This study used two-dimensional numerical and analytical modeling to investigate induced slip and seismicity during hydraulic stimulation in Enhanced Geothermal Systems (EGS). EGS stimulation is performed by injecting water at high pressure into a typically low permeability, fractured, crystalline rock. The increase in fluid pressure causes slip on preexisting fractures, enhancing their permeability. This investigation gives preliminary results from an ongoing project to build an EGS stimulation model. Fluid flow was simulated in a discrete fracture network. The displacement discontinuity method was used to calculate stresses that resulted from fracture slip. The model mimicked what happens during a seismic event because stresses generated by slip on one patch of fracture could induce slip on other patches and lead to a chain reaction. The model was not a true "earthquake" model because it used a simplified representation of friction and neglected dynamic effects. However, the model gave results that captured the first order effects of how pressure and stress distribution affect the tendency for seismic events to occur. Based on the results of the numerical simulations, it is argued that a major mechanism of stimulation is a process in which slip on stimulated regions of fractures induces stress that causes slip on unstimulated regions of fractures. An alternative mechanism would be pressure diffusion into unstimulated regions of fractures. An analytical expression was derived for stimulation of a single, isolated, preexisting fracture that matched the numerical results. Numerical modeling of both a single, isolated fracture and a network of fractures was carried out to investigate the effect of various injection strategies on the magnitude of induced seismicity. Decreasing injection pressure over time and immediately putting a well on production after the cessation of injection were identified as strategies that could reduce the number and magnitude of seismic events.
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