Fracture permeabilityFracture permeability, governed by the Cubic LawCubic law, plays a key role in geothermal energy production by enabling high steam output. The permeability-thicknessPermeability-thickness (kh) product (kh), defined as the permeabilityPermeability multiplied by the thickness of a permeable zone, is widely used to estimate well productivity through borehole testing. Field observations suggest that a kh value of at least 10–12 m3 is required to sustain stable steam production as illustrated in Figs. 11.5 and 11.7 generating approximately 1 MWe of electricity. Such permeability is primarily associated with open fractures, typically observed in: (1) normal faultsNormal fault, (2) strike-slip and pull-apart fault systems, and (3) fractures caused by volcanic processes.