Elastic-plastic behavior and limit load analysis of casings

Most companies producing from high compaction reservoirs continue using casings even if they find that these casings have been elongated or compressed exceeding the yield stress (Furui, et al., 2011). The magnitude of elongation and compression of these casings are often 3 to 5% while the strains at the yield strength are supposed to be 0.3-0.5%. Since casings are actually used beyond casing yield strength, field engineers should take into account the casing properties during plastic deformation. In this study, extension, compression and radial loading tests were conducted from H-40 to V-150 casings up to failure. The tests gave the following results: Under extension tests, the casings were uniformly deformed beyond the yield strength. The uniform deformation continued while the casings were stretched by 4 to 25% until non-uniform deformation was induced. The constitutive relations were constructed simulating the elastic and plastic deformations up to the failure. The radial compression tests showed stability of casings was maintained until the stresses at four cross-points of the stress concentration areas exceeded the yield strength. The higher grade casings did not significantly increase the maximum strength after yielding while lower grade casings significantly increased the strength while being stretched or compressed uniformly after yielding. The analyses show that the casings were uniformly deformed until the maximum strength so that they were usable. The peak load of the uniform deformation is the limit load since they rupture after the uniform deformation. When casings are compressed or extended by geo-technical loadings, thicker casings with a lower grade prevent rupture and kinks while higher grade casings tend to rupture with a large formation deformation. When casings are compressed by directional geo-technical load, the casings start losing the load-support capability after the stresses at four cross-points of the casings exceed the yield stress. Selecting a proper grade and thickness of casings are the key for mitigating casing failure where the data of casing deformation beyond the yield strength are essential information for casing designing using gcomechanical models.