Impact of connected secondary fractures on the seismic reflectivity of large primary fractures

Abstract

Large fractures tend to dominate the hydraulic and mechanical properties of fracture networks and, consequently, of fractured rock masses. Hence, their characterization is of primary interest in a variety of applications. Seismic reflection is a non-invasive tool that allows for the characterization of large fractures due to the high mechanical contrast that they commonly present with respect to their embedding background. Smaller secondary fractures are generally connected to the large primary fractures, thus, creating a network for fluid flow. This, in turn, allows for wave-induced fluid pressure diffusion (FPD) to prevail between the intersecting secondary and the large primary fractures, which may have a pronounced, albeit as of yet unexplored, impact on the compliance and the reflectivity of the large primary fractures. To explore this fundamental problem, we investigate the impact that such FPD process has on the compliance and the reflectivity of large primary fractures. To this end, we consider several canonical models, which comprise an infinite horizontal primary fracture connected to vertical smaller secondary fractures embedded in a background deemed impermeable throughout the seismic frequency range. The individual models differ only with regard to the secondar y fracture proper ties (e.g. length, aper ture, mechanical moduli). For comparison, we also consider a reference model that disregards the secondary fractures. To constrain the effect of FPD on the primary fracture, we e v aluate its ef fecti ve seismic response by means of vertical compressional oscillatory tests over samples of the aforementioned models, to subsequently perform averaging of the vertical components of stress and strain. We use these results to estimate first the P -wave modulus and then to compute the normal compliance and reflectivity of the primary fracture. Our results show that both the compliance and reflectivity of the primary fracture increase by more than one order-of-magnitude with respect to the reference model. These findings point to a very significant enhancement of the seismic visibility of large fractures due to FPD with connected secondary ones.