Friction of Longmaxi Shale Gouges and Implications for Seismicity During Hydraulic Fracturing

Longmaxi formation shales are the major target reservoir for shale gas extraction in Sichuan Basin, southwest China. Swarms of earthquakes accompanying hydraulic fracturing are observed at depths typifying the Longmaxi formation. Mineral composition varies broadly through the stratigraphic section due to different depositional environments. The section is generally tectosilicate-poor and phyllosilicate-rich with a minor portion the converse. We measure the frictional and stability properties of shale gouges taken from the full stratigraphic section at conditions typifying the reservoir depth. Velocity-stepping experiments were performed on representative shale gouges at a confining pressure of 60 MPa, pore fluid pressure of 30 MPa, and temperature of 150 degrees C. Results show the gouges are generally frictionally strong with friction coefficients ranging between 0.50 and 0.75. Two phyllosilicate + TOC (total organic carbon)-poor gouges exhibited higher frictional strength and velocity weakening, capable of potentially unstable fault slip, while only velocity strengthening was observed for the remaining phyllosilicate + TOC-rich gouges. These results confirm that the frictional and stability properties are mainly controlled by phyllosilicate + TOC content. Elevating the temperature further weakens the gouges and drives it toward velocity weakening. The presence of observed seismicity in a majority of velocity-strengthening materials suggests the importance of the velocity-weakening materials. We suggest a model where seismicity is triggered when pore fluid pressures drive aseismic slip and triggers seismic slip on adjacent faults in the same formation and distant faults in the formations above/below. The effect of pore pressure transients within low-permeability shale gouges is incorporated. Our results highlight the importance of understanding mechanisms of induced earthquakes and characterizing fault properties prior to hydraulic fracturing.