Отправлено: 02.09.13 05:12. Заголовок: The petrophysics of shale gas reservoirs: technical challenges and pragmatic solutions

The petrophysics of shale gas reservoirs: technical challenges and pragmatic solutions

Vivian K. Bust, Azlan A. Majid, Joshua u. oletu and paul F. Worthington

ABstrAct: The host rocks of shale gas accumulations act as source, seal and
reservoir. They are characterized by complex pore systems with ultra-low to
low interparticle permeability and low to moderate porosity. The word ‘shale’ is
used in the sense of a geological formation rather than a lithology, so shale gas
reservoirs can show marked variations in rock type from claystones, marlstones
and mudstones to sandstone and carbonate lithological ‘sweet spots’.
The pore space includes both intergranular and intrakerogen porosity.
The density of natural fractures varies markedly, and pore throat connectivity is relatively ineffective.
Moreover, in-situgas pore volume has to take account of both free and
adsorbed gas, an evaluation exercise that is complicated by pronounced variations in water salinity.
All these characteristics present major challenges to the
process of petrophysical evaluation. The petrophysical responses to these issues
are severalfold. First, a broader calibrating database of core measurements is
required at key wells, especially as regards mineralogy, porosity and permeability data,
shale/mudstone sample analyses, total organic carbon, gas desorption isotherms,
and the analysis of extracted formation waters. Second, at least
in the key wells, an extended suite of logs should include an elemental analysis log,
magnetic resonance imager, electrical micro-imager, and a dipole sonic
log. These databases lead to a rock-typing scheme that takes better account
of dynamic properties and fracturability. They also allow reservoir partitioning
based on exclusivity of empirical interpretative algorithms, e.g. quartz content
vs. producibility. These responses comprise key elements of a
functional petrophysical system that encompasses fit-for-purpose interpretation methods, such
as a pseudo-Archie approach, i.e. the application of the Archie equations with
non-intrinsic exponents. This system is presented as a workflow for application in shale gas reservoirs,
for which bulk density retains a major influence on
computed gas in place. The benefits of this approach are especially strong in
reserves reporting of these unconventional gas reservoirs.

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В частности приведен вариант квадрата Вилли с керогеном в матрице (как и у нас по
бажену) и с различием органических и неорганичечских пор и выделением адсорбированного газа