Determining the mechanical properties of heterogeneous rocks from a knowledge of microstructure

Rocks can be described as heterogeneous random materials, in the sense that they are composed of multiple domains of different phases. At the scale of a representative elementary volume (REV) — defined as the smallest volume over which measurements yield values representative of the entire rock sample — such heterogeneous materials can be treated as homogeneous and characterized by macroscopic or effective properties. Determining these effective properties is essential for describing geological processes occurring in reservoirs, aquifers, fault zones, and volcanic environments, as well as the structural changes they induce. However, complex interactions among the constituent phases result in a strong dependence of effective properties on nontrivial aspects of the microstructure, in particular for rocks with more than two or three different phases.

Over the past decades, a fruitful approach to investigating the relationship between rock properties and microstructure has involved predicting effective properties directly from microstructural information. This framework enables quantitative links to be established between microstructural evolution and changes in macroscopic properties. Following this rationale, laboratory-prepared synthetic rocks with specifically designed microstructural attributes have proven particularly valuable. Such materials have provided key insights into the influence of microstructure on mechanical properties, using relatively simple single- or two-phase rock analogs first, and synthetic materials with progressively more numerous distinct phases.

Here, I will summarize results we have collected in the past years by systematically investigating how specific microstructural attributes influence the mechanical behaviour of rocks. Compression experiments conducted on monodisperse sintered glass beads samples show that the stress required to reach inelastic deformation decreases when porosity or grain size alone increase. Using bidisperse and polydisperse sintered glass beads samples, we observe that this stress decreases when the degree of polydispersivity increases. In addition, under high-pressure triaxial compression, an increase in the degree of polydispersity alone leads to a transition in damage evolution from localized to more spatially distributed deformation. These results echo observations from shear experiments performed on heterogeneous fault gouges, where the spatial arrangement of weak and strong mineral phases, in addition to their relative proportions, exerts control on frictional properties and damage evolution during shearing.

More recently, we have employed nanoindentation to resolve spatial variations in elastic properties directly at the grain and crystal scale in natural rocks, allowing for the mechanical characterization of individual phases and quantification of mechanical heterogeneity at the bulk sample scale. These measurements will hopefully provide input parameters for the development and calibration of increasingly realistic synthetic rocks.