The Impact of Heterogeneity on the Anisotropic Strength of an Outcrop Shale

Fjær, Erling; Nes, Olav-Magnar

doi:10.1007/s00603-014-0598-5

Cited by 76 publications

(38 citation statements)

References 11 publications

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“…Niandou et al, 1997;Val es et al, 2004), others do show slight to significant strengthening parallel to bedding (e.g. Fjaer and Nes, 2014;Ajalloeian and Lashkaripour, 2000). All results given in this paper are for compaction and deformation normal to bedding and should not be extended to other stress orientations with respect to fabric anisotropies.…”

Section: Discussionmentioning

confidence: 86%

“…However, static rock strength decreases from orientations normal to bedding through to 30e45 to bedding, then increases again as the maximum principal stress becomes parallel to bedding, sometimes exceeding the bedding normal strength, sometimes equalling it (e.g. Fjaer and Nes, 2014;Niandou et al, 1997;Ajalloeian and Lashkaripour, 2000). Hence, trying to estimate shale strength from velocity in a deviated well would be significantly flawed (Sayers et al, 2009).…”

Section: Discussionmentioning

confidence: 95%

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Empirical strength prediction for preserved shales

Dewhurst

Sarout

Piane

et al. 2015

Marine and Petroleum Geology

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Section: Discussionmentioning

confidence: 86%

Section: Discussionmentioning

confidence: 95%

Empirical strength prediction for preserved shales

Dewhurst

Sarout

Piane

et al. 2015

Marine and Petroleum Geology

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“…The DEM model is calibrated to represent the mechanical behaviors of Mancos shale [ Fjær and Nes , ] under uniaxial compression test. In the laboratory, unconfined compression tests were conducted on 15 mm diameter, 30 mm long specimens drilled at every 15° of inclination.…”

Section: Calibration and Validation Of Numerical Modelmentioning

confidence: 98%

“…This failure criterion consists of the criterion for inherent failure and the criterion for failure along a weak plane as given in equations and , respectively

normalU normalC normalS = \frac{2 S_{0} normalcos ϕ ((), 1 - η \sin^{2} 2 β)}{1 - normalsin ϕ}, when β < 45 ° or β > 71 °

normalU normalC normalS = \frac{2 S_{0 normalw} normalcos ϕ_{w} ((), 1 - η \sin^{2} 2 β)}{normalsin 2 β normalcos ϕ_{w} - ((), normalcos 2 β + 1) normalsin ϕ_{w}}, when 45 ° < β < 71 °

where S 0 is the inherent cohesion, ϕ is the corresponding friction angle, η is a free parameter which can be determined experimentally, and S 0 w and ϕ w are the cohesion and corresponding friction angle of the weak planes, respectively. Parameters determined by Fjær and Nes [] in laboratory are adopted, and the UCS‐normalized major principal stress is given in Figure b. Different from the bimodal curve of tangential stress, four crests can be identified on the normalized maximum principle stress, in which direction wellbore breakouts are more likely to initiate.…”

Section: Propagation Of Borehole Breakouts In Anisotropic Modelmentioning

confidence: 99%

Evolution of stress‐induced borehole breakout in inherently anisotropic rock: Insights from discrete element modeling

Kang

Kwok

2016

JGR Solid Earth

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The aim of this study is to better understand the mechanisms controlling the initiation, propagation, and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings. A two‐dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs. Both isotropic and anisotropic hollow square‐shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale. The isotropic model is validated by comparing the stress distribution around borehole wall and along X axis direction with analytical solutions. Effects of different factors including the particle size distribution, borehole diameter, far‐field stress anisotropy, and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation. Simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks. Reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure. Rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress. Far‐field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings. Results from this study can provide fundamental insights on the underlying particle‐scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock.

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“…Damage behaviors of shale are very important to understand the failure and deformation of the wellbore, as well as the fracturing propagation, permeability change in the shale reservoirs [3][4][5]. There have been many theoretical studies on the rock with structural planes, including the strength criterion [6][7][8][9][10], and constitutive relationships [4,11,12]. However, the strength criterion cannot consider the damage process of the rock, and the constitutive relationships are always built based on some assumptions and they need the examination by the experimental results.…”

Section: Introductionmentioning

confidence: 99%

Cyclic Experimental Studies on Damage Evolution Behaviors of Shale Dependent on Structural Orientations and Confining Pressures

Cheng

2018

Energies

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Damage process of shale is of great importance when considering the wellbore stability and reservoir stimulation during the work of shale gas recovery. As shale has typical transversely isotropic structures and may be under different stress states in the reservoir, the damage process should be studied while considering both loading directions and confining pressures. A series of cyclic uniaxial and tri-axial compressive tests have been carried out on the shale samples with different oriented weak planes and confining pressures. The dissipated strain energy and its ratio to the releasable elastic strain energy have been studied comparing with the fracturing patterns of the samples. Based on the strain energy dissipation, damage variable is defined and the damage evolution equation is built to describe the damage process of shale samples dependent on loading directions and confining pressures. The damage equation shows that the damage of the shale samples increases as a power function of the axial stress. Under higher confining pressures, the damage is limited in the early loading stage, while it increases significantly with the formation of macro shear fractures when the peak strength is approaching. The change of increasing rate is weak for the shale samples (β = 60 • ) as the main failure pattern is slip along the weak planes. This study is helpful for understanding the damage process and failure of wellbore, as well as the stimulation effect of the shale gas reservoir.

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The Impact of Heterogeneity on the Anisotropic Strength of an Outcrop Shale

Cited by 76 publications

References 11 publications

Empirical strength prediction for preserved shales

Empirical strength prediction for preserved shales

Evolution of stress‐induced borehole breakout in inherently anisotropic rock: Insights from discrete element modeling

Cyclic Experimental Studies on Damage Evolution Behaviors of Shale Dependent on Structural Orientations and Confining Pressures

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