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Chapter 5
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The Sample
Support Problem
for Permeability Assessment
in Sandstone Reservoirs
Y. Anguy
Laboratoire Energétique
et Phénomènes de Transfert
L.E.P.T.-ENSAM
Esplanade des Arts et
Métiers
Cedex, France
R. Ehrlich
C. M. Prince
V. L. Riggert
Department of Geological
Sciences
University of South Carolina
Columbia, South Carolina,
U.S.A.
D. Bernard
Laboratoire Energétique
et Phénomènes de Transfert
L.E.P.T.-ENSAM
Esplanade des Arts et
Métiers
Cedex, France
*
ABSTRACT
All sandstone fabrics contain
a characteristic complex of microstructures of a scale that is large with
respect to the size of sample plugs conventionally used to measure permeability.
A fundamental element of this microstructure consists of circuits composed
of oversized pores and throats representing the sites of loose or flawed
packing, and largely controlling permeability at the permeability-plug
scale. Very small changes in the location of a permeability plug with respect
to these circuits can result in significant variation in measured permeability.
In the terminology of geostatistics, this is a classic problem of insufficient
sample support, producing artificially heightened variance over small spatial
scales as a result of a sampling volume that is too small. The varying
pattern of the spatial variability of the microstructure dictates that
permeability plugs must vary in size to contain enough of the microstructure
to ensure adequate sample support. This is impossible in practice, but
a combination of physical data and image analysis can yield permeability
values representing rock volumes of the requisite size. A minimum sample
size can be determined by measuring the scales of structural complexity
using Fourier transforms of the image of the porous microstructure |
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