*2.6. Fluid Flow*

Fluids transport and concentrate all resources. Understanding and identifying the pathways of flow is perhaps the most important challenge for the explorationist. Two fundamental questions arise: First: how are the flow pathways likely to change with the distance of flow? Second: particularly if the pathways are increasingly concentrated, where are they located in a specific subsurface volume? Malin et al. [11] address the first question by showing that two factors control whether flow will concentrate with distance traveled: the spread (standard deviation) of log permeability about its mean, and whether permeability is distributed in a scale-invariant fashion. If the spread is substantial and the distribution scale invariant, flow will become increasingly concentrated with distance traveled. The spread of permeability is notoriously large, and fractures are scale-invariant in their distribution. This is a matter of observation. The reason is probably that the Earth's crust is in a state of near failure and scale-invariant systems are commonly observed near the critical point of failure. Knowing that flow is likely concentrated in any particular volume is a good perspective, but of little use if the actual flow channels in the volume cannot be located.

Sicking and Malin [12] describe how subsurface flow channels can be located from the seismic energy emitted by Krauklis waves trapped on water-filled fractures. The contrasting seismic wave velocity between water in the fractures and the surrounding rock traps seismic energy, and seismic waves bounce back and forth from the ends of the fracture. Episodes of harmonic humming, often minutes or more in duration, can be extracted from what is usually considered seismic noise using specialized processing techniques. Permeable channels are presumed to be where fluid-filled fractures are most abundant. A substantial number of field studies support this hypothesis. Necessary analysis can be carried out by processing existing 3D seismic data.
