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| Titel |
Microstructure and permeability of the Whitby Mudstone (UK) as an analogue for the Posidonia shale (NL) |
| VerfasserIn |
Maartje Houben, Auke Barnhoorn, Martyn Drury, Colin Peach, Christopher Spiers |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105199
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| Publikation (Nr.) |
 EGU/EGU2015-4654.pdf |
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| Zusammenfassung |
| In order to make gas productivity from a shale economically interesting we should find ways
to better connect the in-situ pore network to the natural occurring and mechanical induced
fractures in the rock. When trying to improve gas productivity a first aim is to understand gas
storage and gas flow potential through the rock by investigating the microstructure and
measure the matrix porosity and permeability of the unfractured shales. Using a combination
of methods we have characterized the porosity and permeability of the Jet Dogger section of
the Whitby Mudstone Formation (UK), which we use as an analogue for the Posidonia Shale
(NL). The Posidonia shale is a possible unconventional source for gas in Northern
Europe.
A combination of Precision Ion Polishing (PIPS) and Scanning Electron Microscopy
(SEM) has been used to investigate the microstructure and the pores. Microstructurally the
circa 8 meter thick Jet section of the Whitby Mudstone Formation can be subdivided into a
fossil rich (>15 %) top half with an organic matter content of 7-10% and a sub-mm
laminated (alternating clay-rich, carbonate-rich, not necessarily fossils, layers) lower half
were the organic matter content varies from 0.3-16%. In addition, any possible flow in the
rock has to go through the fine-grained clay matrix (all grains < 2 μm) due to the fact that all
larger grains are completely surround by this matrix. Visible PIPS-SEM porosity
(pore diameter > 100 nm) is in the order of 0.5-2.5% and is not connected in 2D.
Furthermore, overall more than 40% of the visible porosity is present within the clay
matrix (sometimes even up to 80%). Porosity and pore size distributions for pores
with smaller diameters (2 < diameter < 100 nm) were determined using Ar and
N2 gas adsorption. The adsorption porosity was in the order of 1-5%, were we
found 1-2.5% porosity for the top half of the section and 2-5% porosity for the
bottom half. Ar gas permeability of the samples was measured on 1-inch diameter
cores using Ar-gas-permeametry with a pressure step of 0.2 MPa. The permeability
measured was in the order of 2•10-19 – 1•10-17 m2 at a confining pressure of 0.8
MPa. The top half of the section (less porous) showed a slightly lower permeability
(2•10-19 –3•10-18 m2) than the lower half of the Jet Dogger section (1•10-18 –
1•10-17m2).
The PIPS-SEM porosity is apparently unconnected on the scale used for imaging and any
flow through these rocks has to go through the clay matrix. A connected network of
nano-scale pores connected to the micro-scale (PIPS-SEM) pores should be present because
of the fact that we measured a through flow of gas. Relationships between the permeability
and Ar-porosity and permeability and microstructure were, however, not systematic. |
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