Relationships between mechanical and transport properties in Marcellus shale

We explore relationships among bulk modulus, crack density, and permeability through repetitive loading of Marcellus shale. Cumulative cyclic stressing (22–26 MPa with confinement of 24 MPa) is applied at a frequency of 0.05 Hz over 100,000 cycles. Changes in acoustic velocities are used to follow changes in dynamic bulk modulus, Poisson ratio, and crack density and to correlate these with bedding-parallel measurements of methane permeability. The shale is represented as an orthotropic elastic medium containing a dominant, noninteracting fracture set separated by thin laminae. An effective continuum model links permeability evolution to the evolution of the bulk modulus and crack density. Bulk modulus is linearly related to crack density by a scaling parameter representing rock fabric and fracture geometry. The Poisson ratio and bulk modulus of the intact, uncracked shale are approximated from our data. We propose a method for tracking permeability evolution of finely laminated shales using acoustic waves. Plain Language Summary: Shales are increasingly important rocks for oil and gas production and carbon sequestration. Due to their tight and often disconnected internal flow paths, it is often difficult to track changes in permeability—how easily a fluid flows through a rock—that arise within the shale itself. This study shows that for some shales sound waves that can originate on the surface or within bore holes can be used to track changes in permeability. Being able to monitor permeability during liquid or gas production/injection will provide useful information to scientists and engineers for a variety of applications.