Effects of Fracture Connectivity on Rayleigh Wave Dispersion. Issue 3 (14th March 2022)

• Record Type: Journal Article
• Authors: Quiroga, Gabriel E.; Rubino, J. Germán; Solazzi, Santiago G.; Barbosa, Nicolás D.; Holliger, Klaus
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-14
• Subjects: poroelasticity -- surface waves -- fracture connectivity; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB022847
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Passive seismic characterization is an environmentally friendly method to estimate the seismic properties of the subsurface. Among its applications, we find the monitoring of geothermal reservoirs. One key characteristic to ensure a productive management of these reservoirs is the degree of fracture connectivity and its evolution, as it affects the flow of fluids within the formation. In this work, we explore the effects of fracture connectivity on Rayleigh wave velocity dispersion accounting for wave‐induced fluid pressure diffusion (FPD) effects. To this end, we consider a stratified reservoir model with a fractured water‐bearing formation. For the stochastic fracture network prevailing in this formation, we consider varying levels of fracture density and connectivity. A numerical upscaling procedure that accounts for FPD effects is employed to determine the corresponding body wave velocities. We use a Monte‐Carlo‐type approach to obtain these velocities and incorporate them in the considered fractured reservoir model to assess the sensitivity of Rayleigh wave velocity dispersion to fracture connectivity. 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 FPD. These effects cannot be accounted for by classical elastic approaches. This suggests that Rayleigh wave velocity changes, which are commonly associated with changes in fracture density, may also be related to changes in interconnectivity of pre‐existing or newly generated fractures. Plain Language Summary: Low‐intensity seismic energy generated by natural or anthropogenic sources is used to obtain a number of physical properties of the subsurface. Among a wide range of applications, this technique is increasingly employed to characterize fractured geothermal reservoirs and to monitor their evolution. The interconnectivity of fractures is a critical characteristic of such reservoirs as it enables preferential pathways for fluid flow. Conventional models for interpreting such seismic data are based on linear elasticity and cannot account for realistic effects related to the interactions of pore fluid pressure and fracture connectivity. To alleviate this problem, we employ an advanced model that accounts for these so‐called wave‐induced fluid pressure diffusion effects. We find that changes in the connectivity of fractures have a significant impact on seismic surface wave recordings. This opens the perspective of using such observations to monitor the hydraulic evolution of fractured reservoirs during successive production and stimulation cycles. Key Points: We study the impact of fracture density and connectivity changes on Rayleigh wave dispersion considering fluid pressure diffusion effects We consider a stratified reservoir model in which a water‐saturated fractured formation is represented following a poroelastic approach Fracture connectivity, so far largely ignored, has a significant impact on Rayleigh wave dispersion, comparable to that of fracture density