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[1] Grain-scale brittle fracture and grain rearrangement play an important role in controlling the compaction behavior of reservoir rocks during the early stages of burial. Therefore, the understanding of single-grain failure is important. We performed constant displacement rate crushing tests carried out on selected, well-rounded, single sand grains and on randomly sampled grains from different grain size (d) batches of pure quartz sand. Applying a Hertzian fracture mechanics model for grain crushing, the critical load at failure (F c ) data obtained for the selected grains were converted into an accurate estimate of the size of flaws associated with failure (c f ). Similarly, the distributed F c data obtained from the different batch samples were converted into distributions of grain failure stress. Weibull weakest link theory could not explain the observed grain failure behavior. On the contrary, the Hertzian grain failure criterion enabled the conversion of the distributed F c data, for the batch samples, into distributions of c f , assuming spherical grains, or of "effective" radius of curvature (r g ), characterizing contact surface asperities in the case of nonspherical grains. In contrast to the model of Zhang et al. (1990), our work shows that there is no clear physical basis for a grain size dependence of c f . However, since roundness data for dune sands exhibit a similar relation between r g and d, as seen in our grain size batches, it is inferred that the Hertzian fracture mechanics model assuming nonspherical grains with a distributed r g is the most physically reasonable model for grain failure.

Mechanisms such as grain rearrangement, coupled with elastic deformation and grain breakage, are believed to play an important role in the time-independent compaction of sands, controlling porosity and permeability reduction during burial of clastic sediments and during depletion of highly porous reservoir sandstones. We performed uniaxial compaction experiments on sands at room temperature to systematically investigate the effect of loading history, loading rate, grain size, initial porosity, and chemical environment on compaction. Acoustic emission counting and microstructural methods were used to verify the microphysical compaction mechanisms operating. All tests showed quasi-elastic loading behavior accompanied by permanent deformation, involving elastic grain contact distortion, particle rearrangement, and grain failure. Loading history, grain size, and initial porosity significantly affected stress-strain behavior, with increasing grain size and initial porosity promoting compaction. In contrast, chemical environment and loading rate had little effect. The results formed the basis for a microphysical model aimed at explaining the observed compaction behavior. Two extreme cases were modeled: (I) a pack of spherical grains with a distributed flaw size at failure and (II) a pack of nonspherical grains with a constant mean crack size at failure but a distributed effective surface radius of curvature characterizing distributed contact asperity amplitudes. The best agreement with the grain-size-and porosity-dependent trends observed in our experiments was obtained using case (II) of the model. Combining our experimental and modeling results, it was inferred that a grain-size-dependent departure from sphericity of the grains exerts a key control on the compaction behavior of sands.

Time-dependent brittle creep plays a role in controlling compaction of sands and sandstones under upper crustal conditions, influencing phenomena such as production-induced reservoir compaction, surface subsidence, and induced seismicity. Brittle creep also plays a role in determining the mechanical behavior of gouge-rich faults. We performed uniaxial creep experiments on sand to investigate the effects of chemical environment (dry versus solution flooded), grain size (d = 196-378 μm), and applied effective stress (σ a up to 30 MPa), at room temperature conditions favoring grain-scale brittle processes. Creep measurements were complemented with acoustic emission (AE) detection and microstructural analysis to characterize the main creep mechanism. Wet samples showed much higher creep strains than dry-tested samples. AE event counts showed a direct relation between grain failure and creep strain, with higher AE rates occurring in the wet samples. Therefore, we inferred that time-dependent deformation was dominated by subcritical crack growth, resulting in grain failure accompanied by intergranular sliding rearrangements, and that crack growth in the presence of chemically active fluids was controlled by stress corrosion. The sensitivity of the compaction rate of the sands to d and σ a can be expressed as _ ε ∝ d i σ j a where i ≈ 6 and j ≈ 21 under dry conditions and i ≈ 9 and j ≈ 15 under wet conditions. Our results were compared to a simple model based on Hertzian contact theory, linear elastic fracture mechanics, and subcritical crack growth. This model showed agreement between the observed stress and grain size sensitivities of creep, within a factor of 2.

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