Evolution of the Uniaxial Compressive Strength of Porous Rocks from a Granular to a Non-Granular State

Sediments deposited in basins undergo densification driven by compaction and cementation. From an initially granular material, these processes build a dense and coherent rock. Changes in microstructure and phase proportions manifest as changes in bulk mechanical properties. The decrease in porosity resulting from densification is associated to a non-linear increase in uniaxial compressive strength (UCS), and there exist robust micromechanical models for the relationship between strength and porosity. However, no single model appears to capture existing data across the full range of porosity evolution that occurs from sedimentation to low-porosity sedimentary rocks. An outstanding problem is that the microstructure evolves not only in porosity but in pore lengthscales and pore-phase topology. Indeed, the evolution from a microstructure dominated by particles surrounded by fluid to one dominated by pores surrounded by groundmass can be described as a topological inversion. Accounting for these effects may explain some of the variance in existing experimental datasets. Our study aims to provide a description of the microstructural controls on the UCS in which we deconvolve microstructure lengthscales and topology. We achieve this by using synthetic samples. Monodisperse distributions of glass beads were prepared at room-temperature and then heated to a temperature higher than the transition temperature for the time-dependent coalescence of the viscous beads. Changing the sintering temperature or the duration of the dwell and the initial particle size allows us to control the final bulk porosity and the pore microstructure independently. We performed UCS tests on samples with porosity between 0.06 and 0.35. Our results show that the higher the porosity, the lower the strength and that, all else being equal, increasing grain radius decreases the UCS. Finally, we show that our data are not well predicted by the pore-emanated crack model but can be fitted using a grain crushing approach, suggesting that the particle-based microstructure topology is valid even down to low porosities. Based on our results and on microstructural observations made on deformed samples, we propose that the micromechanism of failure of our synthetic samples under uniaxial compression depends on the state of granular densification.