Intermediate Casing Collapse Induced by Casing Wear in High-Temperature and High-Pressure Wells

摘要

Casing integrity is extremely important to downhole zonal isolation and preventing well instability. The reduction of casing strength not only occurs in directional drilling, but is also observed in vertical drilling with a slight deviation angle. Deteriorated casing in most hydrocarbon wells is reported from the onset of casing wear by the presence of friction force during the rotation of drillpipe. The friction on the casing wall causes the reduction of casing strength. Furthermore, the combination of corrosive drilling fluids with the rotation of drillpipe could dramatically degrade the casing strength. Although casing burst and collapse strength have been emphasized by many researchers, little research has presented the mechanical response of the worn casing. The studies that do exist on casing wear are not relevant for field applications because they do not consider the effects of high temperature and the surrounding formation. Therefore, it is urgent to obtain a proper stress profile of worn casing in order to reveal the true downhole information. Based on the boundary superposition principle, we propose an analytical solution for the worn casing model that accounts for the contribution of thermal stress. We focus on the stress evolution in worn casing from the effects of high temperature and the confining formation. The predicted results show that the higher thermal loads largely increase the stress concentration of the worn casing, subsequently weakening the casing strength. The finite element solutions indicate that the radial stress in worn casing is not impacted as much as the hoop stress. The remaining part of the worn casing is subject to compression failure, along with an increase of the burst pressure or the elevated temperature.