Push-out test: Why bother?

Abstract

Bonding between cement and steel is currently believed to be of key importance in well integrity since it prevents the development of microannulus. Push-out test is often used to quantify the bonding strength between cement and steel in laboratory. Our experiments reveal a strong size-dependency of the push-out strength measured in these tests: the strength decreases with pipe diameter. This means that the push-out strength is not an intrinsic material parameter ("bonding strength") and therefore cannot be applied to field conditions before a proper correction is introduced. Such correction should take into account the large difference in diameter between the casing and the pipes typically used in laboratory tests. Finite-element simulations show that the experimentally-observed size-dependency could be, at least partially, explained by cement shrinkage that creates initial normal stress at the interface. The observed push-out strength is then not an intrinsic property of the interface between two materials (which would be an adhesive shear strength) but rather a result of interfacial friction. As the pipe diameter increases, the shrinkage-induced normal stress at the interface decreases, the frictional resistance decreases, and thus the measured push-out strength decreases, too, as observed in the lab tests. Simulations show, however, that shrinkage-induced decrease in push-out strength cannot fully explain the size-dependency since the decrease is much larger in the experiments than in the simulations. Another mechanism that could be at play is the common size effect: interfaces become weaker as their size increases because it is more likely that a larger interface contains an initial flaw of a given size. The decrease of push-out strength with pipe diameter has at least two practical implications: (i) the concept of bonding strength is misleading (unless it is specified explicitly that this parameter refers to frictional resistance at the interface rather than to an inherent adhesive strength) and (ii) the results of push-out tests can be used in well design only if a proper correction for the pipe diameter is introduced.