Geomechanical investigation of casing collapse using finite element modeling: The role of cement sheath integrity

Abstract

Casing collapse has been reported in several oil and gas wells all around the world. Cement sheath failure is one of the primary factors that may result in casing collapse. The growing focus on maintaining well integrity over the long term has prompted the creation of more detailed, realistic, and accurate models. Mechanical modeling of the casing-cement-formation setting can reveal the cement's capacity to withstand in-situ stresses, which is critical to sustaining the zonal isolation of the well throughout its lifetime. To date, the literature lacks a rigorous three-dimensional mechanical model to take into account all the geomechanical factors. This study aims to investigate the role of cement sheath failure in casing collapse by developing such a model. To do so, first, a three-dimensional numerical model was built in Abaqus using input data from a collapsed casing case study. Then, the required input data into the model were determined by following a step-by-step geomechanical procedure. Furthermore, to assess the model's accuracy and quality of the mesh, the numerical model was validated against an analytical model for stress distribution around the wellbore. As a preventive measure against casing collapse in upcoming infill wells, a new cement slurry was formulated, and its mechanical characteristics were measured through triaxial tests. The numerical results of the current wellbore cement and newly designed cement were compared to assess the effect of cement sheath integrity on casing collapse. The results revealed that the casing collapse in the case study can be highly attributed to the cement sheath shear failure and migration of the over-pressured formation fluid to the micro-annuli formed at the cement interfaces with the formation and the casing due to plastic deformation. The procedure outlined in this study can be used to design the wellbore cement according to the geomechanical factors to ensure well integrity.