Experimental Determination of the Elastic Anisotropy of Shales: Comparison of Static and Dynamic Measurements

English Abstract:

Interest in the physical properties of unconventional reservoir rocks, which are often all characterized as ‘shales’, has accelerated due to the rapid growth in the use of hydraulic stimulation for the recovery of hydrocarbons. Similarly, it has long been known that such rocks are anisotropic and this has significant implications for proper seismic imaging. We are currently making measurements of the elastic anisotropy of such materials under the assumption they are transversely isotropic. An array of strategically oriented P- and S-wave PTZ ceramics are mounted directly to a specially machined prism of the core material in order to capture the wave speeds in a number of different directions. The measurements are conducted under hydrostatic confining pressures. Strain measurements are simultaneously obtained to allow for comparison of static to dynamic moduli. We observe high levels of anisotropy of up to 25% or more in both P- and S-wave speeds. More interestingly, we see that the physical properties parallel to layering are nearly independent of pressure while those perpendicular to layering show significant nonlinearity with pressure. Heuristically, this is likely a result of the preferential layering of both minerals and pore shapes.