Frictional and sealing behavior of Marcellus shale

The slip behavior of major faults depends largely on the frictional and hydrologic properties of fault gouge. We report on laboratory experiments designed to measure the strength, friction constitutive properties, healing and creep rates and permeability evolution of a suite of saturated gouges in velocity stepping and slide-hold-slide tests. Gouge samples include ground Marcellus shale, quartz, and a 50:50 mixture of the two. Friction measurements indicate that Marcellus gouge is weak (µ<0.47) and quartz gouge is strong (0.56 <µ<0.61), with 50:50 mixtures anomalously potentially stronger than quartz gouge (0.57<µ<0.61). At constant confining stress of 3MPa and at a variety of pore pressures (0.036MPa, 0.6MPa, 0.8MPa) both Marcellus and the 50:50 mixtures show velocity strengthening behavior for sliding velocities in the range 3 to 10µm/s. Quartz shows velocity strengthening behavior in velocity up-step experiments but velocity weakening behavior for velocity down-steps. Marcellus has the lowest frictional healing rate of 0.0076 that is lower than that for the 50:50 mixture (0.0116) with quartz the highest (0.0154). The frictional creep rates are in the order of Marcellus (0.0522) then 50:50 mixtures (0.0427) and 100% quartz (0.0154). An instant permeability jump occurs with a velocity change followed by a relatively constant residual permeability for Marcellus gouges and 50:50 mixtures, while the permeability of quartz decreases continually independent of the velocity change. Permeability of quartz increases during hold periods and permeability decreases upon reactivation, while for Marcellus gouges and 50:50 mixtures, an instant jump in permeability upon reactivation, followed by a dramatic drop during the slide period, with permeability near constant during holds.