Creep Behavior and Mechanism of Casing in Thermal Well Under Cyclic Steam Stimulation at Medium Temperature

摘要

Based on traditional theory, creep behavior does not occur unless operation temperature exceeds the 30% melting point of steel (i.e. 450°C). Whereas, The casing damage in heavy oil recovery well demonstrated that creep occurred obviously for casing materials during long term service at 280-350°C. In the present study the creep behavior, precipitation feature and deformation substructure were investigated in C-Mn steel with low carbon and Cr-Mo steel with low alloy during creep at 350°C comparatively. The results showed that creep rate decreased initially, then remained stabilization. The creep rate reduced significantly for Cr-Mo steel with low alloy. TEM examination revealed that the carbide precipitation distributed along grain boundary in C-Mn steel with low carbon as-received condition. The typical morphology evolved from pile-up dislocation to nets during creep. However, both spherical precipitation with nanometer and rod-like one with micrometer were observed in Cr-Mo steel with low alloy as-received condition simultaneously. The strong carbide containing Nb and V sheared glide dislocation, then stacking faults and microtwins formed gradually during creep. The transition of creep mechanism could be attributed to the reduction of stacking fault energy (SFE) due to addition of alloy element, which leaded to improve in creep resistance finally.