Sensitivity of seismic attenuation and dispersion to dynamic elastic interactions of connected fractures: Quasi-static finite element modeling study

Abstract

Prediction of seismic attenuation and dispersion that are inherently sensitive to hydraulic and elastic properties of the medium of interest in the presence of mesoscopic fractures and pores, is of great interest in the characterization of fractured formations. This has been very difficult, however, considering that stress interactions between fractures and pores, related to their spatial distributions, tend to play a crucial role on affecting overall dynamic elastic properties that are largely unexplored. We thus choose to quantitatively investigate frequency-dependent P-wave characteristics in fractured porous rocks at the scale of a representative sample using a numerical scale-up procedure via performing finite element modelling. Based on 2-D numerical quasi-static experiments, effects of fracture and fluid properties on energy dissipation in response to wave-induced fluid flow at the mesoscopic scale are quantified via solving Biot's equations of consolidation. We show that numerical results are sensitive to some key characteristics of probed synthetic rocks containing unconnected and connected fractures, demonstrating that connectivity, aperture and inclination of fractures as well as fracture infills exhibit strong impacts on the two manifestations of WIFF mechanisms in the connected scenario, and on resulting total wave attenuation and phase velocity. This, in turn, illustrates the importance of these two WIFF mechanisms in fractured rocks and thus, a deeper understanding of them may eventually allow for a better characterization of fracture systems using seismic methods. Moreover, this presented work combines rock physics predictions with seismic numerical simulations in frequency domain to illustrate the sensitivity of seismic signatures on the monitoring of an idealized geologic CO2 sequestration in fractured reservoirs. The simulation demonstrates that these two WIFF mechanisms can strongly modify seismic records and hence, indicating that incorporating the two energy dissipation mechanisms in the geophysical interpretation can potentially improving the monitoring and surveying of fluid variations in fractured formations.