Laboratory measurement of low permeability unconventional gas reservoir rocks: A review of experimental methods

Unconventional natural gas has become an important source of energy. However, the development of such resources has been challenging as these reservoirs are characterised by low to ultra-low permeabilities. The low permeability does not only present a challenge for commercial gas production, but also for experimental measurements of rock samples. Methods to determine permeability of low permeability rock cores and crushed rock samples directly can be divided into two categories: steady state and unsteady state. Unsteady state methods include the pulse decay, oscillating pressure, and GRI method (pressure fall-off method). In this review we describe and compare each method in detail and discuss the challenges specific to measuring low permeability rocks. A brief overview of alternative permeability measurements is also provided (e.g. indirect measurements, canister desorption test). The review highlights each method's advantages and disadvantages. The steady state method is easy to apply, due to its simple experimental set-up and its straightforward solution using Darcy's law. However, as permeability decreases, flow rate measurements become less accurate. Unsteady state experiments measure pressure and temperature, which can typically be determined more accurately. Furthermore, the set-up of unsteady state experiments can be adapted to increase sensitivity, thus improving measurement accuracy or speed. On the downside, unsteady state experiments are typically more affected by leaks than steady state experiments. The review indicates that steady state and unsteady state methods do not always yield the same results, and that the GRI method measures a different type of permeability to the other experimental methods. The permeating fluid can also significantly affect measurements in very low permeability rocks. Additionally, the experimental measurement of low permeability gas reservoir rocks faces several practical challenges: a lack of universal measuring standards for low and ultra-low permeability media affects comparability between results; different laboratories use different methodolgies for sample preparation; and various analytical solutions have been presented to interpret the experimental data, most of which are based on the validity of Darcy's law and the Klinkenberg effect. The suitability of an experimental method depends on permeability, porosity and adsorption capacity of the rock, and the limitations of the underlying assumptions of the solution. A thorough understanding of the applied experimental and analytical technique, and knowledge of the sample's preparation are necessary to accurately interpret and use any results. (C) 2016 Elsevier B.V. All rights reserved.