摘要：Wellhead growth has occurred in many deep wells in China recently. In the paper, casing axial force and wellhead movement are calculated and compared in regard to moments that free sections of surface casing appear. The parameters considered include preliminary casing tension, free section length and change in wellbore temperature. The moment that surface casing free section comes into being has great influence on wellhead movement, so it is discussed in detail. According to comprehensive investigation on deep well and ultra-deep well accidents, wellhead growth is based on the fact that before production the cement sheath of surface casing has already failed, and during production wellhead grows up due to wellbore temperature increases. The most possible reason of surface casing cement sheath failure is bad cement job quality. Bad cement sheath cannot support casing axial loads and separates from casing. The free section of surface casing will experience large axial force cycle which is harmful to wellbore integrity. Effective measures to remedy wellhead growth is supporting the wellhead with a large base set on ground, rather than pads fixed on conductor.
