structural geology, propagation mechanics and - Stanford School of ...

4. Application to the Aztec sandstone
In order to test this computational estimation method under real-world conditions of
limited sample supply, we applied the algorithm to a single, representative thin section of
Aztec sandstone showing a cm-thick CB surrounded by relatively undeformed matrix
(Figure 5.4). As is typical with foreset deposition in æolian sandstones, bedding consists
of alternating, well-sorted coarse-grain (~0.4 mm) and fine-grain (~0.1 mm) layers up to
several millimeters thick. Poorly sorted mixed-grain layers are also common. Within each
of the three distinct bed types, the distribution of grains and pores is relatively
homogeneous, both in the matrix and in the CB.
Digital images were collected in BEC mode on a standard SEM at 15 kV and a
magnification of 80x. Two images were collected from each bed type, with mosaic
composites used as necessary to ensure representative coverage in coarse-grain beds. A
consistent intensity threshold of 72 out of 256 was used for the binary conversions, and
ten 3-D pore-structure realizations were computed for each of the 12 binary images. Flow
simulations were then conducted on each pore-structure realization to yield 120 total
estimates of permeability relative to porosity. Figure 5.6 shows an example BEC image
with its binary conversion for each bed type from both the matrix and the CB.
4.1. Simulation results
The resulting estimates of porosity and permeability are plotted in Figure 5.7; the
averaged permeability results are summarized in Table 5.1.
Porosity and permeability estimates for the matrix range from about 20-27% and 200-
3,500 millidarcys (mD). Not surprisingly, average porosity for the pore-structure
realizations derived from the mixed-grain bed images is lowest (22%), while average
permeability estimated for the coarse-grain pore-structure realizations is highest (1,392
mD). The fine-grain beds, which exhibit the tightest grain-size distribution, produced the
highest average porosity estimate (25.25%) and the lowest average permeability estimate
(406 mD). The combined average estimates for the matrix are 23.6% and 776 mD. This
simple average bulk permeability value, however, does not account for the inherently
anisotropic, layered nature of the sandstone. Figure 5.8 presents a more realistic approach
to estimating directional permeability in the matrix, yielding best-estimate values of
1,053 mD parallel to bedding and 804 mD normal to bedding.
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