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

• Injection induced seismicity is simulated using PFC2D.
• Cyclic injection mitigates the effect of large magnitude induced seismicity.
• Optimized fracture pattern can be obtained by cyclic injection.

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, re-activation 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.

Naturally fractured reservoir is stimulated by the fluid injection with continuous and sequentially increasing injection rate. Moment magnitudes (Mw) of the induced events are plotted with time, which shows high occurrence rate at the time of peak injection rate and shortly after bleed-off. Post shut-in large magnitude events are simulated. Induced events with Mw < 0.9 (white stars) and occurring before shut-in (0–6.5 h) are confined within the inner blue contour which represents 0.1 MPa fluid pressure iso-line at the time of shut-in. Induced events with Mw > 0.9 (black stars) and occurring after shut-in are located mostly at the outer-rim of the event cloud and a few far distance away from the injection point and at pre-existing joints.Figure optionsDownload as PowerPoint slide