| Abstract |
Dynamic stresses imposed by the waves of distant earthquakes are known to trigger numerous hydrological phenomena in the upper crust. Many laboratory and field studies have shown that fracture permeability enhancement due to seismically-induced colloidal mobilization at the fracture scale can produce pore fluid pressure redistribution at the reservoir scale. Geothermal systems are particularly susceptible to this mechanism due to the constant precipitation of minerals and sediments from geothermal fluids that eventually leads to the clogging of open fractures. In this scenario, fracture unclogging has been evoked to explain the pore fluid pressure redistribution that can lead to frictional instabilities and seismicity as well as a phenomenon inhibiting the formation of obstacles to fluid flow in regions with frequent seismic activity. The evidence suggests that seismically-induced viscous shear stresses in the fluid saturating a fracture in the range of tenths of Pascals is sufficient to initiate colloidal mobilization. In this work, we numerically assess the development of viscous shear stress in the fluid saturating a system of fractures due to the action of seismic body waves. We perform a sensitivity analysis of the wave-induced viscous shear stresses in terms of fluid, fracture, and background rock physical properties as well as seismic wave characteristics. Our results show that seismically induced viscous shearing in the order of those initiating fracture unclogging are plausible for typical seismic strains. For an anisotropic distribution of fractures, viscous shearing increases with frequency and the imposed seismic strain and is extremely dependent on the mode and direction of wave propagation. This implies that, for seismic waves of similar strain amplitudes and frequencies, directivity is a key factor controlling the occurrence of dynamic triggering events. For similar seismic wave characteristics, larger viscous shearing is expected for more viscous fluids, stiffer background rock, and thinner fractures. The latter is particularly important as we found that spatially heterogeneous fracture apertures can produce locally enhanced viscous shear stress. This points out that regions where pore fluids conditions change or with different fracture properties may be more sensitive to fracture unclogging, and consequently permeability enhancement, than others. |