Simulation of burst resistance on casing with damage and deformation using finite element analysis

Abstract

From practice, the strength of casing varies with environment after installation. Mechanical and chemical damage caused by operation and production will degrade the mechanical properties of casing. With weaker casing strength, casing failure will happen unpredictably. To prevent casing failure, it is important to evaluate the casing strength. In this study, the burst strength degradation of casing with damage and deformation is investigated using the Finite Element Method (FEM). Damages include crescent-shaped wear by tool joint and slickline. The deformations include elastic deformation by bending moment and plastic deformation by curved well trajectory. The main objective of this study is to generate mathematical relationships between the burst resistance degradation and damage/deformation with different magnitudes and geometries. FEM is widely used as an approximate numerical method for solving field mechanics problems. In this study, damage and deformations were added to a finite element casing model which went through a verification and validation process. The pressures applied on the model were adjusted until the von Mises stress met with the material strength. The final pressures were recorded as the burst strength. Burst strength data were later used to explore the effect of different damage/deformation on burst strength. In this study, linear relationships between the pressure applied and von Mises stress were observed in simulation cases. From the regression analysis, the relationship between the burst strength and thickness remaining/cut depth was determined to be exponential functions. The relationships between the newly created parameters, cut area and cut arc length were determined to be logarithmic function and piecewise linear functions respectively. From the principle stress analysis, the damage on the casing was found to increase the local tangential stress significantly; it was also found the damage can increase the local axial stress. Based on the results, the initial damage on a casing brings the largest burst resistance reduction as compared to following damage with the same increment on cut depth. Pipes with a smaller outer diameter resulted in a larger burst strength degradation with the same cut depth. A casing with crescent-shaped damage had a smaller burst strength than a uniform thickness casing with the same thickness remaining. This study also provides two possible methods to estimate the burst strength with a given damage geometry. The first method finds the effect of each individual parameter in the damage geometry function on burst strength degradation, and establishes universal mathematical functions between damage geometry parameters and burst strength degradation. The second method develops a stress concentration factor function for tangential stress near the damaged zone in the casing model. The von Mises stress and burst strength can be determined with hypothetical axial stress and radial stress. This study reveals how burst strength changes with crescent-shaped damage and deformations in detail, which can help to better evaluate the burst strength of casing in the field. In the future, more research can be done using higher order elements, more complex loading conditions and updated material models with metal plasticity and damage.