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| Titel |
Experimental Determination of the Fracture Toughness and Brittleness of the Mancos Shale, Utah. |
| VerfasserIn |
Mike Chandler, Phil Meredith, Brian Crawford |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072760
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| Zusammenfassung |
| The hydraulic fracturing of Gas-Shales has become a topic of interest since the US Shale Gas
Revolution, and is increasingly being investigated across Europe. A significant issue during
hydraulic fracturing is the risk of fractures propagating further than desired into aquifers or
faults. This occured at Preese Hall, UK in April and May 2011 when hydraulic
fractures propagated into an adjacent fault causing 2.3ML and 1.7ML earthquakes
[1].
A rigorous understanding of how hydraulic fractures propagate under in-situ conditions is
therefore important for treatment design, both to maximise gas accessed, and to minimise
risks due to fracture overextension. Fractures will always propagate along the path of least
resistance, but the direction and extent of this path is a complex relationship between the
in-situ stress-field, the anisotropic mechanical properties of the rock, and the pore
and fracturing pressures [2]. It is possible to estimate the anisotropic in-situ stress
field using an isolated-section hydraulic fracture test, and the pore-pressure using
well logs. However, the anisotropic mechanical properties of gas-shales remain
poorly constrained, with a wide range of reported values. In particular, there is an
extreme paucity of published data on the Fracture Toughness of soft sediments such as
shales.
Mode-I Fracture Toughness is a measure of a material’s resistance to dynamic tensile fracture
propagation. Defects such as pre-existing microcracks and pores in a material can induce high
local stress concentrations, causing fracture propagation and material failure under
substantially lower stress than its bulk strength. The mode-I stress intensity factor, KI,
quantifies the concentration of stress at the crack tip. For linear elastic materials the Fracture
Toughness is defined by the critical value of this stress intensity factor; KIc, beyond which
rapid catastrophic crack growth occurs. However, rocks such as shales are relatively ductile
and display significant non-linearity. This produces hysteresis during cyclic loading, allowing
for the calculation of a brittleness coefficient using the residual displacement after successive
loading cycles. This can then be used to define a brittleness corrected Fracture Toughness,
KIcc.
We report anisotropic KIcc values and a variety of supporting measurements made on the
Mancos Shale in the three principle Mode-I crack orientations (Arrester, Divider and
Short-Transverse) using a modified Short-Rod sample geometry. The Mancos is an Upper
Cretaceous shale from western Colorado and eastern Utah with a relatively high siliclastic
content for a gas target formation. The Short-Rod methodology involves the propagation of a
crack through a triangular ligament in a chevron-notched cylindrical sample [3]. A very
substantial anisotropy is observed in the loading curves and KIcc values for the three crack
orientations, with the Divider orientation having KIcc values 25% higher than
the other orientations. The measured brittleness for these Mancos shales is in the
range 1.5-2.1; higher than for any other rocks we have found in the literature. This
implies that the material is extremely non-linear. Increases in KIcc with increasing
confining pressure are also investigated, as Shale Gas reservoirs occur at depths
where confining pressure may be as high as 35MPa and temperature as high as
100oC.
References
[1]Â Â Â C.A. Green, P. Styles & B.J. Baptie, "Preese Hall Shale Gas Fracturing", Review &
Recommendations for Induced Seismic Mitigation, 2012.
[2]Â Â Â N.R. Warpinski & M.B. Smith, "Rock Mechanics and Fracture Geometry", Recent
advances in Hydraulic Fracturing, SPE Monograms, Vol. 12, pp. 57-80, 1990.
[3]Â Â Â F. Ouchterlony, "International Society for Rock Mechanics Commision on Testing
Methods: Suggested Methods for Determining the Fracture Toughness of Rock",
International Journal of Rock Mechanics and Mining Science & Geomechanics Abstracts,
Vol. 25, 1988. |
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