Determining gas permeability in tight rocks: How do we know if we are obtaining the right value

摘要

The measurement of gas permeability for core plugs of conventional reservoirs, with values larger than 1 md, is a matter of routine in laboratories around the world. The methodology is well established and there are several core analysis companies that offer check plugs for calibrating equipment and validating the methodology used. However, the measurement on tight rocks is more challenging, the results from different laboratories often do not agree, and there are large number of papers comparing different techniques. Additionally, there are a range of mechanisms that might affect these measurements such as: slippage, Knudsen flow, inertial effects, net stress, etc. The permeability of shales determined by different laboratories can vary up to three orders of magnitude. The determination of gas permeability in tight rocks is fraught with difficulties, thus inducing large uncertainty in their values and the mechanisms involved. To reduce this uncertainty we have designed and built standards based on fundamental flow through capillaries following a concept presented by Sinha et al. [1]. We used small diameter glass and fused silica capillaries embedded in PVC cylinders of 38mm diameter and ~ 50 mm length. The permeability of these standards can be theoretically calculated using Hagen–Poiseuille equation for laminar flow. The permeability of our set of standards range from 5.9 mD to 11 nanoD. The gas permeability of these standards was measured both with steady-state and pulse-decay methods. The Klinkenberg corrected permeability agreed very well with the theoretical value within the tolerance of manufacture of the capillaries. The experimental results and theoretical model also allows us to correlate the Klinkenberg slip factor with the mean free path and the radius of the capillaries for the whole range.