No abstract

Homogenised finite element (FE) analyses are able to predict osteoporosis‐related bone fractures and become useful for clinical applications. The predictions of FE analyses depend on the apparent, heterogeneous, anisotropic, elastic, and yield material properties, which are typically determined by implicit micro‐FE (μFE) analyses of trabecular bone. The objective of this study is to explore an explicit μFE approach to determine the apparent post‐yield behaviour of trabecular bone, beyond the elastic and yield properties. The material behaviour of bone tissue was described by elasto‐plasticity with a von Mises yield criterion closed by a planar cap for positive hydrostatic stresses to distinguish the post‐yield behaviour in tension and compression. Two ultimate strains for tension and compression were calibrated to trigger element deletion and reproduce damage of trabecular bone. A convergence analysis was undertaken to assess the role of the mesh. Thirteen load cases using periodicity‐compatible mixed uniform boundary conditions were applied to three human trabecular bone samples of increasing volume fractions. The effect of densification in large strains was explored. The convergence study revealed a strong dependence of the apparent ultimate stresses and strains on element size. An apparent quadric strength surface for trabecular bone was successfully fitted in a normalised stress space. The effect of densification was reproduced and correlated well with former experimental results. This study demonstrates the potential of the explicit FE formulation and the element deletion technique to reproduce damage in trabecular bone using μFE analyses. The proper account of the mesh sensitivity remains challenging for practical computing times.

Cement sheaths are among the most important barrier elements in petroleum wells. However, the cement may lose its integrity due to repeated pressure variations in the wellbore, such as during pressure tests and fluid injections. Typical cement sheaths failure mechanisms are formation of radial cracks and microannuli, and such potential leak paths may lead to loss of zonal isolation and pressure build-up in the annulus. To prevent such barrier failures, it is important to study and understand cement sheath failure mechanisms. This paper describes a series of experiments where we have used a tailor-built laboratory set-up to study cement sheath integrity during pressure cycling, where the set-up consists of down-scaled samples of rock, cement and casing. Cement integrity before and during casing pressurization is characterized by X-ray computed tomography (CT), which provides 3D visualization of radial cracks formed inside the cement and rock. We have studied how contextual well conditions, such as rock stiffness, casing stand-off and presence of mudfilm, influence cement sheath integrity. The results confirm that the rock stiffness and casing stand-off determine how much casing pressure the cement can withstand before radial cracks are formed in the cement sheath, where the rock stiffness is significantly more important than casing stand-off. Furthermore, it is seen that the radial cracks in the cement sheath continue into the rock as well. However, when a thin mudfilm is present at the rock surface, the cracks stop at the cement-rock interface, and the cement sheath withstands less pressure before failure. The bonding towards the rock is thus of importance.

Cement sheaths are among the most important well barrier elements and cement sheath integrity is thus important to maintain zonal isolation. Repeated pressure cycling in a well might lead to radial cracks and/or formation of microannuli in the cement sheaths. However, most experimental tests and model simulations of cement sheath integrity do not include mudfilms at the cement-rock interface, and the effect of such mudfilms on cement degradation during pressure cycling is thus not well understood. In this paper, we have used our custom-made laboratory set-up to study how mudfilms influence cement sheath integrity during pressure cycling, where X-ray computer tomography (CT) is used to visualize microannuli and cracks formed in the cement. Our results show that the bonding towards the formation is of importance for the cement sheath integrity during pressure cycling, both in the case of a soft and a stiff surrounding formation. Additionally, we see that with mudfilm present at the cement-rock interface the cement sheath is able to withstand less casing pressure before failure compared to a cement sheath without mud. For samples with a mudfilm present, radial cracks were not observed to propagate from the cement sheath and into the surrounding rock in the area covered by mudfilm.

It is crucial to understand cement sheath degradation mechanisms, since the cement sheath is an important well barrier element. Repeated pressure cycling is known to cause radial cracks and microannuli in the cement sheath, and the stiffness of the surrounding rock determines how much pressure the cement withstands before failure. However, experimental data on the effect of surrounding rock (shale vs. sandstone) on cement sheath integrity are scarce. In this paper we present experimental studies on how different surrounding rocks influence cement sheath integrity. We have used our unique downscaled experimental set-up to perform pressure cycling tests with both shale and sandstones, where cement sheath integrity is visualized in 3D by X-ray computer tomography (CT). The obtained results confirm that a cement sheath surrounded by a rock with a relative higher Young’s modulus can withstand higher casing pressure compared to a cement sheath surrounded by rock with relative lower Young modulus. All cracks were initially observed as small defects in the cement sheath prior to expanding to full radial cracks and propagation into the surrounding formation.