Previously available interpretation methods developed for determination of gas permeability from crushed samples are mostly applicable for conventional porous rocks of reasonably high permeability. This paper presents an improved formulation which honors the relevant physics of fluid, transport, and shale-rock conditions for determination of extremely-low nanodarcy gas permeability of shale by pressure-pulse transmission testing of crushed shale-rock particles. The new method is demonstrated by means of typical simulated test conditions. Assuming Darcian viscous flow instead of the nonDarcy flow yields almost twice the actual value of the intrinsic permeability for this case.
The intrinsic permeability of shale is only a characteristic of shale rock features, including its fabric and texture. However, its value depends on the prescribed temperature and stress conditions as these conditions can alter the shale-rock by deformation. In contrast, the apparent permeability measured by Darcy's law is a property which depends both on the shale rock properties, transport conditions, and fluid properties and behavior modified by pore-confinement effects. Thus, the permeability measured using a Darcy-type equation is the apparent permeability depending on the prevailing conditions of fluid, transport, and shale, and not the intrinsic permeability. In fact, this is the primary reason of contradictory values of permeability measured by different laboratories. Beskok and Karniadakis (1999) expressed the difference between the intrinsic and apparent permeability for flow through a single capillary tube by a conveniently simple correction factor based on the prevailing local conditions in shale formations. Civan (2010) extended and generalized their formulation to a bundle of tortuous tubes representing the gas transport paths in shale. A bundle of tubes is a special case of a more general porous media. Transport formulas for a bundle of tubes differ from the more general case by the value of their formation factor (Sigal, 2002, Sigal, 2013). In terms of the cementation exponent for conventional reservoir rocks it is the transformation from m = 1 to value closer to two.
Determination of the skeleton permeability in shale is very critical for improved reservoir management and plays a key role in production forecasts, well placement, and configuration optimization. Although permeability estimates may be obtained from production data analysis and history matching, such approaches are generally representative of larger reservoir volumes and thus cannot capture high resolution heterogeneities. A more informative approach is the use of core, crushed samples, or drill cuttings-based measurements obtained in the laboratory (Brace et al., 1968, Hsieh et al., 1981, Neuzil et al., 1981, Dicker and Smits, 1988, Luffel et al., 1993, Wu et al., 1998, Egermann et al., 2005, Cui et al., 2009, Barral et al., 2009, Civan et al., 2011, 2012, Profice et al., 2012, Xiong et al., 2012, and Tinni et al., 2012).