Shale Permeability Measurements on Plugs and Crushed Samples

Abstract

The economic viability of a shale play is strongly dependent on permeability which is often on the order of nanodarcies. Permeabilities are measured on core plugs or crushed samples using unsteady state techniques. However, the resultant permeabilities are the sources of controversy, because of the inconsistency in the permeability values produced with different techniques and different laboratories. In this research we evaluated the experimental factors which could influence permeability measurements with the GRI technique, and also present some permeability measurements on shale plugs. To evaluate the GRI permeability measurement technique on crushed rock, we investigated the effect of particle size, sieving of the crushed samples, pore pressure, different gases, and initial state of the measurement apparatus. The measured crushed shale permeabilities display a dependency on all these parameters. However, the particle size and the pore pressure appear to be the more important factors. This makes the reported values strongly dependent on the exact measurement procedure. This study was complemented by the imaging of crushed shale samples with a micro-CT scanner. These images showed the presence of microcracks even in samples as small as the recommended GRI particle size (~0.7mm). The permeabilities of several Devonian and Ordovician age shale plugs were measured with a pressure build up technique using nitrogen as flowing gas. A permeability decrease by an order of magnitude was generally observed for the Ordovician shale plugs with an increase of confining pressure from 1000 to 5000 psi. For the same Ordovician shale, the permeability anisotropy was found to be close to 2 orders of magnitude.The permeability of the Devonian shale plugs decreased by a maximum of 3 orders of magnitude over the range of confining pressure. For most shales, the confining pressure dependency of permeability is driven by cracks which is confirmed by a fit to Walsh's crack permeability model. However, we also noticed that it is possible to close the cracks contained in some plugs and obtain a value more representative of matrix permeability.

Gas permeability measurements from pressure pulse decay laboratory data using pseudo-pressure and pseudo-time transformations

“…Permeability estimation also requires an independent measurement of porosity, which in this case was determined using the Boyle's law, together with the initial and final pressure and the volumes of both upstream and downstream reservoirs in the experimental assembly. The permeability measurements obtained here range between 3.5 and 6 microDarcy, which may appear higher than what is typically measured for shale cores (Tinni et al 2012) and may correspond with a higher permeability zone. This is attributed to the scale of spatial variations of permeability Fig.…”

Adsorptive-mechanical properties of reconstituted granular coal: Experimental characterization and poromechanical modeling

“…The process of rock fine crushing may create microfractures in the rock matrix that were not present originally (Tinni et al, 2012). These microfractures can affect fluid uptake and also may propagate during mechanical loading.…”

“…The main advantage of crushed coal is the short time needed to equilibrate the coal with the adsorbate. Additional transient analyses on uptake dynamics are helpful to assess coal grain (matrix) permeability (Cui et al, 2009;Tinni et al, 2012).…”

Determining the permeability of tight rock with gas transient flow

“…GRI method has the advantage of shorter experimental time comparing with other methods. Unfortunately the permeability measured from crushed sample can differ by 2e3 orders of magnitude from companion intact sample (Passey et al, 2010;Tinni et al, 2012). Another limitation of this method is that the microcracks in the crushed particles essentially violate the GRI assumptions.…”