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The occurrence of induced seismic events during hydraulic fracturing of reservoirs to enhance permeability is an unavoidable process. Due to the increased public concern with respect to the risks imposed by induced seismicity, however, the development of a soft stimulation method is needed creating higher permeability with less induced seismicity. We use a discrete element model of naturally fractured rock with pore fluid flow algorithm in order to analyse two scenarios of high-pressure fluid injection (hydraulic fracturing) at depth and associated induced seismicity. The ratio of pumped-in energy to released seismic energy is in agreement with field data. Our results suggest that cyclic reservoir treatment is a safer alternative to conventional hydraulic fracture stimulation as both, the total number of induced events as well as the occurrence of larger magnitude events are lowered. This work is motivated by results of laboratory triaxial indenter tests on granite rock samples where continuous loading leads to a wide fracture process zone while cyclic treatment with frequent starting and stopping of loading fatigues the rock, resulting in smaller damage volume and more persistent fracture growth.

This numerical study investigates hydraulic fracturing and induced seismicity in intact and fractured reservoirs under anisotropic in situ stress using hydro-mechanical coupled discrete particles joints model. A 2 km 2 km reservoir model with granitic rock and joints properties is constructed. Various injection scenarios are tested which involve continuous and cyclic styles of pressure controlled and flow rate controlled injections. Results are compared which include: spatial and temporal evolution of induced seismic events in relation with fluid pressure distribution, moment magnitudes of the induced events, occurrence of post-shut-in large magnitude events, etc. Several field observations on induced seismicity phenomena are simulated which include creation of new fractures, reactivation of the pre-existing joints, post-shut-in seismicity and large magnitude event with non-double-couple source, Kaiser phenomenon, moment magnitude vs. frequency distribution of the induced events following the Gutenberg-Richter law, etc. Cyclic injection results in larger volume of injected fluid but less number of total events and larger magnitude events; hence less seismic energy radiated by the induced events, slower relaxation of the fluid pressure after shut-in, longer and thinner propagated fractures with larger fluid saturated area. The major conclusions of this study are that the presented modeling is capable of simulating induced seismicity phenomena in Enhanced Geothermal System and fluid injection in fractured reservoirs in cyclic way has potential in mitigating the effects of larger magnitude induced events.

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Fracture and friction behaviors of Aue granite (Erzgebirge, Germany) under confined asymmetric load are simulated by using the discrete element method. Clumped particle model is designed to account for stress heterogeneities under compressive load in the rock microstructure which affect both, fracture and friction properties. In contrast to conventional circular bonded particle assembly, clumped particle models consist of groups of neighboring individual particles

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