We performed laboratory experiments on synthetic orthogonal fractures to determine the effect of intersections on fracture interface waves. A seismic array was used to propagate compressional and shear waves along fracture planes as well as along an intersection. Measurements were made for a range of normal stresses (1.4 MPa to 14 MPa). Intersections quench bulk shear waves and produce interface waves independent of the polarization of the shear wave source. Furthermore, these intersection waves are highly sensitive to stress concentrations along the intersection.
A major difference between working with single fractures and orthogonal fracture networks is the existence of fracture intersections. Fracture intersections enable the formation of three-dimensional dominant flow paths and they act as potential sources of additional seismic wave scattering not observed for single fractures or parallel sets of fractures. A main challenge in working with orthogonal fracture sets is how to determine the connectivity or properties of fracture intersections.
Fracture intersections act as either barriers to flow or paths of high conductivity and are difficult to characterize with non-invasive or destructive measurements. The goal of this study is to determine if the intersection between two fractures exhibited a seismic response that differs from that measured along fracture planes. In this paper, we explore the potential use of fracture interface waves to interrogate the properties of fracture intersections and fractures.
Fracture interface waves are generalized Rayleigh waves that propagate along fractures [1-5]. The existence and velocity of fracture interface waves depend on the normal and shear stiffness of the fracture, and on the polarization of the shear wave. The velocity of these waves ranges from the Rayleigh wave velocity for a free surface to the bulk shear wave velocity for non-effervescent interface wave modes. As mentioned, the existence of fracture interface waves depends on the polarization of the shear components of the excitation source , i.e. interface waves exist when the shear-wave polarization is perpendicular to the fracture plane. However, at an intersection, fracture interface waves should always exist for both parallel and perpendicular shear wave polarizations (relative to one of the intersecting fractures) because each component is perpendicular to one of the orthogonal fractures. In this paper, we demonstrate that interface waves along fracture intersections always exist and the waves are sensitive to stress concentrations along the intersection.
Experiments were performed on aluminum samples measuring approximately 100 mm by 150 mm by 150 mm. Aluminum was used to ensure that the effects observed were from the fractures and not the background matrix. Two samples were used in this study: (1) an intact piece of aluminum that was used as a standard; and (2) a “fracture” sample containing two intersecting fractures (Figure 1). Intersecting fractures in the fracture sample were produced by quartering a solid piece of aluminum. After quartering, the fracture sample was machined to the same external dimensions as the intact specimen. The fracture surfaces were smooth, i.e. no apparent roughness is visible to the naked eye.