thickness or wear percentage equivalent to a “uniformly – worn” casing model to reassess casing strength for worn casing. However, this strength estimation is a very conservative method. Therefore, it is required to predict the safe working pressures for casing for the mentioned scenarios. This study presents new analytical models to calculate the degraded burst, collapse and bending strengths for a non-uniform casing with “crescent-shape” casing wear. These models can be used together to calculate the overall stress profile of a casing with wear, corrosion or cracks to get an estimate of the degradation of the strength. A parametric study of the casing wear percentage, wear radius, and non-uniformity has been performed. The analytical models have been validated by a numerical simulator FEM which uses finite element technique.