Permeability standards for tight rocks: Design, manufacture and validation

Abstract

Permeability values measured for low-permeability (tight) rocks by commercial and research laboratories may exhibit an unacceptably large variability even when measured using samples subjected to similar experimental conditions. One cause of this variability is that, for a particular measurement method, laboratories utilize different experimental configurations with instrumental components that have variable manufacturing specifications. These device-to-device variations, and consequent wide ranges in measured permeability values, have created confusion for recipients of the data. Regardless of the cause of the observed discrepancies, there are currently no commercial standards available to evaluate the performance and accuracy of these different experimental devices and calibrate them to an “absolute truth”.In this work, the design, manufacture and calibration of standards for gas permeability measurements of tight rocks (with permeabilities down to the microdarcy range) is discussed. Three identical permeability standards were manufactured and tested in this study. These permeability standards were developed by embedding one micron-sized capillary tube (ID: 25±1μm; OD: 1/16″ (1.588mm)) inside an impermeable acrylic cylindrical core plug with the dimensions of 1.5″ (38.1mm) in diameter and 2″ (50.8mm) in length, which is consistent with what is used in commercial permeability measurement devices.For the manufactured permeability standards, it is evident that (1) there is satisfactory agreement (<±10%) between calculated and measured gas (nitrogen) permeability values (within the experimental error range), (2) the gas permeability values measured for these three different standards are very similar, even though they were manufactured separately, and tested independently at different times, and (3) the repeatability/reproducibility of the measured gas (nitrogen) permeability values is excellent (<±1%). A further observation is that, depending on mean pressure (gas flow rates), flow regimes within the permeability standards (microcapillary tubes) may correspond to gas slip or turbulent flow regimes, affecting the measured (apparent) gas permeability values.Using these new permeability standards, it is possible to calibrate permeability devices used by different commercial/research laboratories ensuring accurate steady-state gas permeability measurements for tight rocks with permeabilities down to the microdarcy range.