The Small-Drillhole Minipermeameter Probe for In-Situ Permeability Measurement

Summary Laboratory measurement of permeability using a Hassler cell is the industry standard; however, consistently removing undisturbed rock samples from friableout crops 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-drill hole 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, sealexpansion, 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 porousmedia: 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-cmintervals 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.