Fractures in Low-Permeability Rocks: Can Poroelastic Effects Associated With Damage Zones Enhance Their Seismic Visibility?

Abstract

Fluid pressure diffusion (FPD) between a fracture and a porous permeable background can increase the normal compliance of the fracture and, thus, its reflectivity. However, many fractured environments of interest are associated with background rocks that can be regarded as largely impermeable for the the typical frequencies employed in seismic surveys. Nonetheless, there is evidence to suggest that the seemingly ubiquitous presence of damaged zones (DZs) associated with fractures may provide the necessary hydraulic communication between fractures and their immediate surroundings for FPD to occur. Here, we assess the pertinence of this phenomenon. To this end, we consider a 1D elastic-poroelastic model, which comprises a poroelastic system consisting of a fracture embedded in adjacent DZ layers. This system is enclosed in an impermeable background represented by two elastic half-spaces. We calculate the frequency-dependent P-wave reflectivity at normal incidence at the background-DZ interface for different permeabilities, thicknesses, and porosities of the DZ. We also evaluate the corresponding normal fracture compliance. Our results show that, when accounting for the presence of a DZ surrounding an individual fracture, FPD effects between these regions induce a higher seismic reflectivity and a higher normal compliance compared to that of a hydraulically isolated fracture. This, in turn, implies that, even in largely impermeable environments, the seismic visibility of fractures can be enhanced through FPD enabled by the presence of DZs.