Loads of casing and cement sheath in the compressive viscoelastic salt rock

Abstract

Abstract The purpose of the research is to calculate loads applied to downhole casing and cement sheath in the viscoelastic salt rock under the action of in situ stress. Considering that materials of the viscoelastic salt rock around the wellbore and cement sheath are compressible, a geomechanical model which includes casing, cement sheath, and salt rock is developed. The Burgers creep model is adopted to describe the long-term rheological behavior of salt rock, and the three components model is used to describe that of cement sheath under in situ stress. Adopting the elastic viscoelastic corresponding principle (EVCP 1 ), the solutions of loads of casing and cement sheath are obtained in the compressive viscoelastic salt (CVS) on the basis of the elastic solutions of the same problem. The limit value of loading of casing is more than that of cement sheath in the given example, which exponentially increases with time. Compared to the limit value of loading of casing in the uncompressible viscoelastic salt (UVS), it is increased by 10.0% in the compressible viscoelastic salt. The limit loading of cement sheath in CVS is about 0.5% less than that of cement sheath in UVS. Taking the surrounding salt as the uncompressible material is very dangerous to assess the damage risk of downhole casing, and likely causes the collapse of casing.