Failure analysis of casing dropping in shale oil well during large scale volume fracturing

Abstract

Shale oil and gas are regarded as typical unconventional reservoir, their recovery often impedes by casing damage. Failure analysis is performed for casing dropping in shale oil well via optical microscope (OM), scanning electron microscope (SEM), full size test and finite element modeling. Morphology observation reveals that thread remains intact under as-received condition after make up and break out. Meanwhile, galling and improper make-up position occur for slipping thread. Full size test exhibits that strength of connection declines from 783.4Kips to 697.1Kips after 9th stage fracturing, with loss of 11.0%. Finite element modeling demonstrates that contact stress reaches to high values at two ends and low values at middle part in the pin and box threads under complex stress. Casing dropping can be attributed to insufficient strength of connection due to improper make-up position, which accelerates by fracturing operation. Finally, dynamic damage effect is suggested to introduce in design of casing string in shale oil well.