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| Titel |
Characterising rock fracture aperture-spacing relationships using power-law
relationships: some considerations |
| VerfasserIn |
Martin Brook, Bruce Hebblewhite, Rudrajit Mitra |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122790
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| Publikation (Nr.) |
 EGU/EGU2016-1908.pdf |
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| Zusammenfassung |
| The size-scaling of rock fractures is a well-studied problem in geology, especially for
permeability quantification. The intensity of fractures may control the economic exploitation
of fractured reservoirs because fracture intensity describes the abundance of fractures
potentially available for fluid flow. Moreover, in geotechnical engineering, fractures are
important for parameterisation of stress models and excavation design. As fracture data is
often collected from widely-spaced boreholes where core recovery is often incomplete,
accurate interpretation and representation of fracture aperture-frequency relationships from
sparse datasets is important. Fracture intensity is the number of fractures encountered per unit
length along a sample scanline oriented perpendicular to the fractures in a set. Cumulative
frequency of fractures (F) is commonly related to fracture aperture (A) in the form of a
power-law (F = aA−b), with variations in the size of the a coefficient between sites
interpreted to equate to fracture frequency for a given aperture (A). However, a common flaw
in this approach is that even a small change in b can have a large effect on the response
of the fracture frequency (F) parameter. We compare fracture data from the Late
Permian Rangal Coal Measures from Australia’s Bowen Basin, with fracture data from
Jurassic carbonates from the Sierra Madre Oriental, northeastern Mexico. Both
power-law coefficient a and exponent b control the fracture aperture-frequency
relationship in conjunction with each other; that is, power-laws with relatively low a
coefficients have relatively high b exponents and vice versa. Hence, any comparison of
different power-laws must take both a and b into consideration. The corollary is that
different sedimentary beds in the Sierra Madre carbonates do not show ∼8× the
fracture frequency for a given fracture aperture, as based solely on the comparison of
coefficient a. Rather, power-law “sensitivity factors” developed from both Sierra Madre
and the Bowen Basin span similar ranges, indicating that the factor of increase
in frequency (F) for a doubling of aperture size (A) shows similar relationships
and variability from both sites. Despite their limitations, we conclude that fracture
aperture-frequency power-law relationships are valid and, when interpreted carefully,
provide a useful basis for comparing rock fracture distributions across different sites. |
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