Failure analysis of a cracked Q125 casing for ultra-deep wells at the Tarim Oilfield

摘要

In this paper, the reasons for cracking in the longitudinal and spiral of Q125 casing in ultra-deep wells (UDWs) at the Tarim Oilfield were investigated for the first time ever. Up to know an evaluation of Q125 casing materials of UDWs were were limited to comparisons with WSP-2T and P110 via performance tests. In addition, an applicability evaluation of UDWs was carried out using MPC software and theoretical calculations. The results show that mechanical damage and stress corrosion cracking are the principal causes for longitudinal cracking. Spiral cracking can be the result of the manufacturing process. It should be added that the material out of which Q125 casing is produced manifests high strength, low toughness and great sensitivity to cracks, stress corrosion and low allowable limit crack size (ALCS) when compared with WSP-2T and P110. Owing to the complicated field environment of UDWs at the Tarim Oilfield, the causes of cracking failure have been analyzed, with the result that WSP-2T casing of a similar strength to Q125 casing ought to be selected in UDWs rather than Q125 casing with high strength and low toughness, which is significant for the development of UDWs at the Tarim Oilfield.