Effects of fracture connectivity on Rayleigh wave velocity dispersion

The use of passive seismic techniques to monitor geothermal reservoirs allows to assess the risks associated with their exploitation and stimulation. One key characteristic of geothermal reservoirs is the degree of fracture connectivity and its evolution. The reason for this is that changes in the interconnectivity of the prevailing fractures affect the permeability and, thus, the productivity of the system. An increasing number of studies indicates that the Rayleigh wave velocity can be sensitive to changes in the mechanical and hydraulic properties of geothermal reservoirs. In this work, we explore the effects of fracture connectivity on Rayleigh wave velocity dispersion accounting for wave-induced fluid pressure diffusion effects. To this end, we consider a 1D layered model consisting of a surficial sandstone formation overlying a fractured and water-saturated granitic layer, which, in turn, is underlain by a compact granitic half-space. For the stochastic fracture network prevailing in the upper granitic layer, we consider varying levels of fracture connectivity, ranging from entirely unconnected to fully interconnected. We use an upscaling approach based on Biot’s poroelasticity theory to determine the effective properties associated with these scenarios. This procedure allows to obtain the frequency-dependent seismic body wave velocities accounting for fluid pressure diffusion effects. Finally, using these parameters, we compute the corresponding Rayleigh wave velocity dispersion. Our results show that Rayleigh wave phase and group velocities exhibit a significant sensitivity to the degree of fracture connectivity, which is mainly due to a reduction of the stiffening effect of the fluid residing in connected fractures in response to wave-induced fluid pressure diffusion. This suggests that time-lapse observations of Rayleigh wave velocity changes, which so far are commonly associated with changes in the fracture density, could also be related to changes in the interconnectivity of pre-existing fractures.