Stimulation mechanism and design of enhanced geothermal systems: A comprehensive review

Abstract By utilizing stimulation technologies, the enhanced geothermal system has emerged as a highly promising technology in harvesting geothermal energy. In this work, we provide a comprehensive review of state-of-the-art research and technologies of fracture stimulation in enhanced geothermal systems, including hydraulic stimulation mechanisms and design strategies. The understanding of hydraulic stimulation mechanisms is critical to the successful development of enhanced geothermal systems. Therefore, we summarized four major types of hydraulic stimulation mechanisms, and compared their impacts on the performance of enhanced geothermal systems. Next, a thorough review of 19 representative enhanced geothermal systems projects across the world is presented, in which the hydraulic stimulation design strategies are analyzed in depth. In the comparison of the projects, we mainly focus on the investigation of the impacts of in-situ geomechanical conditions and reservoir responses during hydraulic stimulation treatments. Based on the findings from the comparison, we proposed a series of feasible criteria for hydraulic stimulation design. Moreover, we reviewed the most recent advances in measurement tools and hydraulic stimulation facilities for practical hydraulic stimulation operations. We particularly clarified the difference between hydraulic stimulation operations in geothermal reservoirs and those in oil/gas fields. We aim to comprehensively answer a critical question: which techniques from the petroleum industry can be applied to the geothermal industry and which cannot? Overall, through this work, we intend to provide useful guidelines for both theoretical research and field operations of enhanced geothermal systems.