Hydraulic stimulation of tight geothermal reservoirs

Abstract

To extend geothermal-energy power generation from hydrothermal systems to places without anomalies in the geothermal gradient, fractures of low-permeability rock- typically at 4-6 km depth to find <150°C - should be stimulated to create enhanced geothermal systems (EGS). However, most of the attempts to establish full-scale EGSs have resulted in high induced seismicity as a result of hydraulic stimulation, leading to project cancellation in some occasions. This has created a lot of insecurity in further investments in this kind of system. To achieve widespread deployment of EGSs alongside existing hydrothermal power plants, we need a thorough understanding of the effect of different hydraulic stimulation strategies on both permeability enhancement and induced seismicity. In this chapter, we compared three stimulation strategies in detail - constant rate, step rate and cyclic injection, for a range of dilatancy angles of the stimulated fracture. We find that the extent of fracture reactivation and pressure buildup depends on the injection scheme as well as the dilatancy angle. For a given dilatancy angle, we find that the cyclic-injection scheme provides the highest permeability enhancement as well as the highest slip, whereas both of them are observed to be the lowest for the step-rate injection.