Changes in Fracture Compliance Due to Roughness

Abstract:

Rock fractures are a source of extra compliance, and the effect of fractures on seismic wave propagation can be quantified in terms of the normal and shear fracture compliances. However, fractures are usually assumed to be smooth discontinuities with no preferential orientation for slippage. This assumption rarely matches with outcrop studies. Irregularities always exist on the fracture faces in the form of hackles, slickensides, or gouges. These features may facilitate movement in one direction while oppose shear deformation in the other direction due to the saw-tooth structure of irregularities. This direction dependence of the shear compliance of the fracture planes may affect the measured ratio of normal to shear fracture compliance for different locations along the fracture. Furthermore, these effects may contribute to the pressure dependence of fracture compliance, because fracture opening or shear sliding would change the number of asperities in contact and, consequently, affect the ratio of the normal to shear compliance. Here, we use numerical modeling to investigate the change in normal and shear compliance caused by the presence of slickensides. The effect of the hackles’ geometry, the friction coefficient between the fracture surfaces, and the offset between the fracture faces will be discussed in this presentation. Our analysis shows that even in the case of low asperity angles or small offsets between the fracture faces, the fracture compliance ratio could be greater than one, in agreement with several field observations available in the literature.