The influence of particle size, microfractures, and pressure decay on measuring the permeability of crushed shale samples

Analysis of early hyperbolic curves yields permeability values one to two orders of magnitude greater than whole curve analysis. The hyperbolic pressure decay segment appears to be influenced by microfractures and other large pores near the surface of samples, whereas the late time segment and whole curve correlate more strongly with the microporous to nanoporous rock matrix. Also, permeability values derived from whole curve analysis are sensitive to measurement duration, and different values are obtained when permeability is determined from different time windows. SEM images of all particle sizes analyzed reveal microfractures with diameters ranging from 60 to 1020 nm, but no correlation was found between microfracture aperture and particle size. The optimal sample mass used in our shale permeameter is 50-100 g, which facilitates resolution of the major elements of the decay curve. Optimal particle sizes are between 1.0 and 1.4 mm.