Multi-finger caliper logs obtained from the wells in the Gulf of Mexico and in the North Sea showed significant casing deformations, while they were still usable without losing integrity. The reason was that most of these casings were uniformly deformed without rupture or without kinks. The magnitude of elongation and compression of these casings were 3 to 5% while the strains at the yield strength were 0.3-0.5%. These observations indicate that casing design should include the steel properties beyond the yield strength while the API casing designcriterion is strictly based on the steel properties up to the yield strength.

Extension and compression casing tests were conducted from H-40 to V-150 casings up to failure by using a 5000 kN loading machine. For compression tests, some casings were cemented within a larger pipe to prevent the lateral buckling simulating the in-situ confinement condition. The tests gave the following results:

  1. Under extension tests, the casings were uniformly deformed after exceeding the yield strength. The uniform deformation continued while the casing were stretched by 4 to 25% until non-uniform deformation was induced.

  2. The strain up to the peak strength was as large as 25% for H-40 casings while it was as small as 4% for P-110 and higher grade of casings.

  3. For compression tests, uniform compression continued exceeding the yield point. The uniform deformation continued up to the peak strength which varied from 3% to 12% from V-150 to H-40 casings. Both the cemented casings and short casings induced axisymmetric wrinkles (local buckling) after uniform deformation.

  4. The higher grade casings did not significantly increase the maximum strength after yielding while lower grade casings increased the strength while being stretched or compressed uniformly after yielding.

The analyses show that the casing failure observed in the fields can be explained from the casing failure tests beyond the yield points. The casings are uniformly deformed until the maximum load so that they are usable up to the maximum strength without significant distortion. In the past, casing failures were tried to be mitigated by increasing the casing grade, however, increasing the grade sometimes deteriorated the casing problems since a higher grade casing could not tolerate the significant stretch or compression once they exceeded the yield stress. Selecting a proper grade and thickness of casings is the key for mitigating casing failure where the data of casing deformation beyond the yield strength are essential information for casing designing using geomechanical models. This paper analyzed the casing deformation data of several North Sea chalk reservoirs and Gulf of Mexico sandstone reservoirs provided by oil companies and determined the usable limit of casing deformation based on the laboratory and field measurements.

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