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Summary

Probe (or "mini") permeameters are becoming increasingly important throughout the world for making high-density, nondestructive permeability measurements on slabbed or nonslabbed cores from geologically complex, heterogeneous formations. At their lower effective limits (about 1 md) these steady-state devices are too slow and inaccurate to be useful, and at permeabilities above about 20 md, inertial resistance often causes significant error in permeability calculations.

This paper discusses the theory, design, and operation of a new instrument based on a pressure-decay technique (PDPK-200* patent pending). Its useful range is from 0.001 to 20,000 md. Measurement time varies from about 2 to 35 seconds. Measurement-position coordinates and slip-corrected (Klinkenberg) and conventional gas permeabilities (both of which are free from inertial flow-resistance effects) are obtained. Computer-stored data facilitate generation of permeability "profiles" (permeability-depth logs), permeability contour plots, or "calibration" of inferred permeability from wireline logs. Reservoir heterogeneity can further be quantified by generation of Dykstra-Parsons coefficients.

Introduction

The concept of flowing gas from the end of a tube (or "probe") that is sealed against the surface of a rock sample to estimate local permeability was introduced by Dykstra and Parsons in 1950. Eijpe and Weber further developed the instrument for use in outcrop studies. Goggin et al. performed numerical simulations to determine theoretical geometrical flow factors, , as functions of the ratio of the outer to inner radius of the probe tip seal and of rock sample dimensions relative to the inner radius of the tip seal. In this definitive paper, they also addressed the effects of gas slippage and inertial flow resistance on probe permeability measurements.

Robertson and McPhee and Halvorsen and Hurst used core plugs of known permeability to determine empirically. The latter investigators and Hurst and Rosvoll described the use of a computer-controlled probe permeameter that is capable of unattended operation. Goggin presented an extensive list of references, including investigations that have been carried out at Heriot-Watt University, Imperial College, and the University of Texas, to name a few.

All the mini-permeameters reported are steady-state devices. Gas at a constant, measured pressure is delivered to the probe tip, and its flow rate is measured. Alternatively, gas flow rate is controlled by a mass flow-controller, and its delivered pressure is monitored. Steady-state is achieved in either case when both pressure and flow rate become invariant with time.

Incentive for the present work derived from the need to measure a large quantity of cores, most of which had permeabilities of less than 1 md. These measurements were taking 20 minutes or more to reach steady-state conditions, after which the flow-controller reading flashed between zero and one in its least significant digit. Therefore, even if all calibrations were exact, the uncertainty in the calculated permeability was at least 50%.

We modified the instrument by removing the flow controller and adding reservoirs of different calibrated volumes. The time rate of pressure decay as nitrogen flowed from any one or all of these reservoirs, through the probe and into the sample yielded a direct measure of the sample's permeability. The time requirement for the low permeability samples was reduced from 20 minutes to 20 seconds, and the uncertainty of all measurements was reduced to 5% or less.