Natural fractures are planar discontinuities that may contribute significantly to a formation’s permeability in either a positive (if they are open) or negative way (if they are cemented). Fractures may result from structural deformation associated with folding or faulting, from shrinkage because of dewatering, dessication, or cooling, pore-fluid pressure that exceeds rock strength, and other causes. They often have the same orientation and hence may result in strong anisotropy of permeability and other properties. Detecting fractures is an objective of S-wave birefringence studies and of imaging of the borehole wall (see Figure I-1). The storage capacity in fractures (fracture porosity), a type of secondary porosity, is usually very small. Fractures can be induced by hydraulic fracturing (hydrofracing; q.v.) to improve fluid flow into a wellbore. See Aguilera (1998).