Summary
Laboratory measurement of permeability using a Hassler cell is the industry
standard; however, consistently removing undisturbed rock samples from friable
outcrops is difficult. Although various conventional surface-sealing
mini-permeameters are developed as an alternative for permeability measurement,
these devices generally suffer from difficulties in maintaining optimal forces
on the tip seal when dealing with outcrop irregularities in the field; outcrop
weathering is also problematic. Because a reliable field method is needed for
studies of friable geological units, this paper presents an innovative
technique for measuring permeability in situ. The design of the small-drillhole
minipermeameter probe is discussed, as well as the accompanying analytical
technique and the size and shape of the instrument’s averaging volume.
Small-diameter holes [i.e., 1.8 cm (0.7 in.)] are drilled into an outcrop with
a masonry drill, followed by drillhole vacuuming, probe insertion, seal
expansion, gas injection, and calculation of the intrinsic permeability through
measurement of the injection pressure, gas-flow rate, and knowledge of the
system geometry. Advantages of this approach include access to a nonweathered
surface, an operator-independent sealing mechanism around the air-injection
zone, and the potential for permeability measurement at multiple depths below
an outcrop surface. To date, data have been collected from four diverse porous
media: upper and lower shoreface sandstone (Escalante, Utah), saprolitic soils
(Clemson, South Carolina), nonwelded and sintered ignimbrite (Bishop,
California), and fluvially reworked tuffaceous sedimentary rock (Bishop,
California). The probe has proved durable and robust, with a single probe
sufficient for making thousands of measurements in a variety of environments.
Data quality supports the conclusion that the drillhole probe is a practical
field instrument.
Introduction
Small-scale permeability heterogeneity plays a substantial role in petroleum
migration and reservoir performance; this parameter commonly ranges over many
orders of magnitude (e.g., 0.01 to more than 10,000 md). Permeability
heterogeneities on the meter-to-micrometer scale associated with beds, laminae,
internal sedimentary structures, and variations in pore morphology are the
source of most retrieval difficulties during enhanced-oil-recovery operations,
thus negatively affecting reservoir recovery efficiency.
Considerable heterogeneity is evident when permeability measurements are
made on small scales, either in the field or on field samples in a laboratory
setting. Traditionally, small-scale permeability measurements are made by
inducing 1D gas flow through a cylindrical core plug in a Hassler sleeve or
cell. Recently, such measurements also are made by inducing multidimensional
gas flow through a sample with various configurations of the conventional
surface-sealing gas minipermeameter.
Cylindrical plugs generally are extracted from continuous core at 30-cm
intervals for Hassler-cell permeability measurement, preserving a majority of
the core while minimizing associated costs. Except for relatively homogeneous
formations, this scale of permeability measurement is in an ill-defined
geologic region, falling within the range of laminae and lamina sets.
Furthermore, core-plug samples tend to be biased toward the more consolidated,
less permeable, and less friable core sections. As an example, the effect of
this arbitrary sampling density on Hassler-sleeve measurements for the case of
tight gas sands is that magnitudes of permeability less than 100 md frequently
result, even when coarser-grained beds that would operate as preferential flow
channels or “thief zones” are clearly present. Currently, the scale of
sedimentary heterogeneity is best resolved by use of the minipermeameter, which
allows investigation of permeability heterogeneity at much greater (and
statistically significant) sampling densities and on much smaller scales than
is possible with the traditional technique.
The literature documents use of the conventional surface-sealing
minipermeameter probe for measurements made on outcrop surfaces, core plugs,
slabbed cores, or large-cut blocks. One motivation for using cores, plugs, or
blocks of rock is that natural weathering processes may greatly affect
permeability values obtained from exposed outcrop surfaces. The weathering
effect has been shown to extend up to several inches below the rock surface.
Beyond the issue of weathering, there are other rationales for discouraging use
of the conventional surface-sealing minipermeameter probe in a field setting.
When applying this probe geometry to natural rock outcroppings in the field, as
opposed to cut specimens in an automated laboratory setting, seal-quality
problems are often encountered because of irregular, rough surfaces and
difficulties associated with manually holding the probe stationary while
applying a uniform normal force of the optimal magnitude on the tip seal.
To enable in-situ measurements of friable geologic units and to overcome
weathering and seal-quality problems, a new minipermeameter probe has been
developed that is specifically intended for application inside a small drilled
hole. The design of the small-drillhole minipermeameter probe is discussed in
what follows, as well as the accompanying analytical technique and the size and
shape of the instrument’s averaging volume. This article concludes with brief
reviews of data collected using the technique.
© 2005. Society of Petroleum Engineers
View full textPDF
(
1,258 KB
)
History
- Original manuscript received:
29 March 2004
- Revised manuscript received:
21 September 2005
- Manuscript approved:
22 September 2005
- Version of record:
15 December 2005
View Cart
