The Compliance of Macrofractures

Numerical modelling studies of wave propagation through fractured media which require the normal and shear compliance of individual fractures to be specified are increasingly common, whilst it is generally acknowledged that laboratory and field fracture compliance data are extremely limited. Using a combination of analytical and numerical modelling it is shown that macrofractures, defined as having lengths of approximately a metre to a few tens of metres, might be expected to have compliance values of order no greater than 10-11 m/Pa. This would result in P wave anisotropy less than or equal to 1% ( =0.01). The normal compliance of water filled fractures is predicted to be an order of magnitude less than gas filled fractures. However, the compliance of water and gas filled fractures can be similar at exploration seismic frequencies if the rock matrix is sufficiently porous and permeable to enable fluid to diffuse in and out of the matrix material. The need for additional field estimates of fracture compliance is emphasised.