Natural Fractures in shale: A review and new observations

Gale, Julia; Laubach, Stephen E.; Olson, Jon E.; Eichhubl, Peter; Fall, András

doi:10.1306/08121413151

Cited by 683 publications

(337 citation statements)

References 176 publications

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Section: Discussionmentioning

confidence: 99%

“…Natural fractures present in shale (Gale et al, 2014) may also open aseismically by increased fluid pressure and associated reduction of effective stress (Rutqvist, 2014). In this context, weakly cemented fractures being common in shales (Gale et al, 2014) could have a permeability that is extremely stress dependent. Also consistent with our modeling, field observations of fracturing along faults have shown that microseismic events can travel several hundred meters upwards (Fisher and Warpinski, 2012;Flewelling et al, 2013;Yang and Zoback, 2014).…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Modeling of fault activation and seismicity by injection directly into a fault zone associated with hydraulic fracturing of shale-gas reservoirs

Rutqvist

Rinaldi

Cappa

et al. 2015

Journal of Petroleum Science and Engineering

139

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We conducted three-dimensional coupled fluid-flow and geomechanical modeling of fault activation and seismicity associated with hydraulic fracturing stimulation of a shale-gas reservoir. We simulated a case in which a horizontal injection well intersects a steeply dipping fault, with hydraulic fracturing channeled within the fault, during a 3-hour hydraulic fracturing stage. Consistent with field observations, the simulation results show that shale-gas hydraulic fracturing along faults does not likely induce seismic events that could be felt on the ground surface, but rather results in numerous small microseismic events, as well as aseismic deformations along with the fracture propagation. The calculated seismic moment magnitudes ranged from about -2.0 to 0.5, except for one case assuming a very brittle fault with low residual shear strength, for which the magnitude was 2.3, an event that would likely go unnoticed or might be barely felt by humans at its epicenter. The calculated moment magnitudes showed a dependency on injection depth and fault dip. We attribute such dependency to variation in shear stress on the fault plane and associated variation in stress drop upon reactivation. Our simulations showed that at the end of the 3-hour injection, the rupture zone associated with tensile and shear failure extended to a maximum radius of about 200 m from the injection well. The results of this modeling study for steeply dipping faults at 1000 to 2500 m depth is in agreement with earlier studies and field observations showing that it is very unlikely that activation of a fault by shale-gas hydraulic fracturing at great depth (thousands of meters) could cause felt seismicity or create a new flow path (through fault rupture) that could reach shallow groundwater resources.1

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Modeling of fault activation and seismicity by injection directly into a fault zone associated with hydraulic fracturing of shale-gas reservoirs

Rutqvist

Rinaldi

Cappa

et al. 2015

Journal of Petroleum Science and Engineering

139

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“…In outcrops, veins may be used to quantify kinematic opening, but outcrops with large numbers of veins covering a range of scales are rare, and barren fractures are not representative as exhumation and weathering may open previously cemented or closed fractures [e.g., Gale et al, 2014]. Furthermore, outcropping apertures do not take into account the impact of in situ stresses.…”

Section: Introductionmentioning

confidence: 99%

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

2016

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Predicting equivalent permeability in fractured reservoirs requires an understanding of the fracture network geometry and apertures. There are different methods for defining aperture, based on outcrop observations (power law scaling), fundamental mechanics (sublinear length‐aperture scaling), and experiments (Barton‐Bandis conductive shearing). Each method predicts heterogeneous apertures, even along single fractures (i.e., intrafracture variations), but most fractured reservoir models imply constant apertures for single fractures. We compare the relative differences in aperture and permeability predicted by three aperture methods, where permeability is modeled in explicit fracture networks with coupled fracture‐matrix flow. Aperture varies along single fractures, and geomechanical relations are used to identify which fractures are critically stressed. The aperture models are applied to real‐world large‐scale fracture networks. (Sub)linear length scaling predicts the largest average aperture and equivalent permeability. Barton‐Bandis aperture is smaller, predicting on average a sixfold increase compared to matrix permeability. Application of critical stress criteria results in a decrease in the fraction of open fractures. For the applied stress conditions, Coulomb predicts that 50% of the network is critically stressed, compared to 80% for Barton‐Bandis peak shear. The impact of the fracture network on equivalent permeability depends on the matrix hydraulic properties, as in a low‐permeable matrix, intrafracture connectivity, i.e., the opening along a single fracture, controls equivalent permeability, whereas for a more permeable matrix, absolute apertures have a larger impact. Quantification of fracture flow regimes using only the ratio of fracture versus matrix permeability is insufficient, as these regimes also depend on aperture variations within fractures.

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“…As illustrated in Fig. 1 , in addition to the hydraulic cracks, there are abundant material discontinuities in the shale stratum such as weak bedding interfaces and sealed natural fractures ( Gale et al, 2014;Gale et al, 2007 ). A hydraulic fracture may cross these discontinuities, bend into them or be stopped by them.…”

Section: Introductionmentioning

confidence: 99%

Crack deflection in brittle media with heterogeneous interfaces and its application in shale fracking

Zeng

Wei

2017

Journal of the Mechanics and Physics of Solids

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a b s t r a c tDriven by the rapid progress in exploiting unconventional energy resources such as shale gas, there is growing interest in hydraulic fracture of brittle yet heterogeneous shales. In particular, how hydraulic cracks interact with natural weak zones in sedimentary rocks to form permeable cracking networks is of significance in engineering practice. Such a process is typically influenced by crack deflection, material anisotropy, crack-surface friction, crustal stresses, and so on. In this work, we extend the He-Hutchinson theory (He and Hutchinson, 1989) to give the closed-form formulae of the strain energy release rate of a hydraulic crack with arbitrary angles with respect to the crustal stress. The critical conditions in which the hydraulic crack deflects into weak interfaces and exhibits a dependence on crack-surface friction and crustal stress anisotropy are given in explicit formulae. We reveal analytically that, with increasing pressure, hydraulic fracture in shales may sequentially undergo friction locking, mode II fracture, and mixed mode fracture. Mode II fracture dominates the hydraulic fracturing process and the impinging angle between the hydraulic crack and the weak interface is the determining factor that accounts for crack deflection; the lower friction coefficient between cracked planes and the greater crustal stress difference favor hydraulic fracturing. In addition to shale fracking, the analytical solution of crack deflection could be used in failure analysis of other brittle media.

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Natural Fractures in shale: A review and new observations

Cited by 683 publications

References 176 publications

Modeling of fault activation and seismicity by injection directly into a fault zone associated with hydraulic fracturing of shale-gas reservoirs

Modeling of fault activation and seismicity by injection directly into a fault zone associated with hydraulic fracturing of shale-gas reservoirs

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

Crack deflection in brittle media with heterogeneous interfaces and its application in shale fracking

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