Effect of pore pressure magnitude on the frictional properties and permeability evolution of fractures in schist

Fluid injection-triggered seismicity has increased dramatically over the last decade with elevated pore fluid pressures acting as a prime culprit. Thus, understanding the effect of pore fluid pressure on the mechanical and hydrologic behavior of fractures and faults will illuminate the contributing and dominant physical processes. We present concurrent measurements of shear displacement and flow to quantify the evolution of frictional strength, stability and permeability of schist during the full seismic cycle. We use a miniature double direct shear (mini-DDS) apparatus to conduct velocity stepping (VS for stability) and slide-hold-slide (SHS for frictional healing). Our results demonstrate that increasing pore fluid pressures can stabilize frictional slip under otherwise invariant effective stresses. This implies that elevated pressures favor stable slip as a material characteristic even in the absence of decreasing critical fault stiffness (thereby increasing stability) as a result of decreased effective stress. However, the magnitude of pore pressure does not control permeability evolution during velocity steps as pore pressure does not control aperture dilation/compaction for an invariant effective normal stress. During SHS tests, it is shown that the magnitude of normalized permeability change increases with hold time and that the rate of permeability change generally decreases with the increment of pore fluid pressure, suggesting that high fluid pressures may limit permeability change during interseismic response, although creep response may still dominate over the long term.