Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to Non-Granular Transition

Under uniaxial compression, a porous rock fails by coalescence of stress-induced microcracks. The micromechanical models developed to analyze uniaxial compressive strength data consider a single mechanism for the initiation and propagation of microcracks and a fixed starting microstructure. Because the microstructure of clastic porous rock transitions from granular to non-granular as porosity decreases during diagenesis, their strength cannot be captured by a single model. Using synthetic samples with independently controlled porosity and initial grain radius we show that high-porosity granular samples, where microcracks grow at grain-to-grain contacts, are best described by a grain-based model. Low-porosity non-granular samples, where microcracks grow from pores, are best described by a pore-based model. The switch from one model to the other depends on porosity and grain radius. We propose a regime plot that indicates which micromechanical model may be more suitable to predict strength for a given porosity and grain radius. Porous rocks can be characterized in terms of the proportion of empty spaces in them, that is, their "porosity." When porous rocks are compressed, small cracks can form and propagate inside the solid rock between the pores until they unite into a large fracture, leading to rock failure. Depending on the initial shape and arrangement of pores within the rock, the small cracks may form and grow in different locations and/or directions. To explore the variety in the formation and growth of cracks, we prepared artificial samples with varying porosity and grain sizes. We found that rocks with large porosity and grain-like structures break differently than rocks with low porosity and non-grain-like structures. In the first type, cracks grow and radiate from the connections between grains, while in the second type, cracks grow from the edge of pores toward the solid surroundings. Since differences in the location and manner in which cracks grow can result in variations in rock strength, we showed how different models may be needed for accurate strength prediction, depending on the porosity and grain size of the rocks. Deformation micromechanism changes from grain crushing (granular, high porosity) to pore-emanated cracking (non-granular, low porosity)A regime diagram provides the expected deformation micromechanism for clastic rock with a known porosity and grain sizeMicromechanical models provide strength estimates for clastic rock based on the micromechanism (grain crushing or pore-emanated cracking)