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 the Sichuan Basin, southwest China. Swarms of earthquakes accompanying hydraulic fracturing are observed at depths typified by 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 (~5 wt.%) the converse. We measure the frictional and stability properties of shale gouges taken from the full stratigraphic section at hydrothermal conditions. Velocity-stepping experiments were performed on representative shale gouges at σc = 60 MPa, Pf = 30 MPa and T =150 ℃. Results show that the gouges are generally frictionally strong with friction coefficients spanning a range of 0.50-0.75. Two phyllosilicate+TOC-poor gouges exhibited higher frictional strength and velocity weakening behavior, capable of potentially unstable fault slips, while only ve...