Numerical investigation on optimized stimulation of intact and naturally fractured deep geothermal reservoirs using hydro-mechanical coupled discrete particles joints model

Abstract

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.

Analysis of induced seismicity in geothermal reservoirs – An overview

“…Due to poroelastic effects, it is impossible to distinguish between the two types of anisotropy from geothermal seismicity alone. In addition, Yoon et al (2014) using hydromechanical coupled discrete fracture network models demonstrated that also the injection scenario affects the shape of the seismic cloud. A clear understanding of the physical processes operating at the border between seismically inactive and seismically active zones (asperity, hydraulic barrier) needs to be developed.…”

“…A dynamic fracture-growth approach is the discrete fracture network model by Yoon et al (2014) (see Fig. 8).…”

“…At this stage, we rely on the synthetic induced seismicity catalogues from Yoon et al (2013Yoon et al ( , 2014 because their hydromechanically coupled model approach using dynamic fracture growth and interaction principles (Fig. 8) captures basic physical principles of the natural fracturing process operating in situ during the development of a geothermal field, and in addition reproduce main features of observed seismic catalogues monitored in EGS stimulation (post shut-in larger magnitude events, development of induced seismic cloud in the direction of maximum principal stress, magnitude-frequency distributions of events for different injection scenarios).…”

Discrete element modeling of fluid injection–induced seismicity and activation of nearby fault

“…Dissimilar to recent studies by Zang et al (2013) and Yoon et al (2014Yoon et al ( , 2015aYoon et al ( , 2015b, the model investigated in this study contains a large fault zone. Not only the seismic events computed from bond breakages, but also the seismic events by fracture slip are taken into account and compared with field observations.…”

“…Flow of viscous fluid in a bonded particle assembly is modeled and fluid volume and pressure driven breakages of bonds in mode I and mode II are simulated Zang et al 2013;Yoon et al 2014). Fluid flow is simulated by assuming that each bonded contact is a flow channel (Fig.…”