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| Titel |
Geological implications and controls on the determination of water saturation in shale gas reservoirs |
| VerfasserIn |
David Hartigan, Mike Lovell, Sarah Davies |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097954
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| Publikation (Nr.) |
 EGU/EGU2014-13583.pdf |
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| Zusammenfassung |
| A significant challenge to the petrophysical evaluation of shale gas systems can be attributed
to the conductivity behaviour of clay minerals and entrained clay bound waters. This is
compounded by centimetre to sub-millimetre vertical and lateral heterogeneity in formation
composition and structure. Where despite significant variation in formation geological and
therefore petrophysical properties, we routinely rely on conventional resistivity methods for
the determination of water saturation (Sw), and hence the free gas saturation (Sg) in gas
bearing mudstones.
The application of resistivity based methods is the subject of continuing debate, and there
is often significant uncertainty in both how they are applied and the saturation estimates they
produce. This is partly a consequence of the view that "the quantification of the behaviour of
shale conductivity-¦.has only limited geological significance" (Rider 1986). As a result, there
is a separation between our geological understanding of shale gas systems and the
petrophysical rational and methods employed to evaluate them. In response to this
uncertainty, many petrophysicists are moving away from the use of more complex
‘shaly-sand‘ based evaluation techniques and returning to traditional Archie methods for
answers. The Archie equation requires various parameter inputs such as porosity and
saturation exponents (m and n), as well as values for connate fluid resistivity (Rw). Many of
these parameters are difficult to determine in shale gas systems, where obtaining a water
sample, or carrying out laboratory experiments on recovered core is often technically
impractical.
Here we assess the geological implications and controls on variations in pseudo Archie
parameters across two geological formations, using well data spanning multiple basinal
settings for a prominent shale gas play in the northern Gulf of Mexico basin.
The results, of numerical analysis and systematic modification of parameter values to
minimise the error between core derived Sw (Dean Stark analysis) and computed Sw, links
sample structure with composition, highlighting some unanticipated impacts of clay minerals
on the effective bulk fluid resistivity (Rwe) and thus formation resistivity (Rt). In addition, it
highlights simple corrective empirical adaptations that can significantly reduce the error in
Sw estimation for some wells.
Observed results hint at the possibility of developing a predictive capability in selecting
Archie parameter values based on geological facies association and log composition
indicators (i.e. V Clay), establishing a link between formation depositional systems and their
petrophysical properties in gas bearing mudstones.
Rider, M.H., 1986. The Geological Interpretation of Well Logs, Blackie. |
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