DEVELOPING AND SUSTAINING FRACTURED ENERGY SYSTEMS: CHALLENGES AND OPPORTUNITIES

Natural geothermal systems are characterized by a source of heat, fluid to transfer heat from the rocks and a network of permeable fractures. Although worldwide use of geothermal energy is increasing, natural geothermal resources provide less than one percent of the global energy production. Nevertheless, there is abundant and compelling evidence that the geothermal resource base is enormous and virtually inexhaustible.

Hot, low permeability rocks with temperatures suitable for electricity generation are present at accessible depths throughout the world and offer the potential for large-scale development. However, the lack of natural interconnected fracture networks has proven to be a major obstacle to heat extraction. Many of the technologies currently used to create fracture systems in geothermal environments were adapted from those applied to low permeability oil and gas fields. Resource extraction in both hydrocarbon and geothermal environments has relied on the creation of new fractures as well as reactivation of existing fractures through hydraulic stimulation. Although these environments share many similarities and challenges, there are significant differences in rock type, temperature and resource management. Unlike oil and gas reservoirs, Enhanced Geothermal System (EGS) reservoirs require fracture networks capable of sustaining long term heat transfer for periods of 20-30 years. Despite progress in geothermal drilling, reservoir characterization and stimulation, the development of conductive, interconnected fracture networks at the reservoir scale has yet to be realized.

Critical needs confronting EGS development include: 1) effective stimulation strategies for fracture formation in hot (>200^oC) crystalline rock; 2) identification and imaging of the interconnected fracture networks; 3) demonstration of long-term reservoir sustainability at commercial flow rates; 4) management of the in-situ stress regime; 5) high-temperature drilling tools and zonal isolation technologies; 6) management of induced seismicity; and 7) predictive thermo-mechanical-chemical models. The US Department of Energy initiative, Frontier Observatory for Research in Geothermal Energy (FORGE) will provide a dedicated subsurface laboratory for the in-situ evaluation of EGS technologies.