Abstract

Fracture intersections play a crucial role in the hydraulic connectivity of flow paths in rock, yet no technique has been developed to characterize the condition of an intersection. An approach, to elastic wave characterization of intersections, is to assume each block that composes the intersection supports a wedge wave and that these wedge waves are coupled through the points of contact along an intersection. In this paper, we demonstrate the use of group theory to predict the number of vibrational modes supported by a fracture intersection. Five predicted vibrational modes are supported by the intersection for the case of a fracture intersection formed from fractures with the same specific stiffness. Laboratory measurements on an intersection between two orthogonal fractures showed the existence of several vibrational modes that are strongly affected by the polarization of the shear wave source and the uniaxial loading conditions

1. INTRODUCTION

Throughout the subsurface of the Earth, fracture networks often control the three dimensional connectivity of the hydraulic flow paths. Fracture networks are composed of multiple fractures and fracture intersections. Although much work has been performed on elastic wave propagation across/along single fractures and sets of parallel fractures [1-4], the effect of intersections on elastic waves has been largely ignored or assumed to have little influence [5].

Typically, single and parallel sets of fractures are characterized by their mechanical properties, e.g. specific stiffness, which in turn provides information about the fracture topology, especially under stress [6-7]. These mechanical properties are linked to the hydraulic response of a fracture, i.e., linked to the spatial distributions of apertures and contacts within the fracture [8-11].

Here, we present results from a group theory analysis on an orthogonal fracture intersection under uniform loading conditions and fracture properties, i.e., a highly symmetric intersection. Several vibrational modes are predicted and described. The existence of these waves is demonstrated by using ultrasonic waves propagating along a fracture intersection on the laboratory scale.

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