The effect of introducing nanoparticles on the fracture toughness of well cement paste

The objective of the work is to understand the change in toughness caused by the introduction of repair materials into the hardened cement paste sheath surrounding a well bore. If the impact on toughness of repair material introduced into the hardened cement sheath is not considered, there exists the possibility that a leaking well may hold pressure after repair, but over the long-term crack development is encouraged. To address this, this paper presents a theoretical study of the effect of introducing nanoparticles on the density and fracture toughness of aged class G oil cement paste. The introduction of repair material in cement paste is studied in the framework of fracture mechanics which is employed to determine the effect of the cement paste microstructure near an existing crack tip. The impact of surrounding cracks, pores, and stiff nanoparticles on the fracture toughness of the material is formulated analytically using an extension to the classical J-integral. There is a linear relationship between the J-integral value and toughness if we assume a crack will propagate linearly from the defect it was initiated from. Such a formulation allows us to perform a forward analysis in the design, avoiding complex numerical models during the nanoparticle selection process in the repair. We find that the introduction of nanoparticles into aged cementitious materials increases its density which, in turn, leads to an increase in fracture toughness. The crack-tip stress concentrations near the stiff nanoparticles encourage further brittle cracking of the material, decreasing toughness. Thus, the resulting fracture toughness of the matrix is a trade-off between increasing density and stress concentration. The mechanical behavior of the cement paste containing nanoparticles was also characterized experimentally by standard three-point bending tests. Nanoparticles are injected in hardened cement paste cylinders by an electrokinetic method and samples are cut to conduct standardized toughness tests. Both the experimental and analytical results agree that an optimum particle content can be found which extends the lifetime of the cement paste without encouraging further crack development in the material.