Hydraulic stimulation strategies in enhanced geothermal systems (EGS) : a review

In enhanced geothermal systems (EGS), the natural permeability of deep rocks is normally not high enough and needs to be increased. Permeability increase can be achieved through various stimulation methods, such as hydraulic, chemical, and thermal stimulation. Among these, hydraulic stimulation is the most commonly used technique to increase both reservoir permeability and the specific area for heat exchange. A comprehensive understanding of the underlying processes towards an optimization of hydraulic stimulation performance while minimizing the potential of unwanted induced seismicity is a critical prerequisite for a successful development of any EGS site. In this paper, we review the hydraulic stimulation strategies that have been developed and implemented for EGS. We begin with a description of the underlying mechanisms through which the permeability and heat exchange area increases are achieved. We then discuss the mechanisms of fluid injection-induced seismicity during and after a hydraulic stimulation operation. After that, alternative hydraulic stimulation strategies, namely conventional hydraulic stimulation, multi-stage fracturing, and cyclic soft stimulation, are reviewed based on current research in theoretical studies as well as, laboratory, and in-situ field experiments. Finally, some representative EGS projects are reviewed, focusing on fluid injection strategies, seismic responses, and reservoir permeability enhancement performance. The review shows the importance and need of (a) a comprehensive geological characterization of the natural fracture system including the nearby fault zones as well as the in-situ stress conditions, prior to the development of the site, (b) a proper design of the well arrangement, such as the positioning of the injection and production wells, and (c) the selection of an appropriate fluid injection strategy for the system at hand