The influence of pore geometry and orientation on the strength and stiffness of porous rock

Griffiths, Luke; Heap, Michael; Xu, Tao; Chen, Chongfeng; Baud, Patrick

doi:10.1016/j.jsg.2017.02.006

Cited by 92 publications

(65 citation statements)

References 63 publications

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“…Despite the platy morphology of bassanite making it difficult to determine the exact micromechanical mechanisms in operation during deformation, there is clear evidence in the microstructures (Figure ) that support differences in mechanical evolution in relation to the loading path. It has been shown previously that different microstructural parameters such as pore geometry and orientation (Bubeck et al, ; Griffiths et al, ) and the nature of grain contacts (Louis et al, ) can produce significant strength anisotropy in porous rock. If microstructural properties evolve significantly during accumulation of inelastic shear strain, as in the deviatoric loading path of our experiments, this could produce a considerable change in the rock strength and in the shape of the yield curve.…”

Section: Discussionmentioning

confidence: 99%

High‐Resolution Mapping of Yield Curve Shape and Evolution for Porous Rock: The Effect of Inelastic Compaction on Porous Bassanite

et al. 2018

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Porous rock deformation has important implications for fluid flow in a range of crustal settings as compaction can increase fluid pressure and alter permeability. The onset of inelastic strain for porous materials is typically defined by a yield curve plotted in differential stress (Q) versus effective mean stress (P) space. Empirical studies have shown that these curves are broadly elliptical in shape. Here conventional triaxial experiments are first performed to document (a) the yield curve of porous bassanite (porosity ≈ 27–28%), a material formed from the dehydration of gypsum, and (b) the postyield behavior, assuming that P and Q track along the yield surface as inelastic deformation accumulates. The data reveal that after initial yield, the yield surface cannot be perfectly elliptical and must evolve significantly as inelastic strain is accumulated. To investigate this further, a novel stress‐probing methodology is developed to map precisely the yield curve shape and subsequent evolution for a single sample. These measurements confirm that the high‐pressure side of the curve is partly composed of a near‐vertical limb. Yield curve evolution is shown to be dependent on the nature of the loading path. Bassanite compacted under differential stress develops a heterogeneous microstructure and has a yield curve with a peak that is almost double that of an equal porosity sample that has been compacted hydrostatically. The dramatic effect of different loading histories on the strength of porous bassanite highlights the importance of understanding the associated microstructural controls on the nature of inelastic deformation in porous rock.

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Section: Discussionmentioning

confidence: 99%

High‐Resolution Mapping of Yield Curve Shape and Evolution for Porous Rock: The Effect of Inelastic Compaction on Porous Bassanite

et al. 2018

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“…Strata orientation clearly acts as a weakness plane (Gatelier et al 2002). However, pore geometry and orientation between the pores and the direction of loading may also generate anisotropy (Bubeck et al 2017;Griffiths et al 2017), especially in volcanic rocks. The relation to the pore or initial crack orientation is particularly relevant since the fatigue process is inherently related to movement of the cracks.…”

Section: Perspectivesmentioning

confidence: 99%

Cyclic and Fatigue Behaviour of Rock Materials: Review, Interpretation and Research Perspectives

Cerfontaine¹,

Collin²

2017

Rock Mech Rock Eng

281

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“…Since the grains in sandstones are typically orientated such that their pore major axis is sub-parallel to bedding (Louis et al 2003;Benson et al 2005;Robion et al 2014;Griffiths et al 2017;Farrell and Healy 2017), a higher permeability parallel to bedding is perhaps expected. Although we do not provide an average orientation for the pore or grain major axes of the Buntsandstein sandstones studied herein, we do note that their average grain aspect ratios are within the range 1.54-1.76 (Table 1).…”

Section: Matrix Permeability Of the Buntsandsteinmentioning

confidence: 99%

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

et al. 2017

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Geothermal projects in the Upper Rhine Graben aim to harness thermal anomalies that have arisen due to hydrothermal circulation within the granitic basement and the overlying Permo-Triassic sedimentary units. We present here a systematic microstructural, mineralogical, and petrophysical characterisation of the lowermost unit of this Permo-Triassic sedimentary succession-the Buntsandstein-sampled from exploration well EPS-1 at the Soultz-sous-Forêts geothermal site (France). Twelve depths were sampled (from 1008 to 1414 m) and cylindrical cores were prepared perpendicular and parallel to bedding. These cores were described in terms of their microstructure, grain size and shape, specific surface area, pore size and pore throat size distribution, mineral content, porosity, P-wave velocity, and permeability. The Buntsandstein sandstones are predominantly feldspathic sandstones, often characterised by pores filled or partially filled (with clays (R3 illite-smectite), dolomite, siderite, and barite) as a consequence of diagenesis, tectonics, and the circulation of hydrothermal fluids. The porosity, dry P-wave velocity, and permeability of these sandstones vary from ~ 0.03 to 0.2, ~ 2.5 to 4.5 km s −1 , and ~ 10 −18 to 10 −13 m 2 , respectively. Our data show that P-wave velocity decreases and permeability increases as porosity increases. P-wave velocities are significantly higher when measured parallel to bedding (by about 10 to 25%), and that saturation with water increases P-wave velocity (by about 5 to 50%, depending on sample orientation). The pervasive pore-filling precipitation has significantly reduced the permeabilities of the Buntsandstein sandstones, which are orders of magnitude less permeable than similarly porous unaltered sandstone. We also find that their permeability can be up to an order of magnitude more permeable when measured parallel to bedding than perpendicular to bedding. Although Buntsandstein units with low matrix permeabilities (as low as ~ 10 −18 m 2 ) require macroscopic fractures to attain the high permeability required to sustain regional hydrothermal circulation, matrix permeability is important for units with low fracture densities and high matrix permeabilities. We anticipate that these data will aid future fluid flow modelling and seismic investigations at geothermal sites within the Upper Rhine Graben. Open Access© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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The influence of pore geometry and orientation on the strength and stiffness of porous rock

Cited by 92 publications

References 63 publications

High‐Resolution Mapping of Yield Curve Shape and Evolution for Porous Rock: The Effect of Inelastic Compaction on Porous Bassanite

High‐Resolution Mapping of Yield Curve Shape and Evolution for Porous Rock: The Effect of Inelastic Compaction on Porous Bassanite

Cyclic and Fatigue Behaviour of Rock Materials: Review, Interpretation and Research Perspectives

Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)

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