While it is well appreciated that releasing stress from samples recovered from the subsurface must result in fracture relaxation and dilation, the impact of this on permeability is not well understood. A triaxial direct-shear experiment was performed to evaluate the impact of changing stress conditions on the permeability of a Utica shale specimen. The intact specimen was equilibrated at 22.2 MPa isotropic stress and 23°C with pre-fracture permeability<0.03 mD. The specimen was fractured at 105 MPa direct-shear stress, as identified by stress drop and simultaneous x-ray imaging but with negligible effect on permeability. Subsequent reduction of isotropic stress to 7 and 4 MPa resulted in a slight increase in permeability to 0.12 mD. The specimen was then removed from the triaxial system but was preserved within its Teflon sleeve. Reinsertion into the triaxial device and retesting yielded permeability of 3.0 mD at 2 MPa that then declined to 0.35 mD at 8 MPa. Concurrent x-ray tomograms showed significant increased fracture apertures after the initial removal of stress and these remained open during stress-reapplication. The release and reapplication of stress resulted in a fracture system that did not reproduce the original, in situ fracture properties.
Stress Cycling and Fracture Permeability of Utica Shale Using Triaxial Direct-Shear With X-Ray Tomography
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Carey, J. W., Frash, L. P., Ickes, T., and H. S. Viswanathan. "Stress Cycling and Fracture Permeability of Utica Shale Using Triaxial Direct-Shear With X-Ray Tomography." Paper presented at the 51st U.S. Rock Mechanics/Geomechanics Symposium, San Francisco, California, USA, June 2017.
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