Strain Based Design and Material Selection Technology for Thermal well Casing

Abstract Field investigation and failure analysis on casing damage were conducted for cyclic steam stimulation thermal wells in Xinjiang oil-field of China. Evaluation demonstrated that stress-based casing design only meet the requirements of drilling and completing operation, but could not accommodate sufficient security for cyclic thermal production. Although casing failure seldom appeared during drilling and completing, lots of its failures happened within the following production stage. These results indicated that the service environment during this production is the dominant factor affecting casing design and material selection. Failure analysis displayed that there are several typical modes, including deformation, necking, shear, crack and connection parting. These typical modes revealed that those casing experienced obvious plastic deformation during production. Accordingly, strain-based design is proposed in which characteristics of deformation behaviors are taken into account, such as plastic deformation, hardening, creep, stress relaxation, bauschinger effect and strain fatigue limit. Two criterions were determined for this design according to plastic capacity and strain fatigue limit of casing material, respectively. Moreover, one of these criterions had been verified through 8 wells experiments after three thermal cycles. Strain-based design aims to present sufficient axial plasticity capacity beyond elastic limit, before ultimate necking and tensile fracture. It is necessary to consider attached measures for integrity of thermal wells. For thread connection, sufficient sealability is recommended to prevent steam leakage, otherwise which could greatly promote shale's expanding and then result in shear failure in casing. As for the pipe ends, external upset or additional hardening process is suggested to apply to maintain the integrity of thread connection area.