Collapse strength of worn casing tubes from wear log inspection

Resumo

Oil wells are drilled by depth intervals with different diameters, which usually get smaller when it gets
deeper. When the bottom of a phase is reached by the drill bit, it needs to be cased, cemented, and tested. The inner
wall of the casing string will always be subject to the drill string’s intense contact and rotation. The friction will
remove material from the casing tube, reducing its strength. The current deep wells with directional trajectories
and severe tortuosity increase the casing wear making its assessment even more relevant for structural integrity
purposes. This work proposes a methodology to estimate the collapse strength of worn casing tubes using
inspection data of oil wells integrated with nonlinear Finite Element Analysis (FEA). The methodology starts by
identifying wear grooves from an ultrasonic log (US) by applying a strategy from the literature. Then, an equivalent
cross-section geometry is proposed, in which the identified wear grooves are inserted, emulating the most likely
original geometry, since the inspected shape is deformed. Although inner radius and thickness data for many cross
sections of the casing string also become available with the inspection, the boundary conditions are not known,
e.g., pressure and temperature. Due to this, FEA performed with this scanned geometry will result in an unrealistic
estimate of the tube strength. The 2D numeric simulation is performed in Abaqus with the equivalent geometry to
estimate the residual strength of the worn casing tube. A nonlinear approach is necessary because the collapse of
a tube is an instability problem. It is observed that the stress concentration in the groove wear zone reduces the
resistance. The results show that the multiple wear grooves observed from the inspection can significantly reduce
the strength of the tube. However, most works found in the literature are concerned with only one wear groove in
their modeling, since the inspection data is not accurately interpreted. A case study is presented to demonstrate
this contrast.