DINWIDDIE, C.L., cdinwid@clemson.edu, and MOLZ, F.J., Environmental Engineering and Science, Clemson University, Clemson, SC 29625; MURDOCH, L.C. and  CASTLE, J.W. Geological Sciences, Clemson University, Clemson, SC 29634

Considerable variability (heterogeneity) is evident when permeability measurements are made on small scales, either in the field or on undisturbed field samples. Such measurements have been made using the mini-permeameter, either on surface outcrops, core plugs, slabbed cores, or on large cut blocks.  The motivation for using cores or blocks of rock is that weathering processes may severely affect permeability values obtained from outcrop surfaces, and the weathering effect may extend several inches below the rock surface. Unfortunately, it is not an elementary task to consistently remove quality core plug samples for laboratory measurements from some sandstone outcrops. However, in this same sandstone, it is often quite easy to drill small, high quality holes.
     In this presentation we will describe a prototype borehole mini-permeameter probe and the associated data analysis methodology for performing in situ permeability measurements inside small diameter holes (say 1.6 cm diameter) drilled into outcrops.  Advantages of this approach are that it eliminates the use of questionable permeability measurements from weathered outcrop faces, provides a superior sealing mechanism around the air injection zone, and allows measurements to be made at multiple depths below the outcrop surface. With existing surface probes it is difficult to maintain a constant force of the optimal magnitude on the surface seal, unless the measurements are made with a mechanical device in the laboratory. Surface roughness and irregularity are also serious problems impacting the results achieved by the current technique.
     We will conclude by discussing how to obtain a spatial weighting function for the mini-permeameter probe and what it suggests about the size and shape of the instrument’s averaging volume. Analysis of field or laboratory data, which consists of air injection pressure and mass flow rate, is based on the numerical solution in cylindrical coordinates of the ideal gas flow equation, assuming homogeneous and isotropic conditions at the scale of the measurements. The overall analytical approach follows that developed by D. Goggin (1988, Ph.D. Dissertation, UT Austin), including corrections for gas slippage It is expected that when developed fully, the new methodology will serve as the basis of a true field technique for measuring the apparent permeability of a broad range of consolidated rock