A thermo-poroelastic analytical approach to evaluate cement sheath integrity in deep vertical wells

Failure of cement sheath due to casing expansion or formations pressure during completion or production stages of HPHT or deep vertical wells is a very common phenomenon. There have been many studies providing approaches to predict cement sheath failure, where theory of elasticity or thermo-elasticity together with the plane strain concept were taken into consideration to obtain representative results. However, sedimentary formations in subsurface layers are exhibiting a poroelastic behavior and theory of elasticity may not be able to fully describe their behaviors when changes in pore pressure and in-situ stresses are taking place. In this paper, an analytical approach based on the theory of thermo-poroelasticity was presented to predict the possibility of cement sheath failure in deep structures. A separate numerical molding was also performed to evaluate the application of the approach developed. The results obtained indicated that a thicker cement can withstand a higher load applied by the formations and protect the casing against a significant collapse pressure. The temperature was also found as a significant contributor in increasing the pressure applied by the formation and casing on the cement due to pore fluid and steel expansions. Although some discrepancies observed between the results of the numerical simulation and the analytical model, it seems that the approach presented is able to provide reliable results considering the fact that interactions of material interfaces could not be included in the analytical modeling.