Release the Kraken – How Geothermal Feedback Forms Fractures

Abstract

Geothermal systems are more than just water circulating in fractures that happen to be open and connected near a heat source. Multiple interacting feedback loops convert mechanical, thermal, and chemical energy during water circulation into changes that widen and extend or narrow and seal fractures and fracture networks during natural system evolution and exploitation. Positive fracture feedback can result in large, rapid changes in permeability. Activation thresholds and negative or competing positive feedback processes form important controls on fracture formation, opening, and sealing patterns. Taken together geothermal feedback processes involving rapid dissipation from multiple energy sources modify fracture permeability in self-organized systems. These systems form their own emergent permeability patterns beyond what might occur in areas of water circulation in fractures formed simply from faulting or jointing. Some of these feedback mechanisms have been interpreted to control important permeability patterns and seals in exploited systems. In several cases feedback mechanisms have been artificially stimulated to result in permeability changes during field operations. Similar feedback processes are also likely to occur outside of self-organized geothermal systems in areas of faulting and water circulation in hot rock and may be important in locally modifying upper crustal permeability even if they do not create fully developed, self-organized hydrothermal permeability patterns or exploitable systems. Better understanding of these feedback processes might help us create new geothermal systems by stimulating the release and conversion of natural-stored potential energies to fracture formation and modification where we can exceed activation thresholds. This is likely to require an abundant water supply and initially available or attainable vertically-extensive permeability (even if it starts out sub-commercial). Stimulation would then apply water-frack methods that have been successfully tested in near-field EGS experiments to greenfield or distal near-field areas with concentrations of thermal and mechanical energy in active fault irregularities.