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A Practical Recommendation for Permeability Measurement in Tight-Sand and Shale Reservoirs


Permeability is one of the critical parameters for evaluating and developing tight-sand and shale reservoirs. It is well-known that unconventional shale and tight-sand reservoirs are highly heterogeneous in mineralogy, fabric and pore structure. Pore-throats of unconventional formations commonly have a wide range of sizes from hundreds to just several nanometers. Due to the nanometer-size pores, gas transport in tight-sand and gas shale has a significant component of diffusion, consequently resulting in strong pore-pressure-dependent gas permeability. Different approaches for a Klinkenberg diffusion correction to gas permeability have been discussed theoretically by recent studies with limited experimental data. With nanometer-size pores, the intrinsic or liquid permeability corrected from gas permeability also changes with different test gases. Furthermore, permeability of unconventional rock depends on in-situ confining stress that also changes as the reservoir pressure depletes during production. Therefore, for rigorous evaluation and development of an unconventional reservoir, it becomes necessary to understand how intrinsic permeability changes with in-situ stress and the pore-pressure and stress-dependence of gas diffusivity, which are difficult and time-consuming to determine separately in the laboratory. However, in present practice, most engineering calculations and reservoir simulation software packages do not take the diffusivity as an input. In this study, a simple but rigorous approach is suggested to practically determine the in-situ permeability (combined intrinsic permeability and diffusivity) with consideration of the effects of pore pressure and in-situ confining stress. The permeability is measured under conditions that closely simulate the in-situ stress path of a producing reservoir and can be simply represented as a function of reservoir pressure and therefore be readily usable for various engineering calculations and reservoir simulations. Discussions on various aspects of the permeability measurements of unconventional reservoirs are presented and applications of the measured permeability are illustrated.